Rohit Wadhwani
Industrial & Manufacturing Engineering NED UNIVERSITY OF ENGINEERING AND TECHNOLOGY.
HYUNDAI ELEVATORS HEAD OFFICE 205, AMBER ESTATE, SHAHRAH-E-FAISAL KARACHI 021-34320601 ENGR. SURESH KUMAR
INTERNSHIP REPORT
HOW ELEVATORS WORK
The most popular elevator design is the roped elevator. In roped elevators, the car is raised and lowered by traction steel ropes rather than pushed from below. The ropes are attached to the elevator car, and looped around a sheave (3). A sheave is just a pulley with a groove around the circumference. The sheave grips the hoist ropes, so when you rotate the sheave, the ropes move too. The sheave is connected to an electric motor (2). When the motor turns one way, the sheave raises the elevator; when the motor turns the other way, the sheave lowers the elevator. In gearless elevators, the motor rotates the sheaves directly. In geared elevators, the motor turns a gear train that rotates the sheave. Typically, the sheave, the motor and the control system (1) are all housed in a machine room above the elevator shaft.
The ropes that lift the car are also connected to a counterweight (4), which hangs on the other side of the sheave. The counterweight weighs about the same as the car filled to 40-percent capacity. In other words, when the car is 40 percent full (an average amount), the counterweight and the car are perfectly balanced. The purpose of this balance is to conserve energy. With equal loads on each side of the sheave, it only takes a little bit of force to tip the balance one way or the other. Basically, the motor only has to overcome friction -- the weight on the other side does most of the work. To put it another way, the balance maintains a near constant potential energy level in the system as a whole. Using up the potential energy in the elevator car (letting it descend to the ground) builds up the potential energy in
the weight (the weight raises to the top of the shaft). The same thing happens in reverse when the elevator goes up. The system is just like a see-saw that has an equally heavy kid on each end. Both the elevator car and the counterweight ride on guide rails (5) along the sides of the elevator shaft. The rails keep the car and counterweight from swaying back and forth, and they also work with the safety system to stop the car in an emergency. Roped elevators are much more versatile than hydraulic elevators, as well as more efficient. Typically, they also have more safety systems. Elevators are built with several redundant safety systems that keep them in position. The first line of defense is the rope system itself. Each elevator rope is made from several lengths of steel material wound around one another. With this sturdy structure, one rope can support the weight of the elevator car and the counterweight on its own. But elevators are built with multiple ropes (between four and eight, typically). In the unlikely event that one of the ropes snaps, the rest will hold the elevator up. Even if all of the ropes were to break, or the sheave systems were to release them, it is unlikely that an elevator car would fall to the bottom of the shaft. Roped elevator cars have built-in braking systems, or safeties, that grab onto the rail when the car moves too fast. Safety Systems: Safeties are activated by a governor when the elevator moves too quickly. Most governor systems are built around a sheave positioned at the top of the elevator shaft. The governor rope is looped around the governor sheave and another weighted sheave at the bottom of the shaft. The rope is also connected to the elevator car, so it moves when the car goes up or down. As the car speeds up, so does the governor. The diagram shows one representative governor design. In this governor, the sheave is outfitted with two hooked flyweights (weighted metal arms) that pivot on pins. The flyweights are attached in such a way that they can swing freely back and forth on the governor. But most of the time, they are kept in position by a high-tension spring. As the rotary movement of the governor builds up, centrifugal force moves the flyweights outward, pushing against the spring. If the elevator car falls fast enough, the centrifugal force will be strong enough to push the ends of the flyweights all the way to the outer edges of the governor. Spinning in this position, the hooked ends of the flyweights catch hold of ratchets mounted to a stationary cylinder surrounding the sheave. This works to stop the governor. The governor ropes are connected to the elevator car via a movable actuator arm attached to a lever linkage. When the governor ropes can move freely, the arm stays in the same position relative to the elevator car (it is held in place by tension springs). But when the governor sheave locks itself, the governor ropes jerk the actuator arm up. This moves the lever linkage, which operates the brakes. The linkage pulls up on a wedge-shaped safety, which sits in a stationary wedge guide. As the wedge moves up, it is pushed into the guide rails by the slanted surface of the guide. This gradually brings the elevator car to a stop.
Doors
The automatic doors at buildings are mainly there for convenience and are absolutely essential. They are there to keep people from falling down an open shaft. Elevators use two different sets of doors: doors on the cars and doors opening into the elevator shaft. The doors on the cars are operated by an electric motor, which is hooked up to the elevator computer. The electric motor turns a wheel, which is attached to a long metal arm. The metal arm is linked to another arm, which is attached to the door. The door can slide back and forth on a metal rail. When the motor turns the wheel, it rotates the first metal arm, which pulls the second metal arm and the attached door to the left. The door is made of two panels that close in on each other when the door opens and extend out when the door closes. The computer turns the motor to open the doors when the car arrives at a floor and close the doors before the car starts moving again. Many elevators have a motion sensor system that keeps the doors from closing if somebody is between them. The car doors have a clutch mechanism that unlocks the outer doors at each floor and pulls them open. In this way, the outer doors will only open if there is a car at that floor (or if they are forced open). This keeps the outer doors from opening up into an empty elevator shaft.
Machine room-less (MRL) elevators:
Machine room-less elevators are designed so that most of the components fit within the shaft containing the elevator car; and a small cabinet houses the elevator controller. Other than the machinery being in the hoist way, equipment is similar to a normal traction elevator. The benefits are: creates more usable space use less energy (70-80% less than hydraulic elevators) uses no oil slightly lower cost than other elevators Can operate at faster speeds than hydraulics but not normal traction units.
Facts:
Noise level is at 50-55 dBA (A-weighted decibels), which can be lower than some but not all types of elevators. Usually used for low-rise to mid-rise buildings. The motor mechanism is placed in the Hostway itself.
How Escalators Work Escalators are one of the largest, most expensive machines people use on a regular basis, but they’re one of the simplest. At its most basic level, an escalator is just a simple variation on the conveyer belt. A pair of rotating chain loops pulls a series of stairs in a constant cycle, moving a lot of people a short distance at a good speed. The term "escalator" is a combination of "elevator" and "scala," the Latin word for steps. The core of an escalator is a pair of chains, looped around two pairs of gears. An electric motor turns the drive gears at the top, which rotate the chain loops. A typical escalator uses a 100 horsepower motor to rotate the gears. The motor and chain system are housed inside the truss, a metal structure extending between two floors. Instead of moving a flat surface, as in a conveyer belt, the chain loops move a series of steps. The coolest thing about an escalator is the way these steps move. As the chains move, the steps always stay level. At the top and bottom of the escalator, the steps collapse on each other, creating a flat platform. This makes it easier to get on and off the escalator. Each step in the escalator has two sets of wheels, which roll along two separate tracks. The upper set (the wheels near the top of the step) are connected to the rotating chains, and so are pulled by the drive gear at the top of the escalator. The other set of wheels simply glides along its track, following behind the first set. The tracks are spaced apart in such a way that each step will always remain level. At the top and bottom of the escalator, the tracks level off to a horizontal position, flattening the stairway. Each step has a series of grooves in it, so it will fit together with the steps behind it and in front of it during this flattening. In addition to rotating the main chain loops, the electric motor in an escalator also moves the handrails. A handrail is simply a rubber conveyer belt that is looped around a series of wheels. This belt is precisely configured so that it moves at exactly the same speed as the steps, to give riders some stability. The escalator system isn't nearly as good as an elevator at lifting people dozens of stories, but it is much better at moving people a short distance. This is because of the escalator's high loading rate. Once an elevator is filled up, you have to wait for it to reach its floor and return before anybody else can get on. On an escalator, as soon as you load one person on, there's space for another.
BASIC COMPONENTS DETAILS Rectifier
A rectifier takes power from an AC source (like a home outlet) and converts it to a lower DC voltage. Radios, television receivers and power tools commonly contain rectifiers.
Rectifier Types:
Rectifiers can be separated into two basic categories: half-wave and full-wave. A half-wave rectifier allows only one polarity (positive or negative) to pass through, while a full-wave rectifier permits both. 
Half-wave rectification: In half wave rectification of a single-phase supply, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, mean voltage is lower. Half-wave rectification requires a single diode in a single-phase supply, or three in a three-phase supply. Rectifiers yield a unidirectional but pulsating direct current; half-wave rectifiers produce far more ripple than full-wave rectifiers, and much more filtering is needed to eliminate harmonics of the AC frequency from the output.

Full-wave rectification: A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to pulsating DC (direct current), and yields a higher average output voltage. Two diodes and a center tapped transformer, or four diodes in a bridge configuration and any AC source (including a transformer without center tap), are needed.
Inverter An inverter and a rectifier perform opposite functions. Both work as power converters within a circuit. However, a rectifier changes current from alternating current (AC) to direct current (DC), while an inverter converts DC to AC. An inverter transforms a low DC charge (such as 9 or 12 volts) to a high voltage AC charge. A power adapter that plugs into a cigarette lighter is a commonly-used example of an inverter.
Case Resistors protect drives from overvoltage faults by dissipating excess regenerative energy as heat. Case Resistors are used in applications where infrequent, low duty cycle, or low horsepower regeneration occurs. In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit. Its essential component is a metal wire or strip that melts when too much current flows, which interrupts the circuit in which it is connected. Short circuit, overloading, mismatched loads or device failure are the prime reasons for excessive current.
A fuse interrupts excessive current (blows) so that further damage by overheating or fire is prevented. o
There are two basic types of fuses which is the fast acting and slow blow type. The fast acting fuse will open very quickly when their particular current rating is exceeded. This is very important for analogue multimeter, which can quickly be destroyed when too much current flows through them, for even a very small amount of time. Even if you are an experienced repairer, sometimes we do made mistake by accidentally touching the probe to the testing points where it should not be touch! The slow blow fuse has a coiled construction inside the glass. Slow blow fuses are designed to open only on a continued overload, such as a short circuit. The function of the coiled construction is to stop the fuse from blowing on just a temporary current surge. Don’t replace a slow blow fuse in place of a fast acting fuse because it may not open fast enough to prevent components damage under a high current condition. It’s not dangerous to substitute a slow blow fuse with a fast-acting fuse, but it will probably open up unnecessarily every now and then when the equipment is first switch on such as when you switch on a Computer Monitor.
In electrical engineering, impedance is a measure of the extent to which a circuit opposes the flow of electricity. All materials have some degree of electrical resistance, which causes some energy to be lost as heat, and reduces the flow of current. In the case of direct current (DC), impedance is the same as resistance, and depends solely upon the materials from which the circuit is made. For an alternating current (AC), however, two additional factors can contribute to the impedance: capacitance and inductance. Together these are known as reactance, which is a measure of opposition to a change in current that depends on its frequency, and on the components of the circuit. Capacitive Reactance A capacitor is a device that can store an electrical charge, and later release it. It generally consists of a non-conducting material, or insulator, sandwiched between two metal plates. As part of a circuit, it allows a charge to build up in the insulator and effectively stores energy in an electric field. As the charge increases, the current is reduced. After a certain time, the capacitor will be unable to absorb any more charge and the current will drop to zero, at which point it will discharge, producing a flow of electrons in the opposite direction. If, however, the AC frequency is high, the current will change direction in less time than the capacitor takes to “fill up.” Since the current is at its maximum at the start of a cycle, a high frequency AC supply will be virtually unaffected by a capacitor. In contrast, if the frequency is low, this will allow time for some charge to build up in the capacitor, causing a reduction in current before the next cycle. Capacitors are used in many popular devices and gadgets, and so capacitive reactance is usually an important factor in impedance. Inductive Reactance Inductance is the tendency of a changing current flowing through a wire to induce an opposing current in a nearby conductor. This happens because a changing electrical current produces a changing magnetic field, which in turn causes electrons to flow in any conducting material within its range. When a wire is wound into a coil, it forms an inductor, and will induce an opposing flow of electrons, or electromotive force (EMF) in itself. The voltage of the induced EMF increases with the rate of change of the supply voltage, so increasing the AC frequency will increase the inductive reactance. Like capacitors, inductors are commonly used components.
Transformer
A transformer is static device which transform energy and power from one circuit to another with same frequency. AC circuits are very commonly connected to each other by means of transformers. A transformer couples two circuits magnetically rather than through any direct connection. It is used to raise or lower voltage and current between one circuit and the other, and plays a major role in almost all AC circuits.
Basic Principle: Mutual Induction It is the phenomenon in which a change of current in one coil causes an induced emf in another coil placed near to the first coil. The coil in which current is changed is called primary coil and the coil in which emf is induced is called secondary coil. Ideal Transformer An ideal transformer consists of two conducting coils wound on a common core, made of high grade iron. There is no electrical connection between the coils, they are connected to each other through magnetic flux. The coil on input side is called the primary winding (coil) and that on the output side the secondary. When an AC voltage is applied to the primary winding, time-varying current flows in the primary winding and causes an AC magnetic flux to appear in the transformer core. The arrangement of primary and secondary windings on the transformer core is shown in figure 9 below. The voltage, current and flux due to the current in the primary winding is also shown. For analyzing an ideal transformer, we make the following assumptions: The resistances of the windings can be neglected. All the magnetic flux is linked by all the turns of the coil and there is no leakage of flux. The reluctance of the core is negligible. Ideal Transformer – Equivalent Circuit The equivalent circuit (i.e., without the magnetic core) of an ideal transformer can be drawn as follows. The equivalent circuit is used for determining the performance characteristics of the transformer. Let’s consider some cases. If a < 1, i.e. N1 < N2 The output voltage is greater than the input voltage and the transformer is called a step-up transformer. If a > 1, i.e. N1 > N2 The output voltage is smaller than the input voltage and the transformer is called a step-down transformer. If a = 1, i.e. N1 = N2 The output voltage is the same as the input voltage and the transformer is called isolation transformer. These transformers perform a very useful function for applications where two circuits need to be electrically isolated from each other.
In summary, • The net power is neither generated nor consumed by an ideal transformer. • The losses are zero in an ideal transformer. • If a transformer increases the voltage, the current decreases and vice versa. Transformer rating: The transformer is usually rated in terms of its input and output voltages and apparent power that it is designed to safely deliver. For example, if a transformer carries the following information on its name-plate: 10kVA, 1100/110volts What are the meanings of these ratings? The voltage ratio indicates that the transformer has two windings, the high-voltage winding is rated for 1100 Volts and the low-voltage winding for 110 volts. These voltages are proportional to their respective number of turns. Therefore, the voltage ratio also represents the turns ratio a. (e.g., a = 10 here) The kVA rating (i.e., apparent power) means that each winding is designed for 10 kVA. Therefore the current rating for the high-voltage winding = 10000/1100 = 9.09A Current rating for low voltage winding = 10000/110 = 90.9 A The term “rated load” for a device refers to the load it is designed to carry for (theoretically) indefinite period of time. Rated load for the transformer refers to the apparent power specified as above, and shown in the name plate information. Note that during actual operation, the transformer may be required to operate at less power than its rated power. Application Example: Power Supply Circuits Transformers are an integral part of all power supplies. A typical application example of AC-DC power supply is shown below, where transformer is used for lowering the voltage to a level more suitable for consumption. This voltage is then rectified and filtered to obtain a DC voltage.
Aside from the ability to easily convert between different levels of voltage and current in AC and DC circuits, transformers also provide an extremely useful feature called isolation, which is the ability to couple one circuit to another without the use of direct wire connections. Actual (Non-ideal) Transformer Recall that we had made several assumptions when analyzing an ideal transformer. An actual differs from an ideal transformer in that is has: Resistive (I2R) losses (also called copper losses) in the primary and secondary windings Not all the flux produced by the primary winding links the secondary winding, and vice versa. This gives rise to some leakage of flux. The core requires a finite amount of mmf for its magnetization. Hysteresis and eddy current losses cause power loss in transformer core. The equivalent circuit of an ideal transformer can be modified to include these effects.
Resistances R1 and R2 can be added on both primary and secondary side to represent the actual winding resistances. The effect of leakage flux can be included by adding two inductances L1 and L2 respectively, in the primary and secondary winding circuits. We had assumed that the core reluctance was zero, however a real transformer has non-zero reluctance. A magnetizing inductance Lm can be added to account for this. The corresponding reactance of the iron core is Xm = 2 f Lm We had ignored core losses earlier. However, in actual transformers, hysteresis and eddy currents cause iron losses in the core. A resistance Rc can be added in the transformer equivalent circuit to account for core losses. Effects of winding resistance, leakage flux and imperfect core are added to the ideal transformer circuit shown below to obtain the circuit for a practical transformer.
Equivalent circuit of a practical transformer The equivalent circuit is useful in determining the characteristics of the transformer. Voltage Regulation Because of the elements R1, R2, L1 and L2, the voltage delivered to the load side of a transformer varies with the load current which is undesirable. The regulation of an actual transformer is defined as: Percent regulation = Efficiency Because of the resistances in the transformer equivalent circuit, not all of the power input to the transformer is delivered to the load. Efficiency is defined as the ratio of output to the input. Since, Input power = Output power + Power losses Efficiency =
× 100
REVIEW:
To convert 220V into 110V we usually use transformer.
Current is same in series while voltage is same in parallel.
RST is the incoming supply, (R=red, S=yellow, T=blue)(respectively High, Medium, Low) UVW IS the motor connections. Red= Phase , Black= Neutral and Green= Ground Difference Between Neutral & Earth:
o
o
A 'neutral' is a reference point or say closing path in an electrical system where the conductors are not carrying current. Ground, on the other hand, is an electrical path designed to carry fault current in case of an insulation breakdown. Both terms are generally used to represent the same thing in electrical circuits since they generally serve the same purpose. In a 3 phase system, the neutral carries the unbalance current. The earth ground is not supposed to carry any current unless there is a fault.
Governor – is a safety switch, work at final limit during emergencies. Travel Cables – Elevator traveling cable is a vital link between the elevator car and controller. All power and signal information is transmitted through the traveling cable. DBG – Distance Between Guide Flooring: 20 21 22 23 24 25 = 1 2 4 8 16 32: All floors are addition of these numbers ( 1 2 4 8 16 32)
STAR – DELTA CONNECTION
A Wye-Delta connection refers to one of four three-phase transformer configurations. Threephase transformers are used extensively in industrial power applications because of greater efficiency and up to 173 percent greater power transmission capability than single phase systems.
Wye-Delta Transformation The transformation is used to establish equivalence for networks with three terminals, where three elements terminate at a common node and none sources, the node is eliminated by transforming the impedances. For equivalence the impedance between the pair of terminals must be same for both networks.
Wye (star) Connection A Wye connection consists of three transformer windings configured like the letter Y. Each leg shares a common connection. The voltage between any two windings is equal, but the voltage between any leg and a common connection is 0.577 of the voltage between legs.
Delta Connection A Delta connection consists of three transformer windings configured in a triangle. The voltage between any two legs is equal.
Wye-Delta Connection In a Wye-Delta configuration, transformer primary windings are in a wye(star) configuration and the secondary windings are connected in a delta configuration. A common application is for stepping down high voltage from the power transmission line to a commercial voltage level.
CONCLUSION → VL = line voltage, terminal voltage, line to line voltage i.e.V AB , VBC , VCA . (VAB = two coils net voltage is one line voltage) → VPH = phase voltage i.e. Van ,Vbn , Vcn . (Van = voltage of phase ‘a’ referred to neutral) o STAR: In star connection current is same i.e. phase current = line current (IPH = IL). In star connection VL = √3 VPH (that’s why we get approx. 400V in two phase that is line voltage). o DELTA: In delta connection there is no neutral. In delta connection voltage is same i.e. VPH = VL .
REVIEW: The conductors connected to the three points of a three-phase source or load are called lines. The three components comprising a three-phase source or load are called phases. Line voltage is the voltage measured between any two lines in a three-phase circuit. Phase voltage is the voltage measured across a single component in a three-phase source or load. Line current is the current through any one line between a three-phase source and load. Phase current is the current through any one component comprising a three-phase source or load. In balanced “Y” circuits, line voltage is equal to phase voltage times the square root of 3, while line current is equal to phase current.
Three Phase Induction Motors Introduction The three-phase induction motors are the most widely used electric motors in industry. They run at essentially constant speed from no-load to full-load. However, the speed is frequency dependent and consequently these motors are not easily adapted to speed control. We usually prefer d.c. motors when large speed variations are required. Nevertheless, the 3-phase induction motors are simple, rugged, low-priced, easy to maintain and can be manufactured with characteristics to suit most industrial requirements. Three-Phase Induction Motor Like any electric motor, a 3-phase induction motor has a stator and a rotor. The stator carries a 3phase winding (called stator winding) while the rotor carries a short-circuited winding (called rotor winding). Only the stator winding is fed from 3-phase supply. The rotor winding derives its voltage and power from the externally energized stator winding through electromagnetic induction and hence the name. The induction motor may be considered to be a transformer with a rotating secondary and it can, therefore, be described as a â&#x20AC;&#x153;transformer- typeâ&#x20AC;? a.c. machine in which electrical energy is converted into mechanical energy. Advantages (i) (ii) (iii) (iv) (v)
It has simple and rugged construction. It is relatively cheap. It requires little maintenance. It has high efficiency and reasonably good power factor. It has self-starting torque.
Disadvantages (i) It is essentially a constant speed motor and its speed cannot be changed easily. (ii) Its starting torque is inferior to d.c. shunt motor.
Construction: A 3-phase induction motor has two main parts (i) stator and (ii) rotor. The rotor is separated from the stator by a small air-gap which ranges from 0.4 mm to 4 mm, depending on the power of the motor. 1. Stator It consists of a steel frame which encloses a hollow, cylindrical core made up of thin laminations of silicon steel to reduce hysteresis and eddy current losses. A number of evenly spaced slots are provided on the inner periphery of the lamination. The insulated connected to form a balanced 3-phase star or delta connected circuit.
Rotating Magnetic Field Due to 3-Phase Currents When a 3-phase winding is energized from a 3-phase supply, a rotating magnetic field is produced. This field is such that its poles do no remain in a fixed position on the stator but go on shifting their positions around the stator. For this reason, it is called a rotating head. It can be shown that magnitude of this rotating field is constant and is equal to 1.5ď Śm where ď Śm is the maximum flux due to any phase. Speed of rotating magnetic field The speed at which the rotating magnetic field revolves is called the synchronous speed (Ns). Therefore, for a 2-pole stator winding, the field makes one revolution in one cycle of current. In a 4-pole stator winding, it can be shown that the rotating field makes one revolution in two cycles of current. In general, for P poles, the rotating field makes one revolution in P/2 cycles of current. Since revolutions per second is equal to the revolutions per minute (Ns) divided by 60 and the number of cycles per second is the frequency f, f= = = = The speed of the rotating magnetic field is the same as the speed of the alternator that is supplying power to the motor if the two have the same number of poles. Hence the magnetic flux is said to rotate at synchronous speed. Principle of Operation Consider a portion of 3-phase induction motor as shown in Fig. The operation of the motor can be explained as under: (i) When 3-phase stator winding is energized from a 3phase supply, a rotating magnetic field is set up which rotates round the stator at synchronous speed Ns (= 120 f/P). (ii) The rotating field passes through the air gap and cuts the rotor conductor which as yet, is stationary. (iii)
(iv)
The current-carrying rotor conductors are placed in the magnetic field produced by the stator. Consequently, mechanical force acts on the rotor conductors. The sum of the mechanical forces on all the rotor conductors produces a torque which tends to move the rotor in the same direction as the rotating field. The fact that rotor is urged to follow the stator field (i.e., rotor moves in the direction of stator field) can be explained by Lenzâ&#x20AC;&#x2122;s law. According to this law, the direction of rotor currents will be such that they tend to opposite he cause producing them. Now, the cause producing the rotor currents is the relative speed between the rotating field and the stationary rotor conductors. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to catch it.
Slip We have seen above that rotor rapidly accelerates in the direction of rotating field. In practice, the rotor can never reach the speed of stator flux. If it did, there would be no relative speed between the stator field and rotor conductors, no induced rotor currents and, therefore, no torque to drive the rotor. The friction and windage would immediately cause the rotor to slow down. Hence, the rotor speed (N) is always less than the suitor field speed (Ns). This difference in speed depends upon load on the motor. The difference between the synchronous speed Ns of the rotating stator field and the actual rotor speed N is called slip. It is usually expressed as a percentage of synchronous speed i.e., % age slip,
s=
Ns ď&#x20AC; N Ns
x 100
(i) The quantity Ns N is sometimes called slip speed. (ii) When the rotor is stationary (i.e., N = 0), slip, s = 1 or 100 %. (iii) In an induction motor, the change in slip from no-load to full-load is hardly 0.1% to 3% so that it is essentially a constant-speed motor. Effect of Slip on The Rotor Circuit When the rotor is stationary, s = 1. Under these conditions, the per phase rotor e.m.f. E2 has a frequency equal to that of supply frequency f. At any slip s, the relative speed between stator field and the rotor is decreased. Consequently, the rotor e.m.f. and frequency are reduced proportionally to sEs and sf respectively. At the same time, per phase rotor reactance X2, being frequency dependent, is reduced to sX2. Consider a 6-pole, 3-phase, 50 Hz induction motor. It has synchronous speed Ns= 120 f/P = 120 50/6 = 1000 r.p.m. At standsill, the relative speed between stator flux and rotor is 1000 r.p.m. and rotor e.m.f./phase = E2(say). If the full- load speed of the motor is 960 r.p.m., then, 1000-960 s= =0.04 1000 (i)
The relative speed between stator flux and the rotor is now only 40 r.p.m. Consequently, rotor e.m.f./phase is reduced to: E2 =
40 1000
=.04E2
or
sE2
(ii) The frequency is also reduced in the same ratio to: 50*
40 1000
=50*.04
or
sf
(iii) The per phase rotor reactance X2 is likewise reduced to: X 2 * 40 or sX2 1000 = 0.04X2 Thus at any slip s, Rotor e.m.f./phase = sE2 Rotor reactance/phase = sX2 Rotor frequency = sf where E2,X2 and f are the corresponding values at standstill.
Speed Regulation of Induction Motors Like any other electrical motor, the speed regulation of an induction motor is given by: % age speed regulation = where
N0 - NF.L. N F.L.
*100
N0 = no-load speed of the motor NF.L. = full-load speed of the motor
If the no-load speed of the motor is 800 r.p.m. and its fall-load speed in 780 r.p.m., then change in speed is 800 780 = 20 r.p.m. and percentage speed regulation = 20 100/780 = 2.56%. At no load, only a small torque is required to overcome the small mechanical losses and hence motor slip is small i.e., about 1%. When the motor is fully loaded, the slip increases slightly i.e., motor speed decreases slightly. It is because rotor impedance is low and a small decrease in speed produces a large rotor current. The increased rotor current produces a high torque to meet the full load on the motor. For this reason, the change in speed of the motor from no- load to full-load is small i.e., the speed regulation of an induction motor is low. The speed regulation of an induction motor is 3% to 5%. Although the motor speed does decrease slightly with increased load, the speed regulation is low enough that the induction motor is classed as a constant-speed motor. Power Factor of Induction Motor Like any other a.c. machine, the power factor of an induction motor is given by; Power factor,
cos =
Active component of current (I cos ) Total current (I)
The presence of air-gap between the stator and rotor of an induction motor greatly increases the reluctance of the magnetic circuit. Consequently, an induction motor draws a large magnetizing current (Im) to produce the required flux in the air-gap. (i) At no load, an induction motor draws a large magnetizing current and a small active component to meet the no-load losses. Therefore, the induction motor takes a high no-load current lagging the applied voltage by a large angle. Hence the power factor of an induction motor on no load is low i.e., about 0.1 lagging. (ii) When an induction motor is loaded, the active component of current increases while the magnetizing component remains about the same. Consequently, the power factor of the motor is increased. However, because of the large value of magnetizing current, which is present regardless of load, the power factor of an induction motor even at full-load seldom exceeds 0.9 lagging.
Equivalent Circuit of 3-Phase Induction Motor at Any Slip In a 3-phase induction motor, the stator winding is connected to 3-phase supply and the rotor winding is short-circuited. The energy is transferred magnetically from the stator winding to the short-circuited, rotor winding. Therefore, an induction motor may be considered to be a transformer with a rotating secondary (short-circuited). The stator winding corresponds to transformer primary and the rotor finding corresponds to transformer secondary. In view of the similarity of the flux and voltage conditions to those in a transformer, one can expect that the equivalent circuit of an induction motor will be similar to that of a transformer. Fig. shows the equivalent circuit (though not the only one) per phase for an induction motor. Let us discuss the stator and rotor circuits separately.
Stator circuit. In the stator, the events are very similar to those in the transformer primal y. The applied voltage per phase to the stator is V1 and R1 and X1 are the stator resistance and leakage reactance per phase respectively. The applied voltage V1 produces a magnetic flux which links the stator winding (i.e., primary) as well as the rotor winding (i.e., secondary). As a result, self- induced e.m.f. E1 is induced in the stator winding and mutually induced e.m.f. E'2(= s E2 = s K E1 where K is transformation ratio) is induced in the rotor winding. The flow of stator current I1 causes voltage drops in R1 and X1. V1
E1
I1 (R 1
j X1 )
...phasor sum
When the motor is at no-load, the stator winding draws a current I0. It has two components viz., (i) which supplies the no-load motor losses and (ii) magnetizing component Im which sets up magnetic flux in the core and the air- gap. The parallel combination of Rc and Xm, therefore, represents the no-load motor losses and the production of magnetic flux respectively. I0
Iw
Im
Rotor circuit. Here R2 and X2 represent the rotor resistance and standstill rotor reactance per phase respectively. At any slip s, the rotor reactance will be s X2.The induced voltage/phase in the rotor is E'2 = s E2 = s K E1. Since the rotor winding is short-circuited, the whole of e.m.f. E'2 is used up in circulating the rotor current I'2. E' 2 I'2 (R 2 j s X2 ) The rotor current I'2 is reflected as I"2 (= K I'2) in the stator. The phasor sum of I"2 and I0 gives the stator current I1. It is important to note that input to the primary and output from the secondary of a transformer are electrical. However, in an induction motor, the inputs to the stator and rotor are electrical but the output from the rotor is mechanical. To facilitate calculations, it is
Starting of 3-Phase Induction Motor The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuited secondary. At starting, the voltage induced in the induction motor rotor is maximum (Q s = 1). Since the rotor impedance is low, the rotor current is excessively large. This large rotor current is reflected in the stator because of transformer action. This results in high starting current (4 to 10 times the full-load current) in the stator at low power factor and consequently the value of starting torque is low. Because of the short duration, this value of large current does not harm the motor if the motor accelerates normally. However, this large starting current will produce large line-voltage drop. This will adversely affect the operation of other electrical equipment connected to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are available for this purpose. Methods of Starting 3-Phase Induction Motors The method to be employed in starting a given induction motor depends upon the size of the motor and the type of the motor. The common methods used to start induction motors are:
(i) Direct-on-line starting (iii) Autotransformer starting (v) Rotor resistance starting
(ii) Stator resistance starting (iv) Star-delta starting
Methods (i) to (iv) are applicable to both squirrel-cage and slip ring motors. However, method (v) is applicable only to slip ring motors. In practice, any one of the first four methods is used for starting squirrel cage motors, depending upon ,the size of the motor. But slip ring motors are invariably started by rotor resistance starting. Methods of Starting Squirrel-Cage Motors Except direct-on-line starting, all other methods of starting squirrel-cage motors employ reduced voltage across motor terminals at starting. (i) Direct-on-line starting This method of starting in just what the name impliesâ&#x20AC;&#x201D;the motor is started by connecting it directly to 3-phase supply. The impedance of the motor at standstill is relatively low and when it is directly connected to the supply system, the starting current will be high (4 to 10 times the full-load current) and at a low power factor. Consequently, this method of starting is suitable for relatively small (up to 7.5 kW) machines. (ii) Stator resistance starting In this method, external resistances are connected in series with each phase of stator winding during starting. This causes voltage drop across the resistances so that voltage available across motor terminals is reduced and hence the starting current. The starting resistances are gradually cut out in steps (two or more steps) from the stator circuit as the motor picks up speed. When the motor attains rated speed, the resistances are completely cut out and full line voltage is applied to the rotor. This method suffers from two drawbacks. First, the reduced voltage applied to the motor during the starting period lowers the starting torque and hence increases the accelerating time. Secondly, a lot of power is wasted in the starting resistances.
GEAR AND GEARLESS MOTORS A geared motor has gears to give different power and speeds where as gearless motor is a direct drive that uses the speed of the engine only for power and speed when driven, such as in boat. Geared Traction machines are driven by AC or DC electric motors. Geared machines use worm gears to mechanically control movement of elevator cars by "rolling" steel hoist ropes over a drive sheave which is attached to a gearbox driven by a high speed motor. These machines are generally the best option for basement or overhead traction use for speeds up to 350 ft/min. Gearless Traction machines are high speed electric motors powered by AC or DC current. In this case the drive sheave is directly attached to the end of motor. Features: Suitable for elevators with or without machine room Energy saving up to 30% Low noise No oil spillage problems Key components are imported Outstanding torque and cogging characteristics
Geared vs. gearless
Figure 1: Geared high-speed twin drive with SCIM
Gearless low-speed single drive with synchronous motor Comparing a conventional gear-based electrical drive system with a gearless drive system, the differences are immediately apparent (Figures 1 and 2). To achieve a power rating of 6 MW, a conventional geared solution requires up to four high-speed drive systems, each comprising a squirrel cage induction motor, disc brake, couplings, and gear reducer equipped with numerous parts, such as motor and gear bearings, seals, tooth wheels and oil lubrication with a re‑cooling unit. The number of wear parts is accordingly high (more than 22) and the MTBF is correspondingly low (only 3 – 4 years).
A gearless electrical drive system, on the other hand, is simple and long-lasting. The same power rating of 6 MW can be achieved with just one drive system, comprising a single 6 MW synchronous motor. The number of main wear parts is no more than two, resulting in an MTBF of up to 30 years, which comfortably exceeds the expected operating life of the overland conveyor system. There is also a considerable reduction in the drive system’s footprint and the amount of instrumentation required. The elimination of numerous mechanical and electrical components increases the reliability and efficiency of the overall conveyor system by several percentage points. It also substantially lowers the maintenance requirements of the gearless drive system compared with a conventional drive system, for which gear reducer maintenance alone can amount to up to 5% of the mechanical maintenance budget. Lubrication and gear reducer cooling systems, together with their maintenance, are not required with gearless drives.
Gearless drive systems main components:
Low-speed synchronous motors Frequency converters with voltage source inverters (VSIs) Converter transformer Advanced conveyor control software.
Conclusion: the benefits of going gearless:
The benefits of a gearless conveyor drive system with low-speed synchronous motor include: Compact footprint and simple design. Higher efficiency than with an induction motor (no slip losses). No gear reducer losses. Significantly fewer components. Drive train free of oscillation. Virtually maintenance free. No cooling, monitoring and supervision of the gear reducer and oil coolers/heaters. No backlash during oscillations and at load reversals. Faster installation and commissioning. Higher system reliability and availability. Proven advanced conveyor control and simulation. Significantly lower production losses thanks to the exceptional reliability and availability .
BRAKES
The most common elevator brake is made up of a compressive spring assembly, brake shoes with linings, and a solenoid assembly. When the solenoid is not energized, the spring forces the brake shoes to grip the brake drum and induce a braking torque. The magnet can exert a horizontal force for the break release. This can be done directly on one of the operating arms or through a linkage system. In either case, the result is the same. The break is pulled away from the shaft and the velocity of the elevator is resumed. In order to improve the stopping ability, a material with a high coefficient of friction is used within the breaks, such as zinc bonded asbestos. A material with too high a coefficient of friction can result in a jerky motion of the car. This material must be chosen carefully.
ROPE DRIVES When power is to be transmitted over long distances then belts cannot be used due to the heavy losses in power. In such cases ropes can be used. Ropes are used in elevators, mine hoists, cranes, oil well drilling, aerial conveyors, tramways, haulage devices, lifts and suspension bridges etc. Two types of ropes are commonly used. They are fiber ropes and metallic ropes. ď&#x201A;ˇ Fiber ropes are made of Manila, hemp, cotton, jute, nylon, coir etc., and are normally used for transmitting power. ď&#x201A;ˇ Metallic ropes are made of steel, aluminum, alloys, copper, bronze or stainless steel and are mainly used in elevator, mine hoists, cranes, oil well drilling, aerial conveyors, haulage devices and suspension bridges. A wire rope is made up of stands and a strand is made up of one or more layers of wires as shown in fig. the number of strands in a rope denotes the number of groups of wires that are laid over the central core. Ropes having wire core are stronger than those having fiber core. Flexibility in rope is more desirable when the number of bends in the rope is too many.
The advantages of rope drives: (a) A larger amount of power is transmitted. (b) A rope can be run in any direction or to any distance. (c) Smooth and quiet running is obtained. (d) Electrical disturbances are absent. (e) Economy is obtained in first cost and in maintenance. (/) There is an absence of slip.
Limit Switch
A limit switch is an electrical switch designed to cut off power to a machine at the extent of its possible movement. Function For instance, a limit switch placed on an elevator would shut off the power if the car goes higher or lower than it is supposed to go within safety measures. Safety Limit switches are installed for the safety of those using the machinery but also for the integrity of the machine itself. For example, keeping an elevator from passing a certain point protects not only the occupants but also the motors and pulleys that drive the elevation of the car.
Traveling Cables Elevator traveling cable is a vital link between the elevator car and controller. All power and signal information is transmitted through the traveling cable. When terminating the cable in the machine room, the need to make electrical connections in the junction box is eliminated, but the resulting longer traveling cable, that start from machine room and end at last stop, the travelling cable had two steel wires inside it that support and prevent it from breakup.
The two basic types of traveling cable are: 1. Round 2. Flat Round cable is typically composed of several layers of insulated conductors of components stranded around a central member. Round cables can contain a large number of conductors - as many as 120. The round construction offers great flexibility with regard to number and size of components that can be combined into one cable. Flat cables can generally be classified as either parallel flat or unitized flat. The parallel flat is construction by laying conductors and/or components side by side and applying an overall jacket or sheath. For unitized flat cable, small round cable cores (as in the rope lay construction) are laid in parallel and covered with an overall sheath or jacket. A single round cable can contain a large number of conductors thereby reducing the number of cables required per elevator. In addition, round cables have a smaller surface area than similar flat cables. These characteristics make round cables ideal for use in high rise, high speed applications.