4 minute read
All about electric motors
There are many types of electric motors used in power transmission. They vary from small brush dc motors for moving light loads to high horsepower ac motors powering conveyor lines in heavy duty mining applications. For more precision applications, servomotors and stepper motors are the motors of choice. Here, compact size and high torque as well as greater controllability are common advantages.
Electric motors can be categorized in a number of different ways; as ac (alternating current) or dc (direct current) motors, or by the type of motion they produce, rotary or linear. Another common approach is to distinguish between commutation methods; for example, self-commutated or external, and by mechanical means (as brushes used in brushed dc motors) or electronic commutation (as in brushless dc motors.)
MOTORS – AC
There are two fundamental types of ac motors; induction and synchronous motors.
Induction motors run at constant speed across a wide range of load settings, from zero to full-load. With synchronous motors, their name indicates that they run synchronously with the frequency of the source. The motor speed is fixed and doesn’t change with changes to the load or voltage.
MOTORS – DC
A dc motor works by generating a magnetic field via electromagnetic windings or permanent magnets. The most common industry naming conventions for dc motors recognize three subtypes: brush motors, permanent-magnet (PM) motors, and universal motors. Many dc motors still employ brushes and wound fields, but PM motors dominate fractional and integral horsepower applications below 18 hp.
For brushed dc motors, the magnet acts as the stator. The armature is integrated onto the rotor and a commutator switches the current flow. It does this by transferring current from a fixed point to the rotating shaft. Brushed dc motors generate torque straight from the dc power supplied to the motor by using internal commutation, fixed permanent magnets, and rotating electromagnets.
Brushless dc (BLDC) motors, on the other hand, do away with mechanical commutation. They use electronic commutation that eliminates the mechanical wear and tear involved with brushed dc motors. In BLDC motors, the permanent magnet is housed in the rotor and the coils are located in the stator. These coil windings produce a rotating magnetic field because they’re separated from each other electrically, which enables them to be turned on and off. The rotor’s permanent magnet field trails the rotating stator field, producing the rotor field.
SERVOMOTORS
Servomotors are one of the most common types of motors used in motion control applications. While there’s some disagreement over exactly what constitutes a servomotor, one factor is the presence of feedback for closed-loop control. Closedloop control gives servomotors precise positioning ability by greatly reducing error. That means they can accommodate complex motion patterns and profiles more readily. They offer precise control of torque and speed and they can also operate at zero speed while maintaining enough torque to maintain a load in a given position.
STEPPER MOTORS
On par with servomotors, stepper motors find extensive use in motion systems. They operate open loop without the need for tuning parameters as in closed-loop servo systems. They’re used mostly in positioning applications and have the advantage of being able to be accurately controlled down to fractions of a degree without the use of feedback devices such as encoders or resolvers.
Stepper motors are generally classified by the number of allowable steps they can be commanded to move. For instance, a 1.8 degree step motor is capable of 200 steps/revolution (1.8 x 200 = 360 degrees, or one full revolution) in full-step mode. If operated in halfstep mode, each step becomes 0.9 degrees and the motor can then turn 400 steps/revolution. Another mode called microstepping subdivides the degrees per step even further, allowing for extremely precise movements.
LINEAR MOTORS
For applications requiring linear motion, a viable option may be a linear motor. One way to think of a linear motor is essentially a rotary motor that has been unrolled. So instead of producing torque via rotation it generates a straight-line force along its length. Like many rotary motors, linear motors consist of a coil and magnets. Although there are many types of linear motors, brushless iron core and ironless designs prevail in automation and positioning applications.
Linear motors offer a number of advantages over belts, screws, and other drive mechanisms, including low maintenance and higher accuracy and repeatability. There are no mechanicaltransmission components – such as pulleys, couplings, or gearboxes – to introduce elasticity and backlash. The system’s accuracy and repeatability are determined by the controls and don’t degrade over time. The lack of rotating or sliding components also means linear motors are almost maintenancefree, with only the support bearings, or linear guides, requiring periodic maintenance.