Scalar and vector VFD control methods

AC motors pair with variable frequency drives to satisfy process and motion applications. Here, we compare how these operations diverge.

Lisa Eitel | Executive editor

AC motors are often used in equipment that runs at constant speed regardless of load, such as fans, pumps, and conveyors. But for designs needing speed control, AC motors pair with variable frequency drives (VFDs) that regulate the motor’s speed via one of two control methods — scalar control or vector control — to vary the frequency of the supplied voltage.

A scalar is a quantity that has only magnitude, such as mass or temperature. A vector is a quantity that has both magnitude and direction, such as acceleration or force.

Scalar VFD control methods

Scalar methods for VFD control work by optimizing the motor flux and keeping the strength of the magnetic field constant, which ensures constant torque production. Often called V/Hz or V/f control, scalar methods vary both the voltage (V) and frequency (f) of power to the motor to maintain a fixed, constant ratio between the two, so the strength of the magnetic field is constant, regardless of motor speed.

The appropriate V/Hz ratio is equal to the motor’s rated voltage divided by its rated frequency. V/Hz control is typically implemented without feedback (i.e. open-loop), although closed-loop V/Hz control — incorporating motor feedback — is possible.

V/Hz control is simple and low-cost, although it should be noted that the closed-loop implementation increases cost and complexity. Control tuning is not required but can improve system performance.

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In a VFD, AC power is converted to DC through the rectifier. The rectified power is then filtered and stored in the DC bus (link). The inverter converts it back to AC power with the proper frequency and voltage. Servodrives are sometimes called servo inverters (for their most significant subsection).

Speed regulation with scalar control is only in the range of 2 to 3% of rated motor frequency, so these methods aren’t suitable for applications where precise speed control is required. Openloop V/Hz control is unique in its ability to allow one VFD to control multiple motors and is arguably the mostcommonly implemented VFD control method.

Vector VFD control methods

Vector control — also called fieldoriented control (FOC) — controls the speed or torque of an AC motor by controlling the stator current space vectors, in manner similar to (but more complicated than) DC control methods. Field oriented control uses complex mathematics to transform a three-phase system that depends on time and speed to a two-coordinate (d and q) timeinvariant system.

The stator current in an AC motor is made up of two components: the magnetizing component (d) of the current and the torque-producing component q. With FOC, these two current components are controlled independently … each with its own PI controller. This allows the torqueproducing component, q, to be kept orthogonal to the rotor flux for maximum torque production, and therefore, optimum speed control.

Like scalar methods, vector VFD control methods can be open-loop or closed-loop. Open-loop vector control (also called sensorless vector control) uses a mathematic model of the motor operating parameters, rather than using a physical feedback device. The controller monitors voltage and current from the motor and compares them to the mathematical model. It then corrects any errors by adjusting the current supplied to the motor, which adjusts the motor’s torque production accordingly. With senseless vector control, it’s

VFD-driven cooling-tower motors

important to have a very accurate mathematical model of the motor, and the controller must be tuned for proper operation.

Closed-loop vector control uses an encoder to provide shaft position feedback, and this information is sent to the controller, which adjusts the supplied voltage to increase or decrease torque. This is the only method that allows direct torque control in all four quadrants of motor operation for dynamic braking or regeneration.

Vector control methods are more complex than scalar VFD control methods, but they o er significant benefits over scalar methods in some applications. For example, open-loop vector control enables the motor to produce high torque at low speeds, and closed-loop vector control allows a motor to produce up to 200% of its rated torque at zero speed, useful for holding loads at standstill. Closed-loop vector control also provides very accurate torque and speed control for industrial applications.

More on how V/Hz control works

As mentioned, the most common type of VFD control is a scalar method called volts per hertz (V/Hz) or volts per frequency (V/f).

AC motors are designed for a magnetic field or flux of constant strength. The magnetic field strength is proportional to the ratio of voltage V to frequency Hz — or V/Hz. But a VFD controls the motor speed by varying the frequency of the applied voltage, according to the synchronous speed equation:

N = 120 · f / P

Where N = Motor speed (RPM) f = Input voltage frequency

P = Number of motor poles

Varying the voltage frequency a ects both the motor speed and the strength of the magnetic field. When the frequency is lowered (for slower motor speed), the magnetic field increases, and excessive heat is generated. When

V/Hz control maintains a constant ratio between voltage (V) and frequency (Hz). The V/Hz control method allows one VFD to control whole motor arrays, which is especially useful in process-type applications.

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the frequency is increased (for higher motor speed) the magnetic field decreases, and lower torque is produced. To keep the magnetic flux constant, the V/Hz ratio must remain constant. This keeps torque production stable, regardless of frequency.

V/Hz control of a VFD drive avoids this variation in the magnetic field strength by varying the voltage along with the frequency to maintain a constant V/Hz ratio. The appropriate V/Hz ratio is given by the motor’s rated voltage and frequency.

For example, a motor rated for 230 V and 60 Hz will always operate best at a V/Hz ratio of 3.83 (230/60 = 3.83).

Traditional V/Hz control does not use feedback and only changes the voltage and frequency to the motor based on an external speed command. For closedloop V/Hz control, encoder feedback can be added to measure the motor’s actual speed. An error signal is generated based on the di erence between actual speed and commanded speed, and the controller generates a new frequency command to compensate for the error.

While it improves speed regulation, closed-loop V/ Hz control isn’t common due to the added cost and complexity of the encoder and feedback hardware.

Performance and benefits of V/Hz control

V/Hz control is a simple low-cost method of operating variable frequency drives and is generally regarded as the most common VFD control scheme. It is suitable for both constant torque and variable-torque applications and can provide up to 150% of the rated torque at zero speed for startup and peak loads. Speed regulation is in the range of 2 to 3% of the maximum rated frequency, so this method isn’t suitable for applications where precise speed control is critical. The most common use for V/Hz control is to drive industrial equipment such as fans and blowers.

One unique benefit of V/Hz control over other methods is that it allows more than one motor to be operated by a single VFD. All the motors will start and stop at the same time, and they will all run at the same speed — which is beneficial in some process applications such as heating and cooling.

Final note on terminology

Note that the terms variable frequency drive and variable speed drive are often used interchangeably, but there is a distinction between the two. A VSD is any drive that can control motor speed — including both AC and DC motors. In fact, VSDs can even operate via mechanical, hydraulic, or electrical means. In contrast, a VFD is equipment used to control the speed of an AC motor … and it does so by varying the frequency of the supply voltage to the motor. DW

Look for the second part of this article series in the May issue of Design World.

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