Make the Most of Variable Frequency Drives

A variable frequency drive (VFD), sometimes also called an adjustable speed drive (ASD), is an electronic device that allows users to change the speed at which a motors runs. The combination of a VFD with a motor is becoming increasingly popular as a final control element because it consumes less power than a motor running at full speed and a control valve. Indeed, the U.S. Department of Energy (DOE) is encouraging the use of VFD for motor control to reduce power consumption [1]. However, obtaining good control depends upon proper selection and installation of the drive as well as understanding how control may differ with a VFD.

The VFD is one part of a total system that includes a motor and a load. The motor acts as a power transducer, converting electrical power to rotational mechanical power. AC induction motors, 600V and below, often are paired with a VFD. (We’ll discuss DC motors later.) These AC motors fall into classes with different torque speed curves (Figure 1). Most drive manufacturers assume use of a Class A motor and that the torque speed curve will be almost linear at the operating point (Figure 2). The VFD will shift the whole curve left or right to change the operating point. Note the “slip” in the figure. Every motor suffers from some slip or difference between rotor and stator fields. This is quite different than stiction, the term used with control valves for the needed stem force to overcome static friction. The load — mechanical conveyor, pump, fan, compressor or the like — has inertia, rotational friction and stiction. The process also has dynamic characteristics that may change when using a VFD instead of a control valve. It takes time to accelerate the load to operating speed and this is proportional to the inertia and the motor torque.

Figure 1 -- AC motor classes: Each class features a distinct torque speed curve. Click on illustration for larger image.

VFD Basics
Today VFDs use a technique called pulse width modulation (PWM). First AC power is rectified to DC and filtered. Next, a solid-state semiconductor called an insulated gate bipolar transistor (IGBT) creates a voltage waveform to the motor that is a series of pulses of varying widths. The result is a varying frequency AC sine wave. The switching frequency determines the shaft rotational speed. Because the power waveform from a VFD isn’t purely sinusoidal, it’s important to only use motors specially designed to run with PWM VFDs — these are “inverter duty” motors, Class F winding. If a conventional motor is used, it may burn out.

A VFD also has other control electronics; these may include current, voltage and speed sensors.
The VFD electronics has limitations that affect control performance. One limitation is current. The inrush peak starting current of an “across the line” starter, one without a VFD, is eight times the full load current. Such a current would damage a VFD’s rectifiers and semiconductors. Another constraint: the drive electronics is designed to prevent the motor flux from saturating the core.

The process transmitter sending its output signal to the proportional-integral-derivative (PID) controller that acts as the outer loop senses the process dynamics; its output is cascaded to the VFD.

Within the drive electronics are algorithms that control the electrical motor power, frequency, voltage and current. The current and speed set the motor torque. So the drive doesn’t control just the speed, it also regulates the torque delivered to the rotor shaft. This torque produces the rotational force applied to the load (pump, etc.) that powers the process.

Figure 2 -- Class A motor: The curve with load is almost linear at the operating point. Click on illustration for larger image.

Properly understanding the dynamics of a VFD control loop requires considering all elements of the system and how they interact.

Drive Control Strategies
A computer program connected to the drive or a human machine interface (HMI) front panel enables inputting data about the load and the motor as well as setting the drive control strategy, which usually takes advantage of proprietary functionality. This strategy together with the load dynamic behavior determines performance. Loops within the drive electronics can be configured to control speed (through an external encoder), voltage, current and, in some cases, motor flux. These are the inner loops of the process control cascade. When tuning, remember that the inner loops must be at least five times more responsive than the outer loop. Another term to describe performance is bandwidth; it’s inversely proportional to the time constant of the controller/motor with no load.