You can divide the world of electronic motor drives into two categories: AC and DC. A motor drive controls the speed, torque, direction and resulting horsepower of a motor. A DC drive typically controls a shunt wound DC motor, which has separate armature and field circuits. AC drives control AC induction motors, and-like their DC counterparts-control speed, torque, and horsepower.
Let's take a brief look at a drive application. In Figure 1, you can see a simple application with a fixed speed fan Using A Motor Starter. You could replace the 3-phase motor starter with Variable Frequency Drive (VFD) to operate the fan at variable speed. Since you can operate the fan at any speed below its maximum, you can vary airflow by controlling the motor speed instead of the air outlet damper.
Figure 2. Operating Principles of Induction Motor
Figure 3. Induction Motor Slip Calculation
Figure 4. Volts/Hertz Ratio
Just how does a drive provide the frequency and voltage output necessary to change the speed of a motor? That's what we'll look at next. Figure 5 shows a basic PWM drive. All PWM drives contain these main parts, with subtle differences in hardware and software components.
Figure 5. Basic PWM Drive Components
Today's inverters use Insulated Gate Bipolar Transistors (IGBTs) to switch the DC bus on and off at specific intervals. In doing so, the inverter actually creates a variable AC voltage and frequency output. The output of the drive doesn't provide an exact replica of the AC input sine waveform. Instead, it provides voltage pulses that are at a constant magnitude. The drive's control board signals the power device's control circuits to turn "on" the waveform positive half or negative half of the power device. This alternating of positive and negative switches recreates the 3 phase output. The longer the power device remains on, the higher the output voltage. The less time the power device is on, the lower the output voltage. Conversely, the longer the power device is off, the lower the output frequency.
Drives vary in the complexity of their designs, but the designs continue to improve. Drives come in smaller packages with each generation. The trend is similar to that of the personal computer. More features, better performance, and lower cost with successive generations. Unlike computers, however, drives have dramatically improved in their reliability and ease of use. And also unlike computers, the typical drive of today doesn't spew gratuitous harmonics into your distribution system-nor does it affect your power factor. Variable speed/frequency drives are increasingly becoming "plug and play". As electronic power components improve in reliability and decrease in size, the cost and size of VFDs will continue to decrease. While all that is going on, their performance and ease of use will only get better.
With the large installed base of SCRs, you might want to know how these operate. An SCR (originally referred to as a thyristor) contains a control element called a gate. The gate acts as the "turn-on" switch that allows the device to fully conduct voltage. The device conducts voltage until the polarity of the device reverses-and then it automatically "turns off". Special circuitry, usually requiring another circuit board and associated wiring, controls this switching. The SCR's output depends on how soon in the control cycle that gate turns on. The IGBT output also depends the length of time the gate is on. However, it can turn off anytime in the control cycle, providing a more precise output waveform. IGBTs also require a control circuit connected to the gate, but this circuitry is less complex and doesn't require a reversal of polarity. Thus, you would approach troubleshooting differently if you have an SCR-based drive.