By Roger Desrosiers
By the 1980s, AC motor drive technology became reliable and inexpensive enough to compete with traditional DC motor control. These variable-frequency drives (VFDs) accurately control the speed of standard AC induction or synchronous motors. With VFDs, speed control with full torque is achieved from "0" rpm through the maximum rated speed and, if required, above the rated speed at reduced torque. VFDs manipulate the frequency of their output by rectifying an incoming AC current into DC, and then using voltage pulse-width modulation to recreate an AC current and voltage output waveform. However, this frequency conversion process causes 2% to 3% loss as heat in the VFD — caloric energy that must be dissipated. The process also yields over-voltage spikes and harmonic current distortions.
Figure 1 shows a circuit diagram of a typical variable frequency drive. Notice how the circuit shows three separate sections. The first section shows the rectifier section, where a three-phase diode bridge rectifier changes the three phase AC voltage to pulsating DC voltage. The second section is the filter section where the pulsating DC voltage is smoothed to pure DC voltage. The third section is the transistor switching section which produces the three-phase AC voltage at the desired frequency.
In the first section (rectifier) you can see the six diodes are connected in a bridge circuit to convert the three phase AC voltage to DC voltage. The actual rectifier is mounted on a single module and can be exchanged by removing the three input voltage wires and the two DC bus connectors. Each diode in the module can be tested for front to back resistance ratio with an ohm meter just as if it were an individual diode. The output of the rectifier section is 12 half-wave pulses 60* apart. The output of the bridge rectifier is connected to two large copper conductors that are called DC bus.
The filter section of the circuit consists of several capacitors that are connected in parallel with the DC bus, and a large inductor is connected in series with the DC bus. The capacitors charge and discharge in synchronization with the input voltage. This causes the half wave signal to be converted to pure DC. The capacitors are used to filter the voltage part of the waveform and the inductor is used to filter the current part of the waveform. The transistor section consists of six transistors, two for each output phase. You can see that one transistor of each phase is connected to the positive DC bus and the second transistor is connected to the negative DC bus. Each circuit is controlled by a base firing circuit that is controlled by a micro-processor chip. At the correct time each transistor is turned on in six distinct steps.
Figure 2 shows an example of the wave form that results from turning the transistor on in six steps The Inverters take the voltage from the DC Bus and using Pulse Width Modulation (PWM) sends a signal which appears to the motor as an AC signal.
Pulse Width Modulation
In the diagram above, a close up view of the waveform that goes to the motor shows the switching frequency of the IGBTs. The switching-pattern shown below is known as pulse width modulation or PWM. As the length of time is increased for the IGBT to be ON and then OFF, the motor responds to it as a sinusoidal waveform. The positive IGBT fires first in the diagram followed by its negative counterpart. Only one motor terminal (U) is shown but the same type of activity would appear on V and W, in figure 2.
Typical control for a variable frequency drive is between 0 and 120 % of its rated rpm. In reality the motor is generally adjusted from 60% to 130%of rated rpm, which provides sufficient control for the system being adjusted. It is important to note that the VFD can only rule the motor for a short period of time at RPMs above 100% because the motor will overheat. You can see how important it is to be able to adjust the speed of a very large fan to provide minimal airflow when the temperature conditions are near set point and larger air-flows when the system requires it. The same conditions apply when controlling a refrigeration compressor. A lower heat load requires a lower pumping compressor and a bigger heat load requires an increase in pumping capacity.