Both the VVI and PWM are voltage-source inverters; i.e., they provide a voltage to the motor similar to what it would normally receive from the power system, except at variable frequency.
Current Source Inverter Working
A current- source inverter, on the other hand, acts as a constant current source to the motor. The constant current is switched to the phases sequentially, as shown in Figure 1. Each phase conducts current for 120° out of each half-cycle. Because current must enter one phase and return through another, obviously two switches must be on at a time. The switching sequence is shown at the top of Figure 1.
During the first interval, switches 1 and 6 are closed, so the current flows into phase A and back from phase B, as shown. Due to the high rise times of the current pulses, inductive spikes will occur on the motor phase voltage every time the current is commutated from one phase to another.
FIGURE 1: Phase currents for an induction motor connected to a CSI drive.
The primary advantage of the CSI drive is that it can regenerate power. This means it can convert the energy stored in the rotating mass of the motor and load back into electricity and return it to the source. That is accomplished by delaying the firing angle of the SCRs in the rectifier section so as to put it into inverter operation.
The drive is also tolerant of commutation failures in the inverter, unlike voltage-fed inverters, which require protective circuitry to avoid commutation problems.
The CSI drive is not a popular drive for general- purpose applications for several reasons. Because the phase currents are zero for 60° out of every half-cycle, cogging can be a problem for the CSI drive, particularly at lower speeds. That limits the low end of the operation to about 15 Hz. The high end is limited to about 10% overrated frequency, so the range of speeds is not as large as other drives.
The CSI drive must be matched to the motor, and for optimal operation, the motor should be specifically designed for the CSI it is going to operate, which increases the overall cost of the system. To provide a constant current source requires a large inductor in the DC bus section that can actually be physically as large as the motor.