Variable Frequency drive (VFD) types

There are three types of Variable frequency drives available in Market. These VFDs differ primarily in the type of rectification by which they convert AC power to DC power.

1. Voltage Source Inverters
2. Current source inverters
3. Pulse width modulated inverters

1.Voltage Source Inverter VFD’S

These types of VFD’s are most widely used in industries as they have multi-motor control facility. The VSI was the earliest solid state VFD. It is also known as "six-step" drive because of the voltage sent to the motor.


 

Variable source inverter operation is relatively simple. In that drives AC input voltage and frequency is converted to DC by rectifiers, then converted back to AC through the inverter section. Desired O/P Voltage and frequency is generated at VFD O/P so as to meet the volts per hertz ratio at the VFD output. 

2. Current Source Inverter VFD’S

These types of VFD’s are also called current-fed inverter these behave like a constant current generator which produces an almost square-wave of current.

These type of VFD’s are used instead of Variable source inverters for large VFD’S about 200 HP as these VFD’S have simplicity, regeneration capabilities, reliability and lower cost.
But there are some implications with these VFD’s are that they have poor power factor at low speeds, and are not suitable for multi-motor operation.

There is another disadvantage of CSIs is "cogging,"  or jerky start/stop motions or pulsing shafts while running. Now days VSI type drives come even upto 500 HP as they have multi-motor operation facility.

3. Pulse Width Modulation VFD’S

These VFDs are used where constant V/f ratio required as they deliver a constant voltage hertz ratio with no line notching and very stable current input for the motor. These VFD’s have main advantage over Voltage and current inverters is that they have high efficiency, constant power factor which doesn’t depend on speed as in case of current inverters. These VFD’s also have multi-motor operation.

These VFD’s have pulse width modulation converter section which consists of a diode bridge to rectify AC power, rather than an SCR bridge. As the input to the inverter section is constant, the inverter controls both voltage and frequency. Inverter section might consists of transistors, GTOs, or SCRs.

But the main disadvantage is that these VFD’s require extra parts for line regeneration capabilities and also have more noise. PWM units create significant audible noise.