Motor Starting - VFD (Supplementary Post)

A while ago I wrote a series of posts on starting methods for three phase motors. One of our readers pointed out that it would be good to include a post on using variable frequency drives( VFD) to start motors. Initially I had left this out as there are a lot of similarities between using a VFD and an electronic soft starter. After a little more thought, VFDs probably do merit their own page - and here it is.

VFDs are complex devices and offer a multitude of advantages during normal motor operation. At the moment we are only concerned with the starting of motors and will not touch on the use of VFD during operation; this in itself would be a mini-series.

Any thing you can accomplish with a electronic soft starter you can probably accomplish with a VFD. However, there are a few additional things you can do with a VFD.

To summarize they keys points of soft starters:

ramp the voltage up during starting to control current and torque
varying ramps which can be matched to mechanical requirements
setting of initial voltage
current limiting control
thermal overload protection
can be used for controlled stopping

If you've not read the Motor Starting - Electronic Soft Start page, you should perhaps read it now, particularly if any of the above sounds strange. I won't be repeating the soft start post, just discussing additional material.

What's additional in a VFD that electronic soft starters don't do?

While a soft starter is designed to bring the motor up to speed and then go out of circuit, a VFD is designed not only to start the motor, but to control (change its speed) it during operation. This is achieved not by controlling the firing angle as for a soft start unit, but by a mechanism called pulse width modulation (PWM).

In PWM, insulated gate transistors (IGBT) are switched on and off at a high frequency (typically tens of kHz). The width and frequency of the on pulse is varied, effectively varying to current [rotating magnetic field] to the motor therefore varying the speed. As the width of the PWM signal can be arbitrarily varied so can the speed. While this has a huge impact on the operational performance and what can be achieved with the motor, it also gives additional options during motor starting.

Not only are we able to effectively vary voltage levels, but we can use PWM to alter the speed curves during start up (remember we are going from zero to operational speed, so we are not setting a fixed speed during starting). Some of the things which can be achieved during starting, included:

adjust both frequency and voltage during starting to provide smooth acceleration
adjust both frequency and voltage during starting to provide high torque at less than full load current
provide almost full torque at zero speed

A couple of precautions to bear in mind. Typical motors design may rely on the speed of the motor to drive cooling air across the motor. If you are running the motor a low speeds for extended periods, the cooling air flow may be reduced and present a problem. While it is useful to think of the output as a sine wave (0 to 200 or so Hz), in reality it is a very high frequency wave. Measuring instruments need to have to correct frequency response, high frequency noise may be an issue and there is the possibility of injected harmonics. VFDs are also more costly.

Despite the negatives, the amount of motor control which can be achieved by the application of VFDs makes them widely accepted and used in numerous applications. If you are using a VFD for operation, then your starting issues will be much easier to deal with.