The Basics of Variable Frequency Drives, Part 1

Variable frequency drives are becoming more and more common within the pump industry. As such, it is important for pump users to understand the basics of VFDs.

A Variable Frequency Drive (VFD) is a solid-state device that inputs a fixed frequency AC current and outputs an AC current at a user-controlled frequency.

This makes a VFD an ideal speed controller for an AC motor, as the rotational speed of the magnetic field inside an AC motor determines RPM, and the speed of the field rotation is a function of the frequency of the electrical input.

Modern VFDs also vary the voltage along with the frequency; hence, they are sometimes also called VVVF (variable voltage variable frequency) drives.

A VFD has three distinct sections, each of which performs a discrete operation. These are:

An input converter that changes AC current to DC current.
A DC Bus that filters, or smoothes, the DC voltage.
An output converter (referred to as the inverter) that changes DC to AC at a user-controlled frequency/voltage.

The working of each of these sections is quite complicated and as such will be a topic for another day, so today we will stay on the subject and look first at how a VFD interacts with an AC motor.

Current In Rush

Electric motors, when started by a simple on-off switch, draw a considerable amount of current for a short time. This can often overload the electrical supply available and cause a breaker to trip or a fuse to blow. A VFD can limit starting current to as low as 100% of full load current( FLA), in effect eliminating inrush. To illustrate the magnitude of the problem, a 460 volt 150HP will have a full load current of approximately 180 amps. It may, however, have an inrush current of over 900 amps for over a second with a DOL start (direct on-line start). The graph below compares the inrush that can be expected with various starting methods.

A VFD can manage the magnetic flux of an induction motor. Magnetic flux is directly proportional to the voltage and inversely proportional to the frequency. By keeping the flux constant, the inrush current does not exceed the FLA rating of the motor, and full torque is maintained.

Motor Torque & HP Output

A motor controlled by a VFD can provide full torque from 0 to 100 % of rated speed . Anything over 100 % and the torque output falls off.

HP is a function of speed and torque; therefore, HP climbs proportionally with speed until the motor’s rated HP is reached, and then it stays constant.

Centrifugals and available HP

The HP required by a centrifugal pump increases with the cube of operating speed, as illustrated by the blue and red lines in the graph below:

The green line illustrates the HP output from a typical VFD.

Clearly, the VFD provides more than enough power from 0 to 100% of the rated speed.

However, at speeds over 100 % the HP available does not increase, but the HP the pump requires continues to climb exponentially. In short , an overspeed application can only succeed if the VFD is rated at a higher output than the required input of the pump.

Pretty sure that is enough for today. Stay tuned for next time, when we will look in some detail at the advantages and pitfalls to avoid when applying VFDs to pumps.

Cheers