Electric motors have been around since the 1800’s creating torque for many applications. As applications got more sophisticated the need to vary the speed of the motor began to surface. Enter the Variable Frequency Drive (VFD). Before we go much further let’s cover the basics of motors to get the foundation of how the VFD works.
A motor is designed to run at one constant speed based on supplied AC frequency and number of poles. They consist of several main parts including enclosure housing, stator windings, rotor, motor shaft, bearings, and lead connection box. The motor speed is based on the number of poles configured in the stator windings.
Since we want to change the motor speed for different applications that is accomplished by varying the frequency. Hence – Variable Frequency Drive. The relationship is the stator speed in revolutions per minute is a function of 120 times the applied frequency divided by the number of poles. Here is the math:
(120 x 60 Hertz) / 4 Poles per Phase = 1800 Revolutions per Minute (RPM)
AC Induction motors can operate at various speeds from zero to above 60 Hertz with a VFD. The motor acceleration ramps up speed can be adjusted to reduce the motor inrush current. Motor inrush current can be from 8 to 20 times the maximum motor torque current rating depending on the motor load. The electrical equipment supplying power to the motor would have to be rated at 20 times the motor current if the motor was started across the main supply power line. For example a 145 Amp motor would require a 3,000 Amp circuit breaker.
Another advantage is the motor deceleration ramp down speed can be adjusted from 0 to 6,000 seconds. In a pump application, the ramp down speed can prevent the pump shut-off from causing water hammering and save on unnecessary wear on the plumbing system.
Other advantages are precise motor speed control with proportional-integral-derivative control (PID) and S-shaped speed up and slow down control. An example is a bottle feed conveyor that must start slowly, speed up and then slow down without damaging the glass bottles.
Any application where the AC motor must be controlled for speed, accuracy or energy savings is an application for a Variable Frequency Drive.