The Basics of Variable Frequency Drives, Part 2

In Part 1, we covered how VFDs work and also discussed their use in controlling AC motors. Today, I would like to discuss some of the pitfalls associated with VFDs.

Sizing for a driven RPM at less than 60 Hz

There is a common mistake made when sizing a motor and VFD around a pump required to operate with a drive running at less than 60Hz. This is best illustrated by an example.

If we have a pump that requires operation at various speeds between 700 and 875 rpm and plan to use a 1750 motor, then we will need to use a setup capable of running at a maximum of 30 Hz. If we follow the pump hp curve (the red line on the right-hand graph below) up to where it intersects the 30 Hz line, we see that the pump would need approximately 100 HP. Some people would stop there and order a 100 hp motor and VFD. WRONG! You actually need to follow the green line up to where it intersects the 60Hz line. In this case, 200 HP; hence, a 200 HP motor is required, not 100 HP.

To further illustrate the point, the graph above to the right shows the required pump HP (the red curve) significantly above the available HP from a 100 HP motor (the green line) at all frequencies/pump speeds.

Note: Even with a 200 HP motor, the drive and motor would still be undersized if the operating speed is increased to over 30 Hz.

Some would say that the cost associated with a 200 HP drive for a 100 HP pump is an expensive way to achieve variable speed, and that would be true. The solution to that issue is to use a 2 to 1 reduction belt drive between the motor and the pump. In the example above, this would allow the motor to run at 60Hz, providing the 100 hp required at a pump speed of 875rpm, while the VFD can still provide variable speeds below 875 RPM.

Heat Dissipation

VFDs are ideal for running electric motors at lower than rated RPM for sustained periods of time. Operators of such equipment are, or should be, aware of the reduced cooling available from the motor fan at these speeds (especially with constant torque applications or at altitude). The picture to the right illustrates the result of not discussing this issue with your motor supplier prior to installation.

The VFD itself is also a heat source and must be cooled. This can sometimes be an issue in dirty or dusty environments where forced ventilation would be difficult.

Power spikes and harmonic distortion

Power spikes in the electrical supply can be a common source of “ nuisance tripping” of the drive’s circuit breakers. The solution to this is to install an input line reactor upstream of the controller.

Input line reactors are electromagnetic devices, with coils that protect variable frequency drives (VFDs) by limiting the rate of current rise, reducing harmonics, and absorbing power line disturbances like spikes and transients. By connecting the reactor in series between the power source and the VFD, it protects the drive from damage and nuisance tripping, improves system reliability, and helps prevent harmonics generated by the VFD from returning to the power grid.

A line reactor filters out pulsed and notched distortion, which can minimize interference with other electronic equipment like computers, PLCs, telecommunications systems, instrumentation, and other VFDs.

VFD Generated Voltage Spikes

A VFD in some applications can generate harmful voltage spikes and electrical noise that can harm a motor. In these applications, an output line reactor, also known as a load reactor, should be installed. As illustrated to the right, this is installed between a variable frequency drive (VFD) and a motor.

The need for a load reactor stems from the operating principle of the VFD. In short, a VFD generates a pulsed output known as a Pulse Width Modulated (PWM) signal to control a motor’s speed. These pulses can interact with the distributed inductance and capacitance of long motor leads, resulting in amplified peak voltages at the motor terminals. The pulses also contain sharp edges and high-frequency noise. The output reactor works by adding impedance to the circuit, which:

Filters electrical noise: It suppresses the high-frequency electrical noise and voltage spikes produced by the VFD’s rapid switching.
Softens the waveform: The reactor “tempers” or “softens” the sharp voltage pulses, creating a smoother and more reliable current waveform for the motor.
Reduces stress on the motor: By conditioning the waveform, the reactor reduces voltage stress on the motor’s insulation, which helps the motor and its lead cables run cooler and extends their lifespan.

The question then becomes, when do I need to use line reactors? Generally, use a 5% impedance load reactor on the load side of a VFD when the motor lead length exceeds 100 feet. Typically, you can use two 5% impedance reactors in series (10% impedance) if the motor leads exceed 1,000 ft in length.

However, whenever possible, discuss the use of a VFD with your motor supplier as VFDs without output reactors can cause motors to run noisier, hotter and have a shorter bearing life.

Having scared you with all the possible problems associated with VFDs, I will leave you today with a promise of a part 3 that explains the significant advantages of VFDs.

Stay tuned!