The U.S. Department of Energy (DOE) is implementing minimum efficiency limits on pumping systems and their components. Meanwhile, end users are demanding systems with ever higher efficiencies. One way pump manufacturers satisfy the needs of both groups is by introducing solutions that make it possible for pumping systems to change speed to maintain peak efficiency when output demand changes.
On pumping systems driven by electric motors, this efficiency boost is typically achieved by adding a variable frequency drive (VFD) to the motor. A VFD is an adjustable speed drive that controls alternating current (AC) motor speed and torque by varying motor input frequency and voltage. Many in the pump industry are familiar with how VFDs work and their efficiency benefits. What is not so commonly understood is how to protect motors from the potentially harmful effects that a VFD can produce and how to reduce these effects in a pumping system.
Protecting a Motor from a PWM Waveform
Many motor manufacturers offer motors designed to run on a VFD or inverter. These motors have been specially designed to be operated when powered by a VFD’s pulse width modulated (PWM) power waveform. PWM is a modulation technique used primarily to control the supply of the voltage and current waveform to a motor. It is often preferred because it is a highly efficient method of motor speed control.
A PWM waveform can, however, create issues within a motor. For example, it can cause a motor winding to experience voltage spikes that are well above the rated voltage of both the motor and standard motor winding limits.
Motors that are to be used with a VFD, therefore, should have improved insulating materials and processes—compared to a standard insulation system—to protect against voltage spikes well above their rated voltage.
According to National Electrical Manufacturer’s Association (NEMA) MG1 Part 31, motors with a voltage rating of 600 volts or less that are used on VFDs should have windings that protect, at a minimum, against a voltage spike of 3.1 times the rated voltage. For motors with a voltage rating greater than 600 volts, the minimum is 2.04 times the motor’s rated voltage.
Protecting Against Unbalanced Charge
PWM waveforms can also impact an electric motor’s bearings. When driven by standard sine wave power, the three phases powering the motor have a balanced charge. In other words, when one phase is at +460V, the second is at -460V and the third phase is at zero. The PWM waveform, however, is not a true sine wave. Pulsing direct current (DC) voltage creates an imitation sine wave, which causes an issue with the charge balance within the motor. A differential charge builds up between the rotor and stator that needs to be balanced. That charge is caused by common mode voltage (CMV).
As anyone who has touched a door knob in winter knows, electricity rectifies this imbalance by finding the lowest resistance path to ground. The shock you get when you touch a doorknob in the winter is a smaller scale version of what happens within the bearing of a motor that is not installed and protected correctly. If bearings are not isolated from the shaft and the system is incorrectly grounded, the bearings provide the path of least resistance that a motor seeks to balance the charge. When this balance happens within a bearing, it is called electro discharge machining (EDM).
When EDM occurs, pieces of material from the bearing can dislodge while the motor is running, which can severely damage the bearing—or at least cause noise, heat and premature failure over time. One way to prevent CMV is to give the motor a low resistance path to balance the charge between the rotor and stator. This is commonly achieved by adding a shaft grounding device to the motor and grounding the motor. On larger motors, additional protection can be gained by insulating the bearing opposite the shaft grounding ring to eliminate circulating currents.
Addressing Root Causes
While helpful, these motor protection features do not get to the root of what causes motors to be subjected to the damaging effects caused by PWM waveforms.
To make a variable speed pumping system robust and operate reliably, it takes more than assembling a pump, motor and VFD together. These components must be integrated into, and operated as, a single system that has been designed to mitigate issues caused when using PWM power waveforms.