Protecting from damage on PWM systems can lead to long-running solutions for customers.
By Patrick Hogg
To satisfy demand for even more efficient pumping systems, pump manufacturers are introducing solutions allowing these systems to change speed when output demand changes.
One common way of achieving this speed control is by adding a variable frequency drive, or VFD, to the motor driving the pump. A VFD is an adjustable-speed drive that controls AC motor speed and torque by varying motor input frequency and voltage.
Users are often familiar with a VFD’s ability to help pumping systems maintain peak efficiency. Less understood, though, is how to protect motors from the potentially harmful effects a VFD can produce and how to reduce these effects in a pumping system.
How a Drive Works
Many motor manufacturers today have dedicated product lines that can operate with a VFD. 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 frequency and voltage supplied to a motor. A rectifier, DC bus, and inverter work together to output a waveform mimicking a sinewave at the desired frequency. PWM is often preferred because it is an effective method of motor speed control.
A PWM waveform can, however, create issues within a motor affecting winding and bearing life, among other things.
First, PWM can cause a motor winding to experience voltage spikes well above standard voltage limits for motor windings. Standard sinewave power on a three-phase AC line voltage, for example, transitions from one peak to the next in about 8 milliseconds (Figure 1). PWM voltage can pulse from minimum to maximum in less than 0.001 millisecond (1 μs).
Voltage spikes above the insulation limits can cause motor insulation to break down more quickly than if motor windings are limited to the rated maximum voltage.
Another phenomenon that increases the number of peak voltages applied to the motor windings is wave reflection. Wave reflection is a function of rise time and cable length. When reflective waves bounce back to the source, they can become additive to the incoming waves. Longer cable lengths between the VFD and motor increase the number of times waves experience this additive property.
Fast-rising voltage pulses can also lead to premature bearing failure. This is because the high switching frequency inverters used to achieve the required PWM waveform induces more capacitive energy. This capacitive energy builds between the rotor and the stator and needs a release.
If the path it takes is through the motor bearings, it can damage the bearing through what is known as electro discharge machining (EDM). EDM causes parts of the bearing’s raceway to dislodge. When this contaminant is rolled over and around the bearing as the motor rotates, it causes the bearing to generate heat and noise, potentially leading to premature failure. This damage is known as fluting.
PWM waveforms can also impact an electric motor’s bearings because of voltage differential between the rotor and stator resulting from the inverter’s fundamental construction and use. When driven by standard sinewave 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 is not a true sinewave. Pulsing DC voltage creates an imitation sinewave, which creates an issue with the charge balance within the motor. The differential charge that builds up between the rotor and stator needs to be balanced. This is what is known as common mode voltage (CMV) (Figure 2).
As anyone who has touched a door knob in the winter knows, electricity rectifies this unbalance by finding the lowest resistance path to the ground. The small shock you get when you touch a door knob in the winter is a smaller scale version of what happens within the bearing of a motor not installed and protected correctly. This, too, causes EDM and overall fluting of the bearing.
If the bearings are improperly protected and the system is not optimally installed, the bearings provide the path of least resistance to balance the charge difference and energy buildup.
Protecting a Motor from PWM
With proper motor selection including winding and bearing protections, it is possible to mitigate some of the effects a PWM waveform can have on a motor.
Motors to be used with a VFD, for example, should have improved insulating materials and processes—compared to a standard insulation system—to protect against voltage spikes well above their rated voltage.
NEMA specifies the maximum peak voltage that inverter duty motor insulation must be able to withstand, as well as the minimum rise time for the power waveform. According to 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 and a rise time greater than or equal to 0.1 μs
For motors with a voltage rating greater than 600 volts, the minimum is 2.04 times the motor’s rated voltage with a rise time greater than or equal to 1 μs.
Bearings also require additional protection to prevent CMV from causing the issues mentioned earlier (Figure 3). One way to achieve these protections is to give the charge buildup a low-resistance path to ground. This is commonly achieved by mounting a shaft grounding device to the motor—either internally or externally—and grounding the motor in accordance with local codes.
On larger motors (100 hp and larger), additional protection can be gained by isolating one of the bearings from the motor shaft. This interrupts circulating currents that add extra energy to the motor shaft.
Typically, this involves coating the bearing mount with an insulating material to isolate the shaft from the bearing. Insulating both bearings is an option, however, when the motor is located in a hazardous environment and must be able to handle shaft voltage. In this case, an insulated coupling is often the preferred option.
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. To fully realize their benefits, these components must be integrated into and operated as a single system designed and installed to mitigate issues caused when using PWM power waveforms.
If a system is already installed, it is more difficult to change the setup. But there are potential system add-ons that can reduce damaging effects. These include:
- Load reactors: inductive devices that eliminate high peak voltages and slow quick-rise times
- Inductive absorbers: inductive devices that can help reduce some damaging bearing currents
- dV/dt filters: that limit peak voltage to a motor
- Sinusoidal filters: basically dV/dt filters with added tuning electronics to match carrier frequency for best output waveform.
Beyond such add-ons, CMV and other issues created by the interaction between the motor and VFD can be mitigated by following proper installation techniques, including the following.
VFD installation: When installing a VFD, it is important to make sure the power cables running from the VFD to the motor are shielded and specifically rated for use with a VFD. Check with the manufacturer to ensure they are the recommended size for the system’s voltage and current limits.
Grounding: The motor will not be completely protected if it alone is grounded. The VFD produces high frequency noise. To reduce its damaging effects, the motor needs to be grounded back to the drive as well.
Grounding the VFD involves more than simply running a cable back to the common ground on the drive. The motor should be grounded back to the drive using a braided type of grounding wire that is, at minimum, the same size as a single power lead and runs in the same
conduit as the power leads. Multiple grounding wires should also be considered to get the best mitigating effect.
Conduit selection: The conduit used to house these power leads and grounding wire should be metal. Also, the conduit should be connected to both the motor and drive without isolating either.
In other words, PVC, plastic, or other insulating materials should not be used to connect metal conduit into a drive or motor terminal box. If any of these materials are used, it is critical the metal conduit that carries the power leads and ground cable is properly connected to the grounding circuit. The VFD manufacturer may also be able to recommend filtering options that can reduce risk even further.
“With proper motor selection including winding and bearing protections, it is possible to mitigate some of the effects a PWM waveform can have on a motor.”
Taking a Systems Approach
To protect against PWM damage to a motor, a motor manufacturer can add extra protection on the winding, shaft grounding, and insulated bearings. Additional protections can be added to a VFD’s output as well.
Manufacturers add these protections because they cannot predict where, how, or when these components will be installed. But there is a way to mitigate these effects before they have a chance to start, and that is by designing and installing these components as an integrated system.
To achieve the most reliable, robust, and a long-running solution, the motor and VFD should be treated as a single system. If the motor is grounded back to the drive, and the wire, conduit, and filter are correctly sized, the result should be a variable speed drive system operating
according to specification.
Patrick Hogg is application engineering manager of general industry and integral horsepower pumping for Nidec Motor Corp. He can be contacted at firstname.lastname@example.org.