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The Surprising Sinewave-Protect Your Motor

Protect your motor

Protect your motor, cable, and bearing without external dV/dt or sinewave filter

MOTOR OVERHEAT, INSULATION, AND BEARING FAILURE

An AC motor is designed to work with sinusoidal signals. Conventional VFDs output Pulse Width Modulation (PWM) signals to emulate sinewave signals to the motor. These PWM signals cause the motor to overheat. A motor’s insulation lifespan drops by half for every 10ºC (50ºF) of overheating.

In addition, over long cables, the voltage spikes on the rising edge of the PWM signal can double and exceed the voltage rating of the motor’s insulation, causing insulation failure. These spikes also cause corona discharge on the motor’s cable, damaging its jacket and insulation.

Imbalanced PWM signals cause voltage potential in the motor’s structure known as Common Mode Voltage. This creates induced common mode current (bearing current) that generates sparks and arcing through the bearing lubricant, causing pitting, frosting, fluting, and eventually bearing failure.

Illustration of a motor overheating due to Pulse Width Modulation (PWM) signals
Figure 1: PWM signals causing motor overheating and damage
Motor bearing damage: fluting, frosting, and pitting
Figure 2: Motor bearing damage: 1. Fluting, 2. Frosting, 3. Pitting

ELIMINATING PWM DRAWBACKS AT THE SOURCE

The advanced design of the Clean Power VFD uses SiC MOSFET technology to replace the 40-year-old Insulated-Gate Bipolar Transistor (IGBT) design used in ordinary VFDs. SiC switches 50x faster than Si-based IGBTs, enabling the use of smaller passive components and integrated filters 100x to 200x smaller than external filters.

The Clean Power VFD outputs three sine-waves signals to the motor with very low harmonics, extremely low common mode voltage, and low common mode current peak. Your equipment, motor, cable, and motor bearings are thus protected, and motors typically run 10-15% cooler without costly external hardware such as dV/dt or sinewave filters.

EXTERNAL LAB TEST SETUP

An autotransformer with an impedance of less than 2% is used to supply the systems under test with a voltage of 480V. Additionally, 500 ft (152 m) of VFD cable connects a 25HP motor to the systems. An electrical generator (not shown) provides the mechanical load of the motor, controlled by modifying the generator field current.

The same tests were performed on three drive systems:

  • Clean Power VFD
  • Conventional 6-pulse VFD without filters
  • Conventional 6-pulse VFD with a passive harmonic filter and a sinewave filter
Diagram of a lab test setup showing the connection from utility power to a motor through a transformer, DUT, and long cable
Figure 3: Lab test setup for motor testing: Utility power, transformer, DUT, long cable, and motor with measurement point

PERFORMANCE COMPARISON RESULTS

Clean Power VFD Output Signals

Figure 4 shows the captured waveforms for the Clean Power VFD operating at 60Hz output frequency under full load
condition. It clearly shows that the Clean Power VFD outputs sinusoidal signals to motor instead of PWM output signals.

Oscilloscope readings of VFD output voltage, current, and motor terminal voltage, including common mode current
Figure 4: Clean Power VFD output signals showing motor voltage, VFD voltage, VFD current, and common mode current

Common Mode Current Comparison

Graph comparing motor common mode peak current for Clean Power VFD, conventional VFD, and conventional VFD with filters
Figure 5: Comparison of motor common mode peak current: Clean Power VFD vs. Conventional VFDs

The figure 5 shows the motor common mode current for the tested systems. The Clean Power VFD performs better in terms of common mode current, keeping the peak value below 0.07A. 

The conventional VFD has the highest common mode current, with a peak value of about 1.3A. Adding the sine wave filter to the conventional VFD can reduce the peak common mode current by 50%, protecting the motor from damaged bearings due to pitting, frosting, and fluting.

Motor Peak Voltage.

The peak voltage is relatively higher with conventional VFDs due to the VFD output voltage and the mismatch between the long cable and motor impedance. Adding the sine wave filter to the conventional VFD reduces the peak voltage significantly. Similarly, the peak voltage is reduced with the Clean Power VFD, allowing it to be utilized for systems with long cables, reducing motor winding failures and premature motor insulation failure.

Graph comparing motor peak voltage for Clean Power VFD, conventional VFD, and conventional VFD with filters across different motor load percentages
Figure 6: Motor peak voltage comparison: Clean Power VFD vs. Conventional VFDs

Conclusion

The test report presents motor-side test results comparing the Clean Power VFD performance with a conventional VFD and a combination of a conventional VFD and a sinewave filter.

The peak voltage causes motor winding failures and premature motor insulation failure, while the common mode current results in damaged bearings due to pitting, frosting, and fluting. The Clean Power VFD’s superiority is due to high switching frequency, high DC bus voltage, and utilizing medium frequency magnetic as integrated output filtering.

The Clean Power VFD eliminates the need for external filters and mitigation solutions to protect your motor, cable, and motor bearing.

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