Motor Insulation Systems (5)

One mechanical stress phenomenon that is more likely to appear on inverter applications than line-started ones is resonance (when a mechanical system oscillates at it’s natural frequency). A common example of resonance is the vibrations noted on the side view mirror on an old car. As the car accelerates from standstill to freeway speeds, engine and frame vibrations are transmitted to the mirror’s mounting base. At some point during acceleration, these vibration frequencies change and the mirror stabilizes again. Motor, pump and machinery designers all take resonance into account when designing their product.

They will add mass, change support struts, or increase mounting base lengths to ensure that the item’s natural frequency is well above 60Hz. When the machine is assembled onto a base, coupled to a motor, and bolted to a concrete pad, the natural frequency decreases, but, by design, remains higher than the running speed when excited to 60Hz. However, as the machine speed is changed with an inverter, the likelihood of stumbling onto the system’s resonant frequency increases dramatically. Once resonance is reached, severe vibrations can occur in the motor, stressing stator coils, brinelling bearings, and even fatiguing bolts and castings to the breaking point. As the coils continue to move, they’ll ultimately chafe through all layers of insulation, and a failure will result. Since this new resonant point is determined not by the parts of the machine, but instead by the assembly of parts, it must be corrected at the system level. This is best done during start-up. Although additional supports can be welded onto bases and belt ratios altered, the most cost effective method to avoid these resonance frequencies is to program an offset to the critical frequency. During acceleration and deceleration the inverter will pass through the critical resonance frequency but the critical frequency offset will prevent the inverter from operating at the programmed frequency band, thus avoiding the mechanical resonance.

Voltage or Dielectric
The Dielectric properties of a material are the characteristics of the material that make the material an electrical insulator rather than a conductor. When there is a voltage difference across the thickness of an insulating material, the voltage causes a Dielectric Stress that opposes the material’s ability to prevent current from flowing through the material. The Dielectric Strength of a material is a measure of the material’s capability to withstand dielectric stress. An insulation
system’s rated operating voltage is determined by the dielectric strength of the insulating materials. If the insulation is subjected to excess voltage, it can fail suddenly and catastrophically. Gradual deterioration can be caused by voltage levels that exceed the insulation rating but do not cause catastrophic failure. The other forms of stress – thermal, environmental, mechanical and vibration – lead to a reduction in the insulation’s ability to withstand dielectric stress. The insulation ultimately fails when it can no longer withstand the applied voltage and the flow of short circuit current causes catastrophic failure.


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