The conversion of power from Alternating Current (AC) to Direct Current (DC) and then back to AC is not a clean transition, as power distortions created during conversion can result in unwanted additional voltage and current, causing overheating and high-voltage stress in the motor. Applications using sensitive clock or timing functions can also become disordered due to the damage to motor and power supply cables.
Ideally, the motor should anticipate a power pulse and regulate the correct amount of current provided to sustain the speed increase. However, where non-linear power (a change in voltage without the same change in current) is necessary, the current does not properly support the motor’s requirements, this contributes to high-voltage stress and generates excessive heat.
To illustrate with an example, when inversion occurs, the voltage must rise from zero to 650 volts, then back to zero approximately 20,000 times per second. During this process, the nominal voltage can overshoot from 650 volts to 2,000 volts or more. A longer length of power supply cable typically experiences greater and more intense voltage spikes than a shorter cable length.
Although voltage spikes last for only a millionth of a second, permanent damage may result with improperly designed cables.
During initial motor start-up, there may be an inrush of current, causing the motor and power supply cable to act as a large capacitor that must be charged up to its normal operating level. When the motor is first energized, there can be a draw of up to six times its full load power requirements. Hence, it is critical that the installed cable is of adequate American Wire Gauge (AWG) size to avoid any significant voltage drop.