Starting a large motor can create huge “ripples” in the power signature of the supply. All loads in the facility may experience transient voltage spikes alternating with voltage dips.
That first voltage spike might be several thousand volts. During its short life, it can puncture insulation on hundreds of conductors. The next spike will be an order of magnitude smaller, and so will the next, until the oscillations stop.
That first voltage dip means a corresponding rise in current. Momentarily, the system will “try” to deliver multiplies of the intended current (heating up conductors).
The voltage and current dance back and forth, damaging equipment. If you have “mysterious” failures of various loads, put a power monitor on a few feeders and have it record the power signature. Most likely, you’ll find this dance going on.
It isn’t the large motor per se that’s causing these problems. It’s the fact it’s starting across the line and demanding that inrush current all at once.
A solution you’ll find with your own home air conditioning system is to start the motor with the stored energy of a capacitor. This can also work for industrial settings. In fact, you’ll find capacitors on punch presses and other production equipment sometimes for this very reason.
This solution has its drawbacks. Two solutions that are better are the softstart and the motor drive. These are adjustable and can be tailored for the exact application. We’ll look at other benefits in the next issue.