Voltage Spikes with VFD

Voltage spikes

Standard AC power that comes from the electric company, has a smooth sinusoidal wave, much like a wave in the ocean. VFD's will convert standard AC into DC power and store it in a capacitor, which works much like a battery. The VFD, working as a radio transmitter, draws power from the capacitors and transmits positive and negative pulses, which simulate AC power to the motor. This type of radio transmission is what causes harmonics, radio frequency interference, stray voltage, dirty power, which is discussed in another article.

Voltage spikes are measured in several ways. How high they go is one measure. How often they occur is another. How fast they rise is very important. In the graph below you can see the smooth wave lines of standard AC voltage. Superimposed over the smooth wave are multiple square waves, which are created with pulsing DC voltage from a VFD. You can see that the time between peaks with standard power is about eight times longer than the time between spikes with a VFD. You can also see that the rate of rise for the square wave is almost instantaneous, while the sinusoidal wave takes it's time to reach the peak voltage. Then you can see that there are 12 square wave pulses in the same time frame as 1 wave of standard power. What can't be seen well in this picture is the height that the square waves spike. It is almost impossible to limit the pulses of the square waves to the 230 volts required. Even with filters on the electronics, the pulses can spike 2 or 3 times the required voltage. This means that a 230 volt motor running on a VFD can see spikes of 460 volts, 690 volts, or even more.

The rapid rate of rise along with the high spike voltages from a VFD, causes partial discharges in the windings of the motor. Partial discharges are when little bits of copper burn off the wires of the windings and disappear into smoke. Any air gaps or weaknesses in the insulation of the windings are the first to go. Even well insulated wires cannot take this abuse for long before the motor is shorted and fails. Square waves from a VFD can come in 6, 12, or 18 pulses, the more times per second these pulses occur, the higher the heat generated. The graph below shows multiple square wave pulses in 1/60th of a second.

"Constant Pressure" has many benefits for your water system. "Constant Pressure" is really the only job that a VFD is trying to accomplish. The Cycle Stop Valve or CSV, was designed to mimic the "constant pressure" delivery of a VFD, without needing the computerized controls that create so many problems. The best technology is not always electronic. The best technology is one that has been reduced to the simplest form that will accomplish the job at hand. The CSV is newer technology and has been used as a reliable replacement for VFD's every day since 1993. The CSV reduces energy consumption of a pump as much as varying the speed with a VFD. The CSV works with standard AC motors and controls, that do not see the destructive rapid rate of rise, and multiple high voltage spikes caused by a VFD.

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