WorldWide Drilling Resource

68 APRIL 2015 WorldWide Drilling Resource ® Long Live the VFD! by Britt Storkson Owner, P2FlowLLC Variable Frequency Drives (VFDs) are incredible technology. A VFD will vary the speed of any three-phase AC (alternating current) induction motor from full on to full off, and even provide motor braking with the correct additional com- ponents. We are seeing more and more variable speed control as the price of silicon semiconductors used to make VFDs comes down and the price of copper and steel used to make electric motors goes up. When used for water pumping, VFDs can eliminate pump panels, soft starts, and phase converters. Most VFDs pro- vide outputs that can indicate motor speed, amperage load, motor voltage, and total energy used. It is nearly impos- sible to damage a motor connected to a VFD. If there is any problem condi- tion detected, the VFD will quit and dis- play a code telling you why it quit. Almost 100 motor control functions are moni- tored, and if anything is not within the specification ranges, the VFD won’t run. All VFDs can run on single-phase power, but must be oversized by a fac- tor of 1.73 because single-phase current per watt is 1.73 times three-phase cur- rent. So if you have a 10-horsepower three-phase motor running on single phase, you will need a 20-horsepower VFD to service the motor properly. The single-phase input current will be 1.73 times the motor nameplate rating, so the input wiring must be sized accord- ingly. There is a caveat to the above state- ment which should be discussed. Some VFDs have phase loss detection on the input voltages. Mostly, this feature is found on more expensive VFDs because it costs the manufacturer more money as it requires additional components. If your VFD has this feature and it is not disabled and you are using only two wires with single-phase power, the VFD will not run even if everything is cor- rectly sized and installed, because the VFD cannot detect a voltage on the “open” leg. All VFDs use a diode rectifier set to convert the AC into DC (direct current). This current is “filtered” by one or more high-voltage capacitors to produce a mostly constant DC voltage. Since con- necting the capacitors directly to voltage source would result a current surge which would blow the breaker or fuse, the current is first routed through one or more resistors until the capacitors fully charge, and is then reconnected to the voltage source. The high-voltage DC produced by this process is used to produce an output waveform which is adjusted to satisfy motor speed and current load require- ments. No input voltage detection at the AC inputs could present a problem if you are using three-phase power and “lose a leg”. If it happens, the motor will continue to run, but the input current will nearly double. Instead of the current load being spread across six rectifier diodes and more or less dissipating the same amount of heat, only four rectifier diodes are being used with almost dou- ble the heat being dissipated per diode. If you are running the motor at or near full load rating, the diodes will over- heat and fail. The circuit breaker or fuse may or may not blow depending on how much current is used and the breaker or fuse rating. If you are running the motor at part load, the heat may be dis- sipated adequately, but this is not a good condition because we don’t know how much heat the four rectifier diodes can safely dissipate. So it is best not to go there. The motor may run fine at part load and then blow the breaker or fuse when the load amperage gets clos- er to full load. There are a number of ways to detect loss of a “leg” if your VFD does not have this feature, but this adds cost and complexity. Then the question be- comes whether the extra cost and complexity is worth the potential bene- fits it provides. It’s a cost versus benefit issue just like most everything in the marketplace. Britt Britt Storkson may be contacted via e-mail at michele@ worldwidedrillingresource.com The first gasoline-powered automobile in America was taken for a test drive in Springfield, Massachusetts, on April 19, 1882.