|Forum topic by TopamaxSurvivor||posted 01-25-2011 02:22 AM||2218 views||0 times favorited||5 replies|
01-25-2011 02:22 AM
3 phase comes in 2 common models in the US for small motors. Delta, which is a triangle transformer connection with the power wires coming off the corners of the triangle. The other and most common, at least in the NW, is a Y. The transformer is connected with the 3 phases coming off the ends of the Y. The center of the Y is grounded so as to create a neutral. From each corner to the grounded center is 120 V. From any end to any other end is 208. All 3 ends make 208 3 phase. Any 2 corners are 208 single phase.
The delta gives 240 volts from any corner to any other corner. All three corners make 240 3 phase. Any 2 corners of the triangle is 240 single phase. You ground the center of any side of the triangle to get access to 120 volts. When you ground the one side, you have 120/240 single phase with those 3 wires. The 4th wire coming from the odd corner gives what is commonly known as a “high leg.” It is supposed to be marked Orange. The voltage to the neutral on the high leg is approximately 190 volts. If you see a panel with a 120 volt circuits and every third breaker is missing, it is 120/240 3 phase with a high leg. The high leg can only be used for 240 single phase or 240 3 phase connections. If it gets connected on a 120 volt circuit, it will probably cause the smoke to come out of the equipment. We all know, electrical devices work best with the smoke left intact!! ;-))
The other common 3 phase voltage is 480/277 Y. Most 3 phase motors will have 9 leads in the connection box for dual voltage connections; either 208/480 or 240/480. If the motor is marked 240, it will run on 208, but it needs a Service Factor (SF) of 1.25. Motors used to have SF of 1.15 or 1.25. I have seen them with 1.0 lately. The SF tells you how much of an overload the motor will handle without any damage.
Connecting 3 phase motors using single phase is much easier with the latest Variable Frequency Drive technology. Basically they erase the voltage coming in and give you anything you want out. A VFD is a much better option than an electronic phase converter. The converter just fools the motor into thinking it has 3 phase and is not very efficient. The other option is a rotary phase converter. It is much better that an electronic phase converter and will start motors under load such as pumps or compressors. The electronic versions will start no load motors such as table saws just fine. With the technology of drives, that is the method of choice today.
-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence