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3ph, 208V vs 240V motor question


dan_m

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I have three phase power in my shop, but the voltage is 208 not 240, and I'm confused about what the implications of this are. After doing some research online, I've read conflicting information as to what a 208/240 rating on a motor means. Some people said that it means the motor will run okay on either input, but others stated that the 208 rating simply indicated the low end of acceptable input voltage, and that a true 208V input will often run below this, causing the motor to draw more amperage than it is rated for. Does anyone know which of these is correct? Also, if the latter is true, is it acceptable to use a 240V motor with more capacity than is needed to accommodate the lower voltage (ie, using a 240V, 7.5HP motor when only a 5HP is needed)? I'm aware that if this won't work I can always get a buck/boost transformer, but it appears it would be cheaper to just get a slightly oversized motor if that will work, or is even necessary. Thanks!

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Most motors have a small box on the side where power connections are made. Inside there are winding leads that can be rearainged to accommodate various input voltages.

208 is from a Wye connection at the transformers, IE each transformer has a common connection to make a neutral, and the other pole is the power output, this gives 120 volts single phase, 208 single phase, and 208 3 phase. A delta has the transformers in a configuration where each pole connects to the next transformer and is also an output, and will give 120 single phane, 240 single phase, and 240 3 phase. a neutral comes from the center tap of one transformer.

You can use the 240 motor as wired in 208 but it will be at reduced power. change the internal connections (and most larger motors are set up this way) and you get full power at 208.

Simply call an electrician after you verify there is a bundle of wiring that can be changed around, to do it for you, because your asking this type of question is proof you may be asking for trouble if you attempt it yourself, and do it wrong.

Steve IBEW jiw 305

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