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Vfd motor wiring question

dhaddox

Plastic
Joined
Jun 23, 2008
Location
West Virginia USA
I have a 3hp 3phase 2 speed motor on a bridgport style milling machine. It says it is 2/4 pole 3400/1700 rpm. My question is should i wire it up to the high speed and max the frequency at 60hz or wire it up 4pole and try to run at 120hz to get the 3400 rpm. Will it have better low speed power with the 4 pole and what will it do to the highspeed power? Any help appreciated. I have attached a pic of the name plate on the motor if it helps any.
Dave
 

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Running a 4 pole motor at 120Hz is no problem and it will give more power over the speed range. The top end power will fall off a bit, using a 208V motor on a 240V supply will help a little.
 
Depends also on what you want to do at the higher speed. At 120Hz, you will have less than 1/2 the normal torque output. Remember, that is a 3HP 60Hz designed motor and unless you can increase the voltage along with the frequency, it will top out at 3HP when you get to 60Hz. After that it is in "constant HP" operation meaning the torque is dropping as the speed increases. The drop is not linear, so at 120Hz you have only a little torque remaining.



If what you want to do at that high speed is not going to need much torque, you may be fine. But if it does, it will likely stall very easily.
 
"If what you want to do at that high speed is not going to need much torque, you may be fine. But if it does, it will likely stall very easily."

The motor is constant HP (same, or nearly the same rated HP at 4 poles as at 2 poles).

Therefore, for a given rpm, if you select 2 poles and then send 60 Hz to it, you will have considerably more torque than if you select 4 poles and then send 120 Hz to it, as for 2 poles you are within (right at, in fact) the motor's base speed, whereas for 4 poles you are at twice the motor's base speed, and within its variable torque range (where torque declines as the speed increases).

Smoothness of surface finish may or may not be the issue, but torque at a given rpm certainly could be.
 








 
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