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#1 mark2007

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Posted 03 July 2007 - 01:18 AM

Hi
Right now we have one machine which runs with VFD. The current specification of motors are 1740 rpm at 59.2 Hz ant 380 volts. The KW is 5.5.This is maximum as given.
I would like to increase the speed of motor by increasing the frequency. I would like to know the potential problems in doing so. My intention to increase frequency by 10% of maximum.

regards



#2 jraef

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Posted 03 July 2007 - 05:28 AM

The torque design rating of a motor is based upon it's design voltage and frequency, denoted by a "V/Hz Ratio". So if you have a motor designed for, in example, 380V 50Hz, it is designed to provide a certain amount of torque at a V/Hz ratio of (380/50) 7.6V/Hz. So as long as you maintain that ratio, all is right with the motor.

That is, until you get to where you have no more voltage! A VFD cannot create voltage that isn't there, so if your supply is 380V, you cannot go above 50Hz without some loss of torque, because at max. voltage with higher frequency, the V/Hz ratio goes down. For example, if the above motor were run at your current stated speed of 59.2Hz, the voltage is still no higher than 380, so your V/Hz ration is (380/59.2) 6.42, already 14% lower than what the motor was designed for. If your load NEEDS full torque at that speed, this means that your motor will overload. If you raise the frequency another 10% to 65Hz, then your V/Hz ratio will drop to 5.84, a good 23% lower than the motor's rating. If your load doesn't need all of the motor's FLT and can live with 23% less, then it will work. If not, it will overload.

If it is a pump or a fan however, load generally increases at the cube of the speed. So if your motor was sized to do a job at 50Hz, and you are running it at 65Hz, the load on the motor will increase by (65/50 = 1.3, cubed = 2.86) 286%! So now your load has almost tripled AND your motor torque has dropped by 23%, so it is highly unlikely to work unless that motor was 3 times as big as it needed to be to start with.
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