Frequency / Current motor curve 2

[SOLBAR]

I have a 460 VAC 200 HP electric motor rated 1800 RPM
I am using a VFD (45 HZ) to run this motor at 1400 RPM
My load is a fan pulling 180 HP at that speed
The problem I am having is that my VFD faults out because of a over current situation.

Is there a graph/curve showing generally what is happening with frequency and current with a constant load or can someone explain what is happening here.

Thanks.

 

[ScottyUK]

You're at 78% speed and drawing 90% of the base power. Voltage normally decreases linearly with frequency, although there is sometimes a bias at low frequency so the voltage doesn't actually go to zero.

Do the maths: decrease voltage to 78% of base volts. Decrease power to 90% of base power. Current has to go up to 115% of base amps to maintain this relationship.

Power factor might skew the numbers a little, but the principle holds: your drive isn't big enough.


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

What he said.

The HP/Speed relationship is essentially linear. So (assuming 60Hz) at 45Hz you are at 75% speed, the HP load the VFD can provide for is 75% of 200HP or 150HP. You have 180HP connected to a drive and motor that can only provide 150HP, it's the definition of an Overload.

Either reduce the load (air flow) or buy a bigger motor and drive.

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[SOLBAR]

Does the curve follow above 60 HZ

This application is belt driven, so if I swap my motor sheaves and run this setup at 90 HZ while maintainig the same load from the fan. Would this approach eliminate the over current problem.

 

[rockman7892]

A vfd can provide rated torque throughout the entire speed range. At rated torque the current should stay the same throughout the entire speed range. As Jraef said however the load hp is greater then the motor hp at the 45hz speed. So therefore at a required 180hp at 45hz the load requires a greater torque which equates to a larger current.

 

[LionelHutz]

If this fan always runs at 1400rpm then the most energy efficient solution would be to change sheaves to run the motor at 60Hz and eliminate the VFD. You will have 200hp available and all will be good. This assumes you can get the fan up to speed without the VFD. FYI, the VFD will be adding around 5kW to 7kW of losses to your system.