over speeding induction motor

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QES

Senior Member
Location
California
What is the current/voltage characteristics when you go over 60Hz or "overspeed" a 3phase induction motor?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
If the motor is being fed with a fixed 60Hz supply and the shaft is forced to turn faster than synchronous speed, then the machine starts operating as a generator. It still consumes inductive VARs to produce the magnetic field, but it produces real VA and feeds them to the supply.

Very approximately:

If the 'overhauling load' maintains a constant speed, then the current (both VARs consumed and VA produced) will adjust in proportion to the connected terminal voltage, thus torque loading and power produced will vary as the square of the terminal voltage.

If the 'overhauling load' maintains a constant torque, then for small changes in applied voltage the output current will vary inversely as the terminal voltage.

Or, are you asking what happens if a frequency greater than 60Hz is applied?

-Jon
 

Besoeker

Senior Member
Location
UK
What is the current/voltage characteristics when you go over 60Hz or "overspeed" a 3phase induction motor?
The speed increases, the available torque reduces, and the available output power stays the same.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
What happens above 60Hz depends upon the capability of the inverter.

If the inverter is at maximum output voltage to drive the motor properly at 60Hz, then the inverter won't have the necessary output voltage to maintain proper V/Hz ratio as the frequency increases. Torque falls off as Beseoker describes with constant power up to some higher frequency, then starts falling off even faster as you hit the magnetic limitations of the machine.

If the inverter is capable of increasing its output voltage as needed past 60Hz, then you will retain the full output torque capability of the drive and motor up to whatever frequency causes the inverter to hit its max output voltage, and then the torque will start to fall off.

One approach that you might consider if if you need the full torque capability of a motor above 60Hz is to connect a dual voltage motor for 240V, but then drive it with a 480V inverter. At 60Hz the inverter is configured to output 240V, properly driving the motor, but the additional voltage is available to drive the motor at higher frequency.

-Jon
 

jdsmith

Senior Member
Location
Ohio
The speed increases, the available torque reduces, and the available output power stays the same.

Are there any problems to watch out for when overspeeding a motor with a VFD? We are in the design phase for a compressor installation that requires 325 HP at 720 RPM and can operate at a max speed of 760 RPM, where it would require 375 HP. The VFD and motor application engineers both say I would be best served by a 10-pole (720 RPM) 400 HP motor on a VFD rather than an 8 pole (900 RPM) motor on a VFD. I don't have a full torque curve for the compressor, but it supposedly requires much less torque than the rated curve of either 400 HP motor. I couldn't get a good answer out of anybody exactly why I should overspeed up to 63.3 Hz rather than run a 900 RPM machine at 51 Hz, assuming that the speed/torque curves of both machines are sufficient for the load.
 

Besoeker

Senior Member
Location
UK
Are there any problems to watch out for when overspeeding a motor with a VFD? We are in the design phase for a compressor installation that requires 325 HP at 720 RPM and can operate at a max speed of 760 RPM, where it would require 375 HP. The VFD and motor application engineers both say I would be best served by a 10-pole (720 RPM) 400 HP motor on a VFD rather than an 8 pole (900 RPM) motor on a VFD. I don't have a full torque curve for the compressor, but it supposedly requires much less torque than the rated curve of either 400 HP motor. I couldn't get a good answer out of anybody exactly why I should overspeed up to 63.3 Hz rather than run a 900 RPM machine at 51 Hz, assuming that the speed/torque curves of both machines are sufficient for the load.

Simple quick answer is that, at 51Hz, your 400HP motor will give only 340HP at rated current.
 

philly

Senior Member
Simple quick answer is that, at 51Hz, your 400HP motor will give only 340HP at rated current.

Would HP really matter as long as the motor could supply the required torque at this particular speed? I know HP is a function of torque and speed.
 

Besoeker

Senior Member
Location
UK
Would HP really matter as long as the motor could supply the required torque at this particular speed? I know HP is a function of torque and speed.
The 8-pole machine at 51Hz gives the right speed but less power thus less torque.
 

philly

Senior Member
The 8-pole machine at 51Hz gives the right speed but less power thus less torque.

So if I understand correctly you are saying that the 8-pole motor will have less available torque then the 10-pole motor which I see as true. Even though running the 10-pole above base speed will cause some of the torque to drop off, there will still be more torque avaliable with this motor. Am I understanding you correctly?

Seeing that these are both the same power motor (400 hp) I tend to look at things from a torque perspective? Is this an incorrect approach for such problems?
 

Besoeker

Senior Member
Location
UK
Seeing that these are both the same power motor (400 hp) I tend to look at things from a torque perspective? Is this an incorrect approach for such problems?
What you need to consider is that the motors will produce 400 hp at base speed and rated voltage. The 8-pole machine doesn't get to base speed so won't produce 400 hp.
For both motors running at around 760 rpm, the one producing more power, in this case the 10-pole machine, will also produce more torque.
 
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