Operating a motor above 60hz

[buddy91082]

Anyone know the cons of operating a motor above 60hz? We will be using a vfd to operate above 60hz, around 25% above rated frequency.

I assume there would be a reduction in the life of the motor.

Thanks.
b

 

[ScottyUK]

Bearing life will be reduced, perhaps severely reduced depending on how far above base speed you want to go. There will be a safe maximum speed for the rotor, and 25% overspeed on a 2-pole machine will approach or exceed it. Larger machines have lower safe maximum speeds.

Talk to the manufacturer.


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

 

[wolf39]

For a squirrel cage motor there should be no problem, unless mechanical problems can be an issue (vibration, centrifugal forces, bearing temperatures, etc.). Why don't you ask the motor OEM?

Wolf

http://www.hydropower-consult.com

 

[ScottyUK]

Oh, torque will drop off above base speed unless you can maintain the V/Hz ratio above base speed. This is a problem for many loads.


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

 

[jraef]

I would strongly suggest using a good "inverter duty" motor as well. Cooling issues may be different than you would think, depending on the motor. Some have fans that will not move much more air at increased speed, but the friction and electrical losses may be higher so the motor cooks. Inverter duty motors usually are designed with these issues in mind.

You didn't state the nature of the load. Keep in mind that with most centrifugal loads (pumps and fans), flow increases with speed and the Power it requires increases at the cube of the speed change. So at 125% speed you might need almost twice the HP, but at above base frequency you are in Constant HP operation for that motor. This may be a problem even if you CAN increase the motor voltage to maintain the V/Hz ratio as ScottyUK says, because the load power requirement is increasing faster than the motor HP output.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[buddy91082]

Thanks. Good information here.

jraef - these are supply and exhaust fans.

Anyone know if reduction in motor life can be quanitifed as a percentage? So is if overspeed by 10%, how much % motor life is reduced, ball park #.

b

 

[waross]

If these are 1750 RPM motors I would not worry. 3500 RPM motors may have more issues that may reduce the life.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wolf39]

waross is right. Again, ask the OEM to design a custom-made motor.

Wolf

http://www.hydropower-consult.com

 

[ozmosis]

As they are supply/exhaust fans, I would monitor the current now at 60Hz. If it is way below the rating of the motor and the motor details are set correctly into the VFD, then increase the Hz by 1 or 2 Hz at a time and monitor the current. As you increase the fan speed the current will increase and therefore flow. Once you get to the optimum frequency or if the current tops out, then stop.

 

[ScottyUK]

Doing that while allowing the V/Hz ratio to fall away will result in the motor running a lot further up the slip curve than normal, and the rotor will get abnormally hot as a result. It will likely contribute to the early demise of the mbearings as heat will be conducted out by the shaft as well as roasting the stator from the inside.


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

 

[DickDV]

Based on numerous motor manufacturers' published data, taking cast aluminum rotor NEMA motors with four or more poles up to 90hz does not result in shortened motor life or hot rotors. In fact, I routinely design motor-power train-drive systems so max load speed is 90 hz on the motor.

Yes, you have to be sure the available motor torque will run the load but this is rarely a problem due to the higher power train ratio.

On two pole motors, i limit max hz to 75hz unless the fan gets changed out from plastic to metal.

Since the same bearings often get used in 2 and 4 pole motors, I don,t see the concern for bearings.

Of course, if you are dealing with motors larger than a 440 frame or if it has a built up rotor, then a call to the manufacturer would be in order.

 

[MaintenanceGeek]

Variation from Rated Frequency:

Alternating - current motors shall operate successfully under running conditions at rated load and at rated voltage with a variation in the frequency up to 5 percent above or below the rated frequency. Performance within this frequency variation will not necessarily be in accordance with the standards established for operation at rated frequency.

Combined Variation of Voltage and Frequency:

Alternating - current motors shall operate successfully under running conditions at rated load with a combined variation in the voltage and frequency up to 10 percent above or below the rated voltage and the rated frequency, provided that the frequency variation does not exceed 5 percent. Performance within this combined variation will not necessarily be in accordance with the standards established for operation at rated voltage and rated frequency.

Effects of Variation of Voltage and Frequency Upon the Performance of Induction Motors:

Induction motors are at times operated on circuits of voltage or frequency other than those for which the motors are rated. Under such conditions, the performance of the motor will vary from the rating. The following is a brief statement of some operating results caused by small variations of voltage and frequency and is indicative of the general changes produced by such variations in operating conditions.
With a 10 percent increase or decrease in voltage from that given on the nameplate, the heating at rated horsepower load may increase. Such operation for extended periods of time may accelerate the deterioration of the insulation system.
In a motor of normal characteristics at full rated horsepower load, a 10 percent increase of voltage above that given on the nameplate would usually result in a decided lowering in power factor. A 10 percent decrease of voltage below that given on the nameplate would usually give an increase in power factor.
The locked-rotor and breakdown torque will be proportional to the square of the voltage applied.
An increase of 10 percent in voltage will result in a decrease of slip of about 17 percent, while a reduction of 10 percent will increase the slip about 21 percent. Thus, if the slip at rated voltage were 5 percent, it would be increased to 6.05 percent if the voltage were reduced 10 percent.
A frequency higher than the rated frequency usually improves the power factor but decreases locked-rotor torque and increases the speed and friction and windage loss. At a frequency lower than the rated frequency, the speed is decreased, locked-rotor torque is increased, and power factor is decreased. For certain kinds of motor load, such as in textile mills, close frequency regulation is essential.
If variation in both voltage and frequency occur simultaneously, the effect will be superimposed. Thus, if the voltage is high and the frequency low, the locked-rotor torque will be greatly increased, but the power factor will be decreased and the temperature rise increased with normal load.
The foregoing facts apply particularly to general-purpose motors. They may not always be true in connection with special-purpose motors, built for a particular purpose, or as applied to very small motors.
Operation of General-Purpose Alternating-Current Polyphase 2, 4, and 8 Pole, 60 Hertz Integral-Horsepower Induction Motors Operated on 50 Hertz:

While general-purpose alternating-current polyphase 2, 4, 6 and 8 pole, 60 Hertz integral-horsepower induction motors are not designed to operate at their 60 Hertz ratings on 50 Hertz circuits, they are capable of being operated satisfactorily on 50 Hertz circuits if their voltage and horsepower ratings are appropriately reduced. When such 60 Hertz motors are operated on 50 Hertz circuits, the applied voltage at 50 Hertz should be reduced to 5/6 of the 60 Hertz horsepower rating of the motor.

When a 60 Hertz motor is operated on 50 Hertz at 5/6 of the 60 Hertz voltage and horsepower ratings, the other performance characteristics for 50 Hertz operation are as follows:

Speed
The synchronous speed will be 5/6 of the 60 Hertz synchronous speed and the slip will be 6/5 of the 60-Hertz slip.
Torque
The rated load torque in pound-feet will be approximately the same as the 60 Hertz rated load torque in pound-feet. The locked-rotor and breakdown torques in pound-feet of 50 Hertz motors will be approximately the same as the 60 Hertz locked-rotor and breakdown torques in pound-feet.
Locked-Rotor Current
The locked-rotor current (ampere) will be approximately 5 percent less than the 60 Hertz locked-rotor current (amperes). The code letter appearing on the motor nameplate to indicate locked-rotor KVA per horsepower applies only to the 60 Hertz rating of the motor.
Service Factor
The service factor will be 1.0.
Temperature Rise
The temperature rise should not exceed 90 °C.

 

[Tmoose]

Are they inverter rated now? We have had a few "standard" ones that the manufacturer said were OK from the persepective of insulation quality and winding encapsulation for electomagnetic wiggling, etc.