Can any of you explain me how much is the maximum power that can be produced by an induction motor run above rated speed? We are having an 130 kW motor for driving a winch. In case when the motor is driven above synchronous speed what is the maximum power it can generate and at what speed. How about the speed and power output characteristics?

Thanks,

N.Vedachalam
India
e-mail: [email protected]

 

N.Vedachalam,

Asynchronous generation (induction generation) is very common. An induction motor has a synchronous speed (approximately no load speed) which is called its base speed. Electric motors run at a slightly slower under load, about 2% less than their no load speed. When you drive an induction motor about 2% faster than its no load speed it will output the same amount of power back into the power system it was using as a motor. The induction motor continues to get a voltage and frequency reference from the line. Power companies and others have used this for well over a hundred years. There is information on this on the web at: http://home.carolina.rr.com/microhydro/induction.html

I hope this helps,
William Hinton

 

Replying to N. Vedachalam's Fri, Feb 18 query:

When operated hypersynchronously (above synchronous speed) a motor's torque/slip curve has a shape similar tothat for subsynchronous operaton. In other words, if speed is increased above synchronous speed, by an amount equal to its rated slip, then, the stator output current should be limited so as to not exceed its rated current.

However, the required driver torque will be greater than its rated torque. In addition, system efficiency will be lower. Power output can be calculated from the motor design parameters.

You probably realize that when operated as an induction generator, its excitation must be derived from the supply it is connected to. If not, a capacitor must provide the necessary magnetizing voltamperes.

Power Output.
If the rated stator current is not exceeded, then the output powe, Po, as an induction generator will be between 80 and 90% of the motor's kW rating, or about 100 to 120 kW.

Speed.
The above is based on a maximum speed, No, related to rated slip,

No = Nr x ( 1 + 2 x Sr ) where,

Nr = rated speed, rpm.

Sr = rated slip, per-unit.

If additional information is required, just ask.

An aside, hypersynchronous and supersynchronous, are not synonymous.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Typically - the maximum power is defined by the maximum operational speed at which the flux density (V/Hz) remains constant.

When you operate the motor above base speed while not increasing voltage at the motor T-Leads... you are effectively decreasing the V/Hz ratio (voltage is held constant while Frequency increases). Since Torque is directly related to the V/Hz (flux density) you will see torque drop off linearly with increasing speed - thus HP is maximized at the motor rated base speed.

The breakdown torque drops off as a square of the flux density and as such at some point the BDT will approach that of the motor operating torque and the motor will stall. So depending on the BDT characteristics of the motor, you will have a practical limit on how high you can push the frequency before you run out of torque and stall the motor.

Now - if you wanted to operate a dual winding motor on the low voltage leads while using a VFD that operates at the high voltage rating - you can parameterize the drive such that the motor base speed is set to double the synchronous speed of the motor. This will in practice maintain a constant flux density to 2x motor base speed and allow doubling of the HP rating of the motor without risk of damage. Of course, you will need a VFD rated at 2x the motor's low voltage current rating, but in cases where high rotational speeds are desired using a standard 4 pole motor, this is a suitable solution that will have no ill effect on the motor or the drive.

 

Your treatise on VFD drives to operate motors in ranges greater than their normal speed is excellent. However your generalization about doubling of rpm requires a cautionary notice. Although certainly possible, the probability of success is very dependent on the mechanical
design of the rotor. Such a design should be carefully evaluated for higher speeds. In other words critical-speed become an issue.

This is especially true for the size given in the original post query. The centrifugal force is considerable, i.e, 4 times the normal value for the case you made. It affects rotor-bar to end-ring connections as well as rotor-bar to rotor-slot retention methods.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Phil - thank you for your comment - while I have applied this method in a variety of applications upwards of the horsepower ratings per to initial post, these applications involved a significant amount of interface with the motor designers to ensure safe operation to 3x base speed for continuous operation. This required removal of the fan on TENV design motors and precision coupling and motor mounting considerations.