need advice: motor speed & voltage decrease

[yhg]

now ,we test the energy saving system for induction motor,we found the problem:when the motor work at light load,the energy saving system reduces the motor voltage,the important thing is the motor speed decrease a little too.

 

 

so my question is :are all the motor speed decrease a little when the voltage decrease? can it keep the motor speed constant when reduce the voltage?

 

thank you !

 

[marke]

Hello yhg

 

Welcome to the forum.

The energy saver operates by reducing the flux in the iron when full torque is not required.

The reduction in flux reduces the iron losses.

Flux reduction is achieved by reducing the voltage applied to the motor stator. The reduced flux reduces the induced voltage in the rotor and so the rotor slip increases to increase the rotor voltage. The system can only operate by increasing the slip, but the slip should not increase beyond the full load slip. ie a motor with a rated speed of say 1489RPM at full load wil operate at a higher speed under no load, say 1497 RPM.

With the energy saver applied. provided that the motor is correctly fluxed, the slip will increase, but the motor should not operate at less than full load speed, in this case 1489 RPM. If the motor slow further than this, the saver is under fluxing the motor and you will have performance problems.

 

Best regards,

[yhg]

Thanks for taking the time to respond.

 

we have test a motor with a rated speed of 2940RPM, when the motor operate without the energy saver under very light load ,it is 2990RPM, With the energy saver applied, the voltage reduce from 380V to 290V, and the speed reduce from 2990RPM to 2986RPM, so the speed decrease a little.

 

so i want to know whether the speed must be decrese when voltage decrease in the light load.

and whether the speed can be keep constant when the voltage decrease a little (i.e 380V to 370v)

[mariomaggi]

yhg,

an asynchron motor in open-loop mode cannot have a very precise speed.

From cold to hot rotor, fro cold to hot stator windings, you could have some speed changes.

Generally speaking, the speed will decrese when voltage decreases.

 

To improve speed stability, if the full available motor power is not used, you could ask a motorist to mill some grooves on the rotor surface, to obtain a quasi-synchron motor.

 

You have to make something like this , but grooves must be only few millimeters deep.

 

Regards

Mario

 

[marke]

Hello yhg

 

so i want to know whether the speed must be decrese when voltage decrease in the light load.

and whether the speed can be keep constant when the voltage decrease a little (i.e 380V to 370v)

 

Yes, if you reduce the voltage, the flux in the stator will alter. For any given shaft torque (load) the speed will change as the voltage changes, however with a standard induction motor, that change will be small.

The slip (difference between running speed and synchronous speed) is a function of the shaft torque and the flux in the iron. The flux is a function of the stator voltage, so the slip will increase with reducing voltage, and the slip will also increase with increasing shaft torque (load).

 

I would not expect that in real application, the difference between 2990 and 2986 will be noticed or have any affect on the machine operation, so I would say that this is really of academic interest only.

The slip will change by a greater amount as the shaft load is altered.

 

Best regards,

[yhg]

thanks for your reply .