Breakdown Torque Of Motor

[rvim002]

Hi,

 

I am looking at a spec sheet of a motor which has a starting torque of 140% and a breakdown torque of 241%.

 

Now, if the load that is attached to the motor is small ( say 50% at the rated speed and even less than that at speeds below it..), does the motor still produce 241% of breakdown torque. ie, as the motor accelerates from 0 speed to rated speed, at the speed at which breakdown torque occurs in the torque speed curve, does the motor practically produce the 241% torque inspite of the motor load being small?

 

Thanks in advance...

[marke]

Helo rvim002

 

Yes, if the motor is started with Full voltage, it will produce the torque from the motor torque speed curve at all the sppeds from zero to full speed.

 

At any speed, there will be some torque used by the load, internal fan and friction. Any surplus torque is acceleration torque and transfers energy into the intertia of the motor and load and accelerates the load to full speed.

 

If the motor torque is very high, and the load is very low, then the acceleration torque will be very high and so the rate of acceleration will be higher than if the acceleration torque was low.

 

The rate of acceleration is a function of the acceleration torque and the inertia.

Best regards,

Mark.

[rvim002]

Thanks heaps Marke... that clears all my doubts..

[mariomaggi]

rvim002,

I agree with marke.

 

Instead, if load, friction and load inertia are very low, therefore the motor will start in milliseconds, it is possible that breakdown torque will be never reached.

 

Best regards

Mario

[rvim002]

Thanks Mario,

 

But it is not the case here.. The load is low.. But not too low for the motor to start in milliseconds.. It will take approximately 3 seconds to start DOL and hence should reach the full breakdown torque when it passes through that speed.

 

Thanks.

[yuri]

Thanks Mario,

 

But it is not the case here.. The load is low.. But not too low for the motor to start in milliseconds.. It will take approximately 3 seconds to start DOL and hence should reach the full breakdown torque when it passes through that speed.

 

Thanks.

 

According to the diagram from an other thread

 

http://www.lmpforum.com/inforum/post-a96-T...-curve.JPG.html

 

I can imagine that with only half of the full load

 

the Locked Rotor Torque of an induction motor may be 75% of the full load torque - FLT,

 

then, when rotor's speed reaches some 15% of the synchronous speed (but, probably, more soon only at 7-8% - because there is only half load) the torque falling to some 60% of the FLT,

 

then, from this point - "pulling up" - untill speed reaches some 75% of the synchr speed (probably, again, only 50-60% - because there is only half load) the rise in torque till some 150% of the FLT - the Breakdown Torque,

 

then abrupt fall in torque till some 50% of the FLT (now the motor being at rated speed/slip).

 

I can imagine that this "breakdown torque" exceeding the "locked rotor torque" (required from an engine accelerating taking place at an point when the speed already has reached about half of the maximal speed) is characteristic not only of el motors but of any engines accelerating their moving parts and the loads attached.