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Torque

Torque

(OP)
Hi,

I know this topic had been discussed previuosly in this forum.I am looking for a direct relationship between torque and power consumed above rated speed for VSD application.We know that the output torque is reduced because of reduced V/F and overfluxing of the iron core.But the power consumption is more dependent on the application and in some cases increases with increased speed.What happened to the output torque in this situation?

Regards.

Guardiano

RE: Torque

If speed increases and torque doesn't decrease in proportion to inverse speed, then power will increase. Not much to it. Happens all the time. At some point, the load torque gets too high for the motor or the drive. Then it either stalls or trips or refuses to increase speed any more.

Is there any special application you are thinking about? And what are your observations or problems?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Torque

There are two limits on the torque that a motor may output.
As the load on a motor is increased, the current increases. When the current exceeds rated current the motor will run hotter. Some overloading may be allowable depending on the ambient temperature as it relates to the maximum design temperature  and the pattern of the overloading. (Is the overload steady or are there adequate "cool down" periods.)
As the overload becomes greater, the heating of the motor increases and shortens the life of the motor. Moderate overloading may reduce the number of years that the motor will  last. Extreme overloads may reduce the expected lifetime of the motor to a few minutes.
As the torque demand or overload increases the second limit may be reached. This is the maximum torque that the motor may produce at the applied voltage and frequency. When this limit is reached the motor stalls.

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

RE: Torque

I'm not sure I follow your question. The torque is always required by the load. The motor will quit accelerating if it doesn't produce the torque required by the load.

Power consumption is proportional to torque x rpm + motor and VFD losses.

Reducing the V/Hz doesn't over flux the motor. You'd have to increase the V/Hz to do so.

RE: Torque

Quote:

But the power consumption is more dependent on the application and in some cases increases with increased speed. What happened to the output torque in this situation?
Motor output torque always matches load torque (steady state).   At some point, you may reach a limit of the motor /drive, then what happens? See comments from Gunnar and Bill.

So you should consider load torque variation with speed somewhat independent of any motor torque LIMIT variation with speed, up until the point when the motor torque limit becomes limiting.

=====================================
(2B)+(2B)'  ?

RE: Torque

Quote:

But the power consumption is more dependent on the application and in some cases increases with increased speed. What happened to the output torque in this situation?
Motor output torque always matches load torque (steady state).   At some point, you may reach a limit of the motor /drive, then what happens? See comments from Gunnar and Bill.

So you should consider load torque variation with speed somewhat independent of motor torque LIMIT variation with speed, up until the point when the motor torque limit becomes limiting.
 

=====================================
(2B)+(2B)'  ?

RE: Torque

Maybe this is an easier way to think about it. In a motor that is being driven beyond base motor design speed, you are operating in a "Constant Horsepower (kW)" mode. So the power consumed will cease to vary* as you increase the speed beyond the rating of the motor. Your V/Hz ratio is going down IN EFFECT because you have run out of voltage as you increase Hz, so your torque decreases. But you are still doing the same amount of WORK, defined as torque at a given speed. If, as mentioned above, the motor torque output drops below the load torque requirment, you CEASE to do work because the motor stalls.

*Other than the fact that you may have some minor additional losses in the VFD and motor.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
  
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RE: Torque

(OP)
Thanks for the comments.What basically account for the extra losses in the VSD and motor? Is it true to say that the efficiency of the motor is reduced above rated speed?

Guardiano

RE: Torque

Yes. There is more windage and the slip increases when flux goes down. So efficiency is lower above rated speed. And reduces the higher in speed you go.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Torque

(OP)
Thanks all.Skogsgurra,ok I understand that efficiency goes down as speed is increased avove nominal.To what extent the efficiency is reduced? 5 % ?

Gurdiano

RE: Torque

It really depends on the amount of overspeed.

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

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