Motor Torque and HP vs Current

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Pony motors are used on boiler fans that are exposed to hot air when one of the two fans is not running. The slow rotation keeps the fans from warping.

On turbine generators a pony motor (turning gear) rotates the turbine and generator rotors to prevent warping a bow in the shaft from their weight.

Maybe the process has some material in this crusher/grinder that creates problems if it is not kept agitated.
 
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O.K. it turns out that there is indeed no electrical advantage to starting this motor with the pony motor. The pony motor is strictly for a mechanical maintnance purpose and for bedding the table of the mill prior to starting the main motor. Thanks for all the help guys.

My one last question is for what happens to a motor that is started while it is spinning backwards? If a motor is frespinning backwards and then started, I have always seen that this trips the breaker that the motor is on. (Must be a very high current value). What is the theoretical reason behind this? Does the motor somehow draw more than locked rotor current in this situation?

Also is it true 100% of the time that the current will be less for a motor that is started while already spinning in the correct direction?
 
Motor Runnign Backwards

Motor Runnign Backwards

The inital (1/2 cycle) inrush current that builds up the magnetic field is essentially the same whether the motor is stopped, full speed, or full reverse speed. But the current is a lot different once the magnetic field generates currents and torques in the rotor.

Look at your Thevanin circuit model. If the motor is spinning backwards, slip is > 1.0 and the rotor equivalent resistance is negative, resulting in more than normal accelerating current. Also, the motor has to apply torque to slow down and stop the reverse spin then accelerate up to speed sp it is drawign high accelrating current longer than normal. Either that intial high current or the extended high current trips the breaker.
 
philly said:
Also is it true 100% of the time that the current will be less for a motor that is started while already spinning in the correct direction?
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Yes, it will be reduced but not a lot unless it is running at close to normal operating speed as the curves below show. The current for this particular machine doesn't drop much until the speed is above 90% of synchronous speed.

MOTORCURVES07-10-08.jpg
 
weressl said:
I think it is more accurate to say no linear relationship.
Click to expand...
True. Sloppy on my part.
I should have said there isn't directly proportional relationship between input current and output torque rather than there isn't a direct relationship between current and torque.

weressl said:
When you look at the calculations for torque, current is ALWAYS a component of it, eg. if other components of the equitation would not change, the current would have a linear relationship to torque.
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Rotor current, yes pretty much.
For input current you have an approximately fixed magnetising current which is there regardless of output torque.
You could subract this fixed current from the stator current for different loading but you can't do it arithmetically since the magnetising current is a principally a reactive (quadrature) component and the torque producing element is an active (in phase) component.
There is an approximately linear relationship between the active component of the input current and output torque.
But not between input current and output torque.
I'm not sure that I have explained this very well.
Here it's evening of the 31st and so far we have had 68 trick-or-treaters.
Entertaining for sure, but not entirely conducive to continuity of thoughts.
Oops - make that 72.
 
Besoeker said:
True. Sloppy on my part.
I should have said there isn't directly proportional relationship between input current and output torque rather than there isn't a direct relationship between current and torque.


Rotor current, yes pretty much.
For input current you have an approximately fixed magnetising current which is there regardless of output torque.
You could subract this fixed current from the stator current for different loading but you can't do it arithmetically since the magnetising current is a principally a reactive (quadrature) component and the torque producing element is an active (in phase) component.
There is an approximately linear relationship between the active component of the input current and output torque.
But not between input current and output torque.
I'm not sure that I have explained this very well.
Here it's evening of the 31st and so far we have had 68 trick-or-treaters.
Entertaining for sure, but not entirely conducive to continuity of thoughts.
Oops - make that 72.
Click to expand...
Spot on - for me.

The onslaught is about to start here, I stopped counting years ago. What I don't like is that it is not just a neighborly activity but parents actually bus kids around in from distances away. I have my house-slaves handing out the condiments......;)

Happy Halloween, although it is beyond me why should a morbid event be happy...:)
 
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