Constant Torque and overload

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NE (9.06 miles @5.9 Degrees from Winged Horses)
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I have two 1 hp motors with gear reduction set up on VFDs and constant torque. Augers. The drives are tripping on overload @ about 30 hz. I can change the sheaves to allow the motor to operate more closely to full rpm but I feel they will still trip on overload once the load rpm approaches the "limit" where the drives trip now. Yes/No?
 

Jraef

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San Francisco Bay Area, CA, USA
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Electrical Engineer
I have two 1 hp motors with gear reduction set up on VFDs and constant torque. Augers. The drives are tripping on overload @ about 30 hz. I can change the sheaves to allow the motor to operate more closely to full rpm but I feel they will still trip on overload once the load rpm approaches the "limit" where the drives trip now. Yes/No?
Aside from all the good stuff in that previous thread, you also need to know that most VFDs have an over Load calculation that takes the driven speed into consideration. So regardless of the speed setting, the fact that it is tripping on OL is, in the case of a constant torque load, indicative of the motor being under sized for that application.

But here's the rub. Often times, VFDs are misapplied in cases like this where the old method of speed control was a Varidrive, or there was no speed control, but there was a gearbox. The issue is, the old method of lowering the working speed, being mechanical, entailed a corresponding INCREASE in torque at the shaft. Then is the gearbox or Varidrive is removed, and the motor HP is unchanged, now even though you have the correct speed, you do NOT have as much shaft torque at the load! What should have happened is that someone should have factored in the mechanical torque advantage and used that to determine the correct motor size when replacing the gearbox with the VFD, in other words, up-sized the motor.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
There were 5hp varidrives in there before. Upper management came by during this winters remodel and said 1hp is all they needed. Operators said the drives were tripping at about 30 hz but the fault history showed no overloads when I got there today. Of course they were shut down so I couldn't do any other checks. My first time at modbus control so I could very well be guilty of something.
 

bob

Senior Member
Location
Alabama
The main problem "Upper management came by during this winters remodel and said 1hp is all they needed" By God it must be true.
It has to be the operators fault.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
There were 5hp varidrives in there before. Upper management came by during this winters remodel and said 1hp is all they needed. Operators said the drives were tripping at about 30 hz but the fault history showed no overloads when I got there today. Of course they were shut down so I couldn't do any other checks. My first time at modbus control so I could very well be guilty of something.
Not much you could do on modbus control that would make it trip on OL!

Gotta love the "5HP only needs to be 1HP" thing... they probably forgot about the mechanical advantage issue as well. With a Varidrive using a 5HP motor at 1/2 speed, it would have had the equivalent shaft torque of a 10HP motor.

A VFD is essentially reducing HP with speed. So a 1HP motor at 30Hz is a 1/2HP motor. If you needed 1HP on the shaft at 1/2 speed, you would need to start with a 2HP motor.
 

philly

Senior Member
But here's the rub. Often times, VFDs are misapplied in cases like this where the old method of speed control was a Varidrive, or there was no speed control, but there was a gearbox. The issue is, the old method of lowering the working speed, being mechanical, entailed a corresponding INCREASE in torque at the shaft. Then is the gearbox or Varidrive is removed, and the motor HP is unchanged, now even though you have the correct speed, you do NOT have as much shaft torque at the load! What should have happened is that someone should have factored in the mechanical torque advantage and used that to determine the correct motor size when replacing the gearbox with the VFD, in other words, up-sized the motor.

When you mention an INCREASE in torque at the shaft when lowering the speed mechanically are you simply saying that when lowering the speed via a gearbox on a constant torque load the torque load required on the motor will be less? By lowering the gearbox speed the motor torque ratio through the garbox is increased so therefore for a constant torque at the load, the motor would see less of a load when lowering the gearbox speed. I guess this is the same as saying that the motor would now have more avaliable torque as seen by the gearbox output?
 
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