Can a VFD Solve This Problem?

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charlie b

charlie b

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The contract drawings call for a 1HP motor to raise and lower a gate, about a 90 second evolution. The contractor mistakenly purchased and installed a 10 HP motor. They want to solve the problem by installing a VFD. My feeling is that motor speed and power requirements are unrelated. Can this work?
 
charlie b said:
The contract drawings call for a 1HP motor to raise and lower a gate, about a 90 second evolution. The contractor mistakenly purchased and installed a 10 HP motor. They want to solve the problem by installing a VFD. My feeling is that motor speed and power requirements are unrelated. Can this work?
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Hi Charlie B

I agree with you. The VFD isn't the issue. And it would be silly/expensive to replace the simple 1HP motor.
 
What problem does the contractor wish to solve?

A 10 hp motor with a 1 hp load will use power similar to a 1 hp motor of the same speed rating, but will have the starting current of a 10 hp motor. Presumably it would also need supply conductors sized for a 10 hp motor even if only 1 hp of power is being used.

-Jon
 
It sounds like it may work but It must be a very expensive motor to not just change it to a 1 HP motor and be done
 
Even if you did this the speed is the same and you would just be torque limiting. The drive itself will be close to $1000 while the motor won’t be over a couple hundred dollars.
 
It's installed and the deadline for project completion is fast approaching. Obtaining a 1HP motor and swapping it out is not compatible with the schedule requirements. There will be a teleconference with the design team and the contractor in the next day or so. I will make sure they discuss the wire size and the trip settings. Thanks to all.
 
But again, what is the _problem_ that needs to be fixed?

The situation is that you are forced by circumstances to use an oversized motor for the job, and _something_ about this oversized motor is making things not work.

If they installed a 10 hp motor of the same speed as the intended 1 hp motor, then perhaps there is a way to 're-rate' the motor by operating it at lower voltage; possibly by even re-connecting the motor for a higher voltage than the actual system voltage. If I may: what is the supply voltage, and is the motor a 12-lead motor?

In any case, if you are stuck with the particular motor, a VFD very likely would solve the problem, not by virtue of being able to control the speed of the motor, but by being able to control the voltage and starting current of the motor. One mode that some VFDs use is to reduce the voltage being supplied to a motor in lightly loaded conditions, reducing magnetizing current and total losses. The VFD would eliminate the initial inrush current of the motor, as well as the locked rotor starting current, and then reduce the losses as the motor is powering the very low load.

-Jon
 
Let's approach it this way. What the VFD can help with is:
  1. Excessive torque from the 10HP motor (you can set the torque limit to that of the 1HP motor)
  2. High starting current from the 10HP motor (you can set a current limit similar to that of the 1HP motor starting current)
  3. Finishing the job on time (assuming ready availability of a 10HP VFD in a suitable enclosure and a re-working of the circuit intended for a 1HP motor)

What it CANNOT help with:
  1. Cost (setting aside penalties)
  2. Physical size of the motor (but assuming since it is already installed, they already dealt with that).
  3. Complexity. The VFD adds complexity to something that likely didn't have it.
 
What the VFD will do is avoid any surge current problems caused by across the line starting of the 10 HP motor. There may also be some concern about the rating of the motor for repeated starting within a short time if the gate goes up and down often. The contactor/motor starter would have been larger for the 10 HP compared to 1 HP.
There may also have been an issue with fitting a breaker for the 10 HP motor and wiring accordingly.

UPDATE: What jraef said....
 
I wonder if the higher inertia of a 10 HP motor (and therefore stored rotational energy) could be a concern when the gate completes each raising or lowering operation. If there' a clutch that disengages after each operation is completed then it may not matter. Or if there's a clutch that slips to limit the torque it might not be a problem except for possibly higher long term wear of the clutch. Otherwise a braking resistor on a VFD might be used to help slow the motor at the end of each operation.
 
synchro said:
I wonder if the higher inertia of a 10 HP motor (and therefore stored rotational energy) could be a concern when the gate completes each raising or lowering operation. If there' a clutch that disengages after each operation is completed then it may not matter. Or if there's a clutch that slips to limit the torque it might not be a problem except for possibly higher long term wear of the clutch. Otherwise a braking resistor on a VFD might be used to help slow the motor at the end of each operation.
Click to expand...

I have not seen breaking resistors used even on high speed security gates. In fact with a larger motor it has higher torque and higher thermal mass so the time constant is improved. Even if Wk2 is slightly increased.
 
synchro said:
I wonder if the higher inertia of a 10 HP motor (and therefore stored rotational energy) could be a concern when the gate completes each raising or lowering operation. If there' a clutch that disengages after each operation is completed then it may not matter. Or if there's a clutch that slips to limit the torque it might not be a problem except for possibly higher long term wear of the clutch. Otherwise a braking resistor on a VFD might be used to help slow the motor at the end of each operation.
Click to expand...
I think it's a valid concern, given that the OEM only required a 1HP motor, so the mass of the gate may be low to begin with. But it could possibly be mitigated with proper programming of the drive, i.e. anticipating the End of Travel via the sensors and adjusting the decel settings.
 
Apparently, my colleague who asked me to consult on this issue did not have the entire story. The problem is not so much the HP rating of the installed motor. Rather, the larger motor came with a different set of speed reduction gearing. The resulting rate at which the gate will be lifted or lowered will be on the order of 45 times faster than originally designed. A VFD should be able to help with that problem, should it not?
 
charlie b said:
Apparently, my colleague who asked me to consult on this issue did not have the entire story. The problem is not so much the HP rating of the installed motor. Rather, the larger motor came with a different set of speed reduction gearing. The resulting rate at which the gate will be lifted or lowered will be on the order of 45 times faster than originally designed. A VFD should be able to help with that problem, should it not?
Click to expand...

Maybe.....

If the gate is moving 45x faster, then the power mechanical power requirement is 45x greater (assuming same constant torque torque is 45x higher. But the motor is 10x bigger and thus rated for 10x the torque. The VFD can slow the motor down and can drive the motor in overload to get more output torque, but....

-Jon
 
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