Can a VFD Solve This Problem?

[retirede]

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

In other words, he needs a 45HP motor and the VFD!

 

[GoldDigger]

The utility of a VFD solution, therefore, depends on two factors not yet in evidence:
1. Can the motor be safely operated at 1/45 the speed by a VFD? This depends on both the motor and the VFD. And whether the motor will be adequately cooled during the 90 second run and whatever cooldown before the next operation.
2. Can the motor, driven at 1/45 the speed, produce the needed torque. If the 1 HP motor was specified to use its full available torque, the 10 HP motor will have to be driven to 4.5 x full load torque. Hope that is not the case.

 

[Jraef]

...
2. Can the motor, driven at 1/45 the speed, produce the needed torque. If the 1 HP motor was specified to use its full available torque, the 10 HP motor will have to be driven to 4.5 x full load torque. Hope that is not the case.

Huh?

If the motor is 10HP, it has 10X the torque of a 1HP motor (assuming the same base motor speed, another fact not in evidence). The VFD will maintain that same torque regardless of speed. So at 1/45th speed, it is still 10X the amount of available torque as the 1HP motor at whatever speed it was operating at.

The bigger problem to me is the turn-down ratio. 1/45 speed on a 1750RPM motor is 1.33Hz. If the motor is designed as an "Inverter Duty" with a 100:1 turn down ratio (probably TENV at best), it would work. Most likely though it is not, so it will need some critical investigation of duty cycle and possibly forced cooling.

At some point the desire to keep this motor is going to outweigh the costs to implement it.

 

[retirede]

Huh?

If the motor is 10HP, it has 10X the torque of a 1HP motor (assuming the same base motor speed, another fact not in evidence). The VFD will maintain that same torque regardless of speed. So at 1/45th speed, it is still 10X the amount of available torque as the 1HP motor at whatever speed it was operating at.

I think he’s saying that the gearbox supplied with the 10 HP motor has a ratio that is 45 times higher that what would have been supplied with a 1 HP.

So the torque available at the gate is 1/45 of the 10 x torque or about 22% of what the correct motor/gearbox would have supplied.

 

[paulengr]

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?

This can’t easily be fixed for a couple reasons.

You have 10 times the torque but then geared down by 1:45 so torque is now 10/45 or about 1/5th of the original but it sure sounds like a typical “gate motor” so probably acceptable from the gate company and works fine, just very fast. Sometimes procurement with those companies can be SLOW though.

Which brings up the problem. This is what we call a constant torque load. And it’s probably a custom motor. On NEMA motors when run near their design torque you can run them down to about half speed (50%). After that the fan is now at 1/4 of the cooling (fan CFM is square of speed) so even though Horsepower is half what it was fan cooling dropped even faster. So the motor is in danger of burning up unless it doesn’t get run very long for very often (duty cycle) and again probably difficult or impossible to get full motor specs from the company that wants to sell you the proper gear motor and not mess with hack jobs and possible warranty claims.

And finally at under 2-3 Hz control over the motor becomes a problem. You are getting close to the point where the drive output begins to look like DC. Flux vector doesn’t work. I’m assuming encoder control might be impossible (buying another motor) despite being your best option and V/Hz is notoriously finicky. So control is going to be a huge problem if it doesn’t burn up first.

Also those gate motors tend to be high torque so slip is low which compounds your control issues.

 

[drktmplr12]

i would ask the contractor to verify with the gear and gate mfr that this is an acceptable way to install and operate their presumably warrantied equipment. when something breaks papers, could fly, and I don't mean dollar bills.

 

[Jraef]

I think he’s saying that the gearbox supplied with the 10 HP motor has a ratio that is 45 times higher that what would have been supplied with a 1 HP.

So the torque available at the gate is 1/45 of the 10 x torque or about 22% of what the correct motor/gearbox would have supplied.

Yep, I see it now, I was thinking backward.