Monitoring VFD Power

[gar]

211213-2334 EST

I might know input horsepower, but I may not know the output HP.

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[petersonra]

211213-2334 EST

I might know input horsepower, but I may not know the output HP.

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Pretty close

 

[Besoeker3]

211213-2334 EST

I might know input horsepower, but I may not know the output HP.

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The motor nameplate gives you the HP output.

 

[gar]

211214-0850 EST

Output HP = output Torque ( in #-ft ) * RPM ( in rev per minute ) / 5252

Is torque proportional to input current? Possibly roughly. Sometimes fairly close, and in other configurations grossly in error.

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[petersonra]

211214-0850 EST

Output HP = output Torque ( in #-ft ) * RPM ( in rev per minute ) / 5252

Is torque proportional to input current? Possibly roughly. Sometimes fairly close, and in other configurations grossly in error.

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In some VFDs at least, the torque is calculated for you and available to be read via an analog input.

 

[winnie]

Since the drive is in sensorless vector mode, buried in its algorithms should be a motor equivalent circuit model that is estimating the actual motor torque.

It appears that AO1 can be assigned to report this estimated torque.

You are already going to have a complex enough problem figuring out the relationship between drag speed, product feed rate, and drive torque to determine if there is a problem. I wouldn't add trying to estimate motor torque based on drive current (or worse motor torque based on power and frequency) on top of this.

The VFD is _already_ doing this calculation.

-Jon

 

[Besoeker3]

211214-0850 EST

Output HP = output Torque ( in #-ft ) * RPM ( in rev per minute ) / 5252

Is torque proportional to input current? Possibly roughly. Sometimes fairly close, and in other configurations grossly in error.

.

It depends. Many of the applications for us were DC shunt motors and, for those, the current and torque varied linearly. For other motors like cage induction motors the power factor was typically 07 or 0.8 but the efficiency was 95% including the variable speed system. That said, my expertise is just in the industrial field.

 

[ptonsparky]

I set the VFD to give me a 4-20 ma output for power. A couple test truck loads of 14 %+ moisture corn showed 88% power. Todays lower moisture of 13% ran us up to 110% power. Slowed up the grain to +90 power and torque monitor showed in the +90 % as well.

I don't know how well the power and torque will relate at lower speeds

 

[Besoeker3]

I set the VFD to give me a 4-20 ma output for power. A couple test truck loads of 14 %+ moisture corn showed 88% power. Todays lower moisture of 13% ran us up to 110% power. Slowed up the grain to +90 power and torque monitor showed in the +90 % as well.

I don't know how well the power and torque will relate at lower speeds

It depends on the application. Many of ours were cube load for pumping stations. Typically the loads were 70% - 100%.

 

[ptonsparky]

Augers. Constant torque.

 

[Besoeker3]

Augers. Constant torque.

Ah, OK. A lot of the paper mills we did were constant torque - some of the time............
There were typically 10 or 15 sections (rolls). Now and again the sections got water logged. and needed a lot of torque to get them started. Big industrial stuff.

 

[ptonsparky]

Tried explaining why I'm monitoring power and not amps to manager. Because I want to wasn't good enough.

 

[winnie]

I'm still curious why power rather than torque?

Jon

 

[ptonsparky]

I'm still curious why power rather than torque?

Jon

I was looking amp vs power earlier and set it up for power. I really don't know which would be better. I do know no load amps was about 12 with Power about 25. I didnt check torque.

 

[ptonsparky]

It would only take a Vfd parameter change to monitor whatever but it takes 7 minutes to unload a truck and no one knows when one will show up.
I'll try to read all three next time i happen to be onsite and lucky.

 

[Besoeker3]

I'm still curious why power rather than torque?

Jon

Torque times power is speed ( RPM. ) At standstill torque is zero speed.

 

[winnie]

Torque times power is speed ( RPM. ) At standstill torque is zero speed.

Agreed.

ptonsparky is monitoring a variable speed auger to catch an abnormal condition.

My guess is that an overload will best be seen in the torque. In the extreme case you cited, power is zero at zero speed. So if something is jammed on starting you wouldn't see much increase in power consumption but torque would be high without rotation.

Jon

 

[Besoeker3]

Agreed.

ptonsparky is monitoring a variable speed auger to catch an abnormal condition.

My guess is that an overload will best be seen in the torque. In the extreme case you cited, power is zero at zero speed. So if something is jammed on starting you wouldn't see much increase in power consumption but torque would be high without rotation.

Jon

For fixed speed it would. For variable speed it would be limited.

 

[ptonsparky]

In my OP,
I did ask what power is at locked rotor. Obviously if gets that far along, I'm SOL.

Power seems to be very responsive and I've already set it up to drop out at feed xxx% which solves the one condition we were concerned with.

The operator came up with a simple one after I was almost done. Restrict access to the truck drivers under that one condition. (I had asked about all access.)

 

[gar]

211217-2924 EST

ptonsparky:

We have previously discussed this. For constant supply voltage and frequency output torque, power, and current are approximately proportional to each other. The current curve does not go to zero at 0 load. Current is still a vastly better indicator of load than power input on an AC induction motor.

I don't readily have any plots that show how current and power vary as a function of frequency, or supply voltage.

My sample current curve comes from Alternating-Current Machinery, by Bailey and Gault, McGraw-Hill.

I really want a torque transducer on the output shaft, not the input to a gear reducer. If you mount a motor and its reducer on a platform with a pivot and a force transducer you can determine output torque, or just use an output torque transducer.
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