Torque settings on an AC drive

[Thedroid]

An AB powerflex 4 drive I'm working with is going to power a 2hp Baldor motor to drive a weigh conveyor. The motor will need to be able to run at slow speeds for extended periods of time. Besides the issues of overheating would changing the torque setting to "constant torque" be better than "variable torque"?

The gearbox has a pretty high ratio, but the motor might see continous operation at 300-400 rpm.

The scale needs the ability to feed from 0 - 15 tons an hour, and will see both ends of the spectrum.

What are some of the reasons to adjust these settings on any drive?

 

[davidbeach]

Conveyors are more a constant torque type of load than a variable torque type of load. Separate cooling fans for the motor can allow sustained full load operation at reduced speeds - all the way down to zero speed.

 

[ozmosis]

Thedroid
Variable Torque settings on a VFD define a number of areas such as the Voltage/Frequency (V/F) characteristic and a typical overload setting.
The V/F for a variable torque load will be designed to follow the characteristics of a centrifugal fan or pump. These typically follow the affinity laws of centrifugal loads meaning they would try and follow a square law. The voltage at maximum frequency will be maximum voltage but as the speed/frequency dropped, then so does the voltage to develop a square of the torque (trying to follow a square of the pressure as flow drops.
Constant torque loads (such as the one described) will require a linear V/F to ensure the motor develops the necessary torque at the speed range below nominal.
If you set Variable torque on a constant torque load then the motor could stall.
Another aspect of VFD's that can be set at constant or variable torque is the overload setting. More often, variable torque loads demand a lower overload than constant. Typically 110% of rating for 60secs, whereas constant torque loads could require much higher (160~~200%).
It is often this factor that would also determine the HP(kW) rating of the VFD. VFD's with a higher overload capability will often provide a lower HP(kW) rating than a VT rated VFD.
Where VFD's can be set to either VT or CT (or some drive manf's call it High overload and Low overload) then the actual power rating of the drive will typically be rated one size lower or you will have to de-rate the drive accordingly.

Another point is if you are using the internal electronic thermal overload to protect the motor. Often, VFD's will automatically set the e-thr to the type of load they are running. If the VFD 'thinks' the load is VT then it will set the IsqT accordingly and possibly be tripping on this protective function if the correct setting is not made in the VFD according to the actual load.

 

[Skogsgurra]

Are there any reasons you think that constant torque would be better?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Thedroid]

We commissioned the conveyor on Friday, and it seemed to run fine on the default setting, which i think is sensorless vector. I wondered if constant torque would be better, because the converor will have to start fully loaded and might only ramp up to a few hundred rpm.

 

[Skogsgurra]

Sensorless vector is just fine. Torque control is for other purposes and none that fits your application. Just carry on the way you are running it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...