Variable vs Constant Torque Application 2

[controlnovice]

We have a batch reactor with an agitator with a 50HP motor and ASD (VFD).

Do we need a Constant Torque or Variable torque motor?

I've called two separate motor companies and each gave me a different answer.

The plant only sets the speed at 50% and 100%. They don't vary the speed. During initial charging, the speed is 50%. Once cooking starts, it is 100% and stays at 100% until the end.

The concern is that the viscosity can increase during the cook, but if it gets too high (close to motor overloading), we may be able to slow down the speed (and maybe hp/amps?) to allow the operators enough time to 'save' the batch. But if viscosity is increasing, you'd want to maintain the torque in the batch to maintain mixing. (???) Can you straighten me out?

I know if the rpm decreases, so does the hp. But I'm unsure how a constant torque motor works and what is correct for this application.

On other reactor's agitators at this plant, they have two speed motors. Would these be constant torque or variable torque?

 

[CJCPE]

A constant torque drive is capable of operating at rated torque from 100% speed down to some minimum speed. The minimum speed is generally determined by the ability of the motor to cool itself at reduced speed. The VFD design may also limit the minimum speed.

VFDs can generally supply more than rated torque at any speed for a limited time to accelerate load inertia and handle short term overloads. The maximum intermittant torque vs speed and time is mostly determined by the VFD design but influenced by the motor selection.

Reactor agitator torque at a given viscosity would vary somewhat like a fan or pump -- torque proportional to square of speed. If the torque at full speed can exceed the motor's rated torque, a constant torque drive will probably be required to allow opreation at reduced speed without overloading. Check the nameplates of existing 2-speed motors on other agitators.

 

[DickDV]

This is a tricky application problem especially for the inexperienced drive/motor person. The subtlety lies in the fact that most blade or vane type agitators are, by design, variable torque machines. The application is, on the other hand, much closer to constant torque due to batch processes being subject to operator and process disturbances such as viscosity changes, batch overcharges, and similar irregularities.

For these reasons, it is always prudent to size the drive/motor package as a constant torque system.

Induction motors are, by design, constant torque devices with large short term overload capacities built in. In this case however, it appears that any process disturbance such as viscosity increases, would occur for longer than "short term" so the motor and drive will have to be sized for extra CONTINUOUS capacity rather than short-term overload capacity.

You will have to make a good judgement estimate of just how much oversizing will be required. Often, moving up one size in motor will be adequate. Then, since you have covered your upset loading with oversizing, you will not be concerned about short-term overload capacity in the motor or the drive. The motor doesn't give you any choice once you've picked the continuous hp but the drive should be sized, not by hp, but by continuous no-overload amps. In most cases, that would be a "normal duty" or "variable torque" rated drive, both of which give you only 10% short-term overload capacity.

Don't get mislead by these "snake oil" drive sizing terms of "variable" and "constant" torque ratings. Drives rated this way are both capable of operating variable and constant torque loads. The only difference is short-term overload capacity. The same hardware will give you, for example, 35 amps continuous output with no overload capacity, 32 amps output with 10% overload for one minute, and, at 27 amps output will give you 50% overload for one minute. That's right, the same piece of hardware!!!

If nothing else, this should clarify why you should size VFD's by continuous and short-term amps, not by hp or kw.

 

[jraef]

My experience with agitators supports DickDV's post. Consult with the ME on the equipment design. If the motor has already been oversized for transient loading, a VT drive will be fine. If it was marginally designed to do only the required task, a CT drive would be a better bet. If you don't know, go with the CT drive and quit worrying about it.

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[aolalde]

Hi, Controlnovice.

I think that your critical power and torque demand is at 100% speed. “At 100% speed, viscosity increases till the end”.

You need to define the maximum requirements at 100% speed worst case scenario (higher viscosity) and get your motor HP and drive continuous current capacity to work properly under such a condition.

The initial charging process becomes very light for a mixer, since it has low viscosity and only 50% speed. That will be then easily covered with a “Variable Torque” motor and driver.

 

[LionelHutz]

DickDV basically described it. In my experience it seems that all VFD's (constant or variable torque) can provide rated torque at all speeds with the constant torque rated VFD's basically giving a higher overload capability.

That example of overload capability hit the nail on the head. I've seen calculations for some of these ratings and it's exactly that. I can draw 35A continuous with this hardware so I'll give a 110% overload capacity drive rating of 32A (call it variable torque) and a 150% overload capacity drive rating of 27A (call it constant torque).

The funny part is that a load that can have a peaking or overloading torque requirement is used on a constant torque drive.

 

[dpc]

As the others have said, if there is any concern, I'd go with constant torque. I doubt if anyone has ever been fired for buying a constant torque drive when a variable torque might have worked. But if you get the variable torque and it can't do the job, everyone will be looking for someone to blame.