VFD control method for back to back loading 1

[place1234]

Sorry for my "broken" English again here
My question is :
I have two VFD driving two motors as back to back loading, that means one motor as test motor and another one as dyno motor ( also as generator).
The principle is first start both test and dyno motor at full speed, they are running freely on the same line. To increase the load of test motor, decrease the speed of the dyno motor until test motor reaches desired load, say 50% of full load. In this case, what is the control method for the dyno VFD? For example, test motor full load 100HP, so 50% of full load is 50HP. Using what method to let the dyno VFD automatically stay at 50HP?

 

[CJCPE]

The control method for the dyno VFD should be torque regulation. You supply a torque reference for negative torque and the VFD operates at whatever speed is necessary to produce the required torque.

 

[DickDV]

It seems to me that your control method should be arranged around what you are testing for. Generally, in testing, you have an independent and a dependent variable.

For example, if you are testing a motor for slip speed at a given torque output, your dyno motor should be set up to regulate to the test torque, allowing the speed, current, and everything else to vary accordingly.

If, on the other hand, you are testing the motor for current draw at a specific slip speed, your dyno motor needs to be set up as a speed regulator to enforce the test slip speed. Then, the torque, current, temp., etc. will go where they have to based on the test speed.

Incidently, in most cases, using hp is not desireable as a test parameter. Much better to use speed or torque and calculate hp.

 

[cbarn24050]

Dont forget that unless you have an active front end on the "dyno" vfd you need to connect the dc busses together so the regenerated power has somewhere to go.

 

[LionelHutz]

I would recommend torque control varying from 0 to 100% torque via analog input or user interface programming.

You will require either a regenerative VFD or connecting the DC busses together. I would not recommend a braking resistor because dumping 37kW of power is awfully wasteful.

Another option is to run the motors to full speed then line connect the dyno load motor. Overspeed the driving motor with the VFD until you reach the load you want. Generally, it only requires about 1 to 1.5hz overspeed to get the required load. I've found that just increasing the frequency in small steps until the required load is reached provides a stable load that varies little as the motors heat-up.

 

[jraef]

place1234,
It sounds as though you are trying to come up with what is called an "electric motor test stand". If you do a Google search on that term you will find a number of articles and products that do this, as well as software packages that can assist you is you want to build the hardware yourself. The process is somewhat involved, beyond the scope of what we could help you with here IMHO.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[place1234]

Torque control using torque signal from a torque transducer is needed.
What I am wondering is when the torque reads 50% x Full load torque ( we want to load the test motor at 100% full load ), dyno VFD will try to speed up. But, actually in this case we want dyno VFD to speed down to increase the torque.
How to control this? In the VFD, is there any "Negative torque" control like " CJCPE" said?

 

[CJCPE]

I think drive suppliers that offer a regenerative VFD with torque control will have negative torque control available. You can buy a system including negative torque control using a signal from a torque transducer, but as jraef points out above, it may be pretty difficult for you to put together a system from "off-the-shelf" components.

 

[place1234]

What about a normal VFD? Do they have "negative torque control"?
As i know, a normal VFD has sensorless vector; V/HZ; Venctor control. All of above are "postive control" methods.

 

[CJCPE]

If the drive is not regenerative and not set up by the factory for connecting the DC buses together, it might not include negative torque control. Sensorless vector, vector and V/Hz control are not inherently limited to "positive control," but drives that are not regenerative and not set up for connecting the DC buses together offer only resistive braking as a negative torque option and therefore don't require much negative torque control functionality.

 

[aolalde]

I suggest the load generator (SCIM) connected direct to the power supply (50 or 60 HZ depending on the location).
The VFD supplies the motor under test increasing the frequency (speed) over the line frequency by the motor speed slip.
The advantage, you will return most of the load energy back to the power supply with no complicated or expensive systems.

 

[ScottyUK]

Neat if the load runs at fixed speed.


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Sometimes I only open my mouth to swap feet...

 

[CJCPE]

Using two VFDs is most useful if you are testing one of the VFDs or the VFD operation of one of the motors.

If you are testing fixed speed operation of one of the motors, you are probably really interested in testing the motor on sine wave power rather than VFD power. In that case, the motor under test should be connected directly to the AC line and the motor serving as load should be connected to a regenerative VFD.

Connecting the motor serving as load directly to the line is most useful for running thermal tests on a VFD.

 

[cbarn24050]

hi aolalde, the problem with your suggestion is you can't start the system easily and if the vfd is running at a lower spedd than the mains the power flows the other way and has nowhere to go so you get a bus overvolt trip.

 

[place1234]

Hi CJCPE ?
I'll connect the two DC bus together with fast fuses in seires.
I still do not understand how to set a "negative control" for a VFD? If this could happen, my plan is to automatically test the load motor at 1/2,1/4,1,1.2 load ( load motor running at nameplate speed, while test motor increase/decrease using this "negative control".

Thanks.

 

[cbarn24050]

I'd be very wary about fitting fuses between the busses, if they open under heavy current flow, and thats when they will open, your buss voltage could go through the roof allmost instantly.

 

[CJCPE]

To set negative torque control for a VFD, the VFD should have positive/negative torque control programmed into the embedded firmware. The torque reference parameter should have a positive to negative range of adjustment. A bipolar analog input should be provided to accept a positive/negative torque reference signal. If external torque feedback is to be used, there should be a second bipolar analog input to accept that signal. The VFD instruction manual should contain information listing the parameters associated with torque control and explaining the functions.

I would be wary of connecting the DC buses of two drives together without guidance from the manufacturer of the drives.

 

[LionelHutz]

If you set the drive to run at 0hz and set the drive to torque limit via the analog input level then with the analog input at minumum you get no torque and as you turn the analog input up you get more and more load torque.

 

[place1234]

I come back to this thread again!
Is it possible to use Torque Control mode on the dyno VFD drive? Torque reference could be the torque reading from the torque meter mounted between dyno motor and test motor.

But I am wondering these:
When "actual torque"< "required torque", my dyno VFD should try to reduce the speed of dyno motor ( to increased the torque).
But in the Torque Control mode , it might increase the speed of dyno motor while it sees "actual torque"< "required torque"? Because in the normal situation, a VFD drive is driving a load, while it sees "actual torque"< "required torque", it should increase the speed . This is my guessing, no experiment taken.

Anyone can clarify this with me, thanks a lot !

 

[Skogsgurra]

It is difficult to understand what your question is. I think that you get lost in the details without understanding the details.

Get yourself a block diagram of a drive and a motor. Identify speed controller , torque controller (Iq) and excitation controller (Id). Look at different limits and external signals influencing the limits and make sure you understand what they do. Do not mind (yet) if you do not understand the modulator or switches. You will pick that up later.

Draw a diagram showing speed/torque and put your two drive's speed and torque in that diagram. Remember that speeds are identical (if coupled shaft-to-shaft) and torques have same value, but opposite signs. Play with different situations and you will soon get the picture.

Gunnar Englund

http://www.gke.org

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