invertter motor load test stand

[place1234]

Hi everybody,
We are thinking setup a load test stand for inverter duty motors. Put one inverter motor as the dyno, the test motor coupled with this dyno motor. Both motors controlled by two VFD drives. This should work in theory, But practically a few questions I need to know.
1. When the test motor reaches FLA( in other way, full load), what settings for the dyno motor? The dyno motor should try to run the opposite directio

 

[abcd3286]

Have some BIG doubts about using another motor as a load.

I would use an alternator (AC) or generator (DC). Use a tank of water with baking soda for a load (place this outside in well ventilated area)

OR use a bank of heaters should keep the shop warm.

OR add another hot water tank and use that for a load.

Dan Bentler

 

[aolalde]

The shafts are mechanically coupled and the phase sequences are fixed to rotate in the very same direction. (NOT reversed).
One motor needs to work as generator returning power to the line. So the load motor is connected to the main power supply (50 or 60 Hz). The other motor is energized with a frequency slightly above the power supply. The machine connected to the utility will work as induction generator and the other works as a motor supplied by the VFD. The full load condition is adjusted increasing the driver frequency until the slip is such that the full load is handled.

Note that the frequency of the motor is around 2% higher than 50 or 60 Hz but the test is very close to one made on a motor fed with 50 or 60 Hz and loaded by a dynamometer.

 

[LionelHutz]

We've got some motors set-up somewhat like you're thinking about. I set it up with a couple of VFD's and one line re-gen controller to operate the test motors. The VFD's operate in a torque limit mode with the frequency set to 0hz. Basically, as the VFD goes into torque limit it allows the frequency to increase. We have pots in the test bench that allows us to vary the torque limit setting.

The VFD's need to be both operated in the forward direction (to get forward operation you actually reverse connect one motor due to the end-to-end coupling).

I decided to do it this way for a couple of reasons. We test soft-starters which can not overspeed the motors and this allows us to apply a load at any motor speed. This also allows a continuous load test.

The other way you could do it is to connect the DC busses of the two VFDs together so the load VFD feeds the power right back into the driving VFD.

 

[place1234]

Hi, LionelHutz ,
Thanks for you explanation. Is there any documentation about this kind of test?
Also, you said both motor (one as dyno, one to be tested) run in the same direction?
I just need further detailed information, if anybody could help, thanks a lot.

 

[electmech]

I have sold systems like this one before a number of times, and using 2 motors coupled shaft to shaft is fine (not to mention MUCH cheaper than installing a dyno). However, there are 3 points that you need to consider:

1) If you are building a test stand, you are likely trying to test multiple units at different ratings, so the current limit will not work if you oversize the VFD to a point that will provide sufficient enough current to cover all scenarios since the current limit is based on feedback from output CTs (i.e. you can only run in V/Hz mode unless you are within the range where your current transformer ratio at the output will be meaningful to the VFD processor).

2) You will probably have to re-commission the VFDs for each new motor that you want to test which has different electrical characteristics.

3) You can only operate the motor that you have coupled to your driver at -+5% (within the slip tolerance). This is important since you must operate the generator at a speed that is faster than the frequency applied to generate at all.

 

[Marke]

Hello place1234

The drive controling your "load" motor should be operated in "torque" mode and should be a fully regenerative drive capable of 100% regeneration continuously. There are a number of suitable options for this. You can use most active front end drives for continuous regeneration.
A suitable option is the Control Techniques SP where you use 2 x SP drives, one to drive the motor, and one as the regeneration unit. There are a number of other options.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[ashelec]

We basically do the same testing by putting a pulley on the test motor and hang weights or pull by force springs. THE torque value is the product of weight/force and radius of pulley.You can change force values and vary speeds to get a complete torque-speed curve.

 

[LionelHutz]

The rotation is simple. When you start the motor under test it will rotate in one direction. So, start the load motor and make sure it rotates the same direction.

If you want to test at various speeds then I suggest back-to-back VFD's. If both VFD's have DC buss connections then you can just connect the busses together and that will complete the test set. This will eliminate the need for a regen capable load drive which adds a lot of money to the drive cost.

If you only want to test at rated speed then you can run the load motor connected right to line power. Run the motor under test up to 60hz and then start the load motor with a contactor. Then, increase the speed of the test VFD until full load is achieved.

There are some other possibilities for a load depending on the sizes you want to test so maybe you could tell us what ranges of motors you're looking to test.

 

[blacksea]

Is dyno (i.e. current controlled brake for example) more expensive vs. AC motor with frequency inverter really?