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?

 

[DickDV]

place, you seem to be getting hung up on this notion of negative torque. If you think about it, any negative torque in the absorber (dyno) is positive torque in the driving motor.

Therefore, it would seem that your test could just as well regulate to positive torque in the drive section as it is the same as the negative torque in the absorber.

 

[LionelHutz]

If you want to use the torque transmitter as the reference then you would need to use the PID control loop in the VFD. You would provide a torque reference via something like the keypad or pot and then the PID loop would make the feedback from the transmitter equal to the reference.

 

[place1234]

After study, I finally found out actually there are 3 ways to doing a load testing at such back to back loading using two VFD supply.
Way #1.
In this case, both VFD drives using V/HZ speed regulation mode.
1.Start both test motor and dyno motor at same time, speed command at nameplate speed, say 60HZ
2.Manually decrease the speed command of Dyno VFD using scaled 0~16383, the drive will try to slow down the dyno motor, you will see the load increase on the test motor, until reach the Full load amps or a specified value.
3. Stop both motor

Way#2
Test VFD using torque control, Dyno VFD using V/HZ
1. Start both test motor and dyno motor at same time, speed command AT nameplate speed, say 60HZ
2. PLC try to vary torque setpoint of test VFD, say 0~100% (represents 0~100% load). As test VFD is in torque control mode, you will see load increase on the test motor.
3. Stop both motor and clear torque setpoint of test VFD

Way #3 (this method may need discussion here)
Test VFD using V/HZ, Dyno VFD using V/HZ too.
1. Start test motor at nameplate speed, say 60HZ.
2. Start dyno VFD, but give a zero speed command
3. PLC try to vary torque current limit, say 0~100%, you will see the load increase on test motor. ( send a negative torque current limit 0~-100%?)
4. Stop together and clear torque current limit.

I've not practiced all these methods but sooner I'll try.
Please correct me if any mistake. Method#1 is the easiet way I think. At a slip maybe only 1~2HZ you will see the test motor reaches 100% load. Will verify this after commissioning.

 

[place1234]

Hi all, finally i've setup this test plant.
I've tested today. I do not know how to upload my schematic drawing here? but principle is described here in this thread. Test VFD drives a test motor which is coupled with dyno motor driven by dyno VFD. A torque transducer is mounted between.

I have tried this:
Both VFD using speed regulation
1.Start test motor @ 50HZ,reaches full speed
2.Now start dyno motor to 50HZ
3.Ruduce the speed of dyno VFD, you will see load increase on test motor.
But I'm having a problem in step, when dyno VFD tyrying ramp up to 50HZ, around 12HZ, dyno VFD had a alarm saying "Drive OL Level2". This means the drive current is exceeded its rated amps (156A). Any suggestion/improvement about this?

I'm going to try using toruqe control:
1. test VFD use speed regulation, run test motor at 50HZ
2. Dyno VFD use "Flux Vector Control + torque regulation". Run dyno motor at 0Hz and vary the "torque setpoint" (or "negative torque limit"?). Torque feedback signal is from torque transducer.
3. Manually increase "torque setpont" from 0 upwards, you will see the load increase on test motor.
Please correct me if any as I've not done this. I'm connecting commond DC bus with a Fast Fuse to feed the regenerative power from dyno VFD to test VFD.

Thanks for all help. I appreciate.

 

[DickDV]

When accelerating up to speed, you have to speed match both drives to avoid one loading the other.

If you are getting the OL Fault, the dyno drive is clearly not sync'ed with the motoring drive.

You may have the speed reference matched but, for example, you may not have the accel ramps matched. That would generate a difference in speeds causing your overload fault.

To be sure the dyno is sync'd with the motoring drive, I would take the dyno speed reference from a speed output on the motoring drive. That way they have to match.

 

[place1234]

DickDV ,
Thanks for your help.
I start the test motor first say at 50HZ, acel time 5s. After reach full speed, I start dyno motor at 50HZ, acel time 5s also. You 've seen the problem, the dyno motor won't ramp up to 50hz as the VFD OL alarm because the speed is not sync'd.
One point to note: the two motors are not the same size.

I've tried to start the both motor together with same speed reference and accel time, but also see a VFD OL alarm on the dyno VFD.

 

[LionelHutz]

You need to use the 0hz and torque or current limit on the dyno motor you start first. It should start but just sit at 0 speed. Then, when you start the test motor the dyno VFD should allow the dyno motor to track it's speed.

The other option is to turn on the "flying start" or "catch on the fly" option in the second VFD you start. That way, it will check the motor and when it sees it spinning it will just match the motor speed instead of trying to start at 0hz.

I guess you could also set the same ramp times equal and start them both at the same time.

 

[place1234]

LionelHutz ,thanks.
1. Can I try this way: start test motor first. Dyno VFD setting: "flux vector control"/"torque regulation" (question: with encoder feedback or without"). Set speed reference at 0hz on the dyno VFD. I want to manally increase the load by vary "torque setpoint" parameter. Does it work? Please clear this question for me, thanks.

2. I've tried "flying start" option in the dyno VFD. But not successed,drive OL Levl2 alarm. There is a "flying start gain", shall I set to small value or big value? Also, using "flying start", does it matter with "accel time"?

3. Set same ramp time ("accel time")and start at same time. That did not work , I've tried. As two motors are dirrent size, still "drive OL Level2" alarm.


4. I heard some case people is using "zero speed" and "variable current limit", what is that? Can anybody explain me better?

Sorry I am lost here, appreciate all can help me.

 

[DickDV]

Judging by your response, you missed my point in that the speed input to the dyno drive must come from the motoring drive. That way they will match and you should not get overcurrent faults. Once up to speed, you can introduce some speed reduction into the dyno drive to load the motor.

I would run the motoring drive as a speed regulator with vector control and the dyno drive as a speed regulator with open loop control. That way you don't get two precision speed regulators fighting each other.

 

[place1234]

DickDV,
Thanks.
The motoring drive is already running at 50hz, then start the dyno drive , give the speed input of 50hz. That way, I will see a overcurrent faults.
Do you mean that both drives start together? I've tried this also, same overcurrent faults.

 

[place1234]

LionelHutz ,
You mentioned "torque limit",which parameter in the VFD is this "torque limit"? I did not see. VFD has a parameter called "Torque setpoint". I'm using two Relinace GV6000 drive. Also, when the dyno drive set as " FVC " control (Flux Vector Control), after give a start command, drive output frequency is same as the test drive, but you will see the "commaned speed" is 0.

 

[LionelHutz]

If you are using the same acceleration time, the same rated speed motors and the correct rotation connection for each motor then you would not get a current fault starting both VFDs at the same time.

Remember, the motor shafts need to turn the same direction when starting each VFD individually. Don't connect the motors so they both go clockwise when run individually.

We're using a regen VFD setup for load testing here. The operation of the load motor is independant of the under test side and we have little problem with this arrangement.

 

[DickDV]

I mean that the motoring drive must lead the dyno drive. The motoring drive run relay provides the run signal for the dyno drive and the analog speed output from the motoring drive provides the speed signal for the dyno drive. Once up to speed, you can trim the dyno drive speed input to load the test motor. Trim range should match the expected motor slip percentage plus whatever overload percentage you desire.