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VFD control method for back to back loading

VFD control method for back to back loading

VFD control method for back to back loading

(OP)
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?

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

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

RE: VFD control method for back to back loading

(OP)
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?

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

(OP)
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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

Neat if the load runs at fixed speed.

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

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

(OP)
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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

(OP)
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 !

RE: VFD control method for back to back loading

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 (n), 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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

(OP)
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.

RE: VFD control method for back to back loading

(OP)
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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

(OP)
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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

(OP)
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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

(OP)
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.

RE: VFD control method for back to back loading

(OP)
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.

RE: VFD control method for back to back loading

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.

RE: VFD control method for back to back loading

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.

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