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?
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
RE: VFD control method for back to back loading
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
RE: VFD control method for back to back loading
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
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
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RE: VFD control method for back to back loading
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
RE: VFD control method for back to back loading
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
RE: VFD control method for back to back loading
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
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RE: VFD control method for back to back loading
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
RE: VFD control method for back to back loading
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
RE: VFD control method for back to back loading
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
RE: VFD control method for back to back loading
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
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
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD control method for back to back loading
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
RE: VFD control method for back to back loading
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
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
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
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
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
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
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
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
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
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