Sensorless , Close Loop or V/F mode

[maypot]

Hi,

We have actually one 400 k W elctric motor 400 V 2955 rpm driving a crusher though a epicyclic gearbox. The motor is diven by a 700 A VFD. The speed of the crusher is controlled by a PLC and the average speed we used to run is around 30 Hz. We actually operates in V/F mode and the torque available at that speed is only 60 % of the rated torque of the motor. I understand that if we switch operation to sensorless vector the tripping will cease.
However, the management is reluctant to operate in this mode claiming that if the crusher is sized, the full torque available at start might damage the gearbox.I undertand that we could limit the maximum torque on the drive.In sensorless vector mode, the drive calculates the torque by actually measuring the output speed and the shaft power.But if the crusher is blocked, the output speed measured will be nearly zero and the drive will actually measure zero torque and continue to drive the load.
I am instead suggesting to implement close loop control through the installation of a PG.

Any comment.

Bob

 

[Skogsgurra]

There is some misunderstanding behind the reasoning that the drive would measure zero torque at low speed while delivering a high torque. The torque in a reasonably well designed vector drive is a current measurement and not a "power/speed measurement". If you are running frequency vector control with an inner torque loop (the normal modus operandi) then the torque limiting should work just fine.

You will have to educate your management. Start by talking to AB, ABB or Siemens about the application. They should be able to give you a correct answer. But do not talk to the salesman - go to the application engineers.

Gunnar Englund

http://www.gke.org

 

[laundry]

Hi maypot.
Normaly I would only run my inverter in v/f,If I was running two motors off one inverter. as you have vector contro on your drive it will map your motors performance, you can then make adjustments to your F L C,There is no resen why you are not able to do this.

 

[DickDV]

I'm guessing here but I suspect that you are getting motor overtemp faults on the drive at 30Hz.

If you have the V/Hz curve set for a straight line, then you should be able to get 100% rated torque out of the motor without going to sensorless vector mode.

The problem would be that the drive is derating the thermal capacity of the motor at 30Hz and faulting on that.

You may be able to alter or customize the thermal derate curve in your drive. Look for something like "break frequency" in the overload setup parameters and reduce that to 30Hz. You must now be very careful to measure actual motor temperature when operating at that speed under full load to make sure you are not harming the motor.

A far better solution would be to bury three Klixon thermostatic switches in the motor windings and take them back to an external fault loop on the drive. That way you can turn off the overload calculations in the drive and fault only on actual measured heat.

You can often get as much as 10% more output from a motor by measuring heat rather than estimating it.

If the drive is not faulting on motor overload or overtemp, disregard the above post.

 

[mil3]

Maypot,
Could you advise what faults you are experiencing? That would help us understand what exactly you are seeing.
There should be absolutely no reason why Sensorless Vector control should not work for you.
#1 - Torque can be set and controlled to never exceed what your gearbox is capable of.
#2 - If your Crusher stalls for any reason, you should be able to program a stall protection custom to your running application and also at the same time your gearbox torque is limited so no damage would happen on it.
Basically you are protected either way. Absolutely no reason why this wont work, but please advise your faults you are experiencing.

 

[jraef]

If you have the V/Hz curve set for a straight line, then you should be able to get 100% rated torque out of the motor without going to sensorless vector mode.

I agree. At 30Hz (assuming that means 60% speed), the difference between V/hz and OL Vector would be in torque accuracy, not capacity. If your torque is reduced that much, you must have the VFD set for "V/Hz squared" mode, typical of Variable Torque applications such as centrifugal pumps or fans. I also agree with the other comments as to using OL Vector, should be fine. And mil3's comments are dead on, any decent VFD should be capable of allowing you to protect your mechanical components, and please describe the faults you are experiencing.

Side note to Laundry;
Where did you get the idea that he has 2 motors on 1 drive?

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[maypot]

Hi all,

Thanks for your help and invalauable comments.
The drive is tripping on torque limit and the maximum torque limit is set to 150 % . I understand that at 30 Hz, the drive is capable on V/F mode to develop only 60 % of the maximum available torque. At higher speed the drive does not trip.

Bob

 

[jraef]

Maypot,
Where did you get that information (60% torque at 30Hz). That is incorrect, unless as I said, you have the drive in Variable Torque operating mode, also known as "V/Hz Squared". If you do, take it out of that mode, it is inappropriate for a Constant Torque load such as that. Also, if you have the torque limit set for 150%, and you are getting torque limit trips, that usually means that the drive was making the motor put out 150% torque, not 60%.

In most drives, torque limit is used to prevent the drive from damaging mechanical components, and it is either continuous (if lower than the drive rating), or time-limited (through a torque limit timeout parameter of some sort). Since you are set for 150%, you will have a time out that is causing the trips. See how much time it is set for. If too short, normal load fluctuations may cause nuisance tripping. If set too long, damage to the motor may occurr. Most VFDs can put out 150% for 1 minute. See if your timeont is less than that. If it is set for 1 minute, then the problem is in your load somewhere. It needs more acceleration torque than what you are allowing it to have from the VFD.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla