VFD Overcurrent

[nhee2]

Yesterday observed at a client site:

VFD controlled motor in current limiting alarm, clamped at 103A but running. Motor is 5HP, 480 VAC, control signal (4-20) demanding 66 Hz (20 ma) and drive stuck at 55Hz, 194V, and 104A (as indicated on the drive display, no measurements were taken). I believe normal current draw at 66 Hz for that particular motor is 83 A.

The motor appeared to have been running in this condition for some time. They shut off the drive for a few minutes, restarted the drive and motor came up to desired speed and normal current draw.

I am assuming that at some point the last time the drive started/ramped up speed, the current limit value of 110% was not enough to let the motor accelerate past whatever breakaway torque requirements were at that speed, and the motor 'stalled' at some speed less than setpoint? It was a Danfoss drive and no manual or design settings were available, and it wasn't our install. But wondering if this is likely a drive settings issue or a motor issue. The motor / drive combo has been in service for many years and to my knowledge has not had this occur previously.

 

[Jraef]

Something is wrong with your numbers. Did you perhaps mean it is a 75HP motor?

Assuming so, the Current Limit set at 110% is likely the rating limit of the drive itself; Variable Torque selected drives usually are limited to 110% OL for 30 seconds or so. Why it would run like that continuously is a mystery though. Unless the drive is actually RATED for 104A and it so happens that 104A is 110% of the MOTOR FLA, so coincidentally the drive was limiting the motor current at what the drive can deliver continuously.

Be that as it may, the WAY a VFD limits current is to fold back the frequency to the motor, which effectively sheds load. So it stopping at 55Hz at 104A is a reasonable expectation. What puzzles me is why that would go away after shutting down and restarting. Load on a centrifugal pump motor is determined by flow. Pretty much the only two ways to increase the load is to have too much flow, ie a pipe breaks and you go “open channel flow”, or there is a mechanical problem in the pump. I’d expect you would know if a pipe was broken and I’ve never seen one fix itself, so I’m going with a mechanical issue in the pump. Might be a bad bearing or too much play in it, ie axial thrust or something, and in restarting, that is now working OK again, for the moment. I’d take it off line at the first convenience and get it checked. Those things also don’t get better, they just hide for a while at best.

 

[rlundsrud]

I'm inclined to agree with the bad bearing Theory, it is similar to a fan motor in a computer making terrible noise but if you bang the case you can get the thing to quit having a harmonic wobble and get the bearing to run straight for a little while.

 

[nhee2]

Something is wrong with your numbers. Did you perhaps mean it is a 75HP motor?
.

Yes - it should have read 75 HP.

 

[nhee2]

I'm inclined to agree with the bad bearing Theory, it is similar to a fan motor in a computer making terrible noise but if you bang the case you can get the thing to quit having a harmonic wobble and get the bearing to run straight for a little while.

I indicated to them i thought it might be a mechanical issue with the motor or pump.

I was puzzled why the drive would run indefinitely at 103 A.

I was also puzzled by the indication that it was running at 55 Hz (if i interpreted the display correctly) while outputting 194 V. Does the current limit override the V/Hz control? Why would it show 55 and not 66?

I think i would understand if it applied 194 V at the desired 66 Hz. Or if it applied whatever the corresponding voltage to 55 Hz. But i was confused by the combination of current limiting, indicated frequency and desired frequency.

However I still have not opened a manual to investigate more.

Thanks for the replies.

 

[Jraef]

I indicated to them i thought it might be a mechanical issue with the motor or pump.

I was puzzled why the drive would run indefinitely at 103 A.

I was also puzzled by the indication that it was running at 55 Hz (if i interpreted the display correctly) while outputting 194 V. Does the current limit override the V/Hz control? Why would it show 55 and not 66?

I think i would understand if it applied 194 V at the desired 66 Hz. Or if it applied whatever the corresponding voltage to 55 Hz. But i was confused by the combination of current limiting, indicated frequency and desired frequency.

However I still have not opened a manual to investigate more.

Thanks for the replies.

A 75HP drive from Danfoss was likely sized for 104-105A, because the standard IEC equivalent to that motor would be a 55kW motor, which at 380V 50Hz would be 105A.

Yes, the current limit will override (I said "fold back", same thing) the Hz command first, but may also have folded back the V/Hz rate as well. If the VFD were set up for "energy savings" because it was designed for a Variable Torque load, the V/Hz pattern would be set to be V/Hz squared*. So at 55% out of 66Hz, that's 83.3% Hz, which means the V would be 69.4% (.833²), and assuming the motor voltage was set for 460V, the baseline V would have been 319V. But given that it was going into Current Limit, it may have lowered it even further when the reduction to 55Hz alone wasn't enough. What that then also does is lower the amount of TORQUE that the motor is producing, and torque is directly related to current.

* A certain amount of the losses in a motor is tied to the VOLTAGE applied to the motor. By reducing the voltage, you reduce a portion of those losses. You also reduce torque at the SQUARE of the voltage change. But in a Variable Torque load like a centrifugal pump, the LOAD on the motor, i.e. the HP required at the shaft, reduces at the CUBE of the speed change. So at 1/2 speed, the pump requires 1/8th of the HP it requires at full speed (.5 x .5 x .5). So knowing that, the rate at which you reduce the voltage with speed can be increased to take advantage of those small energy savings, yet although you start losing more torque from the motor, the demand ON the motor is so much lower than it's OK.

 

[nhee2]

A 75HP drive from Danfoss was likely sized for 104-105A, because the standard IEC equivalent to that motor would be a 55kW motor, which at 380V 50Hz would be 105A.

Yes, the current limit will override (I said "fold back", same thing) the Hz command first, but may also have folded back the V/Hz rate as well. If the VFD were set up for "energy savings" because it was designed for a Variable Torque load, the V/Hz pattern would be set to be V/Hz squared*. So at 55% out of 66Hz, that's 83.3% Hz, which means the V would be 69.4% (.833²), and assuming the motor voltage was set for 460V, the baseline V would have been 319V. But given that it was going into Current Limit, it may have lowered it even further when the reduction to 55Hz alone wasn't enough. What that then also does is lower the amount of TORQUE that the motor is producing, and torque is directly related to current.

OK. So potentially with the control system calling for 100% (66 Hz), the drive automatically reduced frequency to 55Hz to fold back the load current, then subsequently further reduced voltage? Does this mean it would be outputting 194V at 55 Hz at 104A? or some lower frequency? Based on flowrate and observed conditions (cooling from fan) it seemed like it was operating at lower than 55.

Appreciate the education.

 

[Jraef]

It would have done whatever it took to limit current. In some VFDs, like the A-B PowerFlex, you can program it to fold back Hz, Torque (which is done by Volts), or both (or none, which means Trip). Not sure what Danfoss does in that drive, but it appears to be both from your description.

The idea behind this concept is to at least keep doing SOMETHING rather than trip off-line and do nothing. So in a way, if it ran this way for a long time, it was doing exactly that. Had it been another device like a starter it would have just tripped off line on Overload. this is one of the ways a VFD can add reliability to a system. If they don't like it, they can probably change it in programming, but you can probably also set up an alarm contact to at least INDICATE that is is not able to run at full speed because it is in Current Limit, because as I said earlier, SOMETHING is causing this and someone should fix it.