Reliance vfd tripping on "overcurrent"

[cccelcj]

I have a Reliance GV3000 which trips on "OC". I have a 30hp, 480v motor with a 60hp drive. The LRC goes up to 78 amps in about 4.5 seconds. Then the drive trips on overcurrent. I have tried to adjust the acceleration slope having numerous values from 5 seconds to 20 seconds. The drive still trips. I have done the Reliance autotune procedure. I am running in V/Hz mode. The motor is a design B. We were told the motor is not suitable for this compressor. We are able to turn the crank rod with a steel bar. Any input will be helpful. Thanks in advance.

 

[itsmoked]

Well I hope in your zest to use a 60HP VFD on that poor little 30HP motor you first reprogrammed the VFD down to 30HP or you will likely have several problems.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DickDV]

Actually, the 60hp drive is not such a bad idea at all if you are trying to take advantage of all of the short-term overload capability of the motor. A NEMA B motor typically has about 220% overload capability which occurs at around 240% of nameplate current. A 30hp motor is going to be around 37amps FLA so 240% of that is about 89amps. That 60hp drive is probably rated for around 72amps so 110% short-term overload drive capacity should be 79amps. You can see that even with the larger drive, the motor still has more overload capacity than the drive does. So the large drive is no problem, in my view.

However, operating the drive is V/Hz is a problem. Since a NEMA B motor typically has 160% starting torque and that occurs somewhere around 7 times FLA (let's see, that would be 37 times 7 or 259amps). With the drive amps max'ed out at 79 amps, the starting torque will be 160% times 79/259 equals 48.8% of nameplate torque. Pretty wimpy, at best.

Changing the drive to Vector operation is the key. While I am not at all fond of the GV3000 (even there published sensorless performance specs are barely better than V/Hz operation), in this case, running the drive in sensorless Vector mode will keep the motor from being driven over the breakdown torque point and give far more starting torque if set up properly. Make sure you put the motor nameplate data into the drive parameters exactly per the nameplate. Then do the ID run or Auto Tune operation that Reliance specifies. Then be sure to set the drive Max Torque Limit parameter to 200%.

Doing this will get you at least three times and maybe even four times the starting torque of V/Hz control.

You didn't state the base speed of the motor but, if it is 1800rpm (four pole) then the continuous torque available would be 90 ft-lbs. Peak torque with the big drive attached would be around 180 ft-lbs.

 

[cccelcj]

I have entered into the drive the nameplate data from the motor nameplate. The rpm is 1750. Full load current is 36 amps. I am not able to check this now,but I beleive in order to run this drive in the vector mode, I need an encoder, which there isn't one. I might add when I had the motor uncoupled from the load , it started perfectly, drawing about 12 amps. I recall at the time I set it up, the only modes of drive operation were v/hz or vector. I will see if I can find a manual and double check on the sensorlrss vector mode.

 

[Skogsgurra]

"I will see if I can find a manual"...

You should read very carefully what Dick says.

Your problem is exactly what he says. And SENSORLESS vector mode means just that - no sensor, i.e. encoder. Thinking otherwise is just standing in your own way.

And, please, in an application like this, you need the manual. Rockwell Automation home site has them all.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

There may be a problem here;

We are able to turn the crank rod with a steel bar.

That sounds a little stiff for a 30 HP compressor. Is the unloader working properly?
respectfully

 

[itsmoked]

Yeah I am bothered by that a wee bit too. Is this with no existing air pressure? How hard to turn is this? A lot of effort? Little effort? ??

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[cccelcj]

We are able to turn it with very moderate force. We have surely ruled out a problem with the crank shaft. When we unload the motor, the speed and current of the motor are normal. It is as though the slip of the motor is high and we never reach the breakaway torque required before the drive trips on overcurrent. Also, this is without air pressure.

 

[Skogsgurra]

As Dick said: Go vector control. Get a manual. There's probably nothing wrong with your equipment. You just have to do it properly. Get help from a drives engineer if you cannot sort it out yourself. Guessing games here will not help if you do not take advice.

There are lots of basic things that you will be forced to check if you go by the book. Protection setting is another important thing that you haven't said anything about.

I repeat: Get the manual. Follow it. Drives can't just be connected and started. They need a (very basic, but still) systematic approach.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Marke]

Hello cccelcj

First, is this a "new" installation, or has this installation been operating correctly and just started to give problems?

From your description, it appears that there is insufficient torque to drive the compressor. This could be due to a drive problem, or a motor problem, or a compressor problem.

I would suggest that you temporarily connect another motor to the drive and ensure that the drive is able to run this motor up to full speed. Check the current draw on each phase when the motor is at full speed and ensure that all three phases are balanced. I have come across installations where the drive is working on 2.5 phases instead of three and this will make it trip on over current as it is not able to develop full torque.
If the drive is able to control a second motor without difficulty, then the problem is most likely with either the motor, or compressor, or compatibility of the system (drive - motor - compressor)
I would try to start the motor and compressor using some form of starter, DOL or soft starter and see if they spin up OK and if they do, what the current is at full speed. If there is no problem with this, then you may have a compatibility or set up issue.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[JBinCA]

When you say that the LRC goes up to 78 amps, do you mean that the shaft does not turn at all?

I have to agree with Dick, Skoggs, and Marke, that insufficient torque production is probably your issue.

You may also consider trying a faster ramp. It seems counter-intuitive, but the capability of the SCRs is better for higher overloads and shorter durations than for lower overloads and higher durations. This means that certain motor-drive combinations can perform a fast accelleration ramp, but not a slow one.

My guess is that if you're not even breaking stiction, the quicker ramp time won't help you on this one though. You'd need the vector control or a bigger drive to get more low end torque.

Speaking of a "bigger" drive. Where did the OC trip and current limit settings come from? Is there room to move them up? Is the time delay or motor protection curve adjustable? Depending on the drive ratings, you may be able to get over the hump by raising these settings.

Regards,

JB

 

[waross]

If I am unable to turn a 30 HP compressor by hand, (Possible with one hand, easy with both hands, on the compressor drive pulley) I suspect compressor problems. If it has been run backwards the bearings may be damaged.
If there is an unloader problem, you can rock the drive pulley back and forth 15 to 30 degrees each way from bottom dead center easily. The air compressing in the cylinders will give a spring like or bounce action rather than a hard stop. A leaking check valve will often hold the crank at bottom dead center and it will be difficult to move off bottom dead center. With no air in the system, the compressor should turn easily by hand. A bar should not be needed to turn it.
respectfully

 

[EEBubba]

A suggestion might be to take the compressor and accumulator to an unloaded state and have the motor-drive-compressor package load the system. As you load the system up, the amps will rise with the torque demanded of the motor. A call to the motor manafacturer may also be advised so that the torque available by the package can be determined.

What you may be facing is an undersizing of the motor and to produce the required torque. That is that you are demanding too much current by running the compressor to a PSI that the motor can not push. Perhaps back the PSI down and see if the motor can operate the compressor.

Regards,