Induction Motor Protection

[dkle40h]

I would appreciate any information on motor protection, right back to the basics.

We have recently blown our second 220kW, squirrel cage induction motor in a slimes pumping operation. We have three pumps running in series to generate the required pressure head. When the slimes settle it is practise to pump water into the line using one of the pumps to get the slimes flowing. The motor in question is connected to a 525V 3 phase supply, and theoretically should draw 290A. A third motor has been installed but trips the breaker on starting. The breaker is set at 2750A. The DOL starting current is 6.9 times full load according to the manufacturer. We start using star/delta.

I am certain that the problem lies with our protection settings. The contactor can safely open 3000A, once only.

Thanks.

 

[jbartos]

Suggestion: There are two aspects: One is theoretical and the other is practical.
"Theoretical" could involve the proper alignment of motor, circuit breaker, and overload relay curves, perhaps with assistance of the hardware manufacturers.
"Practical" could involve checking or testing hardware for its proper functions as designed. A load bank could be used to load the motor circuit.

 

[GordS]

Are the numbers given correct? If so, I would say the installation is not as per Code (at least in Canada) and would explain motor failures. A breaker protecting a motor using a star-delta start is to be set at 200% motor rated current.(There is some leeway for motors that are difficult to start). The protection specified in the original posting is about 950% of motor rated.

 

[dkle40h]

Hi GordS.

Yes, the numbers given are correct. We expect a starting current of 6.9 times FL (290A) for the majority of the starting period. Given the difficulty to clear the line the motor is heavily loaded during this time.

My opinion is that the protection should be set so that the thermal curve of the motor lies to the right of that of the o/l relay and that the instantaneous trip setting should be in the region of 1.2-1.5 times the expected starting current, which would give around 10 times FL current. This would allow for inrush currents but would still protect the motor against short ciruits. The weakest link then becomes the contactor, which can only open 3000A, and this will have to be replaced. I can only hope that I am on the correct track. The first motor blew after a voltage dip I suspect due to incorrect protection settings, the second remains a mystery.

Kind Regards.

 

[dkle40h]

Sorry, I must mention that the motor manufacturer's have recommended that we use DOL starting, rather than star/delta?! Their reasoning is that the on switch-over the stator field momentarily weakens, and then when connected to a higher voltage (line voltage with Delta) the motor experiences large inrush currents that can damage the insulation. Does this sound right? My school of thought was that star/delta starting of large motors was favourable.

 

[gce1991]

dkle40h

The motor manufacturer is right .it is not advantageous to go for star-delta starter for a higher rating like your case .Also the main problem lies with the voltage selection for this rating of 220KW you should have gone for medium voltage like 3300V or 6600 V .You said that already two pumps were running on this 525V supply I understand there will be a huge voltage dip when you start the third motor .Have you done the voltage dip study for all loads connected to the bus.I suspect the distribution transformer capacity to supply the huge inrush when you start the motor of this rating (What is the transformer rating ?).

I suggest you the following

Connect the third pump to a diffenet bus if available which is less loaded if possible start the pumb first on this bus then switch on the other loads.

If 6600V supply is available make the pump motor suitable for 6.6 kv and run on this bus.

Regards

 

[dkle40h]

ESW, thanks for your comments. I will look into the stiffness of the supply - good point. The procedure is to use the first pump to unblock the line with water only, to loosen the slimes, and only once it is running do they start the second and then finally the third pumps. I have requested modifiction to the line so that the third (and last pump in the series) can be started first, so that they don't have to pump through the other two. We have two 1000kVA transformers feeding all the pumps in the plant.

Regards

 

[dpc]

I'm not sure I understand the hydraulics of this system, but it customary to start a centrifugal pump unloaded. Using reduced-voltage starting into a high head could cause long acceleration times and possible overloads.

A second point is that the 6.9 x FLA for starting current is almost certainly the rms symmetrical current. But there can be a brief asymmetrical current as much as 12 x FLA that last for a few cycles. You may need to increase the instantaneous setting of the breaker IF the motor tripping at the instant of starting. If it does not trip instantaneously, this is not the problem. The reduced-voltage starting should reduce this anyway.

What was the reason for failure of the previous motors?

 

[GordS]

The DOL starting current is give as 6.9 *290 = 2000A. If you are starting with a wye-delta starting current should be about 33% of the DOL value (666A). To my way of thinking, the breaker needs to be about 700A to protect the motor. Previous comments regarding the instantaneous trip adjustement to ride through the asymetrical component, I agree, are valid.

That being said, the breaker is likely just short circuit protection? You have another relay for overload protection? Then you would need to supply the characteristics of the overload relay to properly judge whether the motor is protected from overload. Maybe one of the things to check is whether the thermal damage curve from the manufacturer is based on wye or delta currents and how this relates to the currents being seen by the overload relay.

A previous posting raises a good point in troubleshooting your problem. Why did the motors fail? If the problem is indeeed overloads, the motor should exhibit this failure mode when taken apart - damage throughout the entire winding. The failure mode will help point you in the direction to concentrate your investigation.

I have about 25 motors (150 KW - not too much different from your 200 KW) that we start using wye-delta. We have been doing this for about 25 years and do not experience significant insulation failures as has been implied.

 

[jbartos]

Suggestion: Additionally, if the manufacturer recommends to use the DOL start, then it is implied that the motor is preferably designed for DOL and not necessarily for the star-delta start that generates current spikes. Do you use motors from one manufacturer only?