Motor Voltage drops from 11kV to 8.66kV During start-up

[SAMBOY]

We have a 2.2MW, 11kV, 60Hz Motor for a Compressor.
During start-up at no load (uncoupled)the starting current is 251A then the voltage drops from 11kV to 8.66kV.
What is the effect of the voltage drop to the motor? What are the possible solutions or your recommendations if these will affect the motor?

(Just an added information, The motor is being supplied by a 3.5MVA 13.8kV/11kV Delta-Wye Connected transformer, 4.75 impedance)

We tried to measure the primary voltage of the transformer during start-up and its voltage drops from 13.8kV to 12.3kV.

 

[Skogsgurra]

It depends. If it is an asynchronous motor, I would say that all that happens is that the start will take a little longer. In fact, it will be a softer start than normal and that is beneficial to motor life. The rotor losses during start will not be affected much. So, on the whole, there is not much to worry about.

If the motor is a synchronous motor, then the low voltage can make it difficult for the motor to get up in speed and to lock onto the grid. You then probably need to add capacitors to provide reactive power for the starting.

 

[electricpete]

If the motor were completely unloaded during start, than total rotor heating doesn't change with voltage reduction. But if there is heavy load torque during start, the total rotor heating can increase due to reduced starting voltage.

As part of normal system design, the starting conditions should be specified. Relay settings ideally are set between motor starting conditions and motor starting damage curve. If relay doesn't trip during starting and system designed properly you might assume the starts are not destructive. If you want to verify more you would have to look at motor specifications or other info from the motor manufacturer.

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[aolalde]

The effect of the voltage drop to the motor is; reduced starting torque with the squared ratio of the voltage drop and starting current reduction proportional to the voltage drop.

The torque drop could be critical if the inertia of the compressor and friction torques are high. The motor will extend the acceleration time and if the load torque curve is to close to the motor torque, it will never reach full speed.

As Epete recommends check the compressor specifications and requirements.

The reduction of current is beneficial, if the remaining torque is still high enough to start the compressor.

 

[jraef]

You essentially have a soft starter there, more like a reactor starter than an autotransformer type, your transformer is saturating and supplying the additional series reactance to the circuit. Assuming that the motor can get to full speed under the reduced torque as mentioned above, the only other problem is that it is also causing a significant voltage drop on your transformer primary, which translates to potential problems everywhere else. A 21% voltage drop on the primary is far beyond commonly accceptable limits, but only you can determine if that is a problem in your installation.

Say for instance that you have a control system being fed by a transformer connected to the same primary circuit. Most relay and contactor coils will not drop out unless the voltage drops 30%, but they may not pick up (or will chatter) at a 20% drop. If that is a problem, you should consider using a soft starter to reduce the voltage only to the motor and keep the primary voltage drop to a minimum.

"Venditori de oleum-vipera non vigere excordis populi"

 

[Marke]

I would not expect motor problems from that voltage drop provided that there is sufficient torque at the reduced voltage to start the driven load easily.
My concern would be the influence of that voltage drop on other connected load.

If you are able to accept a lower start voltage on the motor and still get sufficient start torque for the driven load, then you could apply a reduced voltage starter to reduce the start current and thereby reduce the voltage drop on the supply. I suspect that you are getting close to the lowest acceptable start current and if this is the case, you need to strengthen up the supply. - larger supply transformer.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[SAMBOY]

Thanks a lot guys.

Here is the development, During start-up the voltage drop decreases from 8.66kV to 10kV with load (coupled), but this time the problem is the motor never reaches full speed. Can you again help me with this, guys. Thanks

 

[RRaghunath]

It clearly means motor is not developing adequate torque for it to accelerate.

The reason is low voltage at motor terminals. Please check and maitain bus voltage (upstream of transformer), alternately (if there is difficulty in raising the bus voltage), check and maintain the tap of the transformer at the position appropriate to the available bus voltage.
The transformer tap change should require switching it off.

 

[PWR]

If you have 10kV available to accelerate an 11kV rated motor you don't have a voltage problem. How much time are you allowing for the acceleration? Is the compressor truly unloaded? (Many times I've seen the process guys call what they consider to be insignificant loading "no load" In actuality, a small load at start up adds disproportionately to the acceleration time.) Finally, you may simply have a motor poorly matched to the compressor.

 

[electricpete]

A few comments:
- 10kv at the bus might be substantially lower at the motor depending on the length of cable. How long a distance from where you read the voltage to where the motor is?
- Reduced voltage or unbalanced voltage does rob the motor of some torque capability.

I surely do not rule out that voltage may be the problem and the solution may be raising taps as suggested (or adding caps). Remember torque goes with voltage squared and there may be one point of closest approach of motor and compressor torque speed curves that you can't squeeze by.

What specifically happened? Did the motor trip?
Is it possible to unload the compressor and retry?

You might consider during next start monitoring for current balance.

99% of the time the answer will lie in the power system or the matching of load/motor as suggested above. As a very remote possibility, there are some faults which can rob the motor of its ability to deliver torque... namely cracked or open rotor bars. There are a number of ways to test for this.

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[electricpete]

There aren't any reduced-voltage starting controls on this are there? (wye-delta)

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[rbulsara]

I am curious how you ended up with a 11 kV motor when you have 13.8kV supply available? Why not use 13.8kV motor? The transformer in between is adding unnecessary impedance, a server impediment during starting.

I would make serious efforts to talk to the mfr.'s design engineers to find out if this 11kV motor is suitable for use on a 13.8kV system. Most likely it will be. The insulation class should be good for 15kV system as it is.

At the least try changnig taps on the transformer to some higher voltage than 11 kV, say 12 kV and try it.

It is clear that the motor is not developing adequate torque and low voltage only exacerbates the problem. A reduced voltage starter will not help.

You may also have a weak utility supply system.

 

[edison123]

As rbulsara says, a 13.8 KV motor would have been better in this case. But tap changing to >11 KV would only worsen the motor performance at start up as well as at load.

A cheap alternative may be to rewind the motor to 13.8 KV and eliminate the trafo.

 

[SAMBOY]

electricpete/Guys

We finally got our motor to run... We change tap from 11kv to make it 11.6kV during starting and we also installed a 500kVar capacitor. However it took us 38s to reach the rated speed (I starting = 256% Ifl) and the voltage drop on the motor terminal is 14.5% ...considering that this is only a Direct online starting and during no-load starting the acceleration time is only 6s (I start= 180%). Now the puzzle is this.. Can we call this normal? does the voltage of 10kV @motor terminal during start-up causes this over acceleration? what whould be the effect on our equipment re; The Motor and Transformer.

Added info: ( the comperssor can be rotated by hands, the Motor MCA is satisfactory, Insulation resistance in in Giga ohms, motor winding temp during starting is only 45deg C)

 

[edison123]

For a Istart = 2.56 x flc, the voltage drop seems high. This may be due to a) long cables b)small cross-section. In both cases, cables with higher cross-section will reduce the drop. This assumes your incoming supply is strong.

Also, additional 200 KVAR cap at the motor head might be beneficial. (Marke is the expert on this PFC).

 

[rbulsara]

well, 14.5% voltage drop is normal considering the transforemr is also in the circuit. The 38 sec. is too long to me, however its not end of the world.

What type of comeressor is it? Being able to rotate by hand is not enough proof of unloaded machine. Ask your mechanical counterparts and make sure the comrpessor is "unloaded" that "decopression" valves are open for first 20 seconds during starting. Unloaded compressors should not take more than 10-12 seconds to come to speed. Not more than 20 seconds in worst case.

It appears that not much thought/calculations went in during the design of this set up, or you would not be having this issue.

If a mechanically it is unloaded, than I have a deep suscpision that your motor is in fact a 13.8kV motor, sold with a 11kV nameplate because some one insisted on having a 11 kV motor!. Where was this motor bought from? (country)? Are you in the USA? 11kV, 60hz is a odd combination. 13.8kV, 60hz is standard in the USA.

edison123:
I would beg to differ with your comment of effect of higher voltage on motor starting.

 

[edison123]

rbulsara,

Normal voltage regulation of the trafo (even with a poor pf like at start) is about 5%. Considering the 14.5% drop in this case, cables can be a suspect.

A higher voltage may benefit starting due to increased torque but the resultant higher inrush current would only heat the motor. And running at higher voltage only makes poor pf and increased core heating.

 

[electricpete]

I would agree with nbulsara, there is more total rotor heating during a loaded start at reduced voltage than during a loaded start at full voltage. Increasing the voltage makes it thermally easier on the motor.

Interesting that there is such a dramatic difference between loaded and unloaded start times. It would seem to suggest the load torque comes close to the [reduced] motor torque at some point during the start.

What did you mean by 256% FLC during loaded start and 180% during unloaded start? I would expect for either start you get to locked rotor current and stay there until the motor is almost up to spped. Even 256% sounds low.

Do you have nameplate or factor info on locked rotor current? Your starting current should be about 8.66/11 times the locked rotor current at 11kv.

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[SAMBOY]

electricpete/guys,

While i'm still gathering information on where it came from and the nameplate, there is one more thing i'll ask. Is the acceleration time(38 seconds to reach the rated speed) harmful to the motor and other equipment based on the given instances?

 

[Kiribanda]

SAMBOY,

From your data my calculations show that even if your transformer is connected to an infinte bus, you cannot start on DOL with this 3.5 MVA/ 4.75% transfomrer to not to violate allowable 15% voltage drop.

Usually the transformer capacity should be at least 3 times the MVA rating of the motor. Therfore my suggestions are,

1)To continue with DOL starting

1.1 Increase the transformer size to at least 10 MVA with a suitable short circuit impedance which has to be decided
by taking into consideration the 3-phase minimum short circuit level at 13.8 kV bus.

1.2 It is better if the motor is rated to 10 kV (name plate) rather than rated exactly to 11 kV. I beleive that the name plate shows 10 kV and not 11 kV as you have specified. Please check!

1.3 A dynamic motor starting study should be done to fine tune the transfomer parameters and to avoid a long starting time.(Extending this time which is given by the manufacturer will damage the rotor bars)

1.4 Motor must always start with the compressor unloaded.

2) Use a soft starter which cost you very much at this voltage.

Kiribanda