500 HP AIR COMPRESSOR¦S MOTOR TROUBLE 3

[petronila]

Hello Every Body,

A New 500 HP- 1777 rpm- 460 V- 564 Amp, Model MGA 315C, S1, Cat N, Istarting/I rated = 7 manufactured by WEG, was sended to one motor repair shop due to the motor can´t start with satar-Delta starting.(The internal winding connection is Wye) The motor was rewound and mounted, the compressor is belt coupled (7 belts) and the motor starts in 7 sec, with out belts and drawns 122 Amperes at 445 volts.When the motor was coupled with only two belts drawn 800 Amperes and can´t start due to a breaker shoot.

The starting time was increased to 14 Sec and the motor starts( coupled with all belts) and drawns 320 Amperes with load. The originally motor is a 500 HP- 460 Volts-620 Amperes- 1785 RPM, manufactured by SCHORCH,Design A and code A, it was starting belt coupled in 7 secs and drawns 250 Amperes.

I would like to know your comments about this case history.

Thanks in Advance

PETRONILA

 

[rbulsara]

Firstly clarify a few things.

You said the original motor did not start with star/delta starter. They in the last sentence you say it was starting fine in 7 sec. Where was the problem?

Or did you mean that the star/delta starter could not be used (in delta mode?).

What type of starter is there now?

I will wait for your clarification, but I think the 'change' was unnecessary.

 

[petronila]

Thanks rbulsara,

The original motor was a 500 HP,1760 rpm,620 Amperes, y-D Starting, design A, CODE A manufactured by SCHORCH-Germany and was removed for maintenance.The costumer replace it with the WEG´S motor,but this motor can´t start in Y-D the WEG motor was rewound in order to start Y-D. The WEG´s motor : 500 HP- 1777 rpm- 460 V- 564 Amp, Model MGA 315C, S1, Cat N, Istarting/I rated = 7 was mounted, the compressor is belt coupled (7 belts) and the WEG´s motor starts in 7 sec, with out belts and drawns 122 Amperes at 445 volts.When the motor was coupled with only two belts drawn 800 Amperes and can´t start due to a breaker shoot.The starting time was increased to 14 Sec and the motor starts( coupled with all belts) and drawns 320 Amperes with load.
The originally motor was starting belt coupled in 7 secs and drawns 250 Amperes.

The WEG´S winding was necessary due to the winding connection was 4Y.

Thanks for your comments.

Petronila

 

[rbulsara]

Now a mute point, but did you ever try to use the Y connected motor to run Direct-On-Line? It would have ran just fine in star, imho.

It appears that by rewinding it, it produces even lesser torque in star than before and hence need longer acceleration time.

Trying with two belts is meaningless, as two belts may not have enough friction to tranmist necessary power from the motor to the compressor.

It this is a temporary arrangement, there is nothing much wrong with the set up nor you can do anything about it, short of changing the motor again.

 

[petronila]

Thanks rbulsara,

Unfortunatelly the power sources is not enough to handle the DOL Start for it the motor was rewound, but the torque remains constant, I think the added acceration time is consecuence of the motors Designs. The Original Motor was a Design A, and the rewounded motor is Design B, The design A motor can have higher starting and breakdown torque than the design B motor . But something is worrying me and it is the High starting current lasting(800 Amperes in 14 seconds)I think this motor could have in the future a failure due to insulation breakdown.

Thamks for your comments.

Petronila

 

[aolalde]

Hello Petronila:

Checking these motors I found them quite different and then the expected performance should be different.

The original “design A” motor with Wye start-Delta run winding must have higher torques. The inrush current for code A is from 0 to 3.15 kVA/HP max 354*3.15 = 1776 amperes.
The WEG motor is “design B” (general purpouse motor) and the inrush current (DOL) Is is 7 times the full load current , Is=7*564 = 3948 amperes. If the new winding is equivalent when connected in wye the inrush current reduces to 1/3 ; then Isy = 3948/3 = 1316 amperes.

If you measured 800 amperes most probably the line voltage has drop around 60% due to the current value. The voltage drop will reduce the motor torque more, extending the accelerating time required.

To accelerate a rotating system the net accelerating torque Tave ( average Motor torque minus the Load torque), the total inertia of the system (Wk^2 in Lb-Ft^2) and speed change (d-rpm) will define the required time in second to accelerate.

t = (Wk^2 * d-rpm) / (308 * Tave)

500 HP seems to large power for pulley and belt coupling to the load. Expect lot of head aches with the motor bearings. Same with the voltage, 460 volts look to low for 500 HP, the large currents required due to the low voltage will challenge any weak electrical connection.

4160 volts and gearbox reducer seem to be a logical selection for this size motor.

 

[jraef]

aolalde's points about the design differences in the 2 motors explains what is going on in your motor very well.

You may want to double check with the compressor manufacturer if 14 seconds is an acceptable starting time. Design A motors are not very common, but the compressor manufacturer may have used it for exactly the above reason, it could accelerate faster in Y-Delta starting than a Design B motor can, and they needed that faster acceleration for lubrication purposes. Switching to a Design B may be allowing the compressor to run without lube for too long.

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[petronila]

OK Thanks to every body, all is clear.

Regards

PETRONILA

 

[rbulsara]

One comment:

NEMA design A and Code A togther sounds odd. Torque is proportional to the KVA and Code A had lower starting kVA.

The design A has significantly higher torque at full speed compared to that of design B, but not so much at near zero speed (during starting). Not so much as to double the acceleration time.

I think all of nameplate code letters became meaningless once you rewound a Y motor to delta.

Check the link:

http://www.mastercontrols.com/EngInfo/Articles/Barr/TA_RBarr.htm

This link was copied from one of old thread that lost track of.

 

[rbulsara]

Another thought:

Not that it matters but still something to think.

Also compressors typically start unloaded and have lower staring torque requirement. Or why would you start a motor in Y which cuts the rated starting torque in 1/3?

It is another matter that the Y connected motor may not have adequate running torque to sustain the compressor.

I also find odd that a Y connected motor has starting current 7xrated, meaning it draws similar curernt as a typical delta connected motor would.

I am not sure why I am stating all these, but I think there is more history of that Y motor than it appears here.

 

[rbulsara]

70A of load current increase (320 vs 250) suggests that either your load has changed, or the rewound motor has much poorer power factor!!! I suspect the latter.

 

[petronila]

Thanks rbulsara the Link is excellent.

Petronila