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DC 250KW motor Field overheating urgent!
6

DC 250KW motor Field overheating urgent!

DC 250KW motor Field overheating urgent!

(OP)
Hi, i need help i rewind motor field winding but the issue is all 4 fields burnt after some time but only half side of the field the other half of the field was as good as new recheck connection all are correct field voltage as per nape plate is 180 volts with 23.5 amp when the motor runs on DC drive field voltage keep increasing minutes after minutes but Ampere remains same. Drive is working fine i have checked it,
thanks

RE: DC 250KW motor Field overheating urgent!

Quote (drive field voltage keep increasing minutes after minutes but Ampere remains same)


How? It's DC and pretty impossible.

Quote (all 4 fields burnt after some time but only half side of the field the other half of the field was as good as new)


Again, how?

Post pics of motor nameplate and the so called half side failed field winding.

Muthu
www.edison.co.in

RE: DC 250KW motor Field overheating urgent!

If the drive has field current regulator, the field voltage will increase as the temperature (and resistance) of the winding increases.
Only half the field is burnt – the inner windings? May be due to too thin wire being used.
Some field windings are wound with larger wire dimensions in the inner windings as they have less cooling. This makes rewinding difficult without the motor winding data. Nameplate data 180v 23.5A is when winding is warm,

RE: DC 250KW motor Field overheating urgent!

(OP)
[img

RE: DC 250KW motor Field overheating urgent!

(OP)

RE: DC 250KW motor Field overheating urgent!

Never seen the winding+core assembly being removed like that. If you did the same after rewinding, the winding was already damaged by the time it went into the stator frame.

Muthu
www.edison.co.in

RE: DC 250KW motor Field overheating urgent!

Agree with Edison, it's a novel and mostly destructive way to remove the field windings.

It's understandable however because of the sequence in which the coils must be assembled
during the rewinding process.

Initial guess to the original problem: Obstructed or lack of cooling air flow.

It looks like one of the bearings in the device may be a 7220, but have to admit
the name plate is most difficult to interpret.

John

RE: DC 250KW motor Field overheating urgent!

Often all the poles are pulled out so that the access and rewinding of the main poles goes faster.
Problem is how to return the sheets under the poles, which can affect the geometry and behavior of the DC motor under load .
You should always rewind all the windings of the main poles, bearing in mind that they all suffer heating, and if you don't do a Baker test, you can't be sure that the other windings don't have a problem.
It looks like there is definitely a cooling problem and increasing the amount of air will solve the problem.

RE: DC 250KW motor Field overheating urgent!

Sumair An overheating issue on one side of the field pole assembly means one of three things.
1) Winding fault on that portion of the field winding (likely turn-to-turn fault, although could be turn-to-ground). Local area around the fault will get extremely hot, but the coil (as a whole) will not.
2) Poor heat conduction and/or convection on the "hot" side. Most likely culprit is having the pole too close to the adjacent one so either the pressure drop through the gap is too large, or the coils of adjacent poles (in this case, main pole and interpole) are touching. Alternative culprit is having a blockage at (or near) the end where cooling air should be entering the space between windings. This second "blockage: could be a misplaced baffle.
3) The main field winding is in direct contact with the interpole winding (see #2 above). The heat is not coming from the main coil, but rather from the interpole coil which carries a LOT more current (and can exhibit a subsequently higher surface temperature).

As BVSM22 noted, a field supply which is CURRENT REGULATED will hold a current setpoint and allow the applied voltage to vary. As the coil(s) heat up, the resistance increases - which in turn requires more voltage to push the same current through the circuit.

Last thing - it looks like the sling is looped through the end of a winding (either interpole or poleface). In either case, this is a pretty sure bet there will be some sort of damage to one or more windings in the machine - these are not meant to be "lifting points".

Converting energy to motion for more than half a century

RE: DC 250KW motor Field overheating urgent!

Does the motor have a high speed blower? And was it running? I presume the louvres are the hot air outlets. Is the outlet air forceful? Did you fix winding RTD's for the shunt and interpole windings during rewinding to monitor temperatures?

At 23.5 Amps, I would expect custom designed rectangular conductors. If you replaced it with standard round wire, you lost a lot of copper area.

Shunt field winding is like a room heater (in your case, a 4 KW heater) and hence needs a constant cooling even when the motor is running on no load.

And no, you cannot use your weak copper winding for lifting the poles out or putting the poles back in.

Muthu
www.edison.co.in

RE: DC 250KW motor Field overheating urgent!

(OP)

RE: DC 250KW motor Field overheating urgent!

(OP)
Only interpol windings are original shunt and field winding are rewound,
it was in round wire originally SWG 14
Used same 99.99% copper
All the spacer below Field winding cores are intact.
Yes amperage remain same but voltage keep increasing and field temperature touching 100°C at 280Volts 23.5amp in just half an hour while on load, but checked off load it remains same but shunt, armature and even commutator temperature is around 60°C
Thanks for you valuable time

RE: DC 250KW motor Field overheating urgent!

Sumair Please check your circuit connection to THE ORIGINAL DESIGN. My guess is that you now have the coils connected in series (1-2-3-4), instead of two parallel paths (1-3, 2-4). The series connection would give you the correct current as that is what you're regulating on, but would require roughly twice the voltage of the "original design" connection.

And I hate to say it, but errors in stamping data on the nameplate have occurred over the years.

Looking at your last set of images - I would say the field coil is built out too far from the main pole steel and there is correspondingly no room for airflow between main coil and interpole. For reference - it is fairly normal for the the larger OEMs to build their coils such that outside edge of the winding runs outward along the same radial line as the edge of the pole shoe (i.e., the coil does not overhang the pole shoe AT ALL).

Converting energy to motion for more than half a century

RE: DC 250KW motor Field overheating urgent!

Also, please check that motor no load speed, at armature voltage/field current = 440v/23.5a corresponds to not less than the nameplate's 1200rpm

RE: DC 250KW motor Field overheating urgent!

280 V, 23.5 A or 180 V, 23.5 A, which is it?

How many 14 swg? What was the shunt field connection, series or parallel?

What was the no-load speed after rewind at 23.5 A?

Photos of the failed half coils?

So many questions.

Muthu
www.edison.co.in

RE: DC 250KW motor Field overheating urgent!

(OP)
Field connections are 2 parallel
Voltage is 180 and motor starts at 175 volts but by the time goes it increases and after half an hour it reaches 280 volts 23.5 amps
SWG 14 single wire parallel connection
Will answer all the questions you guys have i just need to solve this problem ASAP thanks

RE: DC 250KW motor Field overheating urgent!

(OP)
Coils weights are same as original so there won't be a guage or turns difference

RE: DC 250KW motor Field overheating urgent!

At 180 V, what was the field current and speed?

At 280 V, 23.5 A, what was the speed?

14 swg in 2 parallel is ok provided the polarity of shunt field was maintained. Post a sketch of how you connected the shunt field coils.

Muthu
www.edison.co.in

RE: DC 250KW motor Field overheating urgent!

(OP)
Speed remain same from 180 to 280 volts 500RPM
In the nameplate its written
440 Armature volts
600 Armature amps
180 Field volts
23.5 Amp
But RPM is 500/1200
We are running it on maximum 500RPM on load is there any problem with this?
Posting winding connections in a while

RE: DC 250KW motor Field overheating urgent!

(OP)
Edison can we talk on email?

RE: DC 250KW motor Field overheating urgent!

Ok, field turns and connection are ok since you got the base RPM of 500 at 23.5 A.

Normally, the field voltage starts at about 70 to 80% of rated voltage in the cold start and as the field winding warms up and thermally stabilizes, you hit about 100% rated voltage to compensate the winding resistance rise due to temperature rise.

In your odd case, you keep going beyond rated 180 V to 280 V to hold the field current at 23.5 A. The only reason I could think of at this point is that the copper you used is not electrolytic grade 99.9% pure copper but a commercial grade copper (probably made from scrap copper with its impurities) and its resistance is high to begin with and went into thermal runaway that overheated it.

Do you have original vs rewound field winding DC resistances measured at room temperature for comparison?

I have seen OEM's cutting corners and the machines ending up in my shop with burnt field windings within a short period of commissioning. But in those cases, field current was not stable (due to ever increasing turn shorts) and hence the speed was not stable. Yours seem very odd with a stable current and stable speed.

Muthu
www.edison.co.in

RE: DC 250KW motor Field overheating urgent!

(OP)

RE: DC 250KW motor Field overheating urgent!

(OP)
Tell me one more thing if i add one more blower on commutator side or change the original blower to commutator side will it impact good or bad?
Today i run motor at 18.5 amps it start at 155volts by the time goes it increases to 194 in 75 minutes (off load)

RE: DC 250KW motor Field overheating urgent!

Quote (Do you have original vs rewound field winding DC resistances measured at room temperature for comparison? )


Muthu
www.edison.co.in

RE: DC 250KW motor Field overheating urgent!

appears that your resistance is wrong. I would expect the cold resistance to be around 5.48 ohms and the hot resistance to be around 7.67 ohms. if the rpm is correct, one would assume you have the turns correct. possibly the original coils have two different wire sizes at the beginning and end and that would explain the resistance possibly being wrong and the heating issue.

RE: DC 250KW motor Field overheating urgent!

(OP)
Kflan if there is any slight changes in wire guage the weight wouldn't be same as original weight is exactly same as original and when motor was original it shows 7 ohm on when motor is not running and 7.7 after hot

RE: DC 250KW motor Field overheating urgent!

(OP)
Tell me one more thing if i add one more blower on commutator side or change the original blower to commutator side will it impact good or bad? Or i should change original blower 5hp to 7.5hp

RE: DC 250KW motor Field overheating urgent!

"Kflan if there is any slight changes in wire guage the weight wouldn't be same as original weight is exactly same as original and when motor was original it shows 7 ohm on when motor is not running and 7.7 after hot "

So, resistance variation was 10% for original winding that mean for copper a delta temperature about 25 degree and new one have 180/23.5=7.66ohm cold to 280/23.5=11.91ohm... 55.5% variation that mean 141 degree if copper ideed is used... but seem to be "other copper type".

RE: DC 250KW motor Field overheating urgent!

You need to be more forthcoming with old and your new winding resistances in order for us to help. Otherwise, you are wasting other people's time.

I doubt the original cold & hot resistances are 7 and 7.7 ohms. Going by the nameplate, cold & hot resistances (both original and rewind) should be around 5.6 ohms at 30 deg C and 7.7 ohms at 90 deg C (as per standards).

No, adding a blower will not solve the problem. This is a motor designed to blow cold air in one end and let hot air at the other end. Where do you plan to put an additional blower?

Both old and new windings weigh the same does not necessarily mean both resistances are same. It all depends on quality of copper used.

Muthu
www.edison.co.in

RE: DC 250KW motor Field overheating urgent!

Sumair ,
as colleagues say, something in t resistance values is definitely not right, so you should measure the voltage drop on each excitation coil.
When DC motor is not rotating, is there any additional resistance included in the excitation circuit? This could further reduce coil heating depending on intermittency

RE: DC 250KW motor Field overheating urgent!

(OP)
The copper i used is best quality available here and it says 99.99% copper so there is not a chance of copper impurity, if i change blower from 5.5hp to 7.5 hp will it lower the temperature? Motor is currently running on 18amp and after 2 to 3 hours temperature raised to 90°C

RE: DC 250KW motor Field overheating urgent!

If this is true "Voltage is 180 and motor starts at 175 volts but by the time goes it increases and after half an hour it reaches 280 volts 23.5 amps"
and have same 23.5Amp at 180V and 280V, resistance variation is 55%.
Maybe there was measurement errors... DC voltage is enough filtred or with too high ripple? What Amp meter used?

RE: DC 250KW motor Field overheating urgent!

(OP)
No there is nothing additional in excitation circuit, i can't check this right now, will check it if i can,
If i rewind the motor again should i increase it's guage and lower its resistance to 5.6OHM? Or if anyone could help me with this thing?

RE: DC 250KW motor Field overheating urgent!

(OP)
iop95,
I used three different meters for volts and amperes both shows almost same and the drive is programmed to give 23.5amp no matter whats the voltage

RE: DC 250KW motor Field overheating urgent!

Your field winding turns and connections are right if the motor turns at rated 500 RPM at rated field current of 23.5 A and at rated armature voltage of 440 V.

Wire suppliers make all sorts of spurious claims, trust me.

Comparing OEM and your winding resistances at room temperature is the straight forward way of comparing the quality of your wire (provided you didn't change the wire size).

If OEM's and yours are about 7 ohms at room temp, then OEM screwed up and you can't be blamed for temperature rise since their original cooling system/design and/or field winding design was not good enough to begin with and you just copied their design.

But if OEM resistance is 5.6 ohms and yours is 7 ohms, then you need to admit your wire is of poor conductivity and hence poor quality and you need to take it up with your seller.

Blower air output is dependent on its diameter and speed. Changing the blower motor alone from 5 to 7.5 HP will not deliver higher amount of air. For DC motors of this rating, 3 to 5 HP blowers are pretty much the norm.

Muthu
www.edison.co.in

RE: DC 250KW motor Field overheating urgent!

(OP)
Neither i change the wire size nor turns, as per user's statement they took field resistance in winter around 20°C it was 6.6OHM IN ORIGINAL WINDING rest you know

RE: DC 250KW motor Field overheating urgent!

Quote (rest you know)


No, we don't.

Muthu
www.edison.co.in

RE: DC 250KW motor Field overheating urgent!

(OP)
If i rewind this motor should i increase its guage? From 14 to 13 and make it 5.6 OHM at 30°C?

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