Motor cable leads sizing 1

[SA07]

Hi
Can someone share a table with motors of various kW (from approx. 110- 700 kW 690 V ac connection star) and the corresponding cable leads in the motor connection box to be used? It would be good if the table is from a motor manufacturer and in mm2.

We have several motors 350 kW 379 A 690 Vac star connection with 95 mm2 cable leads that the winding has burnt or supply cable & motor leads burnt in the motor connection box. Thks

 

[edison123]

Recommended motor internal take-off cable size 1 A/sq mm since the cable not as well cooled as the motor winding.

Muthu

http://www.edison.co.in

 

[SA07]

We just had a problem on a 132 kW 690 Vac motor. The star point cables & lugs have heated up. Since we will not change the supply voltage or starting method of our motors, is it possible when we rewind a burnt motor, to do the star point inside the motor on the winding coils? This will avoid more lugs, connections which may have loose connections and heat up.

Is their any inconvenience of doing so?

 

[Padlock01]

Randhir120173
With respect, rather send the motor away for specialist repair and certification for
putting it back into service.

 

[SA07]

Since the motor winding is good, we have cut the cables of the star point, put new lugs and linked the 3 cables as our plant must operate. To replace the motor, we need a crane. This would take time and we might need to stop or reduce our plant power generated. We might be in penalty also.

We send all burnt motors for repair to a workshop.
We were just thinking to avoid such problem of cables heating up on the star point/loose connection, for future rewinding of burnt motors we could ask the workshop to do the star point inside the winding. No need to bring the 3 cables in the motor connection box. It will be less work for the rewinder also.

 

[edison123]

LV motors of that rating normally have all 6 leads brought and are delta connected to reduce the phase current (and hence the smaller winding leads) and for possible a star/delta start/run. I have rarefy seen motors over 20 KW LV motors wound in wye.

Muthu

http://www.edison.co.in

 

[SA07]

Plz see attached nameplate for a motor 350 kW 690 Vac star connected. Most of our motors > 110 kW are 690 Vac star connected.

 

[petronila]

Edison 123

Mostly 6 leads L.V motors without any internal connection, mainly IEC machines, are designed Delta connection for the low voltage and Y connection for a high voltage and with a 1.73 voltage ratio. (i.e. 230/400 or 400/690 ) so when necessary, you can use a Y-D starter for the low voltage. Also, this practice is pretty common in NEMA MV motors (2300/4160 V).

Randhir

Any reputable lead wire manufacturer can provide a Table based on Ampacity and temperature rating (Class B, Class F, Class H or Class R) but always the best is to use the lead wire originally specified by the manufacturer. Something that might help you when rewinding your electric motors rated above 110 kW, Y-connected @ 690 V, is to rewind that motors with only three leads (star internally connected) removing the terminal block when necessary. This practice will allow you to connect the power supply lines directly to the motor leads. The use of a terminal block simplifies the motor connection and avoid using tapes to insulate the power supply lines-lead motor connection, but with the time the terminal block can deteriorate due to many reasons as thermal stresses, excessive torque applied to the terminals, aggressive environment, humidity, etc.

Best Regards

Petronila

 

[che12345]

Dear Mr. Randhir120173
Q1. " ....We just had a problem on a 132 kW 690 Vac motor. The star point cables & lugs have heated up. Since we will not change the supply voltage or starting method of our motors, is it possible..."
A1. I look at the image and would like to submit my observations for your consideration:
a) All three Incoming conductors are over-heated at the terminal connection. The heat is conducted by the conductor which Over-heated the insulation. The insulation is NOT over-heated after approx 150mm? away from the terminal.
b) All three star-point conductors are over-heated at the terminal connection. The heat is conducted by the conductor which Over-heated the insulation. The insulation is NOT over-heated after approx 50mm? away from the terminal.
c) If the incoming or the star-point conductors are under-sized; the heat would be generated along the whole length not within a short length from the heat source; as evident from the image.
A2. It does not matter whether to connect the star-point conductors inside the winding or on the terminal. All IEC LV motors are wired with 6 wires terminated on the 6 terminals. No problem. It may be advisable to maintain the 6-wires terminated on the 6 terminals, in case there is a possibility of using star-delta starting method in the future.
A3. My conclusion is loose connection which result to high contact resistance at all the 6 terminals.
A4. You should request from the motor manufacturer for their recommended tightening torque for their terminals. Following are as "guide", the tightening torque for "dry" steel nuts may be say M10 grade 8 nut 48Nm, M12 grade 8 nut 80Nm; in case the advice from the manufacturer is not arriving in time.
Caution: These suggested "guide" values are for "dry" bolt and nuts. DO NOT/Refrain from lubricating or oiling the terminals or the nuts. Over-tightening would result or a much lower tightening torque value shall be used.
Che Kuan Yau (Singapore)

 

[SA07]

Thks a lot Petronila & Che Kuan Yau for your advice & table

 

[edison123]

So, it's a dual voltage motor, which was not mentioned before. Not clear why your company would buy a dual voltage motor when they could have gone for a 690 V motor which I am sure would have been wound in delta.

Even now, you can ask your rewinder to redesign for delta connection for 690 V and do the delta at the terminal board. The delta winding leads have to carry only 57% of line current i.e. 57% of 347 Amps = 198 A. I would go with 2x95 sq mm copper for each phase leads. This should solve your heat related problems.

Muthu

http://www.edison.co.in

 

[che12345]

Dear Mr. edison123
Q1. " So, it's a dual voltage motor, which not mentioned before. Not clear why ..."
A1. The nameplate shows the winding rating 400V D 690V Y , amount other data.
A2. For LV motors, 400/690V is an IEC Standard voltage rating used throughout the IEC world. It means that the winding is rated for 400V. The windings shall be wired in delta on a 400V supply or wired in star on a 690V source, by different linking as shown in the nameplate.
A3. In a 690V source, it is possible to (rewind) the windings rated for 690V in delta formation or maintaining the existing 400V windings but linked in start formation. It is the conductor to the winding differs. Caution: The current from the source power line and the current at the terminal shall be the same.
A4. All IEC LV motors are wired with 6 leads, terminated on the terminals, link as appropriate to suit the source voltage.
A5. It is my opinion that it is (more essential) to ensure that the [tightening torque] is per the manufacturer's recommended value rather than to [rewind] the existing from 400V to 690V. BTW: When the winding is rewind to 690V in delta formation, it would NOT be suitable for star-delta starting forever; even though there is no such need at the moment.
Che Kuan Yau (Singapore)

 

[petronila]

Edison

It is clear if a Wye design is redesigned to Delta connection you can use smaller lead wire size because the line current will be circulating by two leads. Although this type of redesign seems simple: Keeping the same number of circuits, Delta Turns = 1.73 Wye turns, with the respective wire size change, sometimes you get odd turns so the conversion using the original circuits in parallel may not be feasible and you need to change the number of circuits involving other factors as high volts/coil and current circulations chance (Number of circuits= poles/2) that require extra phase insulation and special extra-long jumper connection, so this "simple" redesign may fall in complications.

Also, the Wye design eliminates the chance of a third harmonic or triple-frequency current that sometimes are present in delta-connected motors

My advice is to keep the winding as was designed (Y connected) unless you need a wye-delta starting or you find a bulk design issue.

Petronila

 

[edison123]

Hi che

Why would a 690 V delta winding with six leads out not be suitable for a star/delta operation?

Hi petronila

OP posted winding failures due to overheat at terminal board and winding heads and hence my suggestion to rewind for delta to reduce current related heating issues. Do you think that instead of dual voltage, if you ordered single voltage 690 V motor, the OEM would have still done the winding in wye for such KW? Conversion star to delta is not such a biggie for a knowledgeable winder.

Muthu

http://www.edison.co.in

 

[che12345]

Dear Mr. edison123
Q1." Why would a 690 V delta winding with six leads out not be suitable for a star/delta operation?..."
A1. If the existing motor with 400V winding is (rewind) to 690V [with six leads out], [it would be suitable for star-delta operation].
A2. Irrespective of whether the winding is rated 400V or 690V , connected {internally} in [delta formation] and with [only three leads {NOT) six leads out], would NOT be suitable for star/delta starting, forever.
A3. Why [rewind] the winding to " 690V with six leads out", while the existing 400V D / 690V Y is a perfect fit for the 690V source? The links shall be as per Y formation, shown in the nameplate.
Che Kuan Yau (Singapore)

 

[petronila]

Hi Edison

My opinion is that conversion is not necessary. The advantage of this practice, if the motor is rewound with 6-leads, is the end customer can either use D.O.L or a Wye-Delta starting at 690 V but these days is not very usual to see a factory starting a 110 kW using a Y-D starting method and always you have risk that the motor repair shop screwup making that redesign, so why run an unnecessary risk?

In this case, I believe there are other aspects that might be related to the problem so the end-user should pay careful attention to: Motor loading, starting frequency, lug clamping, nut tightening, terminal block condition, etc.

Thanks and Regards

Petronila

 

[edison123]

che

I said 6 leads delta, not 3 leads. So ...

Hi petronila

My point is you get to use both the phase leads to carry 57% of current in delta winding as compared to using one lead carrying in 100% of the current in wye connection. So, it makes sense when you rewind the motor, make the winding in delta with 6 leads out.

And it gives the added benefit of wye/delta start/run at 690 V, if ever required. The odds of OP using the 400 V delta connection is practically zero given his supply voltage is 690 V.

Muthu

http://www.edison.co.in

 

[che12345]

Dear Mr. edison123
Q1. "... che . I said 6 leads delta, not 3 leads. So ..."
A1. I take it that your proposal is to [replace] the existing 400V windings by " burning off the insulation, cutting off the conductors and hammer out the 400V windings"; then replace them (with a new set of 690V windings) [with 6 terminals out on the 6 terminal block]. Link it in delta, therefore lower the current on each leads. I disagreed. The manufacturer knows better the correct size of lead conductors.
A2. To replace the existing 400V windings with a 690V windings cost money, down-time and a rewound motor would be of lower efficiency, (not very much if with good workmanship).
A3. Repeat " Why [rewind] the winding to " 690V with six leads out", while the existing 400V D / 690V Y is a perfect fit for the 690V source? The links shall be as per Y formation, shown in the nameplate." There is NO requirement for star-delta starting in this 690V system.
Che Kuan Yau (Singapore)

 

[edison123]

che

when you rewind the motor

Straw man arguments is waste of my time.

Muthu

http://www.edison.co.in