Motor leads sizing 3

[zlatkodo]

Is there any NEMA or IEC standard, recommendation or rule for sizing of LV motor terminal leads?
Zlatkodo

 

[jraef]

Yes, but your query is incomplete.

If you are referring to the leads INSIDE of the motor, i.e. from the windings to the peckerhead (connection box), then for North America that would be NEMA MG-1. If you are referring to the leads going FROM the motor controller TO the motor, that would be, for the US, the National Electrical Code, article 430 or for Canada a similar section of the Canadian Electric Code (but chances are they match anyway).

Others will have to address any IEC requirements, not my area of expertise.

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[electricpete]

I agree with the division between NFPA outside the motor and NEMA MG-1 for T-leads. I presume the question referred to T-leads.

It’s all a bit confusing to me.


Here’s all that NEMA MG-1 says about the subject:

for each of the various parts of the motor[/b] shall not exceed the values given in the following table when tested in accordance with the rating, except that for motors having a service factor greater than 1.0, the temperature rise shall not exceed the values given in the following table when tested at the service factor load....

Note I have bolded “for each of the various parts of the motor”, which under conservative interpretation (but not the only interpretation) might include T-leads and (coil knuckles and series jumpers etc....even though there is no mention of measuring temperature of anything other average temperature of entire winding by resistance and and slot temperature by RTD).

Continuing a conservative approach based on the above, if the motor has class F insulation, then the T-leads must be rated at least class F and should be sized so as not to exceed a total temperature of 40C (motor ambient) plus 105C (class F rise) = 155C at rated load for SF 1.0 motor. It should be important to notice that even though the motor ambient is 40C, the T-lead ambient is higher and so I would expect that needs to be considered in sizing the table (cannot pretend the T-lead itself is in 40C ambient by a conservative approach)

However, this is not the only interpretation. Apparently EASA does not interpret it so conservatively.


EASA Tech Manual section 7.2 gives ampacity of leadwire as a function of leadwire rating and size. The same chart is also available here:

https://secure.easa.com/resources/c...g+Reference+Materials+and+Tools&item=16&sub=1

I tried to correlate the table to the various NEC tables and was not successful. The ratio between EASA ampacity and NEC table ampacity was not constant.
For example consider 90C cable insulation rating:

NEC Table 310.16 Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60_C Through
90_C (140_F Through 194_F), Not More Than Three Current-Carrying Conductors in Raceway, Cable,
or Earth (Directly Buried), Based on Ambient Temperature of 30_C (86_F) gives the following ampacites (90C):
18AWG – 14A
1/0 – 170A

The EASA table gives the following (90C):
18AWG – 18A
1/0 – 155A

So on this comparison, the EASA gives more ampacity than that particular NEC table for the 18AWG, but less for the 1/0. I cannot deduce the pattern or logic.


There is also a note at the bottom of the table “Note: Lead wire sizes for ampere ratings in this table represent recommended minimums. It is best to use the lead wire size originally specified by the motor manufacturer.”
A good caveat, I’m sure.

Then there is another table in section 7.2 that suggests
Class B motor must use 90C lead wire
Class F motor must use 125C lead wire
Class H motor must use 150C lead wire.
And an accompanying note: “Note: The thermal class rating for an electrical insulation system need not
include the lead wire as a primary component. Rather, the lead wire may be considered a secondary component of the system, and hence may have a lower temperature rating as a component than the rating of the insulation system.”

So, it appears EASA’s interpretation is that the T-lead wire insulation need not meet the temperature insulation rating of the motor (they did not follow the conservative interpretation I suggested above). However of course the actual cable sizing should consider the actual cable temperature ratings, so it is not non-conservative in that respect. EASA doesn’t mention anything about ambient temperature of the cable itself. Perhaps the ampacity tables they provided have some empirical correction for that somehow. Also worthwhile to note that T-leads are relatively short, so the approaches we use for long cables may not apply.


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(2B)+(2B)' ?

 

[petronila]

Hello Zlatkodo

To determine the T-Leads you willl need analize use many parameters: 1. Current rating. 2. Number of Leads. 3. Connection Type.4. Cable Material(Hypalon, Silicon, Rubber, Glass, etc.This materials are also linked to the insulation class).

Other good method is calculate the cross section of winding wires and use the right cable (1.25 x wire mm2).

On both cases you will need cable manufacturers tables.

Take account the most L.V IEC motors are designed to operate in Delta-Wye connection and are six leads designed, in this case the leads most support the nameplate current for high voltage rating.

On NEMA Motors the windings could be designed with 3,6,9 or 12 Leads for YY/Y or DD/D then the cables will depends of connection type and number of T-Leads.

Regards,

Carlos

 

[ScottyUK]

Carlos,

Doesn't the winding current remain the same regardless of star or delta connection? The line current in star is equal to the winding current, the line current in delta is [√]3x winding current. I know you know this, that's why I'm puzzled by your comment above.


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If we learn from our mistakes I'm getting a great education!

 

[zlatkodo]

Thanks to all for the helpful answers.
It appears to me that there are not a strict recommendations or standards for the T-leads sizing.
Let's look at 6 leads motors, star or delta.
Minimum size of lead should be equal to the cross sectional area of one turn x number of circuits.
The same is the case in 3 leads motors in star connection.
In the case of 3 leads delta connected motor, the minimum size is a cross sectional area of one turn x number of circuits x 2 / 1.73.
Did I go wrong somewhere?
The question is: whether there are reasons that T-leads must be thicker than the above minimum size?
Zlatkodo

 

[petronila]

Hello Scotty,

I´m speaking from the design point of view.Yes, the current will be the same if is connected in Wye or Delta , but the number of wires will dictates the cable mm2. In the case of 3 cables the T-leads most support full rated current, but if is designed for two star connection then each t-lead will be support the half of rated current.

Regards

Carlos

 

[electricpete]

One situation where delta or wye configuration would be important for process of calculating T-lead current (knowing horsepower, voltage, efficiency, power factor) is the high-voltage/run connection of 12-lead motor (series / delta). The delta is not closed internal to the motor, but only closed when the two motor leads are connected together with each power supply phase. (the other 6 leads are not connected to power supply). This is common configuration for small motors, not for large motors.

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(2B)+(2B)' ?

 

[ScottyUK]

Thanks Carlos, it is clear now.

ePete,

That would be a rare motor over here. Dahlander connection, perhaps with an independent winding for an third speed, is about as complex as we get. Are motors of that design common in the US?


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

Is it common?
Well, it is a configuration described in NEMA MG-1 and you can see it here:

http://www.malloyelectric.com/pages/tech/connections.html

It provides flexibility of dual voltage and ability to perform wye-delta soft start.

At our plant we have a robust power system with no need for soft start (wye delta) to protect the power system. I am aware of one family of 4 motors in our plant that have this configuration. I first stumbled upon them in 2005 when after motor replacement, the motor was tripping the overloads. Vibration analysis showed it something 7% below sync speed. Investigation showed our electricians had mis-wired it in the high-voltage wye configuration because they were not familiar with it (so I guess it's not that familiar).

The next time I stumbled onto these motors was this morning 3/12/12. I was called to the shop with a question about grease relief on replacement motor retrieved from the warehouse for installation. I remembered the application and our previous mistake and I asked the electrician how he would wire it. He had no idea, until I explained it to him.

Why did I got through this big long story? First, because it's quite ironic this occured on the same day as you asked how common it was (I was tempted to say "I just helped install one today", but that would have been quite misleading), and also because this 2nd electrician also wasn't familiar with it, suggests it is a pretty unique configuration within our plant.

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(2B)+(2B)' ?

 

[electricpete]

Three comments I should add:

There's a typo in the link. The right-most 5 in the wye diagram should be a 6.

In the application mentioned above, we don't have any provision to do wye-delta start... we just hook them in hi delta and DOL start (the confusing part for electricians is figuring out whether to use hi-delta or hi-wye). We do have another set of compressor motors that are switched as wye-delta because the skid was provided by vendor that way... never bothered undoing it.

I'm thinking the aspect described above (delta run winding where the delta is not closed inside the motor... only closed where the two leads connect to a given phase), would be a feature of any motor suitable for wye-delta start, wouldn't it?

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(2B)+(2B)' ?

 

[ScottyUK]

Thanks ePete, how odd that you were working on one of these machines just today!

Regarding your question at the end, yes I think so if I understand you correctly. The possible switching permutations for that 12-lead motor had me entertained for a while, but in practice it would be configured as either high or low voltage, then switched as a standard 6-lead Y-D motor.


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If we learn from our mistakes I'm getting a great education!

 

[petronila]

Hello Pete and Scotty,

About the last question concerning a rare motor
"That would be a rare motor over here. Dahlander connection, perhaps with an independent winding for an third speed, is about as complex as we get. Are motors of that design common in the US?"

I just want to add something: Dahlander + Third speed is a three speed motor with 2 windings. The NEMA Dahlander motors are labelled on T-leads with numbers and the IEC with letters, The NEMA Dahlanders are designed for 3 applications: Constant power, Variable torque and constant torque and the IEC only for the last two. So is possible your rare motor could be designed with 12
t-leads distributed like: Six for the third speed (Y-D) and the other six for the dahlander two speed winding. In this case is very important the T-Leads label and name plate data, because will help to don´t have mistakes once you connect.

Attached you will find the dahlander connection draws for IEC.

Regards

Carlos

 

[zlatkodo]

Hi, Carlos,

Other good method is calculate the cross section of winding wires and use the right cable (1.25 x wire mm2).

Please, give us a little more explanation for this.
Zlatkodo

 

[petronila]

Hi Zlatkodo,

Suppose we have the following winding, 36 Slots, 6 groups, 3 coils per group, 15 turns and is winded with 2 wires of 1.0 mm and 1.1 mm of diameter. The conexion is delta-star.

For this example the diameter of first wire is 1.0 mm and the second 1.1 mm, then from a round magnet wire table we have to calculate the mm2 of each wire or you can use mm2 = Phi *diameter ^2/4 for 1 mm the mm2 are 0.78 and for 1.1 mm the mm2 are 0.95 so the total mm2 are 0.78 + 0.95 = 1.73, due to the connection is delta-wye six leads the groups are connected in one single circuit then the end cross section remains the same, so the t-lead will be calculate like: 1.73*1.25(Security factor, some people dont use) so we will have a cable of 2.16 mm2, aproaching you can use a cable of 2.5 mm2 and will be pretty calculate according to the design.

Case 2: The same winding but connected in two circuits (2Delta-2 Star), due to the groups are connected with 2 circuits, now the total cross section will be (0.78 + 0.95)* 2 = 3.46 mm2 and the cable will be 1.25*3.46 = 4.32 mm2 aproach to 5 mm2.

Hope this explanation be useful.

Regards

Carlos

 

[zlatkodo]

Hi, Carlos,
It's clear, but why factor 1.25? Is there any particular reason that this factor is more then 1?
Zlatkodo

 

[petronila]

Hello Zlatkodo,

This factor is not included in standards procedures, is for security and is one practice that you can adopt when you don´t have the cable manufacturers tables, and the 25% factor will avoid any problem with a short cable calculation in the case you don´t know many about the cable or don´t have the information about the cable( No cable table, no material composition, no information) , I am speking from my own experience and what I do when I don´t have that information about the cable. I know the cable is not a fuse but other important think is if you have a large wire and the motor has many startings or temporary overloads the cable will support it and the expected life will be improved.

Regards

Carlos