F Class Insulation Motor With 90'C Cable

[PhilDevine]

In practice I have not found many applications in which motor cables have been derated due to anticipated motor termination box temperatures. As an example: An F-Class insulated motor may operate at temperatures approaching 150'C in with expectant termination box air temps in excess of 120'C. The motor feeder conductors, 75/90'C is sized for 125% of FLA in what is a 30'C ambient, except for the connection box. Any thoughts?

 

[jbartos]

Suggestions:
1. The conductors in the motor terminal box tend to resemble conductors in the air. Such conductors have much higher ampacity rating than conductors in cables or conduits.
2. The connection in the motor terminal box is very short so that the conductor heat propagates outside of the box via the conductor. There, the conductor is in much cooler environment than inside the motor terminal box.

 

[electricpete]

It's a good question.

The only thing I can add from my perspective I don't think the termincal box air temperature in your example would get to 120C. I tend to think it will be closer to outside ambient temperature than to motor winding temperature.

For example 40C ambient and 150C winding temp, my crystal ball says term box air temperature at most 50-60C.

The outside surface of the term box may be expected at most 10C cooler than the inside air. I have never encountered a term box whose outside was so hot that I couldn't put my hand on it.

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[Skogsgurra]

I had the same question a couple of months ago. But instead of the motor box there were some iron powder rings (in a common-mode filter) that got rather hot. There were three rings around the three motor phases and the middle ring reached 74 C steady state.

This is very warm - to the touch - but there was about 10 mm air between rings and cable and the cable was cooled at both ends. The man in charge decided that they couldn't be used. I tried to discuss the matter, but to no avail. So this paper machine is now running with heavy EDM in some of the motor bearings and they will have to change bearings on a couple of 1000+ kW motors within less than a year. That I can guarantee. But the factory found that to be a better alternative than installing the filter and do some extra supervising of the cable temperature for a week or so to get a better knowledge about temperature in the cables.

It is often so. To many people, absolute limits are absolute limits. Even if they are measured somewhere else. The cable temperature never got higher than 55 C, but the 74 C was too much for them. Even if it had nothing with cable temperature to do.

 

[aolalde]

I have seen that the operating temperature of the terminals or conduit box of motors is not the same for different manufactures, HP, Speed , Frame size, Enclosure Type, etc.

Most of them work at reasonable low temperatures but others are very hot. I suggest that when a motor is found to operate with high temperature into the conduit box, derating proportional to the surrounding temperature should be made. This will prevent premature aging and possible failure of the cable insulation

 

[jbartos]

Comment on PhilDevine (Electrical) Mar 22, 2004 marked ///\\In practice I have not found many applications in which motor cables have been derated due to anticipated motor termination box temperatures. As an example: An F-Class insulated motor may operate at temperatures approaching 150'C in with expectant termination box air temps in excess of 120'C. The motor feeder conductors, 75/90'C is sized for 125% of FLA in what is a 30'C ambient, except for the connection box. Any thoughts?
///Why to apply 75/90°C conductor to such the motor? Why not conductors 150°C such as Z, 200°C such as FEP, FEPB, PFA or 250°C such as PFAH, TFE?\\\

 

[busbar]

FWIW, plain-vanilla tubular-copper crimp lugs like Burndy YA- or T&B 54100-series are rated 90°C. In this case, they are not terminated in an overcurrent device.

For US-style 'flying lead' splices in the motor j-box, bolted-stub splice insulation would have to be rated for similar temperatures. For 150°C, there is material like 3M #27 'toaster' tape, but forget about ever peeling it off once warmed up. {#69 tape is rated 200°C — #70 is 180°C}

 

[PhilDevine]

All -I appreciate the insight

Jbartos: In response to your 25MAR04 post: All applications I have run across in both commercial and industrial facilities only use 75/90'C cable to supply low voltage motors. Granted many designers are specifying class-F insulation with class B-temp rise in an effort to extend motor life.

 

[jbartos]

Comment on the previous posting: What I meant was to apply a little flex jumper from a junction box to the motor terminal box that would be rated higher than 75/90°C. Lug technology is available, e.g. 649°C (1200°F) Temperature Rating on web site:

http://www.newportus.com/Products/Wire/BBS_BBP.htm

Many electrical devices close to Diesel Engine have very high temperature cable rating.