wire ampacity and temperature derating

Thunderchild · Sep 6, 2011

We have a number of electric fans at work that have been returned by a customer in very poor condition. For some reason the manufacturer has out two inductors in the motor each in series with one of the supply wires. The wire diameter is 1mm (18 AWG) and these inductors are very badly burnt up to the point some have melted the copper wire ! The fans will easily draw up to 14-15 amps and I've been running the fans in a warm environment (to simulate a vehicle engine bay) to the point that the fans outer case reaches 80C.

What sort of amperage derating is applied to single core copper wire with enamel insulation ? surely 1mm is far too small ? what we cannot understand though is hoe the copper wire is melting !

Sceadwian · Sep 6, 2011

How it can melt is VERY easy to understand, it's reaching it's melting point. Why it is reaching this temperature is a different question altogether. There is obviously something about the working environment that you are not aware of that is causing a portion of it to reach the melting point of the copper. Thermal derating is complex and application specific, so much so that what you've described so far gives no clue to the true thermal environment.

ronv · Sep 6, 2011

I can only guess, but my guess is that the motors are being stalled or the motor itself is shorted. You might stall one (keep it from turning) and see if you get the same coil failure.
If you run the failed motors after replacing the coil do they still work?

Thunderchild · Sep 6, 2011

I will be doing a stall test today and also sending photos to the manufacturer. essentially I have a single strand of wire (in a coil) carrying 15A in a 90 degree Centigrade "environment" - ambient, should the copper wire take this ?

4pyros · Sep 7, 2011

Should not the inductor wire be as big if not bigger than the motor winding wire?

Thunderchild · Sep 7, 2011

well it is bigger than what the motor windings are but they don't get continuous current through them, these get the full motor power all of the time plus any AC/spikes because of the commutations

4pyros · Sep 7, 2011

Are thay mounted in the motor housing? Maybe thay should be heatsinked?

Thunderchild · Sep 7, 2011

yes they are inside and connected to the brush distribution. photos here: https://www.eevblog.com/forum/index.php?topic=4709.msg62324#msg62324

4pyros · Sep 7, 2011

From the pics it seems its a sealed motor. 4 brushes, 2 in parallel? I can not tell if the inductors are in series with both or between the brushes.

ronv · Sep 7, 2011

To fuse 18 AWG wire like that takes 82 amps. Can't tell if the coil form is conductive to ground or not as the insulation would break down much sooner. I would be looking at the bad ones rather than testing good ones because 15 is a long ways from 82. Have you attempted to run the bad ones again with the inductor removed? How are the brushes and brush holders? Winding resistance? Freedom to turn etc.

Thunderchild · Sep 7, 2011

4pyros said:
From the pics it seems its a sealed motor. 4 brushes, 2 in parallel? I can not tell if the inductors are in series with both or between the brushes.

Correct sealed motor, 2 windings in parallel, the inductors are in series with both coils ie: on the power feed

Thunderchild · Sep 7, 2011

ronv said:
To fuse 18 AWG wire like that takes 82 amps. Can't tell if the coil form is conductive to ground or not as the insulation would break down much sooner. I would be looking at the bad ones rather than testing good ones because 15 is a long ways from 82. Have you attempted to run the bad ones again with the inductor removed? How are the brushes and brush holders? Winding resistance? Freedom to turn etc.

is that 82A at room temp ? I think there is a good 90+ celcius in the motor body

shortbus= · Sep 7, 2011

Thunderchild said:
is that 82A at room temp ? I think there is a good 90+ celcius in the motor body

I think your temperature is kind of low for an automotive environment. Most cars today have at least a 200*F/93*C thermostat. So air over the motor would be at least that hot. Then add in heat from the transmission cooler and air conditioner condenser and ambient temp to that. If they are in a front wheel drive car then you need to add the radiated heat from the exhaust manifold, which will be very close to the fan motor.

Thunderchild · Sep 7, 2011

I am not using room temperature infact, I have put the fan inside a box so that it "self heats" and it is running in 60C, that is as hot as I can get it. I am noticing lots of brush wear already at 85C case temperature so I'm in no position to go higher as I'd rather not start a fire. The temperature of the stat you refer to is the hot water in the engine, this is not neccesarily the ambient temperature. The engineer told me to run the motor until 80C (case temp) I think we usually calculate an ambient of 80C, of course the cardboard box it is in don't like heat much !

4pyros · Sep 7, 2011

With 4 brushes you are powering 4 windings all the time, two for each pair of brushes.
Is the motor sealed?
Is it posible its running backwards? IE Blowing air into the direction of travel.

shortbus= · Sep 7, 2011

Thunderchild said:
I am not using room temperature infact, I have put the fan inside a box so that it "self heats" and it is running in 60C, that is as hot as I can get it. I am noticing lots of brush wear already at 85C case temperature so I'm in no position to go higher as I'd rather not start a fire. The temperature of the stat you refer to is the hot water in the engine, this is not neccesarily the ambient temperature. The engineer told me to run the motor until 80C (case temp) I think we usually calculate an ambient of 80C, of course the cardboard box it is in don't like heat much !

Yes that is the engine coolant temp and the radiator is to remove heat to keep water at that temp. Where do you think the heat from radiator goes? Right into the air drawn through the radiator by your fan and over the fan motor.

4pyros · Sep 7, 2011

The engineer told me to run the motor until 80C

For what??
80c is only 176 degrees F most engines will run hotter than that like about 210F. The ambent temp in the engine compartment may be alot hotter and if the fan motor is sealed it may be even hotter than that. Lets say 250F to 300F.

Thunderchild · Sep 7, 2011

shortbus= said:
Yes that is the engine coolant temp and the radiator is to remove heat to keep water at that temp. Where do you think the heat from radiator goes? Right into the air drawn through the radiator by your fan and over the fan motor.

the hot water is coming out of the engine and going into the radiator, the temp drops quickly as the fan pushes air away from it into the cooler. what I don't know is what temperature the air in the engine bay is at due to the dissipation of heat from the engine itself through the air. In this particular case these are oil coolers not water coolers that the fans are mounted to

4pyros · Sep 8, 2011

Are the motors sealed? and is the air flow going the right way?

Thunderchild · Sep 8, 2011

4pyros said:
Are the motors sealed? and is the air flow going the right way?

yes they are sealed (IP68) as far as I know they are going the right way

wire ampacity and temperature derating

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