Thermal Switches

[MrGentry]

I am wanting to put a thermal switch on an electric motor for a project in the pre-engineering class I teach.

I plan to use the thermal switch to turn on 2 120 mm computer case fans fans that should push/pull air toward the motor and cool it. What temperature should I have the switch close at? I have found them from 40-90 degrees. I know 40 is too low, but wasn't sure where to aim.

 

[MrGentry]

And yes, I know I could keep them on all the time and it wouldn't draw much current, but I think it would be cooler (no pun intended) to have an automated cooling system.

 

[mckutzy]

It depends on the motor, what is it rated for?

 

[fowler]

Easy enough to do

Your thermo switch is heated by an engine system

The fan has a posative wire running to the battery
The earth wire runs to a relay then to the battery

The thermo switch operates the relay with in turn operates the fan
If the engine is water cooled then put the switch in the coolant
On the hot side of the radiator

If its air cooled then put the switch in the sump so it takes engine oil temp

What type of engine is this
An industrial motor like off a lawn mower or cement mixer already has a fan built in that will out perform any small computer fan

U must find the specs on your engine as to operating temperature then find a switch that works in that same range

 

[itsid]

If the engine is water cooled then put the switch in the coolant
On the hot side of the radiator

If its air cooled then put the switch in the sump so it takes engine oil temp

What type of engine is this

I think you missed that info

I am wanting to put a thermal switch on an electric motor ...

Well, I wouldn't say 40 is too low to be honest,
since the cooler the motor runs the more efficient it'll be.

The motor can stand serious heat, the only thing that is heat affected really is the insulation, and that is mostly Polyimide these days which has a glass temperature of over 400°C,
so you are pretty safe here

I'd go for at least a 60-70° switch, to run the motor as cool as possible to reduce the resistance though.

'sid

 

[fowler]

Still
It can be air or water cooled
Doh

And still
Does it not already have a fan built in

Does it already have a heat sink in it
U can't really just tape a sensor to the side if the motor
Maybe drill a hole to fit a sensor to the housing around a brush holder

U will also have to investergate your type of sensor
Some won't work in open air

 

[itsid]

I never seen cooled DC motors at all,
AC yes, DC nope.

therefor: no build in fan,
Since he asked about computer fans (12cm), I though we are talking about computer sensors.
And then just glue (heatconductive glue) it on and you're done, yes.. that easy.
Simply thermistors in a nicer package
Still one would need a controll IC for the actual switch, but he only asked about a temperature;
I think he -being teacher in an engineering class- will have a fair understanding on how to read a thermistor and trigger a switch

'sid

 

[MrGentry]

The motor is a 24 volt 650 watt Currie motor. We are going to design a custom mount with a heat sink for the rear. I plan to have the students put a slot in the heat sink to hold the sensor against the motor. Here is the sensors I have been looking at.

http://www.moddiy.com/search.php?search_query=thermal+switch&x=-1324&y=-135

 

[itsid]

that's nice, no controller needed...

If you attach the thermal switch to the cooler I'd take the lower temperature rating (60 rather than 70)

the currie motor (as it appears on a schwinn s650 scooter) does not have any vents.. iis that right?
I don't know at what operating temperatures this thing is rated, but I would assume it doesn't even need any cooling at all to run properly

'sid

 

[MrGentry]

There are no vents. I assume it is a much better motor than the my1020 I had on another project. We are putting the motor in an enclosed environment, so I wanted to put some fans to help move a little air when the motor needed it.

Would the 60 degree switch not be on at room temp? I was afraid of using a low temp switch because I was worried that it would keep the fans on all the time, which I am hoping to avoid.

 

[mckutzy]

Can you contact the manufacture for the spec ideal operating temp, then conform to that.
I'd say run the motor how you are going to, use a inferred thermometer to see what you are running at. Don't run it hard just enough to see if a sharp rise in temp.

If a thermal sensor is going to be used, attach it directly to the stator windings.
If it is to be a permanent item, u can use 5min epoxy. You will however need to secure it to the windings with a cotton string. Coat all binding including the thermal.
When reassembling check for clearance WITHOUT power(spin it by hand) to listen for any ticking or contact with the components.

 

[itsid]

Yes, INSIDE the motor case is great, but it will void any guarantee I guess, so outside is good enough as long as you go for a lower value to compensate the difference.

And I understand those degrees to be celsius, so 60° will not trigger at room temperature (~22°)

'sid

 

[mckutzy]

If it is going to get hot it will quickly, it need to be in the case, or get one with a klixon or other thermal device installed.

 

[MrGentry]

I have no intention of opening the case. Since this is a school project bought with school moneys, I don't want to run the risk of damaging the motor by opening it.

As for the temps on the switches, Celsius makes a lot more sense now that I look at it ha.

My plan is to incorporate the switch inside the motor mount. So it will be sandwiched between the motor and an aluminum block/heatsink/mount. Hopefully that will give enough of a temperature change to trigger the switch.

 

[fowler]

what is this thing going to power

its going in a school of course so i presume its going to be a teaching tool or demonstrasion of some type

is it actually going to get hot enough to trigger the sensor
is it going on a vehical of some sort or on a dislay on a bench

u could well put it in a mount of some sort
but i wouldnt expect the system to respond very quickly to temp changes in the motor

its not like the motor is going to see 22hr days working in a factory
u could get a sensor that will trigger early so u dont have to stand around for ages waiting for the heat to transmit from the motor to its mount
as sid said the cooler then better

 

[MrGentry]

The motor is going to power a "cooler scooter" that my engineering students are designing from the ground up. (Concept sketches, CAD sketches, prototype, build). It won't see long periods of continuous use, but we plan to ride it in the hall ways during open houses etc. to pass out water and promote my program.