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motor with extra wires

Bill D

Diamond
Joined
Apr 1, 2004
Location
Modesto, CA USA
Continuing saga of a Startrite bandsaw. I bought it knowing the wiring was bad. The motor has no nameplate, looks original British made. I think I figured out the power wires. It has two wires for the capacitor and two extra wires hanging loose. These are small gauge, so not power, coming out of the windings with no obvious place to attach them.
I am thinking they could go to a external thermal overload or even another capacitor.
Bil lD.
 
If small, they are very possibly a thermal switch intended to go in series with the "stop" switch for momentary switch "start/stop" setups.. If motor overheats, it opens, removes power from the contactor, and stops the machine.
 
The last motor that I bought had a thermistor built in to the main windings. I wasn't sure what the wires were for at first, but fortunately I was able to find the data on line.

If you just Ohm out the wires thinking it is a switch when it is really a thermistor, you will not get clear results. You might be able to verify the thermistor by lightly warming the motor while observing the resistance.

I decided to use the thermistor in my installation, given that it was there. I was using the motor (which was inverter rated) with a VFD and having a way of sensing winding temperature seemed to me to be a very good idea. I started to work out a circuit that would work with the thermistor, but then discovered that there are thermistor relays that are built specifically for this job. I found one fairly cheap on Ebay that was easy to wire into the VFD and this has been completely successful. Some VFDs already have the capability to work with a motor thermistor. Unfortunately I couldn't use this facility on the VFD I had already bought; I can't remember the reason why and in the end a separate relay was easier.
 
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could be a thermal switch or thermister since this has a weird switch with a built in relay. I understand that when it hits the stop switch on cutting full depth it has be lifted then the momentary "go" switch has to be pressed again.
But I do not see any extra wires near the motor to tie them into.
Bill D
 
Thermistor is a possible, yes. They would take a good bit more "stuff" to use the output, and a switch is "easy", so ????.

Yes, warm it up, and if it does not change resistance, it is not a thermistor. Any label on the wires? I assume they have continuity between them....
 
The "weird relay" is likely a thermistor trip relay. Basically, a thermistor is a resistor that exhibits low resistance fairly continuously, until it gets to a specific temperature, then it goes to very high resistance. There is not a linear relationship, so it's not good for measuring actual temperature because it has a "knee point" at which it quickly goes from low to high. So if you use a thermistor trip relay, and it turns out it has a klixon thermal snap switch, the relay reads it the same way and works the same. But if you assume it's a snap switch and it turns out to be a thermistor, you can damage the thermistor.
59113_fig_01.gif
 
Those can be hard to distinguish unless you are observant.

The switch will have a very low resistance when closed, and if it opens, it instantly goes to very high resistance , The thermistor will have a significant resistance, much more than zero, and will change to high resistance relatively slowly, with resistance changing slowly, then faster as temp goes up.

If you use a high resistance scale on the meter, they can look quite similar.
 
The UK versions used a good quality Brook Compton Parkinson motors. The 240v single phase version used a Starcro 75 and the 415v 3Ph version a Starcro 52. The 3Ph version had a proper control circuit with motor overload , the 240v version is basically nothing more than a latching relay ,holding power till the head down switch breaks the circuit (and no overloads are shown).

Rob
 
I wired up the motor and ignored the extra wires. Motor seems to run fine. No continuity between those extra wires and any other wires or ground. The wiring box was ripped off at some time and it probably had the manufacturers information. Two wires got ripped, lucky about 1/4 inch outside the motor casing.
This is a TEFC motor that also lost the fan shroud so I will have to make something to fit. Maybe the bottom of an appropriately sized saucepan will be the starting point. Hopefully an aluminum or steel one for easy drilling of some vent holes. I think stainless would be a lot of extra work.
Bill D.
 








 
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