How to determine the right trip point of thermal cutoff for 1/4 hp 3A 115V single phase ac motor

Question

The Thermal fuse blew in my Ryobi DP102L drill press motor, while I was drum sanding. There were no markings on the fuse. The single phase induction motor is 1/4hp 3A (no load) 1750rpm on 115V AC, according to the manual. There's no name plate or information available from Ryobi. Comparing it to similar drill press motors online, it's probably Class A insulation, but that's a guess.

I want to replace the fuse with a thermostat type cutoff which will reset so I don't have to tear the motor down again. The temperature rating for Class A is 105degC. The device I'm considering is a CanTherm T22, which comes in a variety of trip points which match the different NEMA classes.

Research online yielded only two results for this specific question. One suggested 25% above the rated temperature. Another, which was an article about thermistors, suggested 10%. The trick of course is to have the thermostat remain closed under normal operating conditions, but trip if I push things too hard (like drum sanding at too slow a speed) then reset when things cool off.

If I understand correctly, I don't want the thermostat to trip right at the rated temperature like this one : T22A10505DFFBG0E. But slightly above like this one: T22A12005DFFBG0E. See full list here: CanTherm T22

Does the device I'm choosing makes sense? Are there other considerations? Are there any issues with the current capability of the T22 which is much higher than the required 3A of the motor?

Thanks.

Answer 1

It appears to me that the current rating for use with an inductive load is 3A. You described the motor as "3A (no load)." I would recommend that you use the sensor in series with a relay coil and use relay contacts to switch the motor. The relay contacts should be rated for 1/4 Hp 120 V AC motor duty.

The sensor needs to be embedded in the motor windings in the same w, in the same locations that the thermal fuse was. Even then, the temperature of the sensor may be slightly lower than the hottest area of the motor insulation. I suspect the motor insulation is rated higher than class A. I believe that class A is rarely if ever used in today's motors.

Assuming class A insulation and selecting a 120C trip point is probably fine for an inexpensive imported motor. Insulation life is a matter of time and temperature. Running a motor with class A insulation continuously at 120C would shorten the life to about 1/4 of the expected life. If that happens for a few minutes once in a while, the life of the insulation would probably not be drastic.

Answer 2

What you really want is a protective circuit that trips before damage to the motor occurs. I think you're barking up the wrong tree looking for a thermal solution - by the time that registers, the motor is already fried. That thermal trip isn't there to save the motor, it's to save the house from a fire from the motor's high temperature (note all the sawdust and/or cutting oil in the area). The thermal shutdown doesn't care if the motor survives, and it isn't measuring the critical locations in the motor that are vulnerable to damage.

Your better plan is to stop the dangerous thermal rise from happening in the first place. The way you do that is by limiting current using an inverse time "thermal trip" circuit breaker. Instead of monitoring the actual temperature of the motor, the breaker monitors a bi-metal strip inside the breaker which is warming (nominally) with the motor's critical parts (the windings). You will find you can overload the breaker significantly and it will tolerate that for a short time, the length of time being the inverse of the overload percentage (hence: "inverse-time"). That reflects the cumulative "warm-up" happening in the motor windings.

The good news is every breaker in your panel is one of these. The bad news is they don't make them in 3-9 amps. But others do, and that's what I'd look for. Look at their data sheet to evaluate the "Trip Curve". It may take a few iterations to find one that won't have nuisance trips.

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This honestly seems like a combination of "unfamiliarity with using the tool" and "asking too much out of the tool". So I would focus much on learning not to overload the tool in the first place.

The best option there, IMO, is a "Kill-A-Watt", an inexpensive "through the plug" combo meter that can read out amps directly (and many other things). But you can set it for amps and simply watch visually the amps you are pulling.

Once you get better at using the machine, you can also hear it from the sound of the motor. The thing about induction motors is they have a certain amount of "slip" by design. The amount of slip is proportional to the force/torque (very closely related to current) acting on the motor. So watching the Kill-a-Watt you will find a tight correspondence between the acoustic "pitch bend" from the motor, and the amps on the Kill-a-Watt. Before long you'll be able to hear when you're overloading it.

The Kill-a-Watt and your fingers will tell you this, but the larger the slip, the worse the efficiency of the motor. When you let the motor bog a lot, you'll find current is through the roof yet useful work on the work-piece isn't much more at all (maybe even less). The lost efficiency is entirely warming the motor. Thus running a motor "at bog all the time" will result in fairly fast damage.