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?



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.

  • \$\begingroup\$ Thanks, Charles. I did some checking and can't find where I got the "no load" modifier for the current rating. My mistake. Sorry. The 3A is likely the rated current. It's not all that powerful a motor. Slows down noticeably when drilling steel or drum sanding. The T22 can handle 20A at 250V (failure is 30A at 250V). CanTherm calls it a high power thermostat. Seems it can handle quite a lot at 115V. Is a relay really necessary? \$\endgroup\$ – MellowDios Aug 1 '18 at 22:40
  • \$\begingroup\$ I understand how the sensor needs to be mounted. I read an interesting article by Baldor which said that while some induction motors are still designed as Class A, they are probably closer to Class B due to the use of more advanced materials in their insulation. That said, this is an offshore motor in a low-priced consumer drill press, with no data plate! Suppose we assume the insulation to be rated higher - say 120degC (Class E). What trip point would you suggest for the T22 then? Thanks. \$\endgroup\$ – MellowDios Aug 1 '18 at 22:41
  • \$\begingroup\$ I added a paragraph agreeing with original temperature selection. The ability to switch the motor current is the critical factor in the current rating of he device. I believe the cos φ 0.6, 3A rating is the one that applies. \$\endgroup\$ – Charles Cowie Aug 2 '18 at 1:33
  • \$\begingroup\$ Been calling Cantherm for clarification, and just got hold of their sales engineer today. He told me that the cos φ 0.6, 3A rating is misleading. Something about having to state it that way for UL approval. He said the T22's current rating at 60 Hz is virtually the same as it is at 50 Hz, around 20A. Also that the T22 is a direct replacement for a thermal fuse, no relay required. He also confirmed that a trip point of 120degC would be reasonable for the type of drive we're discussing. Looks like I'm good to go with the T22, unless you have any other concerns. Thanks again for your help. \$\endgroup\$ – MellowDios Aug 13 '18 at 19:07
  • \$\begingroup\$ Since you are operating the drill press yourself and this is not something that could be overloaded in un-attended operation, you probably do not need to be as conservative as UL. However there is probably some increased risk of failure. \$\endgroup\$ – Charles Cowie Aug 13 '18 at 19:24

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