Single phase motor getting hot in 2 minutes with no load - Page 1
Forum section: electronics / beginners

author: robsims
author_rank: Frequent Contributor
timestamp: August 21, 2022, 02:53:55 pm
content: Hi, i'm a mechanical engineer and a beginner in electronics. Bought a chinese 6 inch bench grinder. The single phase motor is getting hot in 2 minutes with no load. i can not leave my hands on it for longer than 2 seconds. If a motor is getting hot, from the books, that means too much current is passing through the windings. Am i correct. Please i want a scientific explanation. Can i rewind it with a smaller gauge wire, so i reduce the current? Many thanks in advance

author: bdunham7
author_rank: Super Contributor
timestamp: August 21, 2022, 03:55:45 pm
content: Rewinding with smaller wire isn't going to be any solution because in a properly operating motor, the resistance of the wire is small compared to the other impedances that limit current. Does the motor operate normally or is it slow to start with limited torque? Does it have a start or run capacitor?

author: Uunoctium
author_rank: Frequent Contributor
timestamp: August 21, 2022, 04:12:55 pm
content: Can you insert an simple power cost monitor?Of interest is wattage, current, cos phi. Is that identical to what is written at typeplate? And corresponding to frequency & voltage used in suriname? Is 127V/60Hz right?

author: robsims
author_rank: Frequent Contributor
timestamp: August 21, 2022, 04:59:04 pm
content: Many thanks bdunham7 and Uunoctium. The motor has a start capacitor and operates normally. I don't have a power cost monitor. In the instruction manual it is rated 110V-120V, 1,7A and 60Hz. The voltage on the wall outlet is 129V

author: james_s
author_rank: Super Contributor
timestamp: August 21, 2022, 05:26:37 pm
content: Are you sure it's a start capacitor or is it a run capacitor? It's fairly common for motors to get too hot to touch, especially cheap motors that run the stator core near saturation and have inadequate ventilation.

author: robsims
author_rank: Frequent Contributor
timestamp: August 21, 2022, 05:34:01 pm
content: I measured the resistances of the running and start windings. Running winding is 24 ohms and starting winding is 58 ohms. The capacitor is 4 microfarads and attached to the 58 ohms winding

author: james_s
author_rank: Super Contributor
timestamp: August 21, 2022, 05:54:17 pm
content: I suspect that's a run capacitor and that the motor is a PSC type without a starting switch. Does it make a click as it is spinning down? I doubt there is anything wrong with your bench grinder, it's just cheap. They are not designed for continuous use, normally you will turn it on, grind something for a minute or two and then turn it off.

author: bdunham7
author_rank: Super Contributor
timestamp: August 21, 2022, 06:23:45 pm
content: If possible, try running it on a lower voltage like 115 or so. Perhaps you have or can borrow a variac? 129V just might be a bit too much if it is a really marginal (reduced metal content) design. Heating up that much in 2 minutes is pretty extreme for a bench grinder, I'd expect it to be drawing way too much current at no load for that to happen.

author: robsims
author_rank: Frequent Contributor
timestamp: August 21, 2022, 07:06:27 pm
content: Many thanks james_s. Yes it is a PSC motor. It has no switch to disconnect the capacitor. It also has no ventilation. It is cheap. Bought it for $40. How can i improve the duty cycle of it?. What are the characteristics of induction motors for continuous use?

author: robsims
author_rank: Frequent Contributor
timestamp: August 21, 2022, 07:14:47 pm
content: bdunham7 i have a variac. i tried running it with a variac on 115 volt, but it keeps heating up. I think it draws too much amps at no load. In the shop where i bought it there was airconditioning so i didn't know it was heating up so fast. How can i improve it?

author: Zero999
author_rank: Super Contributor
timestamp: August 21, 2022, 07:19:14 pm
content: Another possibility is a shorted turn, in which case it'll need rewinding, which is probably not economical.

author: bdunham7
author_rank: Super Contributor
timestamp: August 21, 2022, 07:23:00 pm
content: No ventilation is normal for bench grinders and they usually manage to dissipate the heat through the case directly. If it still overheats quickly at 115V, perhaps it is a design issue. Can you safely disconnect the winding with the capacitor after it starts running to see if it keeps going and has power? One thought is that the basic design is a capacitor-start motor with a centrifugal switch, but then they thrifted out the switch part. It would help if you had some way of measuring the current it draws when running as well. Safely, of course.

author: Seekonk
author_rank: Super Contributor
timestamp: August 21, 2022, 07:41:28 pm
content: Does it have any nameplate specifications? Could this be a 60Hz motor running on 50Hz? That would require a much lower voltage, like 100V.

author: Ian.M
author_rank: Super Contributor
timestamp: August 21, 2022, 08:22:00 pm
content: Time to put in a claim for a refund and return. Its overheating dangerously and isn't fit for purpose if used in your country. If the supplier lets you keep it, you could consider rewinding it for fun, with a wire gage chosen to get approx 25% more turns on each winding, but its certainly not economic to do so as the cost of the wire and labor far exceed its value.A better idea would be to keep your eye out for a beat-up but working used bench grinder you could transplant the grinding wheels, and possibly guards and rests onto, or if you've got access to a lathe and the skills to use it (well worth learning), and some sheet metalwork skills, make a double-ended belt-driven shaft for it to fit in place of the bad motor (possibly keeping its housing to support the bearings, with a slot added for the belt), and add a second hand motor + drive pully mounted behind it (and properly guarded). Use a universal motor or high voltage DC motor and you can easily add speed control. Beware of grinding dust getting into the external motor - you'll probably need a cowling to direct dust away from it and a large area fine mesh cloth filter on the cooling air intake end.@Seekonk: Google reckons Suriname is 60Hz mains.

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 21, 2022, 10:29:47 pm
content: If it's a run capacitor using a smaller one would keep it from getting hot. If it's a start capacitor perhaps the centrifugal switch failed to disconnect the capacitor from the circuit.

author: robsims
author_rank: Frequent Contributor
timestamp: August 22, 2022, 02:54:44 am
content: I think it draws more than the rated 1.7 amps. Rewinding it myself with more turns seems like a good option to lower the current draw. Any other ideas how i can improve this motor?

author: Zero999
author_rank: Super Contributor
timestamp: August 22, 2022, 07:29:54 am
content: Quote from: robsims on August 22, 2022, 02:54:44 amIf it has a shorted turn, it'll need rewinding with the same number of turns. A shorted turn is when some of the insulating in the windings breaks down, forming a short circuit. The shorted turn not only reduces the number of turns, but acts like the secondary winding of a transformer, but short circuited of course, causing a high current to flow through the rest of the windings, which form the primary winding.As you bought it fairly recently, the correct thing to do is return it for a refund. Even if it's easy to repair, which it isn't, it's not right people should be able to get away with selling inferior quality products.

author: robsims
author_rank: Frequent Contributor
timestamp: August 22, 2022, 09:36:24 am
content: You're absolutely right Zero999. I will return it

author: taste_tester
author_rank: Regular Contributor
timestamp: August 22, 2022, 09:49:29 am
content: While I also believe the right course of action, in general if I was in a situation like this and return was not an option, I'd try a capacitor with an equivalent amount of capacitance in place just to see if behavior is the same, and measure the original capacitor out of circuit.Ever since one occasion where a new capacitor from a reputable brand, and a high end model, almost $3 per cap, completely failed within 10 seconds of power-on, I am always suspicious. They are the least trustworthy components for a reason. If you measure the capacitance and the value is jumping around and occasionally showing "OL" on your DMM, it's bad. This one I just mentioned became too hot to touch almost immediately and remained warm for maybe 5 minutes after desoldering.Luckily in the above situation, the capacitor was in parallel with another equivalent capacitor and the rest of the circuit was fine, it could have been a lot worse.There are plenty of times I had a capacitor that appeared to do its job right, then you take it out of circuit to test and it's a fraction of the capacitance it should be, or the amount of capacitance is uncertain.

author: james_s
author_rank: Super Contributor
timestamp: August 22, 2022, 04:27:30 pm
content: Quote from: robsims on August 21, 2022, 07:06:27 pmMotors for continuous use tend to be physically larger and heavier for the same power, many have cooling fins cast into the case, the stators will have sufficient iron so that they won't saturate, physical dimensions of parts will be precisely made, the windings will have a sufficient amount of copper, etc. You can't really fix a cheap poorly made motor. You might improve it some but it won't be worth the effort.