Variable-speed Single phase motor about to die. - Page 1
Forum section: electronics / projects

author: paul8f
author_rank: Regular Contributor
timestamp: November 24, 2019, 07:52:16 am
content: I suspect this single-phase motor for a blower-fan is giving trouble. It spins ok and sounds ok when running, but the system is bringing on a fault light. It looks like the fan is not spinning fast enough. I don't see any service ports to apply bearing lubricant.Resistance measurements look ok....: resistance between the two electrical input terminals is low, and there is high resistance between the terminals to the metal case.The motor is variable speed, and is driven by an ST TI 235 Triac. How do I figure out if the problem is with the motor or the driver circuit? Any help very welcome(I suppose the only real way to be sure would be to have a tachometer, and to know the RPM for a given applied voltage)

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: November 24, 2019, 09:34:03 am
content: If there's a capacitor in series, replace that first. The failure mode is the capacitor becomes leaky => heats up => burns itself or the motors coil or both.

author: Ian.M
author_rank: Super Contributor
timestamp: November 24, 2019, 11:51:09 am
content: Try a drop of SAE30 machine oil on the bearings applied where the shaft meets the bronze bush. There's almost certainly another one the other side of the motor unless it doesn't have one and only has a bearing the far end of the fan rotor. If they have been running somewhat dry, it will speed up. if it doesn't make a difference you can assume the original lubrication is still sufficient.Oilite bearings are pre-lubricated at the factory, but supplemental oil may be required during their life. If the old oil charge needs to be replaced (to avoid the need for on-going regular lubrication) its a royal PITA and will require stripping down to remove the bearings which (if press fitted) may be left in metal housings or carriers but due to the need to heat them in oil, must be removed from all plastic or electrical parts. See:http://www.loganact.com/tips/sleeve_bearings/oilube5.giffor the process.

author: MagicSmoker
author_rank: Super Contributor
timestamp: November 24, 2019, 12:36:18 pm
content: That's a shaded pole motor and they are nearly indestructible, electrically speaking. If the fan spins freely by hand then the problem is most likely with the triac-based speed controller. I would be especially suspicious of the capacitor in the gate circuit. See, for example, this page:https://www.industrial-electronics.com/emct_2e_5a.html

author: duak
author_rank: Super Contributor
timestamp: November 24, 2019, 07:01:05 pm
content: Is the shaft of the motor horizontal? If so, the weight of the impeller is causing more wear to the lower part of the inner bushing and the upper part of the outer bushing, the one visible in the image. If this goes on long enough, the rotor can drag on the stator. If the electronics checks out, remove the blower assembly, take it apart and check for score marks on the rotor. If it's OK, the bushings should be good enough so clean the pieces, put a couple of drops of oil on the bushings, reassemble it and see what happens. Quite often, the oil goes away and what's left gets thick. Cleaning the old stuff away and relubing does wonders. Also, I see a bit of debris on the winding so I expect there's also some on the impeller too.Best o' luck,

author: james_s
author_rank: Super Contributor
timestamp: November 24, 2019, 07:33:52 pm
content: Those shaded pole motors are ubiquitous, as a test try squirting a few drops of oil around the bushings and see if that helps, they are not even really bearings, just oil impregnated bushings. That looks like a draft inducer in which case they run very hot and the lubrication fails.

author: Circlotron
author_rank: Super Contributor
timestamp: November 24, 2019, 09:13:31 pm
content: Don’t use grease on the bushes like I did once. It will run for a few weeks then the grease gets so thick and gluggy the motor won’t even start.

author: andy3055
author_rank: Super Contributor
timestamp: November 24, 2019, 09:35:39 pm
content: BTW, it is a good idea to clean and dip that field coil in some shellac and bake it. That will keep it going for a long time.

author: paul8f
author_rank: Regular Contributor
timestamp: November 25, 2019, 10:24:17 pm
content: Wow, thanks for all the replies and advice. Of course I went away and ordered a new blower-motorbeforechecking in on my thread....In the mean-time I will check out the triac, capacitor, bushings and bearings for any signs of trouble. Will post back later with any findings...

author: james_s
author_rank: Super Contributor
timestamp: November 25, 2019, 10:31:05 pm
content: Ultimately that's probably the solution anyway. Once the bearings start to fail anything short of replacing them or the motor is usually a stopgap.

author: Circlotron
author_rank: Super Contributor
timestamp: November 26, 2019, 03:11:31 am
content: One thing that might be causing the motor to run slow is maybe the triac is not giving equal duration positive and negative half-cycles to the motor. That would cause DC to flow in the windings and have a definite braking effect in the motor. Try powering the motor direct from the mains and see if it runs at full speed.