Mains AC motor won't run?? - Page 1
Forum section: electronics / projects

author: SL4P
author_rank: Super Contributor
timestamp: February 20, 2015, 01:05:22 am
content: I admit to knowing nothing about mains AC motors...I have a small blower with a 3-wire fan motor - straight across the mains - with a 2.5uF capacitor to the 3rd wire. I assume the cap is to offset the mains cycle in order to energise alternate windings wrt the 50Hz mains frequency... ?Anyway - motor doesn't 'smell' like it's cooked, but doesn't do a thing - at all.Apply mains - no torque or resistance to turning at all. it's just 'dead' and spins freely like it has no power applied at all.What should I see across the windings (3-wires) / capacitor etc...The motor is a generic Chinese unit - seems well-enough made.It's easy to open up the motor completely, and no signs of overheating etc in the windings that are visible.Thanks for any helpful input.

author: tautech
author_rank: Super Contributor
timestamp: February 20, 2015, 01:08:47 am
content: Look for a self-resetting thermo-fuse, probably OC

author: SL4P
author_rank: Super Contributor
timestamp: February 20, 2015, 01:30:19 am
content: Thanks - I did... that's why I opened up the motor carcass... nothing obvious.

author: AlfBaz
author_rank: Super Contributor
timestamp: February 20, 2015, 01:39:37 am
content: are you getting continuity between all three wires. The should be a common, start winding and run winding... Forget which is which but one of the windings should be higher than the other. Check to see if it has a switch inside that cuts the start winding out after it has started, it may be stuck open

author: SL4P
author_rank: Super Contributor
timestamp: February 20, 2015, 01:44:53 am
content: No switch that is obvious.Mains LN come straight in to a terminal block. teo go straight to motor. Third goes to capacitor then to third wire on motor.As with thermal cut-out - no sign ofanythinginside the motor than the 3-wires in and windings to the alternate poles of the stator.I would have expected it to buzz, or offer some sign of non-co-operation, but it's just dead as a doornail.How are the three wires connected? Is it one 'common', and two alternate pole windings (same resistance?) If so, then I'd expect to see ?? ohms, and 2x?? ohms between the various wires in...

author: AlfBaz
author_rank: Super Contributor
timestamp: February 20, 2015, 02:07:22 am
content: Quote from: SL4P on February 20, 2015, 01:44:53 amYes, here's a typical connection

author: johansen
author_rank: Super Contributor
timestamp: February 20, 2015, 02:10:42 am
content: most small fans have the capacitor permanently connected.even the capacitor in series with the aux winding should be enough to hum if the main winding is cut out.so i'd look for a fuse or broken wire.

author: IanB
author_rank: Super Contributor
timestamp: February 20, 2015, 02:12:30 am
content: Quote from: AlfBaz on February 20, 2015, 01:39:37 amThis. With the motor unplugged, what resistance do you measure for the motor windings?

author: SL4P
author_rank: Super Contributor
timestamp: February 20, 2015, 02:30:30 am
content: Yup it looks like one wire (internal to the windings is OC.Yellow-Green == 380RYellow -or- Green to Black == openSo it appears that Black is common in some way to the others - perhaps the centre of two G-Y windings..?Unfortunately - cheap motor the wires godirectlyinto the coils of the windings - no opportunity to dig & probe with a chance of leaving a safe result.

author: tautech
author_rank: Super Contributor
timestamp: February 20, 2015, 02:44:19 am
content: Non self re-setting thermo-fuse.....melted solder joint.

author: johansen
author_rank: Super Contributor
timestamp: February 21, 2015, 03:05:51 am
content: there is very likely a fuse in series with the black wire which is common to both coils. 380 ohms is rather high for even a small motor, its probably 100 ohms for one coil and 180 for the coil in series with the capacitor.

author: SL4P
author_rank: Super Contributor
timestamp: February 21, 2015, 03:08:17 am
content: Coil resistance was measured without the cap in circuit (motor removed from assembly)...Simply across the three wires shown in the photo.Black wire appears not to 'go anywhere'!

author: tautech
author_rank: Super Contributor
timestamp: February 21, 2015, 04:41:32 am
content: Pierce the insulation carefully close to where the wires enter the windings and check each for continuity.Then chase the insulated leads in and check the joints.Shrink sleeve after.Check each for continuity to the laminations, might indicate a short, if so.....nackeredI wasn't kidding when I suggested an open solder joint.

author: SL4P
author_rank: Super Contributor
timestamp: February 21, 2015, 05:26:30 am
content: FOUND IT- I had to cut / open up the windings - and it was buried in the second layer...and OPEN CIRCUIT.I'm not sure if replacing it is a great idea, as the insulation integrity of the motor is compromised - and obviously gets quite hot - so I'm not sure which way to turn (no pun).Perhaps try reassembling WITHOUT the cut-out installed - then fit a new device mechanicall/thermally clamped to the metal frame of the motor itself??

author: SeanB
author_rank: Super Contributor
timestamp: February 21, 2015, 07:50:38 am
content: 130C thermal fuse, you can get one by taking apart an old wall wart with a 50/60Hz transformer inside it ( any old one that is heavy is a likely candidate) and taking the fuse out of the primary winding. It will either look like that or like a small white tube with axial leads. Solder in with a pair of long nose pliers between soldering spot and the fuse on the lead to keep it from melting.

author: rob77
author_rank: Super Contributor
timestamp: February 21, 2015, 08:18:11 am
content: Quote from: SeanB on February 21, 2015, 07:50:38 amor buy a new oneit's something like 50 cents in single piece quantities for those axial ones.

author: SL4P
author_rank: Super Contributor
timestamp: February 21, 2015, 09:43:01 am
content: Problem is I'm not sure I can securely & safely 'rebuild' the insulation around the windings where the cutout will sit.Having cut the black insulatimg wrap and the 'string' - to get into the problem, it was very tightly bound to fit inside the metal can etc... If I go there, I'll obviously do the best i can - perhaps drizzle some 2-part epoxy around the rework to seal it up.

author: Zero999
author_rank: Super Contributor
timestamp: February 21, 2015, 01:00:32 pm
content: Don't replace it with another thermal fuse which will most likely blow at some stage. Use a bimetalic strip which is resettable.

author: SL4P
author_rank: Super Contributor
timestamp: February 21, 2015, 08:16:34 pm
content: Yep - thanks