Electrically seized fans..whats going on there then?

[scatmanjohn]

A technical answer is required for this one fellas if possible.

When a fan/motor will not turn because an electrical fault (ya know what i mean...loud buzzing...fan seized solid...turn the power off and the fan/motor turns freely).

I have always just thought ...its knackered...fit a new one...then never actually thought about what was going on.

I'm sure someone knows the technical answer.

Ta

 

[Dan Robinson]

Well we changed a Profile fan this morning that worked when you rested in on its side (impellor shaft vertical) but buzzed like a rabbit when you refitted it.

Stripped it down, cleaned it put it back, spun very slowly and still buzzed - conclusion?

Warped/Bent shaft

 

[kevindgas]

burnt out windings is the most likely answer


vvvvvvvvvvvvvv ooooohhhhhh

caveman kev

 

[sooey]

Not a technical explanation, just a pure guess from an electronic caveman, I reckon it's something to do with the windings in the motor being turned into an electromagnet by a certain fault condition, short.

 

[TicklyT]

On many motors there are 'windings' on the rotor that are not connected electrically to the outside world. These determine the distribution of the magnetic field around the rotor when the motor is running. Sometimes these 'windings' appear to be long brass rivets or a piece of wire just there holding the rotor laminations together, but they are also play a part in the electrical circuit.

A failure there may cause loss of performance or stop the motor from running completely.
As they are within the rotor, there is no easy way of testing them.
You would probably need to measure milliohms anyway.

 

[scatmanjohn]

Cheers all.

I think i might try asking in the electrics forum

____________________________________
Lynda, moderator

I have moved it for you

 

[scatmanjohn]

Thats awfully kind of you lynda

 

[briwire]

If it is a single phase fan, it may be the capacitor failed. There are two windings in the motor. One has 230v fed to it directly, the other is via a capacitor and this gives two magnetic fields out of synch so that a turning force is formed.

At the factory where I work there are dozens of fans in panel doors which often fail. 99% of the time its the capacitor, about 50p from RS, as opposed to £190 for a new fan.

The Capacitor is uuslly found on the outside of the motor body, usually cylindrical with 2 leads.

 

[Dan Robinson]

Totally out of my league here, but this started off as a boiler fan question - me thinks.

Capaciters are few and far between - Vokera used resistors to change fan speed - but....? Am more than weilling to learn on this one!

 

[plugwash]

single phase cannot directly generate a rotating magnetic field and an induction motor needs a rotating magnetic field to start. Series capacitors are one way of acheving a rotating magnetic field from a signle phase supply. I think they are mainly used on larger motors though not the small fans you would find in domestic equipment.

 

[oilman]

.......... an induction motor needs a rotating magnetic field to start.

It needs two fields which are not in alignment. At the start nothing is rotating, the fields are just shifted.

 

[OLD]

I go for the capacitor if you make the fan spin it will run ok .

 

[TicklyT]

A shaded pole motor is another possibility for a fractional horsepower AC motor. These use a short-circuited winding on an auxulliary stator pole to generate the second phase required in the magnetic circuit instead of electrically. The short-circuited winding (often a band of copper tape fixed round part of the stator) delays the changes in the magnetic field from the alternating current supply.

As for motors only working when laid on their backs, consider bearing wear. If the rotor wobbles about and touches the stator, it won't run.
Turn it on it's back, and gravity and the end float in the rotor puts an unworn part of the shaft in contact with the bearings eliminating some of the play so it can run again (for a while anyway).

It is not unusual for the relatively hard shaft to have worn more than the softer bearings it runs in.
Fragments worn from the shaft get embedded in the soft bearing material where they work like sandpaper, eventually reducing the shaft to an hourglass shape.

Modern DC motors can have a few 'clever' bits in them (eg permanant magnet rotors, and field coils switched sequentially by hall effect sensors etc.etc.) instead of the commutator and brush arrangement as commonly found in electric drills etc, but all have their own potential failures.

It is not unknown for equipment manufacturers to choose a DC motor supplied with AC through a bridge rectifier arrangement, or to utilise the DC power supply for the control board.

 

[plugwash]

.......... an induction motor needs a rotating magnetic field to start.

It needs two fields which are not in alignment. At the start nothing is rotating, the fields are just shifted.

An induction motor (which is a different thing from a DC motor, a universal motor or a synchronous motor) works by creating a rotating magnetic field which then induces magnatism into the rotor. since the field is rotating it then drags arround the newly magnatiesed rotor.

to create the rotating magnetic field requites the superimposition of two fields which are offset from each other in both position and time. The position bit is easy but the time bit is harder. The most obvious way is to use a polyphase (usually 3 phase nowadays) supply but that has practical problems for smaller motors. Other possible methods are to use a capacitor or do some magnetic tricks to add time lag to one side of the motor poles (giving the shaded pole motor)

 

[scatmanjohn]

Thanks for everyones input.

The type of motor i was interested in was what is found on the fan assy on a fan flued central heating boiler. (not the new premix type).

I have never come across one with a capacitor on it.

I have had a few in the past that would spin freely without any power on but as soon as the power is switched on they lock up solid and will not move even if you try to give it a shoove.


Cheers