How to test (single-phase?) ac induction motor

[JohnW2]

Me too - 12v dc, and later ones variable speed, brushless units. Perhaps Eric was on about mainframe computers, but my memory of those was that they used 240v fans, switched via stats.

I'm not sure what he's talking about! He now seems to be suggesting that, although most of us believe that the "12V DC" fans have DC motors, they in fact contain inverters (goodness knows where, in the tiny amount of space available ) which convert the DC into polyphase AC to feed the motor.

It's not a field that I really know anything about, so I'm certainly in no position to 'assert', but I would be very very surprised if what eric is suggesting is, or ever has been, true of standard "12V DC" computer fans

 

[DetlefSchmitz]

FYI, here is a typical IC used for 4-wire fans.

APX9173 datasheet
Maybe it answers a few questions.

 

[JohnW2]

FYI, here is a typical IC used for 4-wire fans.

APX9173 datasheet ..... Maybe it answers a few questions.

Thanks - and, not to my surprise, the answers it gives are as I would have expected.

It now seems that your previous suggestion that eric was perhaps getting confused by the 4-wire connection was not the explanation for what he's been saying, since he now acknowledges that the unit is fed with DC (hence just two of the four wires, but seems to think that DC then gets changed into 'polyphase AC'.

 

[DartmoorHedgehog]

OK, quick update on this.

I'm pretty sure it is a single phase motor after all, with capacitor starting (thanks for the hint @ericmark ).
It appears that the connection from the neutral wire to the motor windings is open-circuit - it measures infinite resistance from that wire to every other motor connection. I've opened up the motor case and can't see any obvious break or heat damage/discolouration so I suspect it's inside the coil somewhere.
I think the timer circuit is working fine from what I can test (appears to put the right voltage to the various motor connections). Which is a shame because fixing that would have been much more my thing - and electronic components are cheap.

A new motor costs most of the price of a whole new unit so I'm not going to waste any more time on this and have ordered a replacement of the same model - then I have the old one for parts if I need it later. Although I don't really consider the time wasted - it was an interesting bit of research and I know more now than I did when I started (I've not really looked into AC motors much before).

 

[JohnW2]

It appears that the connection from the neutral wire to the motor windings is open-circuit - it measures infinite resistance from that wire to every other motor connection. I've opened up the motor case and can't see any obvious break or heat damage/discolouration so I suspect it's inside the coil somewhere.

In that case, as you say, it's almost certainly 'replacement time' and, if, as you say, a whole unit doesn't cost much more than a motor, then that could well be the most sensible way to go.

Kind Regards, John

 

[Harry Bloomfield]

A new motor costs most of the price of a whole new unit so I'm not going to waste any more time on this and have ordered a replacement of the same model - then I have the old one for parts if I need it later. Although I don't really consider the time wasted - it was an interesting bit of research and I know more now than I did when I started (I've not really looked into AC motors much before).

That is what I ended up having to do, when my unit failed. I couldn't find a fittable match for a complete unit and motor, so unable to swap just the motor, I had to swap the entire thing.

 

[SimonH2]

FYI, here is a typical IC used for 4-wire fans.

APX9173 datasheet
Maybe it answers a few questions.

Thanks - and, not to my surprise, the answers it gives are as I would have expected.

It now seems that your previous suggestion that eric was perhaps getting confused by the 4-wire connection was not the explanation for what he's been saying, since he now acknowledges that the unit is fed with DC (hence just two of the four wires, but seems to think that DC then gets changed into 'polyphase AC'.

I've taken one or two apart in the past. The ones I seen are brushless - and that means multiple windings driven by a switched DC or "AC" supply. I would suggest most likely switched reluctance.

 

[JohnW2]

I've taken one or two apart in the past. The ones I seen are brushless - and that means multiple windings driven by a switched DC or "AC" supply. I would suggest most likely switched reluctance.

That makes total sense, but I still wonder where within one of those things they can find the space for the necessary electronics. I think I'll have to look for one I can dissect

 

[SimonH2]

The same small circuit boards the coils are mounted on - yes, very small !

 

[JohnW2]

The same small circuit boards the coils are mounted on - yes, very small !

Thanks. have no reason not to believe you, but I probably won't be convinced until I've found one and taken it apart. I must say that, quite apart from space, I'm surprised that it can sometimes all be achieved for £2.50 retail, sometimes even less

 

[JohnW2]

That makes total sense, but I still wonder where within one of those things they can find the space for the necessary electronics. I think I'll have to look for one I can dissect

The same small circuit boards the coils are mounted on - yes, very small !

Thanks. have no reason not to believe you, but I probably won't be convinced until I've found one and taken it apart. I must say that, quite apart from space, I'm surprised that it can sometimes all be achieved for £2.50 retail, sometimes even less

OK. I found an old laptop one - and,yes,it seems that you're probably right. Seemingly 4 windings, and some electronics on the bard the windings are mounted on - a tiny IC, a couple of capacitors, a resistor and a couple of other components. Whether I would (per eric) call than "an inverter'is something I'm less sure about! ...

Kind Regards, John

 

[Harry Bloomfield]

Whether I would (per eric) call than "an inverter'is something I'm less sure about! ...

That is what is known as a brushless dc motor. The coils are energised in turn, by sensing the position of the rotor, which is fitted with magnets. +5 or 12v, ground, speed output, and a required speed input on the four terminals.

 

[JohnW2]

That is what is known as a brushless dc motor. The coils are energised in turn, by sensing the position of the rotor, which is fitted with magnets....

Yes, I realise that - but, as I said, I'm not sure that I would describe the electronics that do that switching as 'an inverter', nor would I be probably describe that switched output as "polyphase AC" - but perhaps I'm in a minority