DC and AC Motor Confusion

An AC induction motor requires the squirrel cage rotor, this consists of conductive bars imbedded into the rotor.
These represent shorted turns which provide the attraction required to the rotating field in the stator.
You intend to use a wound brushed rotor, the only way this would work work is to turn it into a series field (Universal) motor which would not be practical without rewinding the stator.

A DC (or universal) motor armature works against a magnetic field oriented across at 90' to the line of the brushes.
(Or 45' if it's a four brush motor etc).

So your blower motor needs a field with one pair of north-south poles, and the brushholders fitted in the correct relative position.

That AC stator looks as if it may have three pairs of poles? Very difficult to be sure but the binding looks vaguely hexagonal.


Either way, that stator (like most AC induction motors) is intender to create a rotating field that induces current in the rotor.

Its a very different type of operation to a brushed DC motor.

If you short out the entire commutator on the DC armature, it may work as an AC induction motor, but probably not anywhere near as efficiently as with the original purpose made "squirrel cage" rotor.

I was writing this as rjenkinsgb posted, and is says much the same.

If it's a 2 pole ac stator, so designed to run near 3000 rpm (50 Hz) or 3600 rpm (60 Hz) then you could make it into a universal (ac or dc) motor by connecting one phase of the windings in series with the brushes. The rotor would be used as is. You would have to rotate the brush holder to find the angle that gives the most torque, and fix the brush holder there. It wouldn't be a very good motor.

That rotor is a 2-pole rotor, as it is designed to be contacted with two brushes, and it creates two poles, N and S, around it. That could work with a 2-pole stator.

A lower speed induction motor would be 4 pole, so that it creates 4 poles, N - S - N - S, around the rotor. That would only create torque with a 4-pole stator.

You could also short out all the commutator segments and run it as an induction motor with no brushes. That would run on ac and would be worse than the original squirrel cage rotor.

rjenkinsgb said:

A DC (or universal) motor armature works against a magnetic field oriented across at 90' to the line of the brushes.
(Or 45' if it's a four brush motor etc).

Click to expand...

Sorry, half asleep, pre-coffee!
The brushes are on the SAME line as through the field magnetic polepieces... The armature field is created 90' to the brushes, or half way between with higher brush counts.

This illustrates the two field poles needed for a two brush armature; if they were permanent magnets rather than wound field coils, the arrangement would be similar.

Having a wound field on a "DC" style motor means the strength and direction of the field can be varied.

If both the field windings and armature (via the brushes) are combined on the same supply with the field, both reverse polarity if the supply polarity is reversed, so the direction of rotation stays the same.

With a typical electric drill or small appliance "series" field type motor, the field windings are connected in series with the brushes. That makes a "Universal" motor that works on AC or DC.

This reminds me of my 'Uncle' Alan (actually an older distant cousin), who performed in the clubs, on TV, and minor film roles - he did a gig at a club up in Yorkshire, and they gave him a VHS recoding of his show.

Unfortunately he had a Betamax recorder - so he manually would the tape from the VHS cassette into a Betamax - then found out it wouldn't play (if he'd asked, I'd have told him that).

He then brought it to me and asked if I could transfer it to Betamax, which of course I could, except it was in a Betamax cassette, so I sent him away, and told him to bring it back when he'd wound it back in a VHS cassette. He did, so I copied it for him.

That was a completely stupid thing to do, and so is this thread, which is why the OP couldn't find anything about will searching.

The answer is quite simple NOOOOOOOO!!!!

Nigel;
Your very funny anecdote reminded me of a similar one, also involving a VCR.

A relative of mine called me to ask me because the the video had severe streaking. I told him that the first cure is to clean the VCR’s heads. Did he have a head cleaning cassette? When he answered no, I then advised to open up the VCR, find the drum head and clean it up.

A couple of hours later, he called me again that the image now was worse, much worse! I told him I would drive to his home.

When I arrived and inspected the VCR, the drum head was scratched and abraded to an unbelievable amount. I asked him to show me the materials which he had used to “clean” the head. He retrieved from the waste bin a sheet of coarse sandpaper, perhaps 60 to 100 grit.

But back to the topic, the operation of AC and DC motors is fundamentally different. Absolutely and completely.
There is only one type of motor, the series wound or universal motor, which can operate with both currents.

You could not use the AC motor stator as-is for a Universal AC/DC (series field) motor.
The Universal stator has to posses two distinct separate poles as opposed to the overlapping AC version.
A parallel, or shunt field DC motor will not work on AC as its series version does.

schmitt trigger said:

Nigel;
Your very funny anecdote reminded me of a similar one, also involving a VCR.

A relative of mine called me to ask me because the the video had severe streaking. I told him that the first cure is to clean the VCR’s heads. Did he have a head cleaning cassette? When he answered no, I then advised to open up the VCR, find the drum head and clean it up.

A couple of hours later, he called me again that the image now was worse, much worse! I told him I would drive to his home.

When I arrived and inspected the VCR, the drum head was scratched and abraded to an unbelievable amount. I asked him to show me the materials which he had used to “clean” the head. He retrieved from the waste bin a sheet of coarse sandpaper, perhaps 60 to 100 grit.

Click to expand...

Ouch!!!!

We had a Sanyo VT5000 Betamax (top loader) in for repair (we sold those in huge quantities), and the customer had suffered a chewed up tape, and had crowbarred the top open to get it out. As well as breaking parts of the loading mechanism, he also smashed the heads - we gave him a big estimate, which he accepted, so we repaired it, gave it a good test, and let him have it back. We also seriously warned him NOT to do it again.

A few days later he brought it back again, with a tape stuck in - and we determined that the reel motor had a dead spot, and was stuck in the dead position. So a little 'nudge' at the motor, and the tape unloaded and ejected with no damage to anything. So I replaced the duff motor (at our expense) - which we couldn't get to fail again either!! - and tried his tape.

All was explained - he was copying porn tapes - and this one was so bad we didn't even make a copy for our selves

(from the little we saw, it included donkeys!).