Alcoholic discussion - ac versus dc motors

[robgraham]

Why is that when you are at one's intellectual worst that the more complex of discussions occur - solution of the world's problems, etc !!!

This particular one revolved round ac and dc motors - covering single phase v. 3 ph induction motors and their relative power /torque capabilities and how they work, which was hard work 45 years after university, but what triggered the whole discussion was how Bosch can claim that a lawnmower with a 36v Li-polymer powered dc motor can have the same capability as a 1700w ac motor. OK 1700w is a comfortable

2hp but single phase induction motors are not too good on torque, and if I remember that is where dc motors score but are modern ones going to match a 1700w ac motor ?

Can anyone help or point me at a site on this topic please.

Rob

[Andrew Gabriel]

I think most electric mowers are universal motors (i.e. with brushes and a commutator). Certainly they were 8 years ago when I last bought one -- I had to go some way off the beaten track to find an induction motor mower. I've bought two over the years -- I think they're both German but different makes (both still working fine, although I've had to replace the run capacitor in the 15 year old one).

The torque profile of a universal and induction motor is quite different. An induction motor doesn't have much starting torque, but when it's nearly up to speed, there's quite a lot of torque to maintain speed, which makes it good for a mower. A universal motor has a high starting torque, but low torque when up to speed, which means it easily slows when under load. That isn't good for a lawn mower. Universal motors also spin too fast for direct coupling to a mower blade. You will also find you need around 30-50% more powerful universal motor mower to get the same cutting power as an induction motor. Universal motors seem to get much hotter, but that may just reflect cheap lower quality motors rather than any inherent reason.

How many AHrs is the 36v Li-polymer battery?

[The Natural Philosopher]

certainly 36v and about 50A is not unusual in the largest 'DC' model aircraft motors. Mind you there is no such thing as a DC motor.

They are all 3 phase. - a DC brushed motor simply uses a commutator to generate the AC..

If you are sticking 1700W in, its got to come out as heat, noise or as shaft power.

I doubt efficiencies are any less on a brushed PM versus induction motor, ..

[The Medway Handyman]

Cetrainly all the 'hovver' mowers would be, an induction motor would be far to heavy.

Cost is also a factor, all the el cheapo pressure washers have universal motors because of the cost, commercial machines have induction.

[dennis]

IIRC qualcast use induction motors even in the little rotary mowers.

[Chewbacca]

My Honda electric mower - about 15 years old with a rust free plastic deck has an induction motor. I want it to break as my wife that likes it will let me buy a petrol mower, but it just keeps on going...........

[Fred]

??? In a DC motor the field is DC. The commutator is to ensure the field in the rotor is aligned wrt the stator to provide continuous torque. Not sure where you get 3 phase from. From a standard single pole pair DC motor I would call it "single phase". 2 pole pairs on a DC motor is common place for starter motors. I haven't come across with 3 pole pairs with 3 sets of brushes?

I remain unconvinced about power in and useable power out for mains motors. At present I would have hoped green thinking would have had some meaningful measure of real power out, not just turned into heat. There is still the concept that the more power in the better the motor! Generally small induction motors are very inefficient, DC motors are inherently very efficient. AC (brushed) motors are less efficient than DC and the best thing that could be done would be to rectify the mains first.

[The Natural Philosopher]

So what?

Its still a three phase motor with the poles switched on one (or two) at a time via a 2 brush system on a Nx3 commutator element.

The commutator is to ensure the field

Each winding gets an AC signal. Each winding is switched at 120 degrees phase to the next one: If I told you that and told you nothing else, you would say 'ah, a 3 phase AC motor!!'

From a standard single pole pair DC motor

The simples motor is a single phase. But its not self starting. Therefore the simplest that works is three phase with two magnet poles.It ameks no dss as to whether the current is originally DC with permanent magnets, or AC with a field winding. It will work.

Unless you actyally go for synchronuus motors, the actual AC that drives the MOTOR as opposed to a high frequency ripple on the magnetic fields, is generated by brushes and commutators or an electronic switch.

On a mains motor, the frequency of commutation is nor far off mains frequency for typical motors speeds - about 3000 RPM say. So there is not much extra iron loss using a mains motor or a permanent magnet motor or an induction motor off DC.

All are capable of better than 90&% efficiency if built well, or as low as 50% if stamped out in thousands in Taiwan, and operated well off decent efficiency points.

There is still the

I dont see why they should be really.

[Kevin]

I think you are wrong a dc motor is not a 3 phase or ac , some motors might have three poles but that only 1 configuration , is a five pole motor 5 phase? and a 7 pole 7 phase? and you cant feed a permanent magnet motor AC current so to call a dc motor ac is simplifiing things to far

[Dave Plowman (News)]

No - it gets switched polarity DC. A different thing entirely.

[The Natural Philosopher]

I cannot think of any motor that does not have a multiple of 3 actual windings. But I suppose its possible. So OK, they are 5, 7 whatever phse motors!

and you cant feed a permanent

Yes you can.

I have about 9 of them myself.

I think not. All electric motors have alternating current in the windings.

Realising that takes a lot of bullshit about the so called difference between 'AC' and 'DC' motors away.

Really there are only two basic types of motor: those that are driven, either electronically, or by means of commutators, with a phased AC that synhronises to the motor speed, and those that are synchronous to a fixed frequency AC source.

The only other variable is whether permanent magnets or electromagnets provide the field. And if electrical whether they are series or shunt connected.

Apart from eddy current motors, which are off the topic here.

[The Natural Philosopher]

NO, it is true alternating current in the meaning of the term. Current that has an average value of zero, but has at any given time, a value that is not zero.

Switched polarity DC *IS* AC!!

[Fred]

With 3 brushes equally spaced I suppose?

In some respects you're right but for all the wrong reasons!

[Chris J Dixon]

So how many do you see here?

[Bob Mannix]

Err no, it's *an* alternating current in *your* meaning of the term "True" alternating current is that whis is assumed in AC theory and engineering and in the design of AC motors and equipment and is sinusoidally varying.

[dennis]

I have a PC fan in front of me with a transparent case.. It has four coils so you can think of one now.

[Kevin]

but that's an brushless motor as the coils stay still and they are fed pulses by the electronics

[Kevin]

you have not done much research, most model boat motors are 5 pole along with trains

they still are not 3 phase and no dc motor is, as there are not 3 rotating magnetic/electrical feilds there is only 1 field that is switched in sequence by the commutator

lets see you feed ac into any normal aero style model motor

are you sure its alternating and not just feeding one set of windings then the next etc

[Kevin]

that's just theory no dc brushed motor usually has two poles minimum is usually 3, which is where Nat gets his 3 phase ac from (which is just wrong),I believe 3 are used as they have better starting torque the more poles they have, 3 5 7 are the usual numbers

[Chris J Dixon]

Motors generally have an even number of poles, most commonly 4 or

2. Chris