3 phase AC motors versus brushless DC motors

Hi all,

I'm fairly new to learning anything other than basic DC or AC electric motors or generators with brushes. In fact I just read today about 3 phase power, what it is and how it works and I'm still a little confused.

Anyway, I've been trying to find a good site to explain a 3 phase AC motor and it's construction, how do you set up the windings, how can there be no brushes, etc. Does 3 phase power travel through 2 lines still or does it use 3 or 4, one for each phase?

That aside, my main question, I read something describing how smooth the torque delivery is from 3 phase AC motors, I also read about variable frequency drives and how the intermediate power is DC. I'm looking at building a motor for use in an electric car, so smoothness is very important to me, then I read about brushless DC motors and how they are often used in electric cars.

I was wondering what the key differences are between brushless DC motors and 3 phase AC motors and how 3 phase AC motors are designed and constructed in general. Are brushless DC motors and smooth with their torque delivery as 3 phase AC? On either one, could I say have 20 windings and every other winding powered at the same time (for increased torque)?

Specifically talking about multiple windings and 3 phase AC motors, could I suddenly use all windings for generation of electricity from the built up rotational inertia? Like to slow the vehicle and to charge at the same time. In a direct drive system with a simple DC motor there would be no way to add resistance and charge more when trying to slow down a vehicle, would having multiple windings or 3 phase AC allow me to do this and control how much resistance is acting against the continued motion of a rotor and also generate electricity at the same time?

These question are probably too broad and I probably just have to read more, but I can't seem to find any reasources targetting my particular questions.


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I think a 3 Phase AC motor is optimized for a real sinodial source, so that a real non-discrete rotary field is used. Because of the real used rotary field, there shoulb be a constant torque and so a smooth rotating.

A BLDC-motor uses DC as supply voltage. There you have only 6 discrete rotary field states. And within one state the torque isnt constant. The maximum of torque appears when the rotary field leads 90 el. degree to the rotor.

Best regards


-- www.ibweinmann brushless development kit / brushless controll

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----------------- You need to find a good text. The questions that you ask are not readily answered in a newsgroup. Too much material to cover.

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Don Kelly

I realize that, I've been reading everything I can find. I think I've decided that 3 phase AC is more efficient and smoother torque delivery. I still don't fully understand exactly what they do for brushless DC, since I think if I was making a controller for it I would make it pulse a couple times in different places and try to make it basically like polyphase AC.

Anyway, I have a good understanding of most other motors now, now I need to learn how to build my own variable frequency inverter.

So, I've decided to use ready available DC through a variable frequency inverter and control 2 synchronous 3 phase AC motors. Now I just need to find out about how much iron core content I need for these motors to be 'ok' with a range of fairly low Hz (I'm hoping about 5) to fairly large scale (at least 500 Hz). Does anyone have any ideas on about how big such a motor that can put out about 100HP would be? Or rather, about how much HP in these conditions could be achieved in about an

8-12" diameter motor?

Any good places to look to pick up a variable frequency inverter that can handle about that much power and a couple motors? Where would one looked for such devices used?

Thanks so much, sorry for the broad questions

D> > Hi all,

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The motor itself is simpler in construction and requires less maintenance also compared to 'traditional' DC motors or single phase units.

Well, purists sometimes argue about 'brushless DC' motors aren't really 'DC', they are AC motors with an installed inverter so they can take a DC input and convert it to AC for the motor. Six in one, half-dozen in the other.

A 100 HP motor running at 5 Hz would be quite a lot larger than a 100 HP motor running at 500 Hz. But most variable speed applications don't need rated HP at all speeds. For example, fans or pumps, the power requirements follow a cubic law with speed, so at low speeds the actual power that needs to be developed is rather small.

To maintain 100 HP over a 100:1 speed range means the developed torque must vary 1:100 over the same range. So at the bottom speed (5 Hz), the torque that would have to be developed would be truly huge, and the iron and air-gap flux for that also large.

Perhaps you don't really need 100 HP over the entire speed range?

To avoid saturating the iron, most variable frequency drive systems maintain a constant Volt/Hz ratio. As the frequency drops, and inductive reactance, the voltage is reduced to prevent saturating the iron. The down side of this is that maximum developed power also falls off. But as I said, a lot of variable speed applications like fans/blowers/pumps are a 'good match' for this power curve. Other applications like hoists and conveyors need about the same torque regardless of speed, so the motors (and drives) have to be sized differently.


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Yes, my knowledge is obviously limited. I don't require 100HP across the range, 0-100HP in a linear fashion would be fine, if it's going to be exponential (or cubic) then I'd want more than 0 at the low end, maybe 20HP. I guess I'm not as worried about that as much as just getting a proof of concept to work, but then again, if I can't get the power I need what's the point?

As for 3 phase vs. brushless, this is what I was thinking. It really seemed to hold true, everything I read about brushless DC said it was nothing like DC and used inverters and a microcontroller to be multiphase (hence polyphase AC).

What I want is a 3 phase AC motor with the rotor on the outside and I was thinking of making it a permanent magnet so I'd have a syncronous motor. I was then going to convert DC to 3 phase through an inverter array. From what I read, brushless DC typically has the rotor external to the stator, maybe it is already exactly what I need?

On my small scale proof of concept I'll still with building my own inverter array and 3 phase motor, but it sounds like a brushless DC is what I want really. I'm told that's mostly what's used in hybrid cars as well. But curiously I'm told most trains and trams use 3 phase AC, I've never heard them mention brushless DC, why the disparity? The one goes from DC to 3phase the other from AC to DC to 3 phase.

Thanks for the resp> >I realize that, I've been reading everything I can find. I think I've

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Well, for one thing, trains and trams have been around a lot longer than high power electronic devices. Polyphase motors have the decided advantage of lower maintenance. Diesel-electric used DC generator/traction-motors for years. Today, many are AC generators with high-power rectification and then inverters for polyphase traction motors. The electric 'coupling' between engine and traction motor is just a different kind of transmission. It allows operating the engine at its best speed, while allowing a lot of control over the amount of power flowing to the traction motors at the speed required of the train.

Trade offs between costs of high power electronics and the energy that may be dissipated in them, versus the maintenance costs of DC motors, versus the flexibility of various drive modes and use of dynamic braking. All to fit on a railroad engine frame.


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The purists are absollutely correct.


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