T-Pain Is On A Boat: Re: [Electric Boats] Orest have u considered AC motors

Thursday, April 18, 2013

Re: [Electric Boats] Orest have u considered AC motors

Both BLDC and PMAC motors are multi-phase. Usually 3-phase, but I have seen academic articles extolling motors up to seven phases. The more phases, the more even the torque produced by the rotor in one revolution.

The drive electronics are commensurately more expensive of course, as at a minimum, you need one extra half-bridge MOSFET or IGBT output stage for each added set of phase windings.

On 18/04/2013 17:51, Roger L wrote:

Moderm Permanent Magnet AC (PMAC) motors and Brushless DC (BLDC) motors are very similar to each other. The big difference is is the shape of the current wavework flowing in the stator coils. PMAC motors have the stator windings and controller drive electronics giving an essentionally sine-wave current, making the motor mechanically quieter.

OK, I think I can visualize that. Check me on this if you would: One could take a Permanent Magnet DC motor design and simply reverse every other winding (or magnet) in order to drive the rotor with the alternating polarities of the AC wave. It would probably need some changes to the shape of the the ferromagnetic flux path in order to get decent use from the Permant Magnets, but that is easily model if not so easily done.

Another issue is that a sinusoidal AC driving wave spends part of its time in a low energy state as it passes through zero. But when the flux path was modified that could be made to coincide with the recurring low magnetic energy position of the rotor.

There still exist old-fashioned ac 3-phase motors, but brought (slightly) up to date by using a PWM 3-phase sine-wave inverter unit providing a variable output voltage and frequency to provide much greater control of the motor. These can provide a lower-cost solution for some applications, as you pay essentially the metal scrap value for the 3-phase motor (Although usually you have to pay the cost of rewinding them for operation at lower voltages). Industrial electrical scrap yards have plenty of 3-phase motors in them that are of little value. The problem is that the 3-phase motors were originally designed for optimum efficiency at 50 or 60Hz, and operating them over a wider frequency range gives good speed control, but the efficiency suffers. I have been involved with a group wishing to electrify a British canal narrowboat using this sort of motor, and I'm still not convinced that I have managed to show them the error of their ways!

Perhaps there isn't any error, simply the reality of initial cost versus ultimate efficiency. As an old engineer I've seen many instances of financial reality being more valid than theoretical efficiency.

   Roger L.

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From: electricboats@yahoogroups.com [electricboats@yahoogroups.com] on behalf of Roger L [rogerlov@ix.netcom.com]
Sent: 18 April 2013 13:36
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Orest have u considered AC motors

Uh-oh....If I may join the conversation..... it looks like I may have to go back to school on electric motors. It's a Good Thing that learning something new is always fun.

The only electric motors I am familiar with are the high efficiency kind made with permanent magnets and driven by a pulsed DC wave. And without really thinking about it, I've been making the assumption that this was the type used in electric boats.

But if so, why is AC any different from pulsed DC? In that type of motor I don't know of any reason or advantage to sinusoidally reverse the polarity of the driving wave. (making it AC).
Am I missing something?
    thanks,
          Roger L.
.........
..............

----- Original Message -----
From: Andrew Gilchrist
To: electricboats@yahoogroups.com
Sent: Wednesday, April 17, 2013 8:18 PM
Subject: [Electric Boats] Orest have u considered AC motors

Orest

there is another option – others here my have tried it

You may want to look at an AC motor . A DC motor's amp draw and torque level fall away at higher rpm, an AC motor has a lower start torque – but you don't really need much at low rpm, but the high end torque is better.

If that isn't an option a more efficient better sized prop than the current one could achieve very significant gains either in performance or efficiency

Yours,

Andrew Gilchrist
fastelectrics.com
Australia
0419 429 201