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

Roger

The solution I have seen to linear drop off in amps with rising rpm is a progressive timing advance – it is good if you need acceleration, speed and have power reserves, however efficiency usually falls as advance increases.

Most controllers do not have wave shaping. DC PWM Brushless uses a half sine wave output. A square wave output. The problem is like a bad amplifier the square wave isn’t. Some designers assume the interaction with current and stator will “smooth” – blunt is probably better the leading and trailing edge of the square wave. That is inefficient. At part throttle the pulse width is anything from some ten thousandths of a second to the full second at full power. The switching can result in efficiency losses. Batteries which cannot deliver and shut down power output at near the rate the fets in many controllers are switching. This can give rise to ripple voltage and current which requires larger smoothing caps on the controllers and losses within the batteries. This effect is much greater in multiphase DC controllers (brushless) than for brushed motor controllers. Brushed controllers have similar efficiencies at full and part throttle.

At part throttle Brushless controller losses can change at rates inversely proportional to the degree of throttle in use i.e. 20% throttle 80% of the input is lost as heat at 100% they too are 98% efficient .

This has to be offset against the efficiency losses of the brushed motor which generally has lower efficiency levels across the working range, and narrower power of high efficiency, and generally lower power handling, than a brushless motor.

Yours,

Andrew Gilchrist

fastelectrics.com

Australia

0419 429 201

From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Roger L
Sent: Friday, 19 April 2013 11:12 PM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Orest have u considered AC motors

Andrew wrote:

"The issue of the linear drop in amps and torque as the motor revs rise remains"

Yes, although I'm a motor guy rather than a controller guy....I've seen huge advances in wave-shaping by controllers.

I do believe that most of the problem of torque (amps) dropping with RPM can be overcome by a controller that will increase the input voltage to compensate for the BackEMF it is seeing from the PM motor. It used to be difficult to find a commercial controller that would operate in that manner. But that was because of a flaw or gap in the CE, CSA, and UL underwriting approvals which hadn't been challenged a decade ago. Is that still the case? Are there now controllers that will raise the input voltage proportionately to overcome Bemf as RPM rises?

"The issue that remains is the very poor performance of most brushless dc controllers at part throttle"

Andrew, I do not understand that comment. I'm making the assumption that most controllers have near infinitely variable wave shaping available - either automatically or as input by the operator. So why would throttle setting have any effect at all on performance?

thanks,

Roger L. - call me a retired but still curious old engineer.

.........

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----- Original Message -----

From: Andrew Gilchrist
To: electricboats@yahoogroups.com
Sent: Thursday, April 18, 2013 6:31 PM
Subject: RE: [Electric Boats] Orest have u considered AC motors

One of the big advances in recent years with brushless DCPMs has been in the controllers which enable the throttle curve to be infinitely recontoured,   variable timing depending on load conditions, degrees of softness in the motor start up etc.

The issue of the linear drop in amps and torque as the motor revs rise remains

The issue that remains is the very poor performance of most brushless dc controllers at part throttle
Yours,

Andrew Gilchrist
fastelectrics.com
Australia
0419 429 201

From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Chris Morriss
Sent: Friday, 19 April 2013 1:05 AM
To: electricboats@yahoogroups.com
Subject: RE: [Electric Boats] Orest have u considered AC motors

I do understand where you are coming from here, but you have highlighted the key problem for the application I was looking at. We need to operate the motor over a 10:1 speed range, (10% of nominal to 100%) rather than the 25% to 130% you quote. The problems that I have seen widely reported are with attempting to get power from a 3-phase motor driven in this way at low rpm. Now I have to admit that although I have good knowledge and some practical experience with BLDC motors, I have never used 50Hz 3-phase conventional ac motors with a variable speed drive, but thinking of the physics of it all, I am not surprised that there are problems when using the VS drive to give low, but controllable, rpm.
From: electricboats@yahoogroups.com [electricboats@yahoogroups.com] on behalf of Julian Webb [julian.proto@gmail.com]
Sent: 18 April 2013 15:06
To: electricboats@yahoogroups.com
Subject: RE: [Electric Boats] Orest have u considered AC motors

If you use a good TEFC squirrel cage 3 phase motor you have a unit 90+% efficient over a wide range of speeds and coupled with a matched Variable Speed Drive you can hold torque at 125% from 25%-130% of rated RPM with no loss of efficiency. They are robust, cheap and easily fixed if needed.
They aren't as exotic or trendy as PM units but they're not that far down on torque or efficiency either.
They are considerably heavier, but for canal or slow cruising that isn't normally a consideration.
If a few percent efficiency is of greater importance than the thousands more you'll pay for a PM unit and its controller in any decent KW or dual drive system over 3 phase a.c. VSD and motor then I guess you've probably got it decided already?
On Apr 18, 2013 2:26 PM, "Chris Morriss" <crsm@oroboros.demon.co.uk> 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.
>
>
>
> 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!
>
> ________________________________
> 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
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>>
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>> there is another option – others here my have tried it
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>>
>>
>> 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.
>>
>>
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>> 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
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>