# [EVDL] AC versus DC motor efficiency



## EVDL List (Jul 27, 2007)

Based on my reading here and elsewhere, I have learned that DC motors are generally not as efficient as AC motors. However, I have not been able to find specs on the efficiency range of, for example, the Warp series DC motors. Is there anywhere I can find this information? I would also be interested in information about comparing the overall efficiency(battery to motor output) of say an Azure AC55 + AC controller to a Warp 9" + Zilla 1K controller. I am aware of the cost, rpm, and other differences. Thanks.
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## EVDL List (Jul 27, 2007)

> Stuart Friedrich and Wendy Lyn wrote:
> > Based on my reading here and elsewhere, I have learned that DC motors
> > are generally not as efficient as AC motors.
> 
> ...


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## EVDL List (Jul 27, 2007)

Hello Stuart and Wendy,

This information is difficult to find. And then it is
difficult to compare one system to the other. 
Efficiency is not just a number. It varies with load
and speed. See: 

http://www.acpropulsion.com/technology/ac150_efficiency.gif


So, to get a real world comparison, one system to
another, you have to know your duty cycle profile. 
That is how much time you spend at what loads and
speeds. Typically this sort of thing is done by
modeling the systems and running simulations. 
Needless to say, this is not an easy task. Your best
luck may be finding an university paper on the
subject.

Regards,

Jeff M


--- Stuart Friedrich and Wendy Lyn


> <[email protected]> wrote:
> 
> >
> > Based on my reading here and elsewhere, I have
> ...


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## EVDL List (Jul 27, 2007)

Thanks Lee. I should have known about the graph for the Warp motor because I recall looking at it a while ago. I am considering a direct drive application and was mainly concerned about how much the efficiency changes at high versus low rpm. Based on my rough calculations I would probably be spending most of my time at around 2700rpm (two-lane highway) with occasional use at up to 4000rpm (4-lane highway).

I'm still in the planning stage, but intend to post my general plan at some point to get overall opinions.

Stuart
> Date: Fri, 23 Nov 2007 09:17:02 -0600> From: [email protected]> To: [email protected]> Subject: Re: [EVDL] AC versus DC motor efficiency> > Stuart Friedrich and Wendy Lyn wrote:> > Based on my reading here and elsewhere, I have learned that DC motors> > are generally not as efficient as AC motors.> > If you buy the best, the efficiency of AC and DC motors are the same.> The same fundamental limitations apply to both. All motors are at heart> AC, and you can get over 90% efficiency on either type.> > However, DC motors are usually optimized to be cheap. AC motors for EVs> are optimized to be efficient. Thus, a good AC motor is more efficient> (and more expensive) than a cheap DC motor.> > > I have not been able to find specs on the efficiency range of, for > > example, the Warp series DC motors. Is there anywhere I can find > > this information?> > It's on their website at http://www.go-ev.com/images/WarP_9_Graph.jpg> For the WarP 9" for example, efficiency peaks at 88!
%, and is over 85%> from 1000-4000 RPM.> > > I would also be interested in information about comparing the overall> > efficiency(battery to motor output) of say an Azure AC55 + AC> > controller to a Warp 9" + Zilla 1K controller.> > This is a complex issue. Engineers usually express this as a contour> plot of efficiency versus torque and speed. It takes a lot of testing to> create such a graph, and every little thing you change in the system> affects it, so the one you find in a book may not actually apply to your> system.> > A brushed DC motor has its own simple built-in inverter (its> commutator). They almost always include an internal fan for cooling. So> the efficiency of a DC motor includes its inverter and cooling fan losses.> > AC motors have external inverters, and are generally liquid cooled. So,> their efficiency numbers do not include inverter and cooling losses.> > Inverters for AC motor inverters have two switches in series;> controllers for DC motors have one.!
So DC controllers generally have> half the losses of AC contr!
ollers. 
But the losses are only 1%-5% per> switch, so we're talking about a controller efficiency range of 90-98%> for AC versus 95-99% for DC.> > Given the products readily available, go for a DC system if you want a > lower cost per horsepower, or an AC system if you want higher efficiency.> -- > Ring the bells that still can ring> Forget the perfect offering> There is a crack in everything> That's how the light gets in -- Leonard Cohen> --> Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net> > _______________________________________________> For subscription options, see> http://lists.sjsu.edu/mailman/listinfo/ev
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## EVDL List (Jul 27, 2007)

DC controllers can go into bypass mode which means nearly 100% controller 
efficiency. If you have a series DC motor and a transmission, most of your 
driving will by in bypass. AC has no counterpart. 







On Fri, 23 Nov 2007 09:17:02 -0600, Lee Hart wrote


> > Stuart Friedrich and Wendy Lyn wrote:
> > > Based on my reading here and elsewhere, I have learned that DC motors
> > > are generally not as efficient as AC motors.
> >
> ...


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## EVDL List (Jul 27, 2007)

> George Swartz wrote:
> > DC controllers can go into bypass mode which means nearly 100% controller
> > efficiency. If you have a series DC motor and a transmission, most of your
> > driving will by in bypass.
> ...


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## EVDL List (Jul 27, 2007)

Lee,
I even designed trolley bus solid state controllers to have a bypass of 
sorts. By gating SCR's full on above base speed and using some field 
control, I was able to lower losses and also, important, lower RFI 
generation. I has worked well in 219 trolley buses for over 25 years.

On thing that makes a bypass even more attractive, where the motor and 
battery pack can be matched, is that bypass shuts off the PWM circulating 
currents in both the battery and motor. These AC current components cause 
extra I2R losses that are seldom attributed to the controller when 
calculating controller efficiency, but are, none the less, system losses 
affecting vehicle wh/mile. Losses are greater in low frequency choppers 
such as older SCR designs and some IGBT designs. Losses would be less for 
higher frequency FET choppers, or controller designs having adequate energy 
storage capacitors on input and output.




On Fri, 23 Nov 2007 12:47:22 -0600, Lee Hart wrote


> > George Swartz wrote:
> > > DC controllers can go into bypass mode which means nearly 100%
> controller
> > > efficiency. If you have a series DC motor and a transmission, most of
> ...


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## EVDL List (Jul 27, 2007)

Excellent summation Lee! This page should be posted somewhere for ongoing 
reference, it will save a lot of repetition!

-Phil
----- Original Message ----- 
From: "Lee Hart" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Friday, November 23, 2007 7:17 AM
Subject: Re: [EVDL] AC versus DC motor efficiency




> > Stuart Friedrich and Wendy Lyn wrote:
> >> Based on my reading here and elsewhere, I have learned that DC motors
> >> are generally not as efficient as AC motors.
> >
> ...


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## EVDL List (Jul 27, 2007)

> On 23 Nov 2007 at 8:33, George Swartz wrote:
> 
> > DC controllers can go into bypass mode which means nearly 100% controller
> > efficiency.
> ...


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## EVDL List (Jul 27, 2007)

> "David Roden" <[email protected]> wrote:
> 
> 
> Dear DavId,
> ...


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## EVDL List (Jul 27, 2007)

Why would losses at high-switching freqs be greater in a IGBT than in a 
MOSFET?

At higher voltages when MOSFETs end up with higher on resistances, I have 
seen that they are less efficient than modern IGBTs which is generally why 
we see them used in higher voltage controllers these days.

Also, I don't recall for sure if gate capacitance is higher in MOSFETs, 
(>200v) but I think it is.

-Phil
----- Original Message ----- 
From: "George Swartz" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Friday, November 23, 2007 12:41 PM
Subject: Re: [EVDL] AC versus DC motor efficiency


> Lee,
> I even designed trolley bus solid state controllers to have a bypass of
> sorts. By gating SCR's full on above base speed and using some field
> control, I was able to lower losses and also, important, lower RFI
> generation. I has worked well in 219 trolley buses for over 25 years.
>
> On thing that makes a bypass even more attractive, where the motor and
> battery pack can be matched, is that bypass shuts off the PWM circulating
> currents in both the battery and motor. These AC current components cause
> extra I2R losses that are seldom attributed to the controller when
> calculating controller efficiency, but are, none the less, system losses
> affecting vehicle wh/mile. Losses are greater in low frequency choppers
> such as older SCR designs and some IGBT designs. Losses would be less for
> higher frequency FET choppers, or controller designs having adequate 
> energy
> storage capacitors on input and output.
>
>
>
>
> On Fri, 23 Nov 2007 12:47:22 -0600, Lee Hart wrote


> >> George Swartz wrote:
> >> > DC controllers can go into bypass mode which means nearly 100%
> > controller
> >> > efficiency. If you have a series DC motor and a transmission, most of
> ...


----------



## EVDL List (Jul 27, 2007)

The losses I was referring to are the PWM currents that circulate in the 
battery and the motor. These are more difficult to filter at low switching 
frequencies, mostly related to older SCR controllers, thus more I-squared-R 
losses if they are allowed to circulate. Going into bypass, either 
electronically, or with a parallel contactor reduces these currents to zero, 
and also lowers the heating and RMS current through electrolytic cap banks 
associated with the controller, one common area for controller failure. The 
operating frequency range for IGBT's is lower than for FET's. And, 
contactor bypass benefits an IGBT type chopper most, because the inherent 
IGBT voltage drop is higher than either well designed SCR or FET circuits. 
(no junction drops in FET's, only channel resistance)

The use of bypass, with a series motor that can follow its horsepower 
limited characteristic curve as a vehicle accelerates results in the lowest 
weight motor, low weight being another factor in obtaining maximum vehicle 
range. The use of a low voltage motor, say 96vdc, and a non matched 180vdc 
battery pack, where the controller must always be in PWM mode is not an 
optimum propulsion design in my opinion, and puts high voltage and frequency 
stresses on a motor that maybe was not designed for it, such as a forklift 
motor. (I have overloaded more than a few motors and I don't lack the 
courage to do it again, but sometimes, it's expensive!)

Note also that the motor series field can be bypassed by contactor and 
resistor or by solid state control to achieve field weakening on a simple 
series motor. A SEPEX or compound wound motor, if available, is easier. 

Bypass is just one more design technique for EV's to get better range by a 
few percent, if that's important to anyone.





On Fri, 23 Nov 2007 14:51:59 -0800, (-Phil-) wrote
> Why would losses at high-switching freqs be greater in a IGBT than 
> in a MOSFET?
> 
> At higher voltages when MOSFETs end up with higher on resistances, I 
> have seen that they are less efficient than modern IGBTs which is 
> generally why we see them used in higher voltage controllers these days.
> 
> Also, I don't recall for sure if gate capacitance is higher in 
> MOSFETs, 
> (>200v) but I think it is.
> 
> -Phil
> ----- Original Message ----- 
> From: "George Swartz" <[email protected]>
> To: "Electric Vehicle Discussion List" <[email protected]>
> Sent: Friday, November 23, 2007 12:41 PM
> Subject: Re: [EVDL] AC versus DC motor efficiency
> 
> > Lee,
> > I even designed trolley bus solid state controllers to have a bypass of
> > sorts. By gating SCR's full on above base speed and using some field
> > control, I was able to lower losses and also, important, lower RFI
> > generation. I has worked well in 219 trolley buses for over 25 years.
> >
> > On thing that makes a bypass even more attractive, where the motor and
> > battery pack can be matched, is that bypass shuts off the PWM circulating
> > currents in both the battery and motor. These AC current components 
cause
> > extra I2R losses that are seldom attributed to the controller when
> > calculating controller efficiency, but are, none the less, system losses
> > affecting vehicle wh/mile. Losses are greater in low frequency choppers
> > such as older SCR designs and some IGBT designs. Losses would be less 
for
> > higher frequency FET choppers, or controller designs having adequate 
> > energy
> > storage capacitors on input and output.
> >
> >
> >
> >
> > On Fri, 23 Nov 2007 12:47:22 -0600, Lee Hart wrote


> > >> George Swartz wrote:
> > >> > DC controllers can go into bypass mode which means nearly 100%
> > > controller
> > >> > efficiency. If you have a series DC motor and a transmission, most of
> ...


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