Estimating motor/controller efficiency (to answer: "how powerful is it?")

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Estimating motor/controller efficiency (to answer: "how powerful is it?")

Darin -@- ForkenSwift.com
One of the common questions I get about the car (after "how fast / how far / how much?") is "how powerful?"

I'd like to be able to offer (with caveats) a peak horsepower figure, but want it to be realistic (ie. reasonably account for system losses).

Nominally it's: 48v * 400 A = 19.2 kW / 746 W/hp = 25.7 hp

I've seen the graphs showing curves for the ADC 8 inch and other motors indicating an efficiency range of about 75-85%.  Is it fair to assume the 8 inch pump motor in the ForkenSwift is in that range?

That would put things closer to: 25.7 hp * .75 motor efficiency = 19.3 hp

What about the efficiency of the 400A Curtis on top of that?  I haven't had any luck finding any stats/estimates about PWM controller efficiency.   (I do realize the 400A limit applies only when the controller is cool - that's one of the caveats I mention to people who ask about power.)  Anyone have any figures to throw around?

thanks,
Darin

http://www.forkenswift.com
48 volt / 400A Geo Metro / $1k
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Re: Estimating motor/controller efficiency (to answer: "how powerful is it?")

Jeff Major

Hi Darin,

I'll insert my 2 cents worth...

--- On Fri, 6/12/09, Darin at- forkenswift.com <[hidden email]> wrote:

>
> One of the common questions I get about the car (after "how
> fast / how far /
> how much?") is "how powerful?"
>
> I'd like to be able to offer (with caveats) a peak
> horsepower figure, but
> want it to be realistic (ie. reasonably account for system
> losses).
>
> Nominally it's: 48v * 400 A = 19.2 kW / 746 W/hp = 25.7 hp

Yeah, but what is the actual battery voltage at 400 A?  More important, what is the motor voltage?  Depending on the type of battery, age, and SOC, the voltage will sag.  It is common to use 0.015 ohms as an internal resistance for a 48 volt battery for forklifts.  Your battery is smaller, no doubt, so let's say .02.  Then the battery terminal voltage at 400 amps is maybe about 40 or 42 volts.  You'll see a 1 to 2 volt drop in the controller, contactor, fuse and cables, on a good day.  So, actual motor voltage at 400 amps is likely about 40 volts.

Motor input power is then 40V times 400A = 16 kW = 21.5 HP.  Motor efficiency guess at that V and I, 75 to 80 percent.  Let's use 77.5%.  Then motor shaft output is 21.5 times 0.775 = 16.6 HP.

That'd be my take on it.

Jeff M


     

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Re: Estimating motor/controller efficiency (to answer: "how powerful is it?")

Darin -@- ForkenSwift.com
Hi Jeff - good points.

I try to never pull the batteries below 1.75 v / cell (or 5.25v per flooded pba batt), under high loads, which would mean pack voltage ~ 42.  

[And since high current is only possible when SOC is also high, "on a fresh charge" is usually one of my other caveats when talking about peak power.  The number of caveats I'll offer is proportional to the amount of glaze in the eyes of the person who asked :) ]

Jeff Major wrote
You'll see a 1 to 2 volt drop in the controller, contactor, fuse and cables, on a good day
Does that mean together the controller, contactor (2 actually), fuse & cables are 96% efficient under high load (@ 40.5v / 42v )?

Darin

http://www.forkenswift.com
48 volt / 400A Geo Metro / $1k
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Re: Estimating motor/controller efficiency (to answer: "how powerful is it?")

Jeff Major
In reply to this post by Darin -@- ForkenSwift.com


--- On Fri, 6/12/09, Darin at- forkenswift.com <[hidden email]> wrote:

> Jeff Major wrote:
> > You'll see a 1 to 2 volt drop in the controller,
> contactor, fuse and
> > cables, on a good day
>
> Does that mean together the controller, contactor (2
> actually), fuse &
> cables are 96% efficient under high load (@ 40.5v / 42v )?
>
> Darin
>

The controller (mosfet) has about a one volt drop at such currents, I think.  You'll get additional voltage drops in all the other components in series with the pack or motor.  Hard telling what those are, so I just lumped them together at about another volt and threw it into the mix.  If you're really keen on finding out, put a voltmeter on your motor terminals.

And yeah, 95 to 96 percent might be about right.

Jeff M



     

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Re: Estimating motor/controller efficiency (to answer: "how powerful is it?")

EVDL Administrator
In reply to this post by Darin -@- ForkenSwift.com
The rule of thumb I've used for years is electrical kw in ~= hp out.  
Measure your voltage and current both at the motor or both at the battery,
and multiply.  If the motor's eating 40kw, it's probably giving you about
40hp.  

Seems pretty crude, and I know in the latter case we're ignoring controller
efficiency, but in many cases it's not that far off from what you get after
calculating the efficiency of various parts of a DC drive.  

David Roden - Akron, Ohio, USA
EVDL Administrator

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Re: Estimating motor/controller efficiency (to answer: "howpowerful is it?")

Darrin-12
> Measure your voltage and current both at the motor or both at the battery,
> and multiply.

> David Roden - Akron, Ohio, USA
> EVDL Administrator

I'm not a EE so please excuse the ignorance.  Since the PWM controller is
chopping up the Voltage, isn't a dc measuring device an inappropriate tool
for measuring Volts and Amps?  Shouldn't a power meter be used instead?

Darrin Brunk - Pensacola, FLA


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Re: Estimating motor/controller efficiency (to answer: "howpowerful is it?")

Morgan LaMoore
On Sat, Jun 13, 2009 at 11:05 AM, Darrin <[hidden email]> wrote:
> I'm not a EE so please excuse the ignorance.  Since the PWM controller is
> chopping up the Voltage, isn't a dc measuring device an inappropriate tool
> for measuring Volts and Amps?  Shouldn't a power meter be used instead?

The hobbyist is unlikely to have a true power meter that would work
for an EV motor.

However, to a good approximation, motor current is DC. That means that
if you measure the average (DC) voltage and the current and multiply,
you'll get the power, and any error will be dominated by the
(in)accuracy of your voltage and current measurement.

You do need to have a voltage meter that can properly average the
noisy, chopped DC voltage signal, though. An old analog meter is
probably best/cheapest for this; the noise in the signal will mess up
all but the most expensive digital multimeters, but basic physics will
make the analog meter properly average its reading.

-Morgan LaMoore

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Re: Estimating motor/controller efficiency (to answer: "how powerful is it?")

Darin -@- ForkenSwift.com
In reply to this post by Darin -@- ForkenSwift.com
Thanks everyone for the replies on this.  I'm glad the figures I've been telling people are relatively close to what's been discussed.  Nice to have a rule of thumb for future reference as well.

Darin

http://www.ForkenSwift.com
48 volt / 400 amp Geo Metro < $1k