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Max safe revs for ADC 8"?

Posted: Tue, 03 Mar 2009, 06:49
by Peter C in Canberra
Hi,
My conversion is well underway using a second-hand ADC 8" motor. I didn't get any documentation with it. Can anyone tell me the maximum safe rpms for this motor? Does ADC specify that or is that just what people have found in practice doesn't cause the motor to disassemble itself? I am hoping my as yet untested circuit will have the car's original tacho display show the motor revs.
Thanks,
Peter.

Max safe revs for ADC 8"?

Posted: Thu, 05 Mar 2009, 03:44
by Richo
DC motor manufacturers never specify the max ratings just nominal.
Seems typical max could be around 5500-6000RPM.

Max safe revs for ADC 8"?

Posted: Thu, 05 Mar 2009, 17:06
by Peter C in Canberra
Thanks for that. I had tried to get a response from ADC head office without luck. This morning I instead sent the query to the Symons (evmotors.com.au) and got a very prompt response that included:
"Advanced DC motors can run continuously up to 5000 rpm, and for peak load periods of 1 to 3 minutes, up to 6000 - 7000 rpm."
Assuming my tacho arrangements work I will mark a 'red line' at 5000rpm and not mention to other drivers of the car that they could probably get away with briefly pushing it a bit further. That should build in a bit of a safety margin.
Peter.

Max safe revs for ADC 8"?

Posted: Thu, 05 Mar 2009, 21:19
by Simon
Thats good to know. I had thought that 5500rpm was the safe limit for ADC but dont know where I got that info from. Accidentally revved my motor (a K91-4003) to an estimated 8000rpm when I had first gear but thought it was in 3rd! Image

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 06:06
by antiscab
Peter C in Canberra wrote: Assuming my tacho arrangements work I will mark a 'red line' at 5000rpm and not mention to other drivers of the car that they could probably get away with briefly pushing it a bit further. That should build in a bit of a safety margin.
Peter.


If you did mention that to them, also mention that at higher revs, you develop less power. (peak power is around 3500rpm at 120v for the 8", as per the data sheet).

Matt

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 16:20
by Peter C in Canberra
Thanks, I have the hand graphed sheet from ADC with Amps and RPM on the left vertical axis and horse power and efficiency on the right vertical axis, torque on the horizontal axis, and lines labeled with Amps or RPM for 120 or 108 Volts. The volts are in the form: '120V -.03I' or '120V -.06I'. I am not at all sure how to read this graph. The title on the graph is "Cold Calculated Performance Characteristic 203-06-4001 Electric Vehicle Motor 8" dia. - fan cooled 108 volt -.031 and .061 120 vold -.031 and .061"
What do the -.03I and -.06I mean?
I think the graph is showing that Amps and torque are in almost direct proportion regardless of voltage. Correct?
I'm not at all sure how I get to power or efficiency however. If I read across from a particular RPM (say 4000) across to the '120V -.06I' line I can see the torque I would have is about 51 ft/lb and from that torque I can see the current I would have is about 345A. 120Vx345A is about 41KW which is about 55HP which would I think be the output of the motor if it were 100% efficient but I don't see how I would read that result or the efficiency from this graph.
I'd be very pleased if someone could explain it to me!
Thanks,
Peter.

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 16:45
by woody
Peter C in Canberra wrote:I'm not at all sure how I get to power or efficiency however. If I read across from a particular RPM (say 4000) across to the '120V -.06I' line I can see the torque I would have is about 51 ft/lb and from that torque I can see the current I would have is about 345A. 120Vx345A is about 41KW which is about 55HP which would I think be the output of the motor if it were 100% efficient but I don't see how I would read that result or the efficiency from this graph.
I'd be very pleased if someone could explain it to me!
Thanks,
Peter.
Hi Peter,

I don't know about the other stuff, but I know this stuff :-)

Efficiency = Power Out / Power In * 100%

Power In = 120V x 345A = 41kW

Power Out = 4000rpm * 51 ft/lb = 4000 rpm * 69 Nm

To Convert rpm + Nm torque to power you multiply by 2Pi (radius -> circumference) and divide by 60 (minutes => seconds). Dived by 1000 for kiloWatts. This works out very close to dividing by 9550 which is easy to remember.

Power Out = 4000 * 69 / 9550 = 28.9 kW

So efficiency at this load and speed is 28.9/41 = 70.5%. No wonder the motor gets hot, you're putting 12kW of heat into it :-)

cheers,
Woody

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 17:56
by Peter C in Canberra
Thanks,
That makes sense. It also tallies with the impression I have picked up that these DC motors generally operate at about 80% efficiency across a range of conditions. Presumably these particular conditions are a bit off the optimum.
I still can't see how I would have been able to read this off from the graph. Having axes labelled with power and efficiency implies that I should be able to do so somehow. Anyone else?
Where I am going with this is to understand what would be the most efficient driving conditions.   I won't know what voltage or current the motor is seeing. I will know the battery pack voltage and I can monitor the motor RPM and the battery current at any given time. I guess I can just do it empirically. Whatever gear/RPM gives me the lowest current for any particular conditions (acceleration rate/speed etc) will be the most efficient. I was hoping that I might be able to read off the graph some theoretical optimum to try to maintain.
Peter.

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 18:10
by Johny
Hi Peter. I have collected a few graphs on the ADC motors and some have an efficiency line sweeping over the top. 90% at about 1/4 of their maximum torque, dropping to high seventies at max. torque. I realise this isn't your motor.
Like this:
Image

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 18:11
by fuzzy-hair-man
Hey Peter, you're not the only one who finds those DC power etc graphs confusing, they make bugger all sense to me. Image

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 18:23
by woody
They aren't the usual graphs we're used to looking at for ICEs. The ICE curves don't show efficiency for some reason :-)

The efficiency looks related to the torque on this graph.

I.E. at 40 ft/lb, efficiency is 82%.

This makes sense to me. The more current, the more torque, but the more current, the more magnetisation changes and over-magnetisation effective, but is inefficient - hysteresis curves and all that.

cheers,
Woody

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 18:36
by antiscab
the reason the 8" graph makes no sense,
is because its missing half its curves.
it only has the rpm/torque and the amp/torque curves.
everything else has to be calculated.
at least the graphs for the 6.7" and the 9" have all the curves.

Matt

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 18:44
by Electrocycle
woody wrote: This makes sense to me. The more current, the more torque, but the more current, the more magnetisation changes and over-magnetisation effective, but is inefficient - hysteresis curves and all that.
don't forget I²R !

The motor has some series resistance, and when you double the current you quadruple the heat generated!

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 19:48
by Peter C in Canberra
Aha! Thanks everyone. That's a lot clearer. I was not confused by any preconceptions from ICE curves because I have not ever looked at those. [I can draw load lines on curves for electron valves to design nice hifi amps though.] I'll have a look at the more complete graphs for the next motor up and down from my 8" and interpolate to get an idea of the likely situation with the 8" or see if I can derive the missing curves on the 120V/8" graph.
So, are the following reasonable interpretations of the example graph above?
-For this particular voltage, max. efficiency occurs with torque in the range 10-30
-That torque occurs at 3500 to 4500rpm
-Higher RPMs would lose power, torque and efficiency so there would be no point selecting a lower gear for higher RPMs.

If I were to look at a bunch of DC motor curves for a range of voltages would I find that the max. efficiency occurs around about the same 3500-4500 range?
Should I tend to select gears to stay in about this range, recognising that I'm not losing much if I stray either side?
Anyone know what the -.03I and -.06I mean?
Thanks,
Peter.

Max safe revs for ADC 8"?

Posted: Fri, 06 Mar 2009, 20:34
by Johny
Peter, I think because the efficiency curve is plotted horizontally it follows the torque on the X axis. Higher torque, lower efficiency.
That would make sense, higher current, lower efficiency.
The RPM line just shows the torque available at given RPM. Eg. You can not get over 80ft/lb torque at over 2000RPM (at that voltage).