# Perm and Agni ratings



## frodus (Apr 12, 2008)

240V ratings on a perm/agni? You know about volts/rpm ratio?

I'd say 100V or so is about the max I'd run those things. They're not BUILT to go that high of a voltage, and even if they could spin that fast without spinning apart, the torque would be so low, it would be unuseable.

The data they have on their website IS useful data. If you think that power/current is too low, then look for a larger motor.



I vote you do the online data collection, I mean, since you don't trust manufacturer specs and figures.


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## 2cycle (Jul 2, 2009)

frodus said:


> hahahahahaha, are you serious? 240V ratings on a perm/agni? Do you even know anything about brushed motors? volts/rpm ratio?
> 
> I'd say 100V or so is about the max I'd run those things. They're not BUILT to go that high of a voltage, and even if they could spin that fast without spinning apart, the torque would be so low, it would be unuseable.
> 
> ...


I trust what they are giving me for info, it's just not what I'm looking for. And no I'm not a motor guy, but you type like you are so tell me this, when you speculate that torque would be so low it would be unuseable throw me a speculative torque curve based on rpm and I'll be the judge of it's usefulness to me. 
I told Major I would do what I could for the EV racing within my resources and I'm trying to gather some info to start with but seem to hit dead ends everywhere I look. For e-racing to thrive it's going to take more than a couple secretive EV guys with a chip on their shoulder. We all need to work towards a common goal and believe it or not it's going to take a hundred wrong choices for every couple right ones to pan out. 
Don't get me wrong I already love electric motors, how else would I start my gas burners. All joking aside, I will have my test room built soon enough, and seeing how you vote me to do the testing I'll hit you up as the first ever subsciber to "E-Dyno online" .


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## frodus (Apr 12, 2008)

Ok, Ok, lemme back up. Sorry for my tone, but there's a few things about brushed motors you should know before you start. Just asking flat out if it can do 240V kinda tells me you need to do some more research on how exactly motors work. Here are some reasons why an Agno won't go 240V.

Brushed PM motors have a volts/RPM ratio. This means, for every volt, you have a certain RPM that the motor turns. If this motor turns at 4000RPM at 72V, then at 240V, it would turn 13,000+ RPM. There are two problems with this:
1) Mechanically, the motor would not take this sort of force. The motor armature windings would start to push outwards and the motor would try to spin apart.
2) You get arcing at higher voltages with a motor that is designed with 72V as its normal voltage. The motor is essentially spinning too fast for the brushes, it arcs. You'd have to advance the brushes to avoid this, but then you run into problem 1 again.

Now, ignore the agni/perm torque curves and look at a real torque curve for a motor 
www.evfr.net/synkromotive/components/motors/K91.pdf
(its a series wound, but it will be somewhat similar)

Notice the torque increase? what happens to RPM? Watch RPM increase? Torque Decreases.

More notes:
- As volts increase, RPM increase. 
- As the load increases, the torque required increases thus causing an increase in current.
- Torque is highest near 0RPM and decreases exponentially as RPM increases.
- HP has a bell shaped curve and increases to a point, then starts to decrease.
- Above a certain RPM, even if the motor could handle the RPM/arcing, you are still decreasing as RPM increases. You'd get to a point that, even if you had a geared transmission, you'll reach a HP limit of the motor and be unable to generate enough force to overcome air/road friction.


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## E4mula (Mar 31, 2009)

Just to clarify a bit, the PM motors have a fixed field strength so you don't see the same rpm drop under load as with the series motor. The torque of the motor is going to be somewhat proportional to the current applied.
With the series motor, the field current is changing with load giving you field strengthening/weakening.


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## frodus (Apr 12, 2008)

Yeah. I don't have a good torque curve for a Permanent Magnet Brushed motor, so its all I had for an example.....

The agni curves only show a small RPM window, so its hard to get an idea of what things do over the entire RPM range.


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## E4mula (Mar 31, 2009)

If you look at the charts on the Agni website, they show the motor at different voltage levels (at least on the 95 motor, didn't look at any others).
For each voltage level, there is roughly the same amount torque. This is regardless of voltage applied or the rpm of the motor. There's not much of a torque curve to speak of.


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## major (Apr 4, 2008)

E4mula said:


> For each voltage level, there is roughly the same amount torque. This is regardless of voltage applied or the rpm of the motor. There's not much of a torque curve to speak of.


Hi guys,

PMDC motors have constant flux, so linear torque per amp. For this motor, http://agnimotors.com/95_Series_Performance_Graphs.pdf the torque constant is 0.13 Nm/amp. This does not change with voltage or RPM.

I have been unimpressed with these axial PM brushed motors in the past because they have lacked the durability needed for EV propulsion application. They are air cooled and carry a decent continuous power rating and very high efficiency. But when put in EV traction duty with low speed overload on acceleration, too often they fail. This was particularly the case with the Etek and why B & S gave up on them, IMO.

C. Lynch took his design to India and Agni. It seems when Agni and or Cedric put in the effort, their motors can do a pretty good job, to my surprise. The Team Zero Agni bike won the first NA TTXGP, same bike as won the first IOM race. It uses 2 Agni motors. The subsequent race it did not fare so well. Competition got better and the Agni engineer wasn't present. Several other TTXGP bikes run pairs of Agnis and don't fare very well. However, the 3rd place guy at Laguna Seca, de Ridder, rode a twin Agni powered Crystalite bike. His numbers weren't too bad at all. 

Again, my opinion, brushed PM motors can do only a mediocre job at propulsion on a medium size bike, and even then 2 are needed. I have not seen a serious attempt at a brushed wound field motor and wonder if I will. I think that high performance battery powered motorcycles will be AC motor powered, either induction or PM.

Unfortunately there is little to choose from as far as available AC motors suitable for this application. It takes creativity, resourcefulness or money. You can buy the propulsion system from Czysz for a mere $42,500.

My view from the paddock,

major


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## 2cycle (Jul 2, 2009)

Thank you Major for shedding some light on the topic. The Agni motor seems to be an off the shelf motor of choice with at least a few bikes. I have been looking for options to start testing with and the ETek's, Perm's and Agni's will be the easiest to test with I believe. I have found some Reuland AC motors I'd love to play around with but haven't even checked on price because I know they will be spendy. 
I strongly believe the Ebike motors of the future will look nothing like what you see today. Developments in specialty racing motors will be a big enough business soon and good things will soon follow.


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## frodus (Apr 12, 2008)

To be a little more clear on what I mean about the torque dropping off. Yes there is a torque constant, but it does also have to do with applied voltage, because of the Counter EMF and the winding resistance. 

The current into a brushed PM motor is I = (Esource - Ecemf) / Rwinding
Esource = the applied voltage
Ecemf = counter electromotive force
Rwinding = resistance of the windings

The Counter EMF acts against the applied voltage.

At rest, the Ecemf is 0 and I = Esource / Rwinding. This is the max the torque will ever be. When Dynoing a motor, they usually get that torque constant at locked rotor (i.e. little or no rotational speed). They just apply amps and see how much torque the motor produces. 

As the applied voltage increases, the Ecemf increases as well, so (Esource - Ecemf) starts to decrease. This causes the current to decrease (see equation above). So as RPM increases, Ecemf increases and it will eventually cause the entire equation to go toward 0.


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## major (Apr 4, 2008)

frodus said:


> ........that torque constant goes out the window as soon as the motor starts to turn........ So yes, it does change with voltage and RPM.


Hi frodus,

My statement is correct. 


major said:


> the torque constant is 0.13 Nm/amp. This does not change with voltage or RPM.


Now different loads and different conditions on the motor may change the current. But for that motor, you always get 0.13 Nm/amp, regardless of those other conditions. 

So for instance, if you have a 400 amp current limit and "floor" it, the motor produces 52 Nm of torque, regardless of the RPM, until the current drops below 400. Obviously, the motor current is linked to the RPM and applied voltage, but the torque constant is always relevant, motor rotating or not.

Regards,

major


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## frodus (Apr 12, 2008)

Major,
I didn't mean to say the torque constant goes out the window, my mistake (edited). I meant to say that the output torque having a linear relation to input current goes out the window. 

My point: the actual output torque has to do with the applied voltage (as well as that torque constant). I didn't mean for it to be interpreted as the torque constant having anything to do with applied voltage. Appologies major if it seemed as though I was disagreeing. I was trying to explain (as you are) to the OP that torque is related to input current, which is related to input voltage and armature resistance. I mistakenly typed "torque constant" and meant to say "output torque". You're completely right, that constant never changes.



To the OP,
To go a little more in depth with my reasoning, The torque output changes based on applied input voltage. The higher the voltage, the higher the Ecemf, and the more the current aproaches 0.


The equation for output torque:
T = Z*F*I/(2*pi)

T = Torque
Z = number of conductors on the armature
F = Flux per pole
I = Armature current = (Esource - Ecemf) / Rwinding

Armature current is based on input voltage, and as that increases, Ecemf increases, causing armature current to aproach zero, so you'll see the torque drop off.


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