# Question About Back EMF and motor voltages



## Duncan (Dec 8, 2008)

Hi,
I am hoping somebody knowledgeable like Major will answer this

DC motor voltages
As far as I can see the normal "forklift" or "EV" DC series motors appear to be
rpm limited rather than voltage limited

150v seems to be more than enough to drive all of them at 1000amps and 5000rpm,

If this is so where do we get "HV" motors like the Warp 11 HV at 288 volts?

The HV motor gets ~ 10% less torque at a given current than the non HV 11 inch motor

As Power = Volts x Amps and Rpm x Torque 
For the same Torque and Rpm an HV motor will need more amps and therefore a *lower *voltage???

Or am I missing something


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## kennybobby (Aug 10, 2012)

Typically to make a higher-voltage motor you would wind it with more turns using a smaller gauge wire. This increases the resistance of the windings to allow for use with an increased voltage. The greater number of turns also increases the back emf.


Duncan said:


> ...The HV motor gets ~ 10% less torque at a given current than the non HV 11 inch motor
> 
> As Power = Volts x Amps and Rpm x Torque
> For the same Torque and Rpm an HV motor will need more amps and therefore a *lower *voltage???


To get more current you would need to raise the voltage.


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## major (Apr 4, 2008)

First off: Published performance characteristic curves for the DC motors commonly used in EV conversions typically suck. Most from what I have seen are sloppy and inaccurate.* Compound that with extrapolation to like twice the shown range and you can get some numbers which are dubious. 

With that said, let's use the two curves from this thread: http://www.diyelectriccar.com/forums/showthread.php?p=385833#post385833 
You have the Warp11 and Warp11HV. So let's not extrapolate and use 160 lb.ft. point from each curve. Both curves indicate 72V (assumed constant at the motor terminals because it doesn't say any different). So; the Warp11 shows 500A and 1300RPM. W11HV shows 550A and 1200RPM. About that 10% difference in torque/amp, right Dunc?

From the torque and RPM you can calculate the hp. 39.6hp for Warp11 and 36.6hp for W11HP. V*I gives you the input power in for each; 36kW & 39.6kW respectively. Then eff = Pout/Pin. 82% for Warp11 and 69% for W11HV. Obviously these efficiency numbers disagree significantly with the eff curves shown on those graphs.

Both motors are around 1200-1300 RPM at 500-550A at 72V. So applying the ratio rule, they would be around 5000RPM at 288V at 500-550A.

I don't know what the design difference is between the two motors and won't speculate. I also don't know what is wanted by the thread title.


> Question About Back EMF and motor voltages


 What's the question?

Excuse me but I refer to generated voltage (Eg) rather than back EMF.

Eg = the number of series conductors in the armature times the flux times the angular velocity. Often represented by Kt*Θ*_w_. 

Tem = the number of series conductors in the armature times the flux times the armature current. (Tem is the electromagnetic torque. Shaft torque must account for the rotational losses) Tem = Kt*Θ*Ia.

Eg = applied voltage - resistive voltage drops and brush voltage = Vm-Ia*Req-Vbr. 

Almost all the DC motors I've seen used for these EV conversions have the same armature construction; 4 pole bar wound single turn simplex wave. So the Kt term is proportional to the number of comm bars. Because we're talking about well saturated series motors, the flux is then proportional to the air gap area generally approximated for comparison sake as D*L. (diameter and length of the armature core)

So when comparing motors one should take into account the armature size and bar count before making assumptions regarding the voltage, RPM, current, torque relationships.

*The Kostov motor curves are an exception. They are nicely done.


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## Tesseract (Sep 27, 2008)

major said:


> ... Because we're talking about well saturated series motors, the flux is then proportional to the air gap area generally approximated for comparison sake as D*L. (diameter and length of the armature core)...


...and therefore torque is linearly proportional to current and RPM is linearly proportional to voltage (again, as noted, when the motor is deep into saturation).

Or what I said in this post, so I guess I am to conclude that Duncan apparently does not not believe what I wrote and/or does not think I am "knowledgeable"...


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## Duncan (Dec 8, 2008)

Hi Tesseract
_Duncan apparently does not not believe what I wrote and/or does not think I am "knowledgeable"_

It's not so - I posted my second question before you posted your reply!

I'm understanding this a bit better now - the higher voltages make a lot more sense


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## Duncan (Dec 8, 2008)

Hi Guys
Lets see if I have got this correct

With a bigger motor I get more torque at a given amperage
(assume same length - 9 inch v 11 inch diameter)

Therefore at a given speed the bigger motor will have lower generated voltage (Eg) 
(not exactly as the efficiencies will vary)

Volts x amps = rpm x torque - so keeping Rpm and Current constant if I get more torque I must have a lower voltage

Tesseract quoted a Warp 9 as 50v/1000rpm at 1000amps
A Warp 11 has 30% more torque at 1000 amps so its coefficient should be about 37v/1000rpm at 1000amps????
30% more torque therefore 30% less volts??

Major's calculations - about 1250rpm at 500 amps and 160 Ft/Lbs at 72v
Gives 57v/1000rpm at 500amps - and presumably 114v/1000rpm at 1000amps

My car has a 130v pack (40S) - an 11 inch Hitachi and a 500 amp controller
If I use 57v/1000rpm and 500 amps I would start to run out of voltage at less than 2000rpm

It doesn't feel that way - if anything it seems to go better at above 2000rpm (54Kph)

So I feel the 57v/1000rpm at 500amps is too high,


I need more torque (to beat these pesky IC cars) so I am upgrading the controller,

If I use the 37v/1000rpm at 1000amps I will be able to keep full current (750amps) until nearly 4000rpm (108Kph)

If I use the Warp-9, 50v/1000rpm at 1000amps - I will start to run out of current at just below 3000rpm

So am I full of shit or am I starting to get it??


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## major (Apr 4, 2008)

Duncan said:


> With a bigger motor I get more torque at a given amperage
> (assume same length - 9 inch v 11 inch diameter)
> 
> Therefore at a given speed the bigger motor will have lower generated voltage (Eg)


Assume the two motors have the same Kt (which is unlikely). A bigger motor has more flux. So far a certain value of current will produce greater torque. And the larger motor will generate higher Eg at a given RPM.


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## Duncan (Dec 8, 2008)

major said:


> Assume the two motors have the same Kt (which is unlikely). A bigger motor has more flux. So far a certain value of current will produce greater torque. And the larger motor will generate higher Eg at a given RPM.



Small motor
_500 amps_ - 67v = 33,600 watts
1000rpm - _321Nm_ = 33,600 watts

Bigger motor
_500 amps_ - 91v = 45,425 watts
1000rpm - _434Nm_ = 45,425 watts

Yep when I lay it out you are right - I got the numbers the wrong way round

So if a Warp-9 is 50v/1000rpm at 1000amps
an 11 inch will be about 67v/1000rpm at 1000amps

And my 130v pack (which sags to 115v) will start to current limit 
at 500amps at - just over 3000rpm - 81Kph
at 750amps at - just over 2000rpm - 54Kph

Yes unfortunately I think it probably is 
(the 500amp - haven't tried the 750amp yet)

At 4000rpm I will be limited to about 350amps

So If I have a controller that can do the higher voltages I should be thinking about changing my current four strings of 40 cells to two strings of 80 cells
260 volts - then limiting the motor voltage to ~ 160v which will give me 750amps to just over 3000rpm 
and about 550amps at 4000rpm


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## skooler (Mar 26, 2011)

Thought it best to emphasize this.



major said:


> So when comparing motors one should take into account the armature size and bar count before making assumptions regarding the voltage, RPM, current, torque relationships.


I'll also throw in that motors with interpoles typically have a smaller armature due to extra space being required in the motor casing between the field windings (I assume thats where they go? major, can you confirm?) and therefore typically less torque.

The reason for less torque with interpoles is that the air gap between the armature and the windings is closer to the shaft.

Hence why the warp 11hv has less torque at the given current than the warp 11.


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## Yabert (Feb 7, 2010)

Duncan said:


> ...then limiting the motor voltage to ~ 160v which will give me 750amps...


120 Kw out of your tiny Headway pack?


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## Duncan (Dec 8, 2008)

Yabert said:


> 120 Kw out of your tiny Headway pack?


I have 160 off 16Ah cells
So 
160cells x 3.2v x 16Ah x 15C(burst) = 122.9Kw
(Said with tongue firmly in cheek)


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## dougingraham (Jul 26, 2011)

The WarP 11 HV has an armature that looks closer in diameter to the WarP 9 armature because of the interpoles.

I am clearly battery limited with my setup. I see the torque drop off at around 4000 rpm. I have 51 cells of 100AH. I need more voltage. I have the motor current set to 1000 amps and the battery current set to 800 amps. I have plenty of range and the car weighs about 100 lbs more than it did with ICE so I don't want to add anymore weight. So I am going to at some point replace the pack with a higher voltage one. Something like swapping out 51 100 AH cells for 86 of the 60AH cells. Capacity and weight will be the same so range will be the same but the higher voltage will give me a wider torque band through the increase in voltage. Even though the current limit on the 60AH cells would be 600 amps the higher voltage will more than make up for this. And I should never see the voltage sag below the max motor voltage.

Upgrade one would be high voltage pack. This should raise my power from 122kw (164hp) into the motor up to 192kw (257hp) into the motor. And that should be more than I ever need for a daily driver. It is already a fast car. I have about 2.5 times the torque it had as an ICE. But it runs out of poop at 4k rpm and the original car was just hitting its stride at 4k rpm.


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## dougingraham (Jul 26, 2011)

Duncan said:


> I have 160 off 16Ah cells
> So
> 160cells x 3.2v x 16Ah x 15C(burst) = 122.9Kw
> (Said with tongue firmly in cheek)


Yep, battery limited. I assume you have this as a 40S 4P arrangement so 64AH at 128 volts?


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## Duncan (Dec 8, 2008)

dougingraham said:


> Yep, battery limited. I assume you have this as a 40S 4P arrangement so 64AH at 128 volts?


Yes
Except the old 16Ah cells I have are not really 16Ah I have measured them as between 15.6Ah and 13Ah
So I am treating this as a 13Ah x 4 = 52Ah pack
And only using 75% = 39Ah


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## major (Apr 4, 2008)

skooler said:


> Thought it best to emphasize this.
> 
> 
> major said:
> ...


Hi skool,

Yeah, you have to make room in there for the interpoles and comm coils, however, when talking about torque per amp, don't forget about the Kt. Do the Warp11 and W11HV have the same # of comm bars?


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