# [EVDL] Basic DC Motor Characteristics and Graphs



## EVDL List (Jul 27, 2007)

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## EVDL List (Jul 27, 2007)

>
> Dear All,
>
> I am struggling to understand a few basic DC series motor concepts... I
> know that current=torque and speed=voltage (roughly speaking), but that
> the resistance goes up with speed and therefore the current you can
> pull/torque lessens?
>
> Lets assume i have a 100V battery pack capable of delivering 500A with no
> voltage sag, and i have unlimited traction and no controller (for the sake
> of simplicity!) If i flip the switch to the motor, it will take 500A and
> give me say 100ft-lbs torque, and will be drawing a few volts at low
> speed.

If the pack can source 500 amps without sagging, what is limiting it from
producing 2,000 amps or 20,000 amps?
Voltage sag (or meltdown) is what normally limits the current a pack can
provide.

>
> As speed increases, so will the voltage drawn from the pack - upto its
> limit of 100V. will the motor continue to draw 500A until 100V if my
> batterys can supply it, or will the current draw decrease?

The pack voltage is the pack voltage. If it doesn't sag, then the motor
will see the FULL pack voltage (100 V) and draw a current related to it's
back EMF.
If it's a traction motor, with no controller to limit current, then it
will draw several thousand amps at stall the current will drop as the RPMs
come up (and the back EMF increases).

What you are describing can only happen if you have a controller in the
circuit, or the pack sags significantly.

>
> I am trying to get a speed-hp curve for a certain max voltage and current,
> but im not sure if i can assume it to be a straight line (i.e. if
> torque/current draw remains constant)... or how i can calculate this!

You can't get there from here. What you want are called torque curves and
they are usually supplied by the manufacturer of the motor, or someone
else, who have come up with the curves by TESTING the motor.

P.s. they are called "curves" because they are not a straight line.

Many of the EV vendors have torque curves available for the motors they
carry.

>
> Any help is greatly appreciated.
>
> Many Thanks,
>
> Scott
>
>
>
>
>
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## EVDL List (Jul 27, 2007)

Scott,

Motor manufacturers collect data on a particular motor and produce a
performance curve. The below chart shows many curves for DC motors, and
I just cut and pasted this message from this list for an example.
Others on the list could provide input on how to collect data to draw
your own curve.
^
www.zev.dk/misc/torque-curves.gif
based on some old test data that I forget where I got. I think it's ADC
themselves that did it long ago
while their curves are quite similar the main difference between the
motors must be how much current they can safely take and that would
presumably be decided by the wire thickness used in each. finding that
data is probably going to be difficult. if anyone has it please let me
know
Dan
^

Alan 

-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Scott Littledike
Sent: Friday, February 15, 2008 2:30 PM
To: [email protected]
Subject: [EVDL] Basic DC Motor Characteristics and Graphs


Dear All,

I am struggling to understand a few basic DC series motor concepts... I
know that current=torque and speed=voltage (roughly speaking), but that
the resistance goes up with speed and therefore the current you can
pull/torque lessens?

Lets assume i have a 100V battery pack capable of delivering 500A with
no voltage sag, and i have unlimited traction and no controller (for the
sake of simplicity!) If i flip the switch to the motor, it will take
500A and give me say 100ft-lbs torque, and will be drawing a few volts
at low speed.

As speed increases, so will the voltage drawn from the pack - upto its
limit of 100V. will the motor continue to draw 500A until 100V if my
batterys can supply it, or will the current draw decrease?

I am trying to get a speed-hp curve for a certain max voltage and
current, but im not sure if i can assume it to be a straight line (i.e.
if torque/current draw remains constant)... or how i can calculate this!

Any help is greatly appreciated.

Many Thanks,

Scott





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## EVDL List (Jul 27, 2007)

Thanks everyone for your replies. Perhaps i havent made my question totally clear/i am missing a fair bit!

I have motor torque curves for the motors i am considering. These show that at say 500A, the motor will produce 100ft-lbs of torque, at 72v that corresponds to peak hp at say 2000rpm, any further increase in speed will then result in a lower power...

I want to know what will happen if my voltage is above that used in the manufacturer graph - if i was able to provide that current at say 144V (and the controller limited current at 500A), would the motor keep pulling 500A/producing 100ft-lbs upto the speed corresponding to 144v (if linear, 4000rpm) ?

I am trying to find out if you keep providing more voltage, will the motor keep pulling the max current your battery system can provide? (until the motor overheats/blows). If not, how can I predict the drop in max current draw as I increase voltage beyond that specified?

Many Thanks,

Scott
_________________________________________________________________
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http://www.searchgamesbox.com
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## EVDL List (Jul 27, 2007)

Interesting that you should bring this up right now. I'm working on my 
senior design project, a 120V 3A DC bidirectional motor speed control. 
We have to model the whole circuit in SPICE before we can build it, 
including the motor. I dont know if you have any experience with SPICE, 
but I found a model for a DC motor here:
http://www.ecircuitcenter.com/Circuits/dc_motor_model/DCmotor_model.htm

I have modified the information given on that page to work with LTspice, 
a free SPICE simulator from Linear Technologies. The graphs I get for 
the simulated motor look very good (at least for a small motor). I would 
be willing to send the LTspice file to anyone interested in modeling a 
motor.

The problem you presented is this: 100V, 500A limit, step input (on/off 
switch).
Motors will pull a large startup current, up to the limit when first 
switched on (this depends on the rotational inertia of the motor+load as 
well as any friction). Then as the motor reaches speed, the current 
drops off because the back EMF comes up. When the current limit is 
reached during this initial acceleration, the voltage will sag.

I will make an attempt at ASCII art here (dont laugh too much)

C |
u | _____
r | / \
r | / \
e | / \________
n | / 
t |/____________________
Time

R | _____________
P | /
M | /
| /
| /
|/________________
Time

V | _______
O |\ /
L | \ /
T | \_______/
S |
|____________________
Time

-Jon Glauser

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## EVDL List (Jul 27, 2007)

> I have motor torque curves for the motors i am considering. These show
> that at say 500A, the motor will produce 100ft-lbs of torque, at 72v that
> corresponds to peak hp at say 2000rpm, any further increase in speed will
> then result in a lower power...
>
> I want to know what will happen if my voltage is above that used in the
> manufacturer graph - if i was able to provide that current at say 144V
> (and the controller limited current at 500A), would the motor keep pulling
> 500A/producing 100ft-lbs upto the speed corresponding to 144v (if linear,
> 4000rpm) ?

Yes, pretty much. It will be a bit less because of the higher losses from
spinning faster.

>
> I am trying to find out if you keep providing more voltage, will the motor
> keep pulling the max current your battery system can provide? (until the
> motor overheats/blows). If not, how can I predict the drop in max current
> draw as I increase voltage beyond that specified?

Yes, the torque vs current remains basically the same as you raise voltage
(minus a little for the extra losses, friction, windage, etc.). The RPM
pretty much follows the voltage if you keep the torque and current the
same, so doubling the voltage will about double the RPM.

By using the controller to limit current, you basically keep the motor at
a fixed torque output until it spins up fast enough to reach the point
where the controller is full on and not limiting current anymore.
At that point, if you keep accelerating, the current will start to fall.

Of course this assumes that the motor doesn't either melt down or go into
self-disassembly mode (fly apart)

There are some formulas that can roughly model a motors performance
outside the graphs, however you have to calculate come motor constants
etc, by sucessive approximation based on the data from existing graphs at
several different voltages.
Not really worth the effort in my opinion. If you have to graphs you can
guestimate the results just about as accurately, and spend a lot less time
doing it.

>
> Many Thanks,
>
> Scott
> _________________________________________________________________
> Free games, great prizes - get gaming at Gamesbox.
> http://www.searchgamesbox.com
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>


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