Sizing of DC motors

dave226

Hi, I am working on an EV and the problem is in sizing or selection of dc motors. I have to select few small PM DC motors instead of a big one, the purpose is to pull a wheelchair, I have the mechanical quantities with me required to turn a wheel:
Torque required is 43 N-m
Power required = 135 W
w(angular velocity)= 3.14 rad/sec

I want to use 24 v supply for the parallel combination of the dc permanent magnet motors, now first thing how to select the motors and while selecting the motors what should I do?
1) Is it correct to consider the amount of current flowing through the parallel combination of motors as equal in each motor?
2) and is sum of all the torques produced by each motor is equal to my mechanical torque required?


please help me out, thks!!!

xez

I'm not an uber-expert on using motors in applications like
this, so review my comments with a somewhat careful
perspective.

a) Using several motors electrically in parallel
or mechanically in parallel can be complicated, and I
suggest you consider your engineering of the drive and
control system carefully. I imagine that maybe you're
trying to have individual motors for all four wheels, or
maybe two motors each on each of two primary large
wheels. Anyway you mentioned having them electrically
in parallel, and it seems like that could be a bit difficult
since in the case of different wheel paths or sizes or
tractions you could have some motors working much
harder than others, or some might lose traction or stall
while others continue to work.
If you're turning, for instance the outer circumference
wheels will travel a larger distance than the inner
circumference ones. If you're making an ultimately tight
turn the inner wheel may need to rotate very little or none
at all, or may be best driven *backwards* while the outer
wheel goes forward etc. In cases where one wheel has
a bad traction or stall it can be useful to independently
detect that situation and adjust the motor drive for that
section independently since the stall would be very bad
for the motor and battery (and maybe other mechanics),
as well as having a possibly free-spinning "peeling out"
wheel accelerating to high speed might be bad too in
the case of lost traction.

b) The angular velocity specification you mention may
be relevant to some aspect of your design, though it's
not clear what aspect you refer to -- presumably motor
shaft rotation rate, and that would have whatever effect
on the driven wheels that the transmission / gear / drive
ratio would imply. Anyway the issue for the motor is
what torque / power load it must be under when going
at your specified angular rotation rate. If it has to have
the specified 43Nm torque @ 3.14rad/s at 135W electrical
input then that's certainly a set of parameters you could
look up in a motor specification graph / table to ensure
your candidate motor is suited for that operation.
Also account for duty cycle/factor, maintenance, lifetime,
heat dissipation, weight, voltage, current,
et. al. in the consideration. It wouldn't be uncommon for
a motor that delivered 3.14rad/sec @ 43Nm load to
be able to spin much faster, though, under lesser torque
loads given the same input voltage drive, though, so
you'll need some kind of speed regulation drive if you want
it to maintain that angular rotation rate depending on
all loads up to the rated maximum, and including the
case where traction may be lost and the motor is unloaded.

c) Starting torque is usually a distinct consideration
since it's easier for some motors to deliver a maximum
torque under a certain load condition whereas when they're
running at high speed or stopped they'll have a different
available torque. If your torque specification is the
minimum acceptable over all needed
ranges of motor speed that's fine, but understand that
the parameter isn't a constant over all possible speeds,
so sometimes you'll get more than you need depending
on the motor and its load / design.

d) Well if you were to literally wire the motors in
electrical parallel they'd have the same voltage across
them by definition of parallel, but the current each
takes would be depending on that motor's load and speed
and temperature and subtle variances in its construction /
winding resistances etc. relative to even nominally
identical parallel units. In general motors of the same
specification will deliver SIMILAR power/torque at
SIMILAR voltage/current levels, but don't expect identical
results, and certainly mechanical issues like friction / wear
or varying loads will cause sometimes very different
performances. Usually it's ideal to have a distinct
motor controller channel for each motor, i.e. something like
an H bridge driver, and also to have individual monitoring
of motor terminal EMF and motor current for each
motor, and perhaps also individual shaft rotation speed
sensors for each motor unless you can get the speed
data reliably enough from the EMF/current.

e) Well IF you have several motors acting TRULY in
concert then, yes, the sum of their mechanical powers
or torques will contribute to driving the load. That's
somewhat of a big IF, though, and it depends on your
drive-train 'transmission' design as well as the motor
controller electronics that are driving each of the group
to help ensure they're operating in most efficient concert.
If you're reaching the power output limit of a given motor
then it's reasonable to see how the further output of
that motor would diminish and another motor that is
ganged together with the same load would be able to
do additional work up to its limit of available torque at
the current rotation rate, etc. etc. so several motors
wired and geared truly in parallel could all contribute
to driving a load. However you'd want to be careful
in the design so that you don't end up with too much
of disproportionate power outputs among the set of
motors such that the ones that were a bit stronger wouldn't
end up doing much more of the work and always
performing near maximum capacity where others would
be under-loaded. If the design is operating conservatively
within the motors power / thermal / speed / torque
limits and the curves on them are relatively flat over
the operating envelope, and they're all quite similar
in mechanical / electrical variances, you'll probably be
able to have success with a more simple load sharing
parallel type arrangement. Though with
simple paralleling you will give up opportunities to
monitor more closely individual motor
health / performance, to reduce the usage of some motors
entirely in the case of light load conditions, to individually
control motor direction and/or speed, etc.
The individual speed/direction control can be handy
for steering, and handling turns and traction differences.

As to how to select them, well, it's not too difficult
once you know the drivetrain design, thermal limits,
duty rating, maintenance or lifetime requirements, and
the power/torque/speed/efficiency,
size, weight, cost, availability you'll need, and you
have figures for some of the key mechanical parameters.

You're looking into something around a 1/5th HP
motor, as you said, ideally DC permanent
rare earth magnet, capable of achieving the power output
and speed you specify at 24VDC input.

Whether they should include things like controllers,
encoders, be relatively more or less environmentally sealed,
et. al. is all a matter for your design to specify.

Danger

Welcome to PF, Dave.
I'm a bit confused about what it is that you're trying to accomplish. Unless you have some independent steering mechanism, you can't run your motors strictly in parallel. Electric wheelchairs steer by differential speed and/or direction of the drive wheels, and are usually set up to be controlled by a joystick.
I have 3 wheelchair motors (2 with worm-drive gear heads). They're 12VDC, 1/2 hp. Each one has 6 wires coming from it, and 2 sets of field coils inside. Depending upon how the wires are connected, you can have the coils in parallel for maximum torque, or in series for maximum speed with half of the torque. In its natural habitat, that connection option is governed by the joystick. It's almost like having a 4-speed transmission (2 forward & 2 reverse speeds).
Excellent post, Xez.

dave226

thks a lot Xez and Danger.......I 'preciate ur kind helps....

Danger

You're quite welcome.