DC motor design rules of thumb?

[BobG]

I asked this in basics, but didnt get any opinions. Trying it over here
if you dont mind.

Anyone know of a site that describes DC motor tradeoffs? I know torque
is proportional to current and turns, and there are practical problems
with wire size and resistance that limit number of turns, etc. Lets say

I have a 3 pole armature and a 7 pole armature wound with the same
number of turns and wire size. Will they have the same torque? Which
one will have greater rpm? I know the inductance goes up with turns.
Seems like there should be a rule about L/R time constant and RPM
somewhere in the rule of thumb list. Some big hefty motors cant be PWMd

faster than a kilohertz or so... is this because of large L?
I have am 8.5W motor that reads 12 ohms and 7 mH. Guess thats 1 amp
stall torque at 12 volts... but that exceeds the continous power. If
its rated at 8000 rpm thats milliseconds per rev. Kind of clobbers my
R/L time constant idea huh?

 

[R Adsett]

In article <1123613575.009787.234880@g44g2000cwa.googlegroups.com>,
bobgardner@aol.com says...

one will have greater rpm? I know the inductance goes up with turns.
Seems like there should be a rule about L/R time constant and RPM
somewhere in the rule of thumb list. Some big hefty motors cant be PWMd
faster than a kilohertz or so... is this because of large L?

That diesn't sound right. I can believe there might be a problem with
insulation on some motors but that has more to do with edge speed than
PWM frequency. Although I can see how the two might get confused.

There is an advantage to getting the switching speed above 20kHz or so in
that it get out of the audible range. Not much advantage to going much
above that since all you do is increas the switching losses unless you
have a low inductance motor.

If the power requirements get high enough you might have a problem
getting the power stage to switch that fast but that's not a motor issue
per se.

The only thing I see limiting switching frequency are motor insulation,
power stage limitations and regulatory requirements on EMI emmissions

Have I missed something?

Robert

 

[BobG]

Thanks for responding. The Curtis golf cart controllers pwm at 1.5KHz
or 15KHz... switch back and forth. Wonder what the freq selection
criteria is? Any golfers out there? Do you hear the motor whine when
its slow or fast??

 

[Martin Riddle]

Armature rpm will determine what the best PWM freq is to use.

"BobG" <bobgardner@aol.com> wrote in message news:1123625885.854226.126580@g47g2000cwa.googlegroups.com...

Thanks for responding. The Curtis golf cart controllers pwm at 1.5KHz
or 15KHz... switch back and forth. Wonder what the freq selection
criteria is? Any golfers out there? Do you hear the motor whine when
its slow or fast??

 

[R Adsett]

In article <6GaKe.1819$lT.203@trndny05>, martinriddle@hotmail.com says...

"BobG" <bobgardner@aol.com> wrote in message news:1123625885.854226.126580@g47g2000cwa.googlegroups.com...
Thanks for responding. The Curtis golf cart controllers pwm at 1.5KHz
or 15KHz... switch back and forth. Wonder what the freq selection
criteria is? Any golfers out there? Do you hear the motor whine when
its slow or fast??
Armature rpm will determine what the best PWM freq is to use.

Actually, no. The lower frequency is to allow control during plugging
(effectively driving the motor in reverse). The controller has a finite
lower limit to the duty cycle width it can apply and when that is applied
at the default frequency the motor current becomes too large to control
and possible too large for the controller to handle without failing. By
cutting the frequency in the fashion they do the maximum duty cycle is
cut to about 10% and control can be maintained during plugging.

The reason you hear it at low frequencies is that the controller must
assume that it is in plugging until it can establish otherwise, anything
else is unsafe. This is on every direction change or power cycle and on
the simple Curtis controllers those are the same thing.

They are more subtle points to consider as well but that's the gist of it
and they are not the only controller to do this.

It's nothing to do with the motor but rather control and power section
limitations. If you could provide a sufficiently fine grained PWM you
could plug at 15kHz and would not need to cut back to 1.5kHz

I don't know of any reason armature RPM would determine PWM frequency.

Robert