# Determine speed of DC motor

Discussion in 'Electronic Basics' started by [email protected], Mar 9, 2009.

1. ### Guest

I have a DC motor with the following specifications:

RPM @ 0 Torque: 3500
Stall Torque : 0.01 Nm
V_ss : 8V

Background information:
The motor will be used to drive a miniature monorail in a Tug-of-War
type competition.

Goal:
Since this is the only motor I can use, I want to ensure it is running
at it's maximum power. How do I determine what speed the motor will
actually rotate at? Do I have any control over this?

2. ### Guest

A tachometer (either photo-reflective or mechanical) can be used to
measure the motor's speed.

The speed will be determined by the capabilities of the power supply
(voltage & current available) and the load applied. You COULD run the
motor on 12 or 24 volts for more speed, but a the cost of shortening
the motor's life - possibly to a matter of seconds. Since the motor
is specified, do the competition rules also limit the capacity of the
power source?

In a Tug-of-War, the winner is determined by strength, not speed. You
can gear down the motor to provide less speed and greater torque.

You need to specify what rules apply to power supply,, gearing, etc.

John

3. ### Guest

The power source is fixed, I'll have no control over that.
My intention was to gear down for as much torque as I can get with the
materials provided, but I want to ensure that the motor runs at it's
maximum efficiency and I'm not sure how to determine that.

4. ### Guest

First of all, there are no competition rules. We're given a set of a
materials and told to build a monorail. That's really all there is to
it.
I was the one asking for help, so wouldn't I disclose as much
information as possible?

5. ### Rich WebbGuest

Don't take Phil's attitude personally. He seems to enjoy hanging around
s.e.b and ranting at people who ask, well, basic questions -- although
there are frequently good bits of advice amongst the flying spittle.

6. ### NobodyGuest

The speed depends upon the applied voltage and the load. If the voltage is
fixed, then the speed will decrease as the load increases. The above
figures state that, for an 8V supply, the speed will be 3500 RPM at no

For tug-of-war, you don't need power, just torque, which means that you
want the lowest possible gear ratio. IOW, this is a mechanical problem,
not an electronic one.

OTOH, if you're allowed to introduce some electronics between the power
source and the motor, then it would be an issue of finding the maximum
current which the windings can handle and constructing a constant-current
switching regulator to provide that current regardless of speed.

7. ### Guest

So what is the mathematical relationship between load and motor speed?

I want to be able to calculate the speed that the wheels will turn.
My gear ratio has not been finalized.

8. ### Bob EngelhardtGuest

There are really 2 limits for motors: a maximum speed above which things
start to come apart & a maximum current above which the windings start
to smoke.

Your motor is spec'ed at 3500 rpm, yet it's a small motor (small diam,
small centrifugal forces) so it'll probably do more. More is better:
power = speed x torque. If you have more than one of these, you could
experiment with higher speeds. Just increase the voltage.

Torque is proportional to current. So maximum current is the current at
maximum torque. Lock the rotor & adjust the current until maximum
torque is reached. That is the maximum current. Motors are designed
for a certain temperature rise. I.e., the maximum current is determined
by the temperature limit. Excessive current => excessive temp => smoke.
If your contest is short, you could run higher currents. Rated
maximum current is for continuous running.

First you need to know how much torque you can put to use. I.e., before
your driven wheel(s) start to slip. Then gear down to that torque.
In use, control the voltage to give the maximum current.

I now see that speed hasn't come into this, so maybe I'm missing
something. Oh, wait ... here's how it comes in: when your tractor is
/moving/, the gearing-down for torque & the voltage used to get maximum
current/torque could cause the motor to exceed it's speed rating. So
when it moves, you would have to monitor speed.

HTH,
Bob

9. ### Phil AllisonGuest

<
"Phil Allison"

First of all, there are no competition rules.

** Blatant lie.

We're given a set of a
materials and told to build a monorail.

** Then the limited resource of materials ARE the rules.

Imbecile.

And you have kept THAT secret.

I was the one asking for help, so wouldn't I disclose as much
information as possible?

** No Google Groper ever fucking does.

A simple matter of fact - you included.

....... Phil

10. ### Phil AllisonGuest

<

So what is the mathematical relationship between load and motor speed?

** Too obvious to even mention.

I want to be able to calculate the speed that the wheels will turn.

** I want \$1M.

My gear ratio has not been finalized.

** You are one, totally arrogant little prick

- aren't you.

...... Phil

11. ### Guest

In other words, you have no idea.

12. ### NobodyGuest

The simplest theoretical model suggests it should be linear: speed is
increases, so does the current, so does the voltage lost due to winding
resistance. IOW, draw a straight line from [email protected] to [email protected]

This assumes that either the impedance of the power supply is low (i.e.
you still get 8V at stall current) or that any voltage drop has been
accounted for in the quoted stall torque figure.

But there are enough other factors that you would be better off
measuring it.

13. ### bwGuest

Nice response, esp the temperature limits. I suspect the OP is at the Power
= torque x RPM level.

14. ### Bob EngelhardtGuest

Yeah, I had a similar feeling - that most of what I said would go right by.

Bob

15. ### Jasen BettsGuest

speed is proportional to the voltage across the motor without its
resistance.

(treat the motor as a zero resistance ideal motor in series with a
real resistor)

constant current will give you constant torque (if you can ignore
friction and sources of drag)

16. ### Jasen BettsGuest

you don't have sufficient information.

stall current (or DC resistance) would help.

also are you allowed to overdrive the motor?

what are the rules? is building a "crawler" (with extremely low gear
ratio) all that's needed to win?