Any way to find out safe operating specs on unmarked DC motor?

Other Hardware General Electronics
1

Hello,

I have an unmarked DC motor of which I would like to determine the safe operating specs.

Any way to calculate the rated/max Amperage/Voltage, without any information to start with?

I happen to know that it's rated at 3 - 6V, can I at lest use that to calculate max Amperage so that I don't fry it?

2

I happen to know that it's rated at 3 - 6V, can I at lest use that to calculate max Amperage so that I don't fry it?

You can't fry it unless you apply excess voltage.

[u]Ohm's Law[/u] says current depends on voltage and the resistance (or impedance) of the load. We rarely know the resistance, but we usually know the current rating.

If you have a multimeter you can measure the resistance and calculate the "stall current" or "start-up" current. Once the motor starts running, the resistance increases and current drops.

If the battery or power supply can't supply the rated current, the voltage will drop and the motor may not run. And, you could fry the power supply.

3

And, if the power supply can't deliver the start up current, but can keep the motor running, then a capacitor -- a very large -- capacitor -- in fact, a Super Capacitor -- can supply the current during start up. But, this won't take care of long stalls. So, its a matter of application.

4

There's no way to calculate it without a lot more specifications from the datasheet.

You can just try it out at different voltages and see what happens. Measure the temperature of the motor. If it gets too hot* then reduce the voltage.

*too hot is also something that the data sheet should give you. Motors can run hot - hot enough to fry an egg without frying the motor. But there are temperatures where either the glues holding it together fail or the magnets in the motor demagnetize.

5

MorganS:
There's no way to calculate it without a lot more specifications from the datasheet.

You can just try it out at different voltages and see what happens. Measure the temperature of the motor. If it gets too hot* then reduce the voltage.

*too hot is also something that the data sheet should give you. Motors can run hot - hot enough to fry an egg without frying the motor. But there are temperatures where either the glues holding it together fail or the magnets in the motor demagnetize.

Yes, and once you find a voltage where the motor seems to run OK, without getting "too hot", then put a current meter in series with the motor and the power supply, and then stall the motor -- i.e., stop the shaft from turning. This, of course, may be difficult for larger motors--AND DANGEROUS!! So be careful!!!. The current you read [assuming you are using a power supply that can deliver enough current], will be the "stall current", and this will be the maximum current the motor will ever need [at that voltage], so whatever you use to actually drive that motor [in it's target application], will need to be able to deliver that current.

6

DVDdoug:
You can't fry it unless you apply excess voltage.

First off, thank you for the information. I thought I read somewhere that excess amperage could cause the coils inside to melt; or can I not get excess amperage without producing excess voltage?

MorganS:
You can just try it out at different voltages and see what happens. Measure the temperature of the motor. If it gets too hot* then reduce the voltage.

Thank you for the info. Mind elaborating on what "too hot" is? It gets just a tad warm, but there is a smell when it is running. Not quite burning smell, but ... grinding gears, maybe?

ReverseEMF:
Yes, and once you find a voltage where the motor seems to run OK, without getting "too hot", then put a current meter in series with the motor and the power supply, and then stall the motor -- i.e., stop the shaft from turning. This, of course, may be difficult for larger motors--AND DANGEROUS!! So be careful!!!. The current you read [assuming you are using a power supply that can deliver enough current], will be the "stall current", and this will be the maximum current the motor will ever need [at that voltage], so whatever you use to actually drive that motor [in it's target application], will need to be able to deliver that current.

Thank you for your reply. At 6V, I'm getting about .60A with no load, and .80A when stalled. If I run the motor through a transistor, and a Pro Mini can only supply about .40A per pin, could that blow a Pro Mini board?

In the past I've had really bad experience with Pro Mini and DC motors, to which I still haven't figured out the problem. I would very much like not to destroy another board.

Any tips on connecting DC motor being controlled by light sensor using Pro Mini, both powered with the same power supply?

Thanks all for your inputs!

7

toxicxarrow:
I thought I read somewhere that excess amperage could cause the coils inside to melt; or can I not get excess amperage without producing excess voltage?

If you run the motor at its rated voltage, it should never draw enough current to damage the internal wires--even when the motor is stalled. Run it at a higher voltage and all bets are off. If you don't know what the rated voltage is, then careful experimentation is the way. And, that puts you into "too hot" territory--as in, if the motor gets too hot, the voltage is too high [see below].

toxicxarrow:
Mind elaborating on what "too hot" is? It gets just a tad warm, but there is a smell when it is running. Not quite burning smell, but ... grinding gears, maybe?

See post #4

toxicxarrow:
Thank you for your reply. At 6V, I'm getting about .60A with no load, and .80A when stalled. If I run the motor through a transistor, and a Pro Mini can only supply about .40A per pin, could that blow a Pro Mini board?

A pro-mini can safely supply a MAXIMUM of 40mA on each pin [that's 0.04A -- NOT 0.4A], but the datasheet specs everything at 20mA. So, no hope of driving that motor directly from one of the Output pins. Also, I forget what the max current is that the ProMini's onboard regulator can deliver, but it has safety features that should prevent damage, should it not be able to supply that 800mA [and I doubt the regulator can]. So, you will need to use a transistor to drive that motor, and you will, probably, need to power the motor from a separate power source.

toxicxarrow:
In the past I've had really bad experience with Pro Mini and DC motors, to which I still haven't figured out the problem. I would very much like not to destroy another board.

Any tips on connecting DC motor being controlled by light sensor using Pro Mini, both powered with the same power supply?

If you're connecting the motor(s) directly to the ProMini outputs, then you're destroying them because you're exceeding that 40mA limit. Otherwise, it could be due to spikes. Check these out for how to safely drive motors--substitute your actual motor voltage [6V right?], for the voltage indicated:

8

ReverseEMF:

I appreciate the info, thanks a ton!

9

toxicxarrow:
I thought I read somewhere that excess amperage could cause the coils inside to melt; or can I not get excess amperage without producing excess voltage?

And to elaborate: Yes, excess amperage will, indeed, melt the "coils" inside the motor. But, this should never happen at the specified motor voltage, as long as nothing else goes wrong--such as motor windings shorting together.

10

ReverseEMF:
And to elaborate: Yes, excess amperage will, indeed, melt the "coils" inside the motor. But, this should never happen at the specified motor voltage, as long as nothing else goes wrong--such as motor windings shorting together.

Thanks! I think I got it; because of ohms law/ math stuff, because voltage and amperage relate to each other, as long as the voltage is within range, even at stall load the motor wont exceed its capable amperage... Does that sound right?

11

toxicxarrow:
Hello,

I have an unmarked DC motor of which I would like to determine the safe operating specs.

Any way to calculate the rated/max Amperage/Voltage, without any information to start with?

I happen to know that it's rated at 3 - 6V, can I at lest use that to calculate max Amperage so that I don't fry it?

At a first guess, measure the winding resistance, and calculate the stall current. Assume the nominal
current will be 10% to 15% of the stall current. For smaller motors may be more like 25%.

The current is usually limited by heat dissipation, so watch out for overheating.

The voltage limit is set by the mechanical maximum speed (ie the speed at which the bearings and
commutator start to wear at accelerated rate) - this is harder to guess, but in general larger motors
run at lower rpm, so 15krpm for a little toy motor, 6krpm for something you can hold in your hand,
3krpm for larger motors.

Another key spec for a DC motor is the motor constant, the ratio of torque to current (or speed to
voltage) is called the motor constant k, measured in Nm/A or V/(rad/s) [ the values are the same
in SI units ].

This is something you can measure by running without a load and measuring the rpm with an rpm
meter. speed in radian/second = pi * rpm / 30

12

ReverseEMF:
If you run the motor at its rated voltage, it should never draw enough current to damage the internal wires--even when the motor is stalled.

An interesting theory - in practice this is how you thoroughly cook a motor. What compounds
the thermal dissipation issue is that motors rely on airflow to help cool them and at stall there is
no airflow.

13

MarkT:
An interesting theory - in practice this is how you thoroughly cook a motor. What compounds
the thermal dissipation issue is that motors rely on airflow to help cool them and at stall there is
no airflow.

Well, would that be a poorly designed motor? I guess it depends on how it's meant to be used, huh?

14

ReverseEMF:
Well, would that be a poorly designed motor? I guess it depends on how it's meant to be used, huh?

No, its an efficient motor. Normally (in anything but toy motors) temporary overload is tolerated
and a thermal sensor cuts out the power if overloaded or stalled for too long (often these days
the temperature is modelled in software to same effect).

An efficient motor will convert > 90% of the input electrical power to mechanical power, only a small
fraction heats the windings. For that to happen the winding resistance has to be a small fraction of
the nominal voltage rating / nominal current rating, typically 10% or less.

If that is the case the stall current is 10 or more times the running current, so the dissipation in
the windings at stall can be 100 times the normal running dissipation, with no air flow cooling. There
is no realistic way that indefinite stall can be tolerated in a well designed general purpose motor.

15

MarkT:
An interesting theory - in practice this is how you thoroughly cook a motor. What compounds
the thermal dissipation issue is that motors rely on airflow to help cool them and at stall there is
no airflow.

So, any decent motor, if stalled at max voltage for too long is basically screwed? What if we halve the max rated voltage? Is there any scenario, hypothetically, where you can indefinitely stall a motor while applying continuous voltage and have the motor turn out fine after a certain amount of time; days, weeks, years, decades?

16

Lots of scenarios. Number 1, run a lower voltage. 2, choose a motor many times larger than you need. 3, add a cooling fan.

17

MorganS:
Lots of scenarios. Number 1, run a lower voltage. 2, choose a motor many times larger than you need. 3, add a cooling fan.

Good to know, thanks!

18

The only practical method of determining the capabilities of an 'unknown' motor is to connect it to some form of dynamic brake then measure rotation speed and applied torque for various voltages. The resultant curves will show a 'humped' form the peak of which is it's maximum efficiency at each measured variable.

19

jackrae:
The only practical method of determining the capabilities of an 'unknown' motor is to connect it to some form of dynamic brake then measure rotation speed and applied torque for various voltages. The resultant curves will show a 'humped' form the peak of which is it's maximum efficiency at each measured variable.

So that's what those alien-like graphs mean...

20

toxicxarrow:
So, any decent motor, if stalled at max voltage for too long is basically screwed?

Yes..... it is like sticking a finger into hot water momentarily...... not too long. Too long.... and there will be scolding .... damage.