Thanks Max, Is this then as simple as using a 4 ohm resistor with a 12v battery to give the 3 amps? what should the wattage of the resistor be?

Michael

 

The how do you know the value of the resistor if you haven't measure the approx value of the motor, which is going to be in series with the voltage dropping resistor?
Max.

 

Sorry, I seem to be missing something. Could you explain very basically what I need to do?

I did measure the leads of the motor for its resistance (not attached to a power supply). As expected, it gave various readings depending on which position the shaft was turned to, the highest reading was 27.3, the lowest 01.2 with the DMM set to its lowest resistance setting.

Michael

 

Unfortunately this means you require a very high wattage resistor using this method so it may be more practical in this case and go with the low value of 1.2Ω.
If controlling this type of motor with any kind of drive, especially portable, it is usually optimum to either have a built in soft start, or some kind of current limit.
Max.

 

The vast majority of the readings came back in the range of 01.2 to 06.0 It was just around a particular shaft position where the resistance suddenly jumped to 16.4 then the next incremental turn to 27.3 and finally 33.7 which I should of said was the highest.

What do these resistance readings tell me about the motor?

Michael

 

Getting various readings this way is normal due to different contact levels and commutator connection from the brushes. Ensure you are not taking a reading whilst turning the shaft as the motor generates a voltage when turned.
Max.

 

Try this:

You have to measure the voltage across the motor and the current through the motor.

You HAVE to keep the AMMETER or a resistor in the circuit at all times meaning, you can;t use your DVM ammeter and then use it as a voltmeter. You need two of them.

Measuring the voltage across a resistor and using V/R will give you Amps. Your DVM ammeter has a burden voltage, that's why it has to stay in the circuit.

So, you measure I in whatever way you have to and measure Vm at the motors terminals. Vm/I is the motor resistance.

 

Thanks for your input on this. If I were to use a resistor (instead of the ammeter) what value should it be?

Michael

 

Thanks for your input on this. If I were to use a resistor (instead of the ammeter) what value should it be?

Michael

It is not the value so much as the high wattage required that makes it a little impractical if using 12vdc source. It sounds as though 1.2Ω is about right for that motor.
Max.

 

One estimate you can do is use your ohmmeter, and measure the resistance of the motor, but subtract the value of the leads shorted.

Use a resistor about 1/4 to 1/2 the value and oversize the power of the resistor.

Ideally, a variable power supply would be the best way.

 

Max, could you explain how you worked out that I needed 1.2 Ohm resistor?
I can use a lower voltage battery supply if that would work better

Michael

 

That was your reading of the armature of 1.2Ω, if you use a resistor to limit 12v to ~3.amp, then you would need a 4Ω-5Ω series resistor.
Lower voltage would be better, as long as it supplies the current.
Max.

 

I understand the part about needing a 4 or 5 Ohm resistor with the 12v battery.
But what is the significance of knowing the 1.2 Ohm armature resistance?

Michael

 

In order to calculate the total current for ~3amps.
Max.

 

Just buy a bunch of 1 Ohm 5 watt resistors, you can use them in series to adjust the current.
Measure the voltage across them individually, the reading will be in volts-per-amp, since it's one ohm.

You can see why the DMM resistance measurement is kinda useless, the readings are all over the place.
The high current reading with the resistors will be way more accurate.

Keep it simple.

 

Just use a 10Ω 5-10w resistor and a 12v supply!.
Just be an amp or so.
Max.

 

Thanks Sensacell.

Ok Max, could you just explain how I set the circuit up with my motor to test?

Thanks

Michael

 

The motor looks like it could have been originally used in a car HVAC blower. Why not just tell us where you bought it and maybe we can gather more information from that? A link to the place you bought it?

 

Hi shortbus,

I attempted to find the spec of the motor before posting here, but have had no luck. The supplier is Hill House Products. The 12vdc motor is one of a pair of throwing motors from a tennis ball machine which uses 2 counter rotating wheels to fire a ball fed between them (a motor on each wheel). I intend to use them for the same purpose, but need to find the stall current so I can identify an adequate motor driver.

Michael

 

Thanks Sensacell.

Ok Max, could you just explain how I set the circuit up with my motor to test?

Thanks

Michael

If you decide to use a 10Ω resistor, connect it in series with the motor from a decent 12vdc source, Ideally you would have two meters, one across the supply, the other across the resistor, but if only one, measure the voltage across the resistor at different points on the comm, with a locked shaft.
If only one, measure the voltage across the 10Ω and move the shaft to a position that gives maximum voltage across the resistor, at this point, record the reading and also the actual voltage at source.
From Ohms law the current can be calculated from the voltage across a 10 ohm resistor, the motor resistance can then be seen as the difference, to produce that current.
For e.g. if the resistor is 10 ohms, and you measure the highest drop across the resistor as 10v, and the supply is = 12v then the resultant motor resistance is (12v - 10v )/1a = 2Ω.
For the above the 10v across a 10Ω resistor = 1amp circuit current.
You realize the motor will only see the zero rpm current momentarily at switch on, for a very brief moment, unless stalled by load.
If you go into full reverse, then it is a whole new ball game.
Max.