Mystery motor - how to measure?

[loply]

Hi folks,

I just picked this motor up at a boot sale, the seller reckons it's came from one of those kids motorised scooters, where apparently it was running off 12v DC.

I want to use it on a project I've been working on to motorise the leadscrew on my lathe w/ a PWM controller. I had been thinking about this for a while but didn't have a chunky enough motor. I kinda thought a small one from China off eBay would do if it was geared down to 60rpm or so, but I'm concerned it will result in variable leadscrew speed if there's a lack of torque, so this big one is more reassuring!

Excuse my complete ignorance but I'm not sure if voltage even matters? Are DC motors designed for a specific voltage or will it run fine on a range?

Secondly, for measuring the current I was thinking about hooking it up to a big 12v motorbike battery that I have handy. I presume the battery will be able to supply whatever current the motor requires, and then I can measure the current it's drawing so I'll know what size power supply I need to buy?

And lastly! Any suggestions about how I can find out the RPM? Is the only way with a tacho of some kind?

Cheers,
Rich

[John Rudd]

Hi folks,

I just picked this motor up at a boot sale, the seller reckons it's came from one of those kids motorised scooters, where apparently it was running off 12v DC.


Excuse my complete ignorance but I'm not sure if voltage even matters? Are DC motors designed for a specific voltage or will it run fine on a range?

Secondly, for measuring the current I was thinking about hooking it up to a big 12v motorbike battery that I have handy. I presume the battery will be able to supply whatever current the motor requires, and then I can measure the current it's drawing so I'll know what size power supply I need to buy?

And lastly! Any suggestions about how I can find out the RPM? Is the only way with a tacho of some kind?

Cheers,
Rich

I assume this is a permanent mag motor? The torque it develops is a function of voltage..What voltage range were you thinking of?
The current the motor draws depends on the load you place on the motor.

Shaft speed could be measured with a tacho..

[loply]

I guess it's a permanent mag motor, it has two wires and feels like any other DC motor I've dealt with when you twiddle the shaft 

I was thinking of running it on 12-18v depending on the current required. I have a few little 12v DC adapters lying around but mostly they only give out around 1-1.5a... I don't know if this is enough?

If not then I was planning on buying a laptop power charger which can be had up to 4/5a but are usually 15v or 18v... I don't know if this voltage would be too high for the motor since it allegedly was ran at 12v in it's OEM life?

[nearnexus]

Try running it off of a 12 volt car battery charger, if you have one that puts out about 5 amps or more.

Depending on resistance, A higher voltage will probably create more heat.

You can run some motors at a higher voltage - eg early 6 volt VW beetle starter motors can be run from 12 volt when people converted the system to the higher voltage.

To measure the RPM you will need a laser or mechanical tachometer.
 

[dickda1]

Laser tachometers are very cheap and easy to use.

Hate to keep posting ebay stuff, but this is what it looks like:

http://www.ebay.com/itm/New-Digital-Laser-Photo-Tachometer-Non-Contact-RPM-Tach-/130550327907?pt=LH_DefaultDomain_0&hash=item1e6567ea63#ht_1501wt_1002

-Dick

[John Hill]

These are my thought, which might not be worth much!!

OK, I doubt that insulation would be an issue at low voltage so the principal limitation on safe working voltage will be maximum current carrying capability and hence wiring temperature.

So, I expect you could safely hit that motor with two or more times over voltage for a short period of time, maybe very short.

I expect that if you could put a light load on the motor and increase the voltage from zero it would start and get faster with increasing voltage, at some point the rate of increase of RPM with voltage increase will slow and I suggest that would be close to maximum safe working voltage.

Safe working voltage also varies with load as a light load would allow the motor to turn faster which would mean more back EMF generated which in turn limits the current and hence the heating of the windings.  So it is all pretty complicated if you want to get technical.

For me, 12 volts is probably a good guess for that sort of motor and if it does your job at that voltage well and good.

[75Plus]

There is a possibility that you have a 24 volt motor. I recently scrapped a scooter, branded Razor, that used two 12 volt batteries in series. The speed controller circuit included voltage limiting on the LOW end. Once the battery voltage dropped below 21 volts the motor would no longer run.

Joe

[hopefuldave]

Alternative to a tachometer... attach a piece of threaded rod with a nut on it to the motor spindle , hold nut with wrench:

Imperial thread: run motor for x seconds and see how far the nut moves(Y"), multiply Y by tpi (so how many turns) and divide by x (giving turns per second), multiply by 60 (turns per minute = RPM)?

Metric thread: run motor for x seconds and see how far the nut moves(Ymm), divide Y by pitch in mm (so how many turns) and divide by x (giving turns per second), multiply by 60 (turns per minute = RPM)?

Just a thought, saves buying a tacho - although such a purchase needs *no* justification 

Dave H. (the other one)

[andyf]

You can use a "cycle computer" or speedo as a tacho by setting it so it thinks it is reading the speed in kph of a pushbike with a wheel circumference (not diameter) of 1667mm. Then, a readout of 156.7 kph indicates 1567 rpm. Surprisingly, some models will read up to 199.9kph (1999 rpm). Some models actually have an rpm function. Here's one, though I don't know its max reading:
http://www.ebay.co.uk/itm/LCD-Bike-Bicycle-Cycle-Computer-Odometer-Speedometer-/140521739875?pt=UK_Sporting_Goods_Activity_Monitoring_MJ&hash=item20b7bf9663
I'm sure the Chinese seller will also supply to the US.

Andy