Brushed 8.4V DC motor help

Please see the attached photos.
I have a DC motor that will not start when power is applied.
However with power still applied if I spin the motor with my fingers it will start running just fine.
All of this is at no load, and tested with a fresh 9V battery.

This is a new motor, and the brushes have only a minor curve to them. If I could get the motor to start I would invest the time to run the motor to get the brushes “broken in”

My meter measures 1.5 -2.0 ohms between the pos and neg for all rotor positions. I do notice a cogging when moving the rotor with no power applied.
I can’t figure out why this motor won’t start. Again this is all with no load, and I am just holding the motor. Its gets warm but not hot. I also only run the motor for maybe 10 seconds just to be sure its not slowing down.
Any troubleshooting tips or suggestions are appreciated.

motor_x_section-a.JPG

additional photos

additional photo

Hi.

Lack of power. What kind of power supply did you apply on it ?

I=V/R

8.4v/1.7ohm=4.9amp. ???

But you need to see any label with model reference on the motor to check for sure.

I can't figure out why this motor won't start.

I would think that 9v battery can't supply sufficient starting current for the motor.

I pressed that 9V in the picture directly to the terminals to test it.

I believe that a NEW 9V alkaline battery is able to provide the starting current for a motor with NO LOAD.( I have tried 2 different 9V now)

I realize there is an inrush current because there is no back EMF at 0 RMP but I have now tried it with a 12V car jumpstart battery and the same result. It did spin much faster when I spun it with my finger.

There is no name plate or model number on this motor.

any suggestions where else I could find help?

These motors are designed for NiMH packs able to supply many amps, a 9V battery is just hopeless at powering them.

12v jumpstart battery.

what current did it give , are you sure the batteries are not flat

OMG are you kidding me. a 12V car jump start battery is more than enough. PS, the 9V is more than enough to start it.

I am going to throw this out there. Anyone have any other suggestions except that the battery isn't able to provide the starting current.

I am open to suggestions to post this elsewhere, if you can suggest another forum

Your 9V battery will not supply enough current to run your motor.

Try connecting three or four 9V batteries in parallel, and see what happens.

Here is an example datasheet for a 9V battery. These batteries contain 6 or 7 tiny cells, each cell smaller than an AAA battery.

http://data.energizer.com/PDFs/522.pdf

You will see that the capability of these batteries to provide current, maxes out somewhere between 200 mA and 500 mA.

And here is a datasheet for the sort of non-rechargeable 9V Duracell battery that you appear to have:

http://ww2.duracell.com/media/en-US/pdf/gtcl/Product_Data_Sheet/NA_DATASHEETS/MN1604_6LR61_US_CT.pdf

You'll notice that the graphs show how long the battery will last for power outputs between 10 mW up to 1000 mW, and in the 1000 mW case, not long.

You'll also notice that the graphs show how long the battery should last for current outputs between 2 and 250 mA.

You'll also notice that the graphs show how long the battery should last, delivering current into a load resistance of 100 to 1000 ohms.

You say your motor has a winding resistance around 1.5 to 2 ohms. Assuming V=IR applies ( which is debatable ), you might need a current of 4 to 5 amperes.

Your Duracell battery is incapable of providing that.

You will also see that the last graph shows a resistance for the battery starting at about 2.3 ohms for a new battery and increasing subsequently.

You can therefore estimate the current that will flow in your circuit, and the apparent voltage applied to the motor.

Battery terminal voltage = 9V - I x R1 where R1 = 2.3 ohms, battery internal resistance. Load current I = Battery terminal voltage / R2 where R2 is the motor resistance, say 2 ohms.

You can solve these equations for the battery terminal voltage and the current, and you get

I = 2.09 A Battery terminal voltage = Motor terminal voltage = 4.18 volts. if you disregard the resistance of the connecting wires.

4.18 volts is probably not enough to create the magnetic fields required to make your motor start spinning.

badgers: OMG are you kidding me. a 12V car jump start battery is more than enough.

Not if its flat. It may measure 12 v but once connected it could drop to 3 v depending on battery type.

Read last few posts regarding battery resistance.

At these currents a bad connection will show the same effect. Does not matter how big it is if its flat it will behave like your pp3

edit if your jumpstart battery reads 12 its flat. it should read 14.6 V

sec ond edit.

if its a drycell emergency type they can be up to 30V but at 500ma it still wont turn your motor

Boardburner2: It may measure 12 v but once connected it could drop to 3 v depending on battery type.

If it's one of those "Harbor Freight" cheapy units, it's likely a 12 volt SLA of some sort; if it's ever been used for jumpstarting a car, or if it hasn't been properly stored or charged, then yeah - it's likely to sag under any kind of real load.

Best way to test it would be to get a battery load tester, connect it on, and switch it to load the battery down (usually done with a heater element inside the unit); you'll quickly know if it is good or not.

I think the real problem that the op is having is understanding that motors run on amps not volts.

you people are killing me. its so not the battery and it is amazing that everyone thinks that.

its like a group think experiment, everyone keeps convincing themselves its inadequate battery. This isn't a huge motor. Come up with something original. its just not the battery voltage drop under load is to great to start a motor free spinning.

PS its a perm magnet stator.

Come up with something original. its just not the battery voltage drop under load is to great to start a motor free spinning.

Well, maybe the motor just doesn’t like you. Nobody but you has your motor (which could have some type of internal electronics, picture doesn’t tell anything), your batteries (which you probably haven’t tested for volts/amps while under load), and your test equipment (which you may/may not know how to use). Your attempt to run the motor using a 9v transistor battery raises questions.