I have a 200 v 2.5 amp brushed dc motor that the field magnets had parted ways with the housing! I did not record the orientation of the magnets when I disassembled it. There are two magnets on each side so when I reattached them I arranged them n/s/n/s and put a North Pole and South Pole on each end. The spec. for the motor says 2k rpm but I get 5k at only 50v! Haven’t increased the voltage beyond that. What have I done wrong?
Thank you for any assistance as I am lost.

 

Hi,
Giving the motor type/model and pictures will boost forum response.
Good luck............

 

Some types of motors can have a runaway effect with improper voltage, high or low. But yes, we need details on the motor.

 

The cat. No. Is 9473-70039. There is no manufacturer name so I was unsuccessful at tracing that number.
hp-.45kw
rpm-2000
frame-56cz
arm volts-200
amps-2.5
type-3327p
encl.-IP55

 

Here is a guess: You changed the fields made it a Four POLE motor instead of a Two pole motor. But only partly. because there are still only two brushes.So while the speed has increased, probably the torque has not. The field alignment is very important on brushed motors.

 

Here is a guess: You changed the fields made it a Four POLE motor instead of a Two pole motor. But only partly. because there are still only two brushes.So while the speed has increased, probably the torque has not. The field alignment is very important on brushed motors.

Thanks MisterBill,
please excuse my ignorance but what would be the correct orientation of the magnets? There are four, two on each side but I don’t know how they should be aligned.

 

Here is a guess: You changed the fields made it a Four POLE motor instead of a Two pole motor. But only partly. because there are still only two brushes.So while the speed has increased, probably the torque has not. The field alignment is very important on brushed motors.

Agreed that this is a likely scenario.
But it could be backwards. A two pole motor should run twice as fast as a four pole with the same armature voltage.

 

Agreed that this is a likely scenario.
But it could be backwards. A two pole motor should run twice as fast as a four pole with the same armature voltage.

I believe that you are correct! And it appearsthat I got it wrong. Which proves that I am not infallible. BUT I already knew that.
So it may be that the way the magnets are positioned now created much wider poles.
Somewhere I have a motors textbook that goes much farther into motors than I have ever been. Probably somebody else already knows all of that and can explain.

But now, looking back and seeing post #3, it becomes clear that what happened is a case of the lost field effect, where aserious reduction in the field intensity leads to an increase in armature current and a runaway condition. So the way the magnets are now positioned REDUCED THE MAGNETIC FLUX, which reduced the effective back EMF, which allows the armature current to increasetowards runaway speed.
Many Thanks toB-JOJO-S for the reminder.

 

So the way the magnets are now positioned REDUCED THE MAGNETIC FLUX, which reduced the effective back EMF, which allows the armature current to increasetowards runaway speed.
Many Thanks toB-JOJO-S for the reminder.

I only know it happens, though I still don't understand why. I read it when perusing a manual on the Falcon-50 Jet. I had some free time years ago and was into aviation. At one point (and probably still do) I had a poster of the cockpit. That was way back when a 747 flew out over Long Island Sound and suddenly burst into flames. Flight 800.

 

I only know it happens, though I still don't understand why.

In a DC motor , the shunt field winding provides the motor's magnetic field. If the field becomes disconnected, the main magnetic flux drops just the residual remains.
This results in the counter EMF just about disappearing so only the residual remains.
What happens then is a drastic drop in field flux and a drop in BEMF and rapid increase in armature current, Hence dangerous run-away condition.
Most large DC motors operating heavy machinery have field loss protection & shut down. in case of this occuring

 

I believe that you are correct! And it appearsthat I got it wrong. Which proves that I am not infallible. BUT I already knew that.
So it may be that the way the magnets are positioned now created much wider poles.
Somewhere I have a motors textbook that goes much farther into motors than I have ever been. Probably somebody else already knows all of that and can explain.

But now, looking back and seeing post #3, it becomes clear that what happened is a case of the lost field effect, where aserious reduction in the field intensity leads to an increase in armature current and a runaway condition. So the way the magnets are now positioned REDUCED THE MAGNETIC FLUX, which reduced the effective back EMF, which allows the armature current to increasetowards runaway speed.
Many Thanks toB-JOJO-S for the reminder.

Thanks for the lesson on theory, I don’t have any knowledge or experience with emf or bemf so can anyone help me on the orientation of the field magnets? There are two on each side(4 total) and no space between a pair so in the center it is a north-south union which doesn’t make sense to me to get a clean north-south pole arrangement.

 

Is there any chance you can post a pic of the motor internally when the magnets are positioned or better, removed?