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A DC Motor/Generator Question


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  #1  
Old 03-20-2018, 08:00:28 PM
Leon N. Leon N. is offline
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Default A DC Motor/Generator Question

Will spinning a DC generator in the opposite direction result in a reversed output polarity? Can someone refresh my memory? Reason I ask, recall back awhile ago we had a fello Staker who inadvertently hooked up his JB 12 volt battery with the positive and negative terminals reversed. However, his JB started and ran. So I presume from starters point of view it did not matter. Now what about a DC generator, does the polarity reverse if you spin it in the opposite direction?

Would this matter when hooking up one of those revolving-armature Onan machines like the old CWs or the Single cylinder JA's?
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Old 03-20-2018, 08:21:42 PM
cornbinder89 cornbinder89 is offline
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Default Re: A DC Motor/Generator Question

No, brushes are often angled for one direction only and shouldn't be reversed
Output is determined by the relation of field to armature. Most (wound field) motors and generators will work the same on either polarity. You flash the field to establish residual magnetism in the pole shoe, then the generator will generate in same polarity that is was flashed.
Starter motors will turn the same direction (again, wound field motors) regardless of polarity.
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Old 03-20-2018, 08:43:15 PM
ArodaPowerCo ArodaPowerCo is offline
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Default Re: A DC Motor/Generator Question

I think the answer is... On a DC generator, the windings rotate through the magnetic field lines of flux in one direction and are "timed" with a commutator to cause the current to flow in only one direction. There are two ways to change the polarity of the DC produced. Reverse the rotation and thus the direction the current induced in the windings, or reverse the polarity of the magnetic field the windings are passing through.

THIS might help.
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Old 03-20-2018, 08:51:59 PM
tmg50 tmg50 is offline
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Default Re: A DC Motor/Generator Question

I do not k now for sure about generators bot a shunt or series wound motor turns the same way as you are reversing both the field and armature. Usually one is reversed to change direction. If you use a permanent magnet motor reversing polarity changes direction. A permanent magnet motor will generate when the shaft is turned with polarity changing with direction.
steve
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Old 03-20-2018, 09:13:46 PM
Leon N. Leon N. is offline
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Default Re: A DC Motor/Generator Question

To clarify my question. If one has, for example, a simple shunt wound DC generator employing a commutator with brushes, with no field flashing feature, and it is spun in the opposite direction, does the output voltage polarity change?
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Old 03-20-2018, 09:35:09 PM
AngrySailor AngrySailor is offline
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Default Re: A DC Motor/Generator Question

Why not just reverse the polarity at the external connections?

Edit: no, I don't think so. The brush polarity must be shifted for polarity change if I recall

Edit 2: *brush orientation (angular) sorry for edits, pic related. That's all PEI moonshine...
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Old 03-20-2018, 09:38:39 PM
Vanman Vanman is offline
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Default Re: A DC Motor/Generator Question

If a shunt generator is driven in reverse, it will not build up, and it will not generate. The polarity of the field must be reversed with respect to the armature, just as is required to reverse the direction of rotation of a shunt motor.

This has nothing to do with the position of the brushes. Brush position is moved only slightly from neutral only on machines without interpoles. This is a compromise to compensate for the shift in the magnetic neutral plane, caused by armature current, and which changes every time armature current magnitude or direction is changes.

The purpose is to secure good commutation, and it is only useful if the machine operates at a fairly steady load. Unless a guy is standing there to adjust it every time the load changes lol.
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Old 03-20-2018, 10:10:44 PM
Leon N. Leon N. is offline
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Default Re: A DC Motor/Generator Question

With all due respect I do not think anyone so far has answered my question that makes sense to me for a couple of reasons.

WRT to Vanman's comment, the voltage excitation occurs when a coil of wire is moved through a magnectic field regardless of the direction of rotation. I do believe WRT DC motors employing a field coil, for example, the Prestolite starter which employs a series wound field coil for high torque, the only way the starter will change direction is if the polarity, that the direction of current flow through the field changes WRT the current direction through the brushes to the armature.

So another way to look at my question, if the relationship (polarity) direction of the current flow through the gens shunt field changes WRT the armature, then would not the gens output polarity change? Otherwise just spinning a shunt wound gen in the opposite direction will not change the output polarity.
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Old 03-20-2018, 10:15:36 PM
ArodaPowerCo ArodaPowerCo is offline
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Default Re: A DC Motor/Generator Question

If my electrical theory is correct, on a shunt wound generator reversing the rotation will not change the polarity because if you change the polarity of the load you also change the polarity of the field. Might do some wonky stuff as it fights residual magnetism?

Or maybe I have it backwards.

My brain hurts.
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Old 03-20-2018, 10:36:07 PM
Leon N. Leon N. is offline
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Default Re: A DC Motor/Generator Question

OK, I think I have found the answer by googling the following question:

Will changing the rotation of a DC generator change the output polarity?

Google says: The Fleming right hand rule applies. Yes changing the direction of rotation will change the output polarity of a separately excited generator because for example two negatives mutiplied together yield a positive result. That is the direction of current flow is changed through both the field and the armature as a result of reversing direction.

Now this gets interesting when talking about DC motors. Guess it means changing the polarity applied to a shunt or a series wound DC motor will not change the direction of rotation.
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