DC Generator brush neutral

[TheElevatorman]

Brain fart..
So I know how to set the neutral. Showing a new apprentice. Explain that we place the brushes to the minimum voltage point to reduce sparking. Made a drawing showing the conductors cutting through the flux... and he asks... "How do you get voltage out if it's always set at the zero point..."
Ummm.... never thought that far I guess.
Tried searching before posting, but can't find an explanation on how it works.
Teach this old dog a new trick

 

[BrianE22]

When you say "zero point" are you referring to the geometric zero or the electrical zero? You can only get an approximate electrical zero because the conductors are separated by small amounts so not all of them can be moving parallel to the flux lines.

 

[TheElevatorman]

Electrical neutral.
I know it won't be zero volts, but we move rigging to get the lowest voltage

 

[dArsonval]

Calling out a D.C. machine's mechanical neutral position or neutral plane setting as "Zero Point"
is an erroneous description or term. One may be seeking the lowest reading on a volt meter, but
the indication is never actually zero.

It can also be noticed while setting brush neutral that the reading can shift as the brush rack/rigging is tightened and secured.
Additionally, after setting the position, one can turn the machine's shaft and note yet a different meter reading.
Making certain the brushes are fully seated and intimately mating with the commutator are also necessary prerequisites
for positioning an "ideal" setting.

John

 

[xnuke]

See the discussions on pp. 18-19 in this Link. I think it gives a good explanation that answers the original question as to how output voltage is still produced continuously even though there is minimal (ideally zero) voltage across the adjacent commentator segments covered by the brushes at the moment of commutation.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

I used to know this one.
The difference between a generator and a transformer.
In a generator the EMF is developed by a conductor moving in a magnetic field.
In a transformer the EMF is developed by the change in the strength of the flux.
The difference between relative motion and varying field strength is such that the relative maximum EMFs are developed at 90 electrical degrees to each other.
In a simple example, the null point of generator action will correspond to the maximum EMF point of transformer action.
And the reciprocal, the point of maximum developed EMF of generator action will correspond to the point of minimum developed EMF.
So maximum voltage as a generator corresponds to minimum voltage when excited by AC and acting as a transformer.
Null point or zero point?
Simple theory tells us that there will be a zero point.
Why can't we find it?
It is a point, and the commutator segments and brushes are not a point.
The zero point is most likely midway between two commutator segments. You cannot find it with the brushes.
You may be able to find it by attaching voltmeter leads directly to opposite commutator segments.
Or:
Construction details, either design or faulty construction or non uniform materials may result in an non uniform magnetic field.
The windings on the left may not be reaching zero at the exact same point as the windings on the right.
At the null point the brushes may be in contact with two commutator segments.
This may be responsible for a slight variation as the rotor is turned.
The fewer poles and commutator segments the greater this effect.
A large variation as the rotor is turned:
I have used the null point test successfully as an "In Situ" growler test.
Set up for a brush neutral test. Leave the brushes fixed. Turn the rotor.
Any shorted or open windings will show up as a large meter deflection.
Short circuits and why do we need compensating poles?
Current in the rotor will set up a magnetic field at right angles to the main field. This field is normally much less than the main field, but the result is that the direction of the flux through the rotor is deflected and with it the null point.
The greater the load, the more current, the stronger the resultant field and the greater the effect.
The result is more sparking to the point of arcing at the brushes.
Commutating poles or inter-poles develop a third flux field to counteract the reaction field and restore the null point to the proper position.
A short circuit will generate a magnetizing force much greater than the original field.
This is how an an Amplidyne works.
The brushes are shorted.
A small current through the field will develop a strong magnetic field at right angles to the original field.
As the rotor cuts this second field, a voltage is induced. A second set of brushes at 90 degrees will is used and this power may be used to power a small motor or to excite the field of a generator driving a much larger motor.
In one model that I remember, zero to plus or minus 2 Watts of power into the field will control 1500 Watts of output.
In a normal generator this effect will be limited by saturation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Bill West]

Hi Elevator. I think your apprentice is being caught in a word trap over the commutator bars. The desired zero voltage is between the bars that are under a given brush. The desired load voltage is between the bars at one brush and the bars at the next brush.

Because the brushes bridge at least two bars they are shorting a small portion of the armature winding. Placing the bushes at the point where the net field is producing little or no voltage in that portion of the winding minimizes the shorted current and thus minimizes the sparking.

Bill

 

[waross]

Sorry Bill.
You are correct that there is little or no voltage between the bars that are under a brush.
That is not the question.
When the generator is running, there is maximum voltage between opposite polarity brushes.
When the generator is stationary and excited with AC instead of DC there is little or no voltage between brushes of opposite polarity.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[TheElevatorman]

Waross, that is almost the question I was trying to ask.
All our generators are wound compound, with a series and shunt field. All voltages are DC (with the exception of the AC motor spinning the DC armature) We are taught that the place where the armature windings are at 90 degrees to the field flux is when the least amount of voltage is produced. That has been explained to us as "neutral" since the winding is not cutting a line of flux, but moving parallel with the lines. (Since the hoist motors are bidirectional, a shortcut that "some" techs use is shifting the generator neutral to get more or less voltage to the hoist motor to obtain a particular speed.)

If the explanation that we were given in class is the brushes, through the commutator, are connected to windings that are parallel to the flux and not producing voltage.... how is voltage produced at the brushes when a stronger generator field is provided.

Thanks all

 

[waross]

Your motor will also have interpoles. These are almost never shown on diagrams and are of the armature circuit.
A motor diagram will typically show the armature as A1 and A2 or A+ and A-.
If you look at a motor closely you will see that one of the A leads goes to the brush gear. there is a connection from the brush gear to the interpole and the lead from the inter pole is the other A lead.
As current through the armature increases, the flux field is distorted and the zero voltage point shifts.
The inter poles counteract this shift to keep the zero voltage point under the brushes.
Don't confuse the interpoles with the series poles. The series pole windings are wound on the same poles as the shunt windings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

how is voltage produced at the brushes when a stronger generator field is provided.

Answer:

are connected to windings that are parallel to the flux and not producing voltage

If the field strength is doubled, or 5 times or 10 times;
0 x 2 = 0
0 x 5 = 0
0 x 10 = 0
When you are talking about brushes and zero volts. don't confuse adjacent brushes with opposite brushes.
Don't confuse DC excitation with AC excitation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Apprentice asks:

How do you get voltage out if it's always set at the zero point..."

Would I be wrong in suggesting to look at the drawings of a simple machine in a motor or generator textbook? I see a drawing that shows the armature windings being in series with each other, and the voltage they generate being cumulative; the voltage from one side of the commutator to the other [180° around the commutator] is therefore at maximum, whereas the voltage between adjacent commutator bars is nearly zero...or am I missing something?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

cr, The difference is the excitation.
With DC excitation and a stationary rotor there is no voltage anywhere.
When the rotor turns the flux is cut by the windings and a voltage is developed EXCEPT in the windings that are moving parallel to the lines of force.
With AC excitation and a stationary rotor the induced voltage is by transformer action and there is a null point where there would be maximum voltage developed by generator action.

DC excitation and rotation.
AC excitation and stationary.
Apples and oranges?
Potatoes and fish?


Bill
--------------------
"Why not the best?"
Jimmy Carter