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WTRVS (Industrial) (OP)
9 Oct 11 10:45
I've learned from a senior electrician how to set a neutral point for a DC-motor. Later, I heard from another technician that this method is not so accurate and therefore I would like ask if this method is correct.

1) Isolate the brushes from the commutator by inserting insulation paper between the brushes and the commutator.

2) Apply 24VAC to the field winding

3) Measure mV on the brush holders

4) Spin the brushholders until you reach a value that is closest to zero Volt

Now what I've heard is that this method is not accurate because the machine is standing still and has no load. When the motor is running under maximum load, the armature reaction results in a higher distortion of the magnetic field and therefore the adjustment of the neutral point is not correct anymore?

I've found this same method on: http://www.patchn.com/SMF/index.php?topic=312.0;wap2

Also interresting but hard to understand because there are not so many details on the different methods:
http://www.wmea.net/Technical%20Papers/Setting%20Neutral%20via%20AC%20Curve%20Method%20on%20DC%20Machines%20-%20Flander.pdf

Is the method that I've learned correct?

Thanks!
Skogsgurra (Electrical)
9 Oct 11 11:24
The neutral point is just that: neutral. And the procedure you describe is correct. Only that I usually use grid voltage (230 V in Europe) instead of 24 V AC. Makes the measurement 'sharper'.

If the motor has compensation windings, you can usually keep the neutral poin as is also when the motor is loaded. Without compensation winding (pole face windings), you may need to adjust the bridge a few mm to reduce brush fire. That is quite OK and was done routinely when DC motors were common. They are the same today and the procedures are the same.

You can also run the machine in opposite directions and adjust the bridge until you have the same speed in both directions when applying same voltage. And variations thereof.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

Helpful Member!  waross (Electrical)
9 Oct 11 11:56
Wondering about "Insulating the brushes from the commutator? We used to isolate the field and apply grid voltage to the field as Gunnar suggests. Read the voltage at the brushes and adjust for null voltage. As a second check the armature may be rotated with the voltmeter on the brushes. Any variation of the voltage indicates a shorted or open armature. Neglect possible small changes as the brushes transfer from one commutator bar to the next, damage will be indicated by the voltage rising and falling as the bad winding passes from one pole to the next.
This gives you a good initial setting. Even a compensated motor may need a little trimming under load. Move the brushes in a direction so as to tend to cover the sparks. Too much movement and sparking will start at the other side of the brushes.
You must use some judgement for reversing motors. We had some motors that only ran in reverse less than 5% of the time. The loads in reverse were often lighter. We set the brushes for forward rotation only and let them spark a little in reverse.
We had a motor which ran the same amount forwards as in reverse. The load was heavy both ways. We adjusted the brushes for the same in either direction.
The main point is to reduce, limit or avoid damage to the commutator and brushes.
Regardless of the type of motor the null voltage test is a good starting point for the initial brush setting.

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

WTRVS (Industrial) (OP)
9 Oct 11 12:28
Thank you Gunnar & Waross for your reactions.

When we were adjusting the neutral point of a DC-machine we did not succeed to get the voltage to zero (think arround 100mV) but we adjusted it as close as possible to zero. Is this acceptable or do we absolutely need 0mV. This was a Indar 900hp anti-compound machine with compensation windings, running as a motor.

(The terminals were marked A-E-H-F and J&K for the field so I assume that the H refers to compensation windings?)

Thx
waross (Electrical)
9 Oct 11 12:38
You need the nul point, which is close to zero if not zero. It's always a good idea to do a couple of revolutions to check the armature integrity. The rotation test for the same faults as a growler test.
H, more likely the series field. Compensation windings are seldom shown and the connections are not brought out. The armature connections include the compensating windings. At least in NEMA land.
 

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

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