Setting the neutral point of a DC-motor
Setting the neutral point of a DC-motor
(OP)
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: ht tp://www.p atchn.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:
htt p://www.wm ea.net/Tec hnical%20P apers/Sett ing%20Neut ral%20via% 20AC%20Cur ve%20Metho d%20on%20D C%20Machin es%20-%20F lander.pdf
Is the method that I've learned correct?
Thanks!
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: ht
Also interresting but hard to understand because there are not so many details on the different methods:
htt
Is the method that I've learned correct?
Thanks!





RE: Setting the neutral point of a DC-motor
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
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Setting the neutral point of a DC-motor
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
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"Why not the best?"
Jimmy Carter
RE: Setting the neutral point of a DC-motor
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
RE: Setting the neutral point of a DC-motor
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