Electronic – Serial diode to universal motor and brush lenghts differs on intermittent run, why

brushed-dc-motordiodesmotormotor controlleruniversal-motor

A universal motor supposed to be run in two speed stages: full speed / reduced speed and motor will run on intermittent duty like 3 seconds ON / 3 seconds OFF.
So simply I added a serial diode (BY255) to universal motor to run the motor in reduced speed; but one brush wears out faster when other wears out very slow, why?
I suspected from switching transients but surge supressors like NTCs are not a good idea since they need cooling time, and the costs are important if you will talk about PCB circuits which has at least 3-4 components.

Additional details:

  • Source: 220 V 50 Hz
  • I measure 120 V on motor (rectifed True RMS value)
  • BY255: Medium Current Plastic Rectifier (Voltage – 200 to 1300 volt, Current – 3.0 amperes)
  • Brushes wears out fine & equal when on full speed intermittent duty
  • When i change the direction of diode, then other brush starts to wear out faster. Diodes' anode sided carbon brush wears out faster (the brush electrons flowing into, conventioal current flowing out)
  • Motor's contionous work values are 0.45 A, 83 W, 6300 RPM with light but right load.

Edit: I tried another motor with another type brush but continous duty; results are same: one brush (anode side) wears out very fast while the other brush wears out very slow.

Best Answer

Intermittent duty means a lot of high current operation, and sparking at the brushes as the motor operates at something other than its rated speed while accelerating.

Sparking occurs when the commutator breaks the current, and can be minimised by ensuring this happens when the induced voltage is zero, which happens at a different brush position depending on the motor's speed. So, the brush position on large motors is adjustable, and you have to set it to the neutral point to minimise sparking, (pdf) but possibly factory set for best operation at normal speed on your motor.

So operating the motor at other speeds, the neutral point is in a different place, and you will see sparking during starts. which will wear the brushes. Now sparks are small arc lights, hot plasma accelerating across the electrical field and colliding with the electrodes (in this case, the brush and the commutator at each end). And arcs eat anodes - twice as fast as they eat cathodes, in the linked article.

In 1876 Paul Jablochkoff, a Russian living in Paris, produced what was considered at the time to be a big breakthrough, [... in electric arc lighting.] His device was known as the "electric candle", since it involved two parallel carbon rods in an upright position separated from each other by a layer of Plaster of Paris. As the carbons were consumed, the positive rod was eaten away at twice the speed of the negative rod so alternating current was preferred to equalise the erosion. In other later arc lamps the positive carbon rod was made twice the size as the negative rod.

So at any one moment, one brush is positive while the commutator is negative, and at the other brush, the commutator is positive while the brush is negative. Now "arcs eat anodes" means the positive brush will sustain more damage than the other. (The copper commutator will also sustain some damage, but it is evenly spread as it rotates).

At full power, your motor operates off AC, so the brushes are taking turns (50 times per second) in being positive, and so the wear is even.

However a series diode upsets this fairness : one brush is always positive, and thus wears faster than the other.

So what you are probably seeing is a combination of two effects : unusually bad sparking by operating the motor through a lot of acceleration (high current and away from its neutral point) and differential erosion of the positive brush because your diode "speed controller" operates the motor off DC.

Either swap the brushes periodically, or reverse the diode every day (use a DPDT switch to make this easy), or install a kinder sort of speed control - like a triac based dimmer, though cheap ones may not be robust enough to drive inductive loads like motors.