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Wednesday, November 8, 2017

Beogram 4004 (5526): Restoration of DC Motor - Oil Infusion of the Bearings and New Spark Snubbers

The DC Motor of the Beogram 4004 (5526) that I am restoring right now was in need of restoration. It seems almost all Beogram DC motors need this treatment at this point in time. First I did the usual oil infusion of the dried out Oilite bearings. This requires completely disassembling the motor. This shows the motor extracted from the enclosure:
And taken apart:
The bearings are the two small 'donuts' on the back pad. I immersed them in motor oil and pulled a vacuum:
Immediately, bubbles emerged from the bearings indicating leaving air to make room for oil that was now able to diffuse into the bearings. After about 48 hours this process finally stopped, indicating that this Beogram DC motor had very thirsty bearings (often it is already over after 24 hrs). I installed the bearings in the motor housing. I used a specially designed tool to press in the ring that holds the upper bearing:
This shows the bearings back in place:
After completing assembly, it was time to give this motor a 24 hrs RPM stability test using the BeoloverRPM device:

Aside from precisely adjusting the RPM, the BeoloverRPM also allows the capturing of the RPM in 10s intervals during extended periods of time. The blue curve in the graph below is what I measured over approx. 24 hrs after the oil infusion:
While not too bad in terms of absolute deviations (they would not have been audible to most people with 'normal ears') this curve was not acceptable. The downward spikes indicated that the spark snubbers of this motor needed replacement.

I took the motor apart again and extracted the rotor again:
The three yellowish 'devices' around the rotor are the spark snubbers. Essentially they are semiconductor devices that become conductive after the voltage across them exceeds a certain value.  They are connected in parallel to each of the three coils of the rotor. It appears that they fail in an initially intermittent way, while usually culminating in a full short circuit at the end of the decay process. The latter causes the motor to be very weak, while the former causes occasional RPM drops, similar to dry bearings. I removed the snubbers ring and installed new modern TVS diodes (small black blocks around the shaft). These diodes need to as close as possible to the coils to not interfere with the brushes when the motor is assembled:
After assembly I tested the motor for another 24 hours and the result was much better (red curve in the above graph). This motor is in business again!








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