EMT 930 motor
 

EMT 930 motor alignment

The motor of the EMT 930 is supplied by the mains transformer. So the motor gets always the right voltage, independent of the mains voltage. Although it runs with three phases it is connected only to one coil of the transformer. By means of a capacitor and a resistor an artificial phase is created.

The needed capacity (about 1.75µF) was measured in the factory with a special measuring unit. Then the capacity was adjusted as close as possible by some capacitors in parallel. So hum of the motor is minized.

Over the years the capacity is drifting away and you may hear an annoing hum. At the same time you may recognize an increasing motor temperature and you may feel the motor vibrating.

For an alignment is needed:
Millivoltmeter, oscilloscope, soldering iron

How to align?
ATTENTION, YOUR ARE WORKING UNDER HIGH VOLTAGE!!!
DON'T CHANGE THE ADJUSTABLE RESISTOR!!!

In case of a safety cage at the motor you need to remove it to get access to the capacitor.
Connect the voltmeter and/or oscilloscope to the output.
Place the needle into an empty groove and try to get the mains switch in a position between the left most and the middle (not marked and normally not stopping there). In this position the motor should be switched on but the platter is not rotating. Depending on the sensitivity the hum will now be visible on the oscilloscope/voltmeter.

It will normally be ok to add small capacitors to the large one to minimize the hum. But it is also possible that the capacity is already too large and you may not achieve a better value.
In this case the large capacitor needs to be replaced. Please order the complete alignment set (9 930 902 which contains the large capacitor). Then replace the large capacitor and add small ones in parallel until the hum shows a minimum value.
Don't forget to mount the safety cage again and enjoy a hum free EMT 930.

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