Problem:
"U90" Motor inoperative on Unimat SL1000 and blowing fuse due to short circuit. Traced to short circuited motor suppression capacitor fitted internally to motor. Figure 1 shows the electrical arrangement of the motor.
Solution:
Physically similar motor suppression capacitor not available. Motor therefore modified to fit electrically compatible suppression capacitor external to motor case.
Figure 2 shows the general arrangement of the motor before modification.
Figure 3 shows the general arrangement of the motor after modification.
Steps:
CAUTION: Do not attempt this modification if you are not competent to carry out wiring changes on mains-operated equipment. 230/260V ac mains is LETHAL. Do not attempt this modification unless you fully understand what needs to be done - what follows is NOT a fully detailed step-by-step procedure.
1) Open motor and remove defective motor suppression capacitor. (The original mains wiring will also need to be removed and the field-coil connections to the brushes unclipped.)
2) Obtain suitable aluminium or other metal box to house new components - motor suppression capacitor, on/off switch and three-way terminal block.
3) Cut holes in box to fit over bearing boss on rear of motor case and to match original cable entry to motor case.
4) Mount box on rear of motor using, e.g. 3 mm CSK bolts, shakeproof washers, and nuts in three places.
5) Install new motor suppression capacitor, three-way terminal block and new double pole on/off switch in box.
6) Connect wires from new motor suppression capacitor to motor earth point and to solder tags on brush holders. This is a fiddly job.
7) Connect field-coil line and neutral wires to on/off switch. (May need to extend field coil wires to reach new switch.)
8) Re-attach field coil connections to the brushes. This is a fiddly job and you will need a pair of looooong-nosed pliers to refit the connectors.
9) Connect earth wire from three-way terminal block to motor earth tag.
10) Reassemble the motor. Take care not to trap or pinch any of the wires which are inside the motor.
11) Connect line and neutral wires from three-way terminal strip to on/off switch.
12) Fit AC mains line, neutral and earth leads to 3-way terminal block.
13) CAREFULLY check all wiring to ensure that it is correct and that there are no short circuits.
14) Switch on and enjoy.
DISCLAIMER. This information is a summary of how I solved a problem on my Emco Unimat U90 motor. It is not an authorised modification and I assume no responsibility whatsoever for any difficulties which may be encountered if attempted by anyone else. You are on your own.
John  McGinty
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