Electrokinetica The Electro-mechanical Museum

Small appliance repair area

Fractional horsepower motor repair

Motor as found

Motor as found

Finished and working

Finished and working

This small Italian-made induction motor has seen plenty of action! It was found in need of a strip and cleanup having been used as a makeshift bench-grinder by mounting an abrasive wheel on its shaft; an arrangement that had ensured lots of dirt and grit had got inside, particularly into the bearings. These are sleeve bearings lubricated by oil rings; two little wells in the end bells are supposed to hold a supply of ordinary mineral oil although what we found in them was more like lapping compound. You won't be surprised to hear there was quite a bit of clearance in the bronze bushes; they will need replacement soon because the rotor almost bottoms out in the stator, so far has the shaft worn through them. There is also a disadvantage in running any machine with the rotor out of centre, i.e. in this case with unequal airgaps above and below the rotor; the magnetic flux will be unequal at the various poles and a loss of torque and efficiency will result. The motor actually runs very well off load, and is very quiet so long as a small amount of side load is applied to the shaft. The insulation is excellent and it will make a useful drive for something in the future. If we find a job for it, we will probably put new bearing bushes in first. One job it certainly won't be used for is grinding! In the meantime the oil has been drained from the wells to prevent it getting spilled into the windings. Oil-ring bearings are strictly 'right-way-up' devices!


The internals in detail

Stripping down motor

Stripping down motor

Cleaned ready for reassembly

Cleaned ready for reassembly

The motor is of single-phase squirrel-cage type, with a copper cage winding on the rotor and four-pole stator. Split-phase starting with centrifugal switch (a.k.a. switch-start) is used, for which the separate coils of the start and run windings are easily seen with the motor dismantled. The pairs of coils physically overlap and create a rotating field by presenting to the supply different ratios of resistance to inductance, causing the currents in the two circuits (and their resulting fluxes) to differ in phase. The starting switch is of the 'three brush' design: The rotor carries three pivoted brush arms, held inwards by a spring but able to fly outward when running at speed. These arms are interconnected but isolated from the rotor. The supply to the start winding is fed through a type of stationary commutator with two segments, mounted on the end bell, against which the brush arms run under their spring tension. With three brushes and two segments, there is always a path from one segment to the other via two of the brushes whilst the motor runs up to speed. The arms then fly out away from the segments (often emitting sparks in the manner of a catherine wheel) and disconnect the start winding. On test, the starting switch cut out rather too early, before the motor had reached a good fration of its running speed. The springs on the brush arms, which had looked rather lazy to start with, were tightened further making the start more satisfactory with load applied.

Starting switch detail

Starting switch detail

Identical motor in catalogue

Identical motor in catalogue

Partially reassembled

Partially reassembled

Tracing the motor's origins

The motor does not bear a makers name and may therefore have been made for an importer. The holes in one of the feet contain paxolin washers apparently intended to have terminal pillars fitted through them. This fact, plus the oil-ring bearings, style of insulation and starting switch details appeared to date the motor to around 1930, so a search was conducted for 'own-brand' motors of this size in British catalogues of the period. The 1929 SUNCO catalogue came up trumps, illustrating a motor of identical design complete with terminal pillars. The catalogue lists other models from the same range; DC series, DC shunt, universal, and even a 'polishing and grinding' version, with heavy duty totally enclosed bearings.

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