Why Bridgeport Single Phase Motors are so Expensive

We did a simple replacement of noisy bearings on a 120 V 3/4 hp Bridgeport motor. The owner had commented that it was the strongest 3/4 hp motor he had seen. When we opened it, the reason was clear. It is a repulsion induction motor with both the field and armature wound. No power is fed to the armature but segments of it are shorted by the brushes. The field induces currents in the armature, making portions of it attract or repel the field poles. As the armature rotates, the excited windings change, keeping the magnetic poles at the optimum location. Note the number of slots and commutator bars. They could get by with fewer, with some degradation of performance, but they apparently were looking for best possible performance. The production cost would be serious because it is like combining the most expensive part of a standard induction motor with the most expensive part of a high quality DC motor.

Bill

9100 said:

120 V 3/4 hp Bridgeport motor.

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Interesting find.

Wonder how much of that - if any - applies to their 1 HP and 2 HP motors?

Anyone know?

Bill

I'm a little surprised, because I thought they would have chosen an electrically reversible version.

It is reversible. I didn't look at the circuit, but it has what looks like a typical drum switch and runs both ways. I did see that there are two windings per armature slot with the leads from the slots connected to adjacent commutator bars. The brush holders are spot welded to a sheet steel disk but two brushes on each side are shorted together by leads and the sets are shorted together by the mounting disk. However, the disk has large cutouts between brush sets which probably makes the current through that path negligible. The number of slots in the field and armature are different. When you line them up on one side, they are off alignment on the other. When the motor was assembled with new bearings, spun by hand was almost silent in the forward direction and made the familiar buzz in reverse from the brushes being seated forward.

I would have liked to look at the connections more but the mission was to install the bearings and get it together.

Bill

Typical repulsion motors need a lever to reverse. It moves the brushes. There are slots on the brush holder in pic 1, And I see a slot in pic 2 on the motor case, which may or may not have anything to do with reversing.

The repulsion induction (not repulsion start) motor will electrically reverse, but IIRC has other wire connections to brushes, with an aux winding. I'd have to look that up.

The owner of this motor has another he wants to re bearing sometime. Maybe I can get it for long enough to follow out the wiring. It definitely reverses electrically, not by shifting brushes, and there is no starting capacitor or centrifugal switch. The only way I can see to reverse is by shifting the pattern of the field. If I can get hold of the other motor, I will see if it will reverse while spinning the other way. I suspect it will.

Bill

I didn't find anything about reversing but the field is wound like a typical induction motor and you could make it run either way by changing the feed point. A straight repulsion motor has poor speed regulation, tending to overspeed when unloaded so they also have a set of squirrel cage bars under the armature windings. If I get into another one, I will I will look for them.

Bill

With the induction winding, it is a repulsion-induction motor, a very nice type, that does reverse electrically.

I seem to recall that yes, they do have an aux field that if reversed will reverse the motor. Just don't recall if it has to be in the roþor circuit.

A friend has a factory optional 1/2 hp 110 v BP M head single phase motor, will have to look at it..

Modifying a standard 56C frame AC motor shaft (Single or 3 phase), by welding an extension on (build up.. and recut keyway). Only takes a couple hours.. Aftermarket providers of single phase BP J head motors, have a special run of long shaft motors made up.. Thus expensive... With a fairly small demand.. And motor mount adapter plate must also be provided

Never seen a factory 1 or 2hp set up, that way... Just plain ol reliable brushless motors..

Ok.. The repulsion motor can be wound with two series fields. one perpendicular to a line between the shorted brushes, as a main field, and one along the line of the brushes, the compensating field. Reversing either one will reverse rotation.

So, yes,electrical reversing is perfectly possible .

A friend of mine has one of those 3/4HP single phase - pancake motors on his Bridgeport. It seems to have plenty of power. It will be for sale later this summer, I think.

JH

James H Clark said:

A friend of mine has one of those 3/4HP single phase - pancake motors on his Bridgeport. It seems to have plenty of power.
JH

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Yes probably 1HP of it

Peter