Pump motors
By Robbie Rhodes
All of the pump motors I've seen in grand pianos were 6-pole motors, therefore they turned at a bit less than 1200 rpm on 60-Hz power, or 1000 rpm on 50-Hz power. The same motor could be used on either 50 or 60 Hz, but a larger motor pulley was installed for the lower mains frequency to compensate the slower speed. I think the Ampico and DuoArt motors were rated for 1/6 horsepower.
These motors were designed for these special needs:
1. Operation with the rotor shaft vertical and the pulley at the bottom, which requires a good thrust bearing on the bottom end bell.
2. Quiet running, with low hum level. The magnetic design was quite conservative, meaning that the flux density in the steel didn't approach saturation. That's why a piano motor usually bears a horsepower rating of perhaps half as much as a modern industrial motor of the same case size (and often runs quite hot).
To reduce the cost of fractional-horsepower motors most are designed to start without an external starting capacitor; this type is called "split phase". The "capacitor start" design is more expensive but has much greater starting torque (which isn't needed in a piano).
All the designs allow swapping the starting winding wires in order to start the motor in the opposite direction. The centrifugal starting switch opens the circuit to the starting winding as the rotor approaches running speed.
Often the motor burns up simply because the connection to the starting winding is bad, perhaps due to a loose or corroded terminal screw, or due to failure of the starting switch. When the motor power is applied without a functioning starting winding, the rotor has no torque at all, and just sits there humming. In less than a minute the main winding will overheat and smoke, unless the motor is protected with a simple thermal time-delay circuit breaker.
I'm not surprised at the $300 quote you got to re-wind the motor. I can imagine that it would take the motor repairman about one day to clean out the burned windings, build the winding fixture, wind the new coils, and fit them into the stator slots. And _then_ he must carefully drive wedges into the slots to prevent excessive hum and buzzing. It's a real craft.
Hopefully someone in this group has a serviceable old pump motor you can obtain. Have your local motor-man clean and lubricate it and check the starting switch and wiring before it's installed in the piano.
If you wish to experiment with a modern substitute try a single- phase 1150-rpm fan motor rated for 208-volt service. It should run quietly and nicely on 115-volts. A motor rated for 115-volt mains may hum too much.
-- Robbie Rhodes
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(Message sent Sun 21 Jan 1996, 00:52:47 GMT, from time zone GMT-0800.) |
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