Magnetic Centre of Rotor and Motor Hunting

An old hand showed me an old bronze bearing motor centering itself. He explained that there is a 'sweet spot' where the motor will run best as the fields settle in with each other. A ball bearing motor can't do this, the spot must have been already sorted out and the motor manufactured so the rotor is in the best place.

The procedure I've used in the past is to run the motor light to allow it to find it's magnetic centre. Measure the gap between the 2 coupling faces. This measurement is used for the spacers in the coupling. Trying to force the motor to run off it's magnetic centre will introduce stress in to the drive bearings

That's cool, Tony. Is that a ball bearing motor?

No they were mill type motors. Roller pedestal bearings at each end.

How was the rotor able to hunt? Was the shaft able to slide inside the bearings? Usually the inner race is tight on the shaft.

In this case roller bearings (sleeve bearing are exactly the same) at each end so at max there could be 10mm of lateral travel. Once the motor is running it holds it's self steady at the magnetic centre. Once stable the gap in the coupling can be measured. Spacers made to this dimension hold the rotor steady at start up, once running there is no thrust at all on the driven machine.

I'll admit I was sceptical about it until I worked alongside a guy from Parsons Pebbles setting up one of our mill motors to a new gearbox.

Where did you come across cantering a motor? It's not an everyday thing.

I started in a machine shop that had some old machines. The drill was flat belt drive and the motor was probably sleeve bearing, would hunt for the good spot. I loved the old machines. One old lathe had two Studebaker transmissions on it to give the speeds and reverse.