How fast can a motor go?

The rotating speed of the stand was nowhere near enough, so we made an emergency plan with a Variable Speed Drive on the original (4 pole) motor - up to about 200 Hz. This was still not fast enough (we reached the constant power flux limit) so we procured a new 2 pole (22 kW) motor.

I must emphasize that this was an absolutely standard off-the-shelf (Chinese manufacture actually) motor from our sister company who supplies these motors. There was absolutely nothing special about it.

Ironically - the workshop people ran the motor at 150 Hertz without me being aware. When I heard about it - I was also 'astonished' that it managed this without breaking up - but after some thought, I realized that it was not as unlikely as one would think - even those of us with many years of rotating electrical machine design experience.

Provided the bearings can sustain the speed - the maximum (burst) speed of a rotor is determined by the strength of the laminations, the rotor bar extensions and short-circuit rings. High-speed copper cage fabricated machines often have retaining rings over the short-circuit rings, so keep them from flying apart.

Most small Squirrel-cage induction motors are made using Cast Aluminum cages, with the method of construction - and usually the materials - behind identical for 2, 4, 6, 8 pole machines from fractional kilowatts to hundreds of kilowatts.

The maximum speed is based on the maximum centrifugal forces on the rotor, with the limit being linked to tangential speed - in the range of 100 to 250 meters per second (depending on the application, and of course whether special materials are used). A rotor burst occurs when the hoop stress of the rotor is equal to the ultimate tensile strength of the rotor material. Die-cast Aluminum rotors are generally more capable of higher speed operation because they have no rotor bar extensions, and the short-circuit ring acts as a form of retaining ring.

So, a motor with a rotor outer diameter (OD) of say 350 mm, will have 100 m/s tangential speed at around 5500 r/min. A motor with a rotor OD of 180 mm will have a tangential rotor speed of 100 m/s at around 10500 r/min - and so on. IT IS QUITE LOGICAL TO SEE WHY THE SMALL 2 POLE MOTOR OPERATED WITHOUT ANY ISSUES AT AROUND 10 000 R/MIN.

About larger motors: Of course their rotors typically use fabricated copper cages, and a 5000 HP unit would have a rotor OD of say 800 mm, depending on speed. This 800 mm rotor OD would have a tangential speed of 100 m/s at about 2400 r/min.

About the bearings: For Rolling Bearings - the operating speed is limited by the speed rating / reference speed. A 6004 Deep Groove Ball bearing (with a inside diameter of 20 mm - has a reference speed of 11 000 r/min. A 6006 bearing of 30 mm inside diameter has a reference speed of 8000 r/min.

Don't do this on your own or without experience, but there is a lot of technical logic behind it, and I would be very interested to know what the major OEM's of smaller motors have to say about it?

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