Siado
Senior Heliman
Location: Port Orchard, WA
| I haven't played with electrics since college, but I work on some very large machines for a living...somewhere to the tune of 15,000 lb rotors on these things. Rotors are constructed one of two ways: Salient pole or Turbo rotors. The pole of the rotor is the protrusion from the shaft which the windings are wound around. On a salient pole rotor, the pole pieces are fitted to the shaft by way of a dovetail fit. This means that under excessive RPM, the contrifugal force can cause the pole pieces to separate from the shaft. On a Turbo rotor, the pole pieces are machined as one piece with the rest of the rotor, giving it its name since it can withstand much higher RPM. I honestly couldn't tell you which way our little model motors are made, but I would imagine for cost reasons, they are Salient pole designs.
Even if they are turbo rotors, the windings themselves are epoxied in place around the pole pieces to hold them in place. At high RPMs, the epoxy could loosen or crack, allowing a few turns of copper to move outward. This could cause enough imbalance in the rotor for it to eat itself.
Fortunately, under load, most series wound, cumulatively compound wound, and and differentially compound wound (whew!) DC machines have what is known as a "pull-out" torque at which the machine will speed up so far it cannot generate enough torque to continue to speed up. However, if you are raising the voltage, then you are moving that value much higher than designed.
As trevorz said, the biggest concern is the heat dissipation capability. Most machines are capable of handling up to 150% rated load for up to 5 minutes with no effects. However, overheating the windings could also cause the epoxy to break down, weakening the strength of the rotor overall.
This may all be useless theory, but just things to keep in mind when pushing the limits.  |
