As a student in the University of Idaho’s electrical engineering program in the early nineties, I can remember our study of synchronous motor operation in a 300-level laboratory classroom. One of us students asked “What happens when a synchronous motor loads to the point where conventional synchronous operation is lost?” - and the answer, in retrospect, seems to have been somewhat cursory: “In a slipped pole condition… the rotor’s magnetic field poles ‘slip’ from their stable attractive arrangement with the machine’s current-induced stator poles, and wind up ‘bouncing off’ the repulsive poles on the other side of things… large transient currents are induced, and circuit breakers pop…”
That was it… no further discussion. This only seems important now in revisiting the synchronous motor/generator pair (on basis of which our entire electrical power grid supply operates) as being adaptable toward a mode of operation which capitalizes on what may be termed the “cooperative energy” available within just such a “slipped pole” mode of operation.
In looking into this, let us begin with synchronous motor/generators as they conventionally run. Ordinarily, a synchronous motor operates such that the magnetic poles of the rotating field windings appear “pulled along” via magnetic attraction to the opposite polarity poles of the stator magnetic field. Perhaps the defining characteristic of this conventional “attractive mode” of operation would be that the Back Voltage (or Back EMF) induced by the magnetic flux of the rotor poles works so as to oppose the very current components which keep motor torque ‘up to snuff’ under loaded operating conditions. In fact, this very opposition accounts for how “Conservation of Energy” considerations remain balanced in these conventional modes of operation.
Alternatively, if we consider a synchronous machine set up to run in a “pushing-mode” - with stator north-poles repelling rotor north-poles (and south-poles repelling south-poles) - whole new vistas open up. The rotor-field induced portion of the machine voltage appears aligned (as a phasor voltage) such that, under intelligent design conditions, it can actually cooperate in producing stator-currents which increase available machine torque.
With the traditional “Back EMF” now working to “help” (rather than “hurt”) system performance, design considerations include taking precautions to ensure that, at large RPM, increased machine inductive reactance works to shift operation toward more conservative configurations. Thus, rather than providing direct opposition to the flow of such positive-torque inducing currents (such as in the conventional mode of synchronous machine operation), investigation of the “slipped-pole” configuration may allow for design and implimentation of over-unity motor-generator pairs readily compatible with the power generation and distribution system currently in place.