Jraef...
No.. it had nothing to do with induction motors! Instead it's a self-starting synchronous motor. Following is its desciption taken from Prof. I.L. Kosow's book, "Electrical Machinery and Controls"! I "fiddled" with two in my very 1st job as a EE... a long, long, time ago!
Super-Synchronous Motor (Prof. Irving L. Kosow)
The super-synchronous motor does not operate at hyper-synchronous speed, but its title is a misnomer. It would have been better to have called it a super-torque motor. The motor was developed by General Electric in order to provide a synchronous motor which was self-starting under heavy loads. The super-synchronous is described as a special construction having five slip-rings and employing a wound-rotor in combination with the d-c field winding.
It is a well known fact that unless equipped with an "Amortisseur" winding, a synchronous-motor has no-starting torque, but, the super-synchronous motor was designed to take advantage of the fact its Pull-Out torque is between 250 and 300 per cent of full-load torque. The super-synchronous motor is capable of developing that torque on starting, but in a unique manner.
It requires a special construction, however, and it is probably the costliest motor of its kind for a given horsepower rating between 400-500 Hp, at 440V. The rotor is the standard cage-type rotor with the d-c field winding brought out two (2) slip-rings on the rotor shaft. It is coupled directly to the mechanical load which it must drive.
The entire stator, however, is free to rotate on trunnions, in the same manner as an a-c dynamometer. But, whereas the latter is limited in its angular displacement, the stator of the super-synchronous motor is free to rotate on bearings at synchronous speed. The stator-armature winding, therefore, is also excited through three (3) slip-rings and is usually started at a reduced voltage by means of three-phase reduced-voltage methods.
Its uniqueness is that a large brake is provided around the outside of the stator-frame to apply a braking action and to secure the stator in its running position. Because the rotor is coupled to the load, when a reduced polyphase a-c voltage is applied to the stator with the brake released, the induction motor torque produced by the rotor poles reacts against the "stator" conductors; this reaction imparts to the stator a torque that is opposite in direction to the direction of rotation of the load
The stator picks up speed as the a-c stator voltage is increased; and, as the stator reaches synchronous speed, full supply voltage is applied in addition to the d-c field excitation. The stator pulls into synchronism with the rotor at a standstill, held by the inertia of the fixed heavy load coupled to its shaft. At this instant, the motor is operating as a synchronous motor without load, generating a counter-emf which limits its stator current.
The brake is now slowly applied to the rotating-stator. Since a synchronous-motor must run at synchronous speed, the reduction in stator-speed must be made up by rotation of rotor-speed in the opposite direction, i.e., for a synchronous speed of, say 1,800 rpm, a stator-speed of 1,790 rpm counter-clockwise requires a rotor-speed of 10 rpm clockwise.
The torque-angle, therefore, increases to provide maximum torque, i.e., pull-out torque, in starting the heavy applied load. The armature-current, although high, is limited by the generated emf in the stator. Reducing the speed of the stator by increased braking increases the speed of the rotor, until the stator is at a standstill and the rotor is rotating with the full applied load at synchronous speed.
And now, you know the rest of the story!
Regards, Phil Corso
Ps: Its application reduced machinery-train length by several feet, because it eliminated an electrical-coupling between the motor, and the drive-machine!
