The systems described in Secs. 9-5 6 and 9-5 7 use an eddy-current magnetic coupling in combination with a stored-energy flywheel to obtain limited sustained opera­tion (i. e., 10 sec to 2 min) after failure of the normal source of power. An eddy-current coupling consists of rotor and stator assemblies, quite similar in many aspects to the common squirrel-cage induction motor. The rotor assembly is an input shaft on which is mounted a field coil and pole pieces, the coil being energized from a d-c source through slip rings. The stator is simply a hollow soft-iron cylinder with the output shaft attached to one of the cylinder bases

The air gap between the rotor pole pieces and the inner surface of the stator is small. As a result, the magnetic field established by energizing the rotor coil is concentrated in the soft-iron cylinder As the energized rotor rotates within the iron cylinder, the magnetic flux of the rotor sweeps through the stator cylinder and induces eddy currents The eddy currents in the stator set up magnetic fields that interact with the rotor fields, and, as a result, torque is developed which tends to drag the stator along with the rotor. There must be relative motion between the rotor and stator to develop any torque If there is no relative motion, no eddy currents are produced and no torque is created. The amount of torque produced by the coupling is a function of the rotor field strength and the speed difference between rotor and stator The output torque (stator) increases with increased rotor excitation and also with increased slip.

The use of a rotating field coil and slip rings with brushes creates maintenance and reliability problems that can be eliminated by using a brushless stationary field coil In this type of eddy-current coupling, the excitation coil is rigidly mounted in a frame. The input shaft carries a smooth cylindrical drum designed so that there is a small air gap between the stationary field assembly and the drum The output shaft carries the rotor, which is fitted into the input shaft cylindrical drum and separated by a small air gap. The flux path is from the stationary field poles to the cylindrical drum on the input shaft to the rotor and then axially m the rotor back to the cylindrical drum and to the field poles. The flux actually traverses two air gaps, as opposed to only one when slip rings and brushes are used. The output torque is developed by the interaction of eddy-current-induced magnetic fields on the inner surface of the cylindrical drum with the main field concentrated in the rotor. The main field flux is prevented from being short-circuited m the cylindrical drum by a nonmagnetic strip that separates the two halves of the drum. The stationary field design has increased reliability and reduced maintenance. The efficiency is less than that of a brush and slip-ring coupling because the double air gap requires more excitation for equal output torque

As noted earlier, the eddy-current coupling is similar to a squirrel-cage induction motor. In an induction motor a rotating magnetic field is established in the air gap by means of a polyphase winding on the stator. In the eddy-current coupling of the rotating field type, a rotating field is established by mechanical rotation of the energized rotor assembly by a prime mover The soft-iron cylindrical rotor of the coupling is analogous to the squirrel-cage rotor bars of an induction motor. In addition to these similarities, the slip-torque characteristic of an eddy current coupling is similar to that of a squirrel-cage induction motor. The slip-torque characteristic of an eddy-current coupling can be modified in the same manner as that of an induction motor. Use of high-resistance material for the soft-iron cylinder affects the slip-torque curve of an eddy-current coupling in the same manner as the use of high-resistance rotor bars affects the slip-torque curve of a squirrel-cage motor.

Eddy-current couplings are noted for their low ef­ficiencies, especially for large differences between input and output shaft speeds. Whenever the output speed is different from the input speed, heat is generated. This loss, called slip loss, is essentially equal to the difference between input shaft power and output shaft power. Slip loss is the major source of heat in an eddy-current coupling, and the heat must be dissipated by cooling fluid or air. At rated torque and output speed, the slip loss of a typical eddy-current coupling will be about 2 to 4%. Considering other losses, such as friction, windage, magnetic drag, and excitation, the peak efficiency is about 92%. At reduced speeds the slip loss increases, and the efficiency becomes essentially equal to the ratio of output speed to input speed.

The application of an eddy-current coupling in a nonmterruptible power supply with a drive motor, stored — energy flywheel, and output generator requires a large difference between the input and output eddy-current coupling shaft speeds. This results m a large coupling slip loss and hence poor efficiency, this can be relieved by operating normally without energizing the eddy-current coupling, see Sec 9-5 7

If the poor efficiency experienced when operating with the coupling normally energized is discounted, extremely close speed control and hence output frequency control can be attained with an eddy-current-coupled system. Upon coast-down, after losing the prime mover input power, the eddy-current coupling, used in conjunction with a flywheel, is able to dissipate the flywheel stored energy at a finely controlled rate just sufficient to maintain a constant output shaft speed (thereby providing an acceptable generator output) for intervals as long as several minutes.