Regenerative braking occurs whenever the speed of the motor is more than that of the synchronous speed. In this method, the motor runs as a generator, and the load is used to provide the required power to the supply. The main criterion for regenerative braking is that the speed of the motor becomes greater than the synchronous speed. This condition will lead to the motor acting as a generator and the direction of both the current and torque reversing. This method can be utilized where the load on the motor has very high inertia.
When the applied voltage to the motor is less than the back electromotive force (EMF), then both the armature current as well as armature torque reverses and the speed reduces. As generated EMF exceeds the applied voltage, the power transport takes place from the load to the supply.
The factor by which, the induced E.M.F gets reduced due to short pitching is called pitch factor or coil span factor denoted by Kc. It is given as
Kc = cosα/2
α = 60°
Kc = cos60/2
Kc = cos30°
Kc = √3/2
A phase sequence is an order or sequence in which the currents or voltages of different phases attain their maximum values. In a three-phase system, there are only two possible phase sequences: R-Y-B (positive Phase Sequence) and R-B-Y (Negative Phase Sequence).
The rotor of the synchronous machine carries a field winding that is supplied with d.c. through two slip rings by a separate d.c. source. Synchronous Rotor construction is of two types, namely :
Salient (or projections) pole type.
Cylindrical type (or non-salient pole type)
In this type, salient poles are mounted on a large circular steel frame which is fixed to the shaft of the alternator.
The rotors of salient-pole rotor machines have concentrated winding on the poles and a nonuniform air gap.
Salient pole generators have a large number of poles. sometimes as many as 50, and operate at lower speeds.
The synchronous generators in hydroelectric power stations are of the salient pole type and are driven by water turbines.
The rotors are shorter axial length but have a large diameter.
A DC motor can be operated from a single-phase AC supply.
The direction of the torque produced by a DC machine is determined by the direction of current flow in the armature conductors and by the polarity of the field.
Torque is developed in a DC machine because the commutator arrangement permits the field and armature currents to remain in phase, thus producing torque in a constant direction.
A similar result can be obtained by using an AC supply, and by connecting the armature and field windings in series.
A P-N junction acts as a voltage-controlled switch. During forward biasied the p-n junction diode acts like an ON switch. When reverse biased, it acts like an OFF switch.