The Concept of Rotor Slip

1.4 1.5

1.1.1 The Concept of Rotor Slip

Three-phase wound rotor induction machines can be used to furnish, at their slip- ring terminals, a three-phase source of variable and frequency. For this application, the wound rotor is connected to normal voltage and frequency supply. The voltage induced in rotor and its frequency are directly proportional to the operating slip [2]. = = where E, is the rotor electromagnetic fields EMF where motor is at standstill and f s is the stator frequency. When the rotor is running in the same direction as its synchronously rotating magnetic field, the slip is : = − For small slip, the rotor frequency is low. If the direction of the synchronously rotating magnetic field is opposite to the rotor direction, the slip is : = − − − = + The slip will be greater than unity, the rotor voltage will be larger and its frequency will be larger than of the stator. The slip rings of the wound rotor can thus become a power source of variable voltage and frequency. The three-phase induction motor essentially consists of two part : stator and rotor. The stator connected to a three phase power supply. Due to the phase shift of 120 ⁰ between each successive phases, a rotating magnetic field is produced and governed by the following expression [2]. = 1.3 1.1 1.2 1.6 1.7 1.8 Where f is the system frequency, P is the number of poles and n sync is the speed of the magnetic field’s rotation. There are two different magnetic field passes over the rotor which can be placed inside the stator. One is called a wound rotor. A wound rotor has a complete set of three- phase windings that are mirror images of the windings on the stator. The three phases of the rotor winding are usually in Y-connection and at the end of the three rotor, wires are tied to slip rings on the rotor’s shaft. The rotor windings are shorted through brushes riding on the slip rings. Wound rotor induction motors therefore have their rotor currents accessible at the stator brushes, where that can be examined and where extra resistance can be inserted into the rotor circuit [2]. = = − The equations 1.6 shows the mechanical power is shaft power, while the power converted from mechanical to electrical. = The equation 1.7 shows the simplest equation for the motor circuit. The only element in equivalent in the circuit where the power consumed is in the resistor R 2 s, while the rotor copper losses are the power which would be consumed in a resistor of value R 2 . = When the R 2 s in the motor circuit is increase, value of current I will decrease. The equations 1.8 as show power crossing the resistance from the stator circuit to the rotor circuit. It is equal to power absorbed in the resistance R 2 s [2]. where : n s = slip speed of the motor n sync = speed of magnetic fields n r = mechanical shaft speed of rotor

1.2 Problem Statement