While the rotor is run at synchronous speed n s, the rotor field is kept unexcited to begin with. The short-circuit characteristic of the machine is obtained by means of the short-circuit test conducted as per the schematic circuit diagram of Fig. The plot of core loss versus open circuit voltage is shown in Fig. The core loss which comprises eddy current and hysteresis losses which vary as square and 1.6th power of open circuit voltage. Their separation into the two components is easily carried out by running the machine at synchronous speed under an unexcited state. The power corresponding to these losses is drawn from the prime mover running the machine and can be measured by means of a dynamometer or torque meter. The open-circuit test conducted at rated voltage gives the constant loss of the synchronous machine comprising no-load (OC) core-loss and mechanical loss due to windage and friction. 8.12, is called the air-gap line and would indeed be the OCC if iron did not get saturated. The straight-line part of the OCC, if extended as shown dotted in Fig. At low values of I f when iron is in the unsaturated state, the OCC is almost linear and the mmf applied is mainly consumed in establishing flux in the air-gap, the reluctance of the iron path being almost negligible. The OCC exhibits the saturation phenomenon of the iron in machine. the relation between the space fundamental component of the air-gap flux and the net mmf/pole acting on the magnetic circuit (space harmonics are assumed negligible). 8.12 which indeed is the magnetization characteristic, i.e. It may be noted that I f is representative of the net mmf/pole acting on the magnetic circuit of the machine. The readings of the open-circuit line-to-line armature voltage, V OC = √3 Ef, are taken for various values of I f, the rotor field current. In this test the machine is run mechanically at synchronous speed n s to generate voltage at the rated frequency, while the armature terminals are open-circuited as in Fig. Open-Circuit Characteristic of Synchronous Machine (OCC): However, it will soon be shown that it is sufficient to determine one point on the short-circuit characteristic (SCC) of the machine (I SC – I f relationship) as it is linear in the range of interest (for I SC up to 150% of the rated current). Since the magnetization characteristic of the machine is nonlinear, it is necessary to determine the complete open-circuit characteristic (OCC) of the machine (V OC – I f relationship). Where V OC = open circuit voltage and I SC = short-circuit current on a per phase basis with the same field current. (8.22) that for a given field current under short-circuit condition (I a = I SC, V t = 0),īut E f = V OC (open-circuit voltage, i.e. As the motor speed pick-up, the current starts reducing.It is immediately seen from Eq. The motor draws maximum current when it starts accelerating from its standstill position. In other words, the slip of the motor is unity when the rotor is at standstill.Īt standstill, the Speed of the motor Nr=0Īt the start, the slip of the motor is 1. The difference in the synchronous speed of the motor and the actual speed of the rotor is unity. When the motor is at standstill, the relative speed of the rotating magnetic field and the rotor is equal to 1. What is the standstill condition of the induction motor? The slip ring induction motor is used for driving the rotary kiln, bucket elevator, etc. When the motor accelerates up to its base speed the external resistance is short-circuited and the motor is equivalent to a squirrel cage induction motor. The external resistance is added to the rotor conductor and the resistance is reduced as the motor is accelerated. The starting torque of the motor gets increased because of the increased rotor resistance The slip ring induction motor is used for driving the loads which demand higher starting torque. What are the applications of the squirrel cage and the slip ring induction motor? If the motor drives the same load at reduced voltage, it will draw more current and will get overload tripped. If the induction motor runs below its rated voltage, the flux will reduce and as a result, the torque delivering capacity of the motor will get reduced.