Transcript SMJE 2103
SMJE 2103 Induction Motor Induction Motor Scope of discussion • • • • • • Structure Basic concept Equivalent Circuit Power and Torque Speed Control Testing Induction Motor - Structure - Squirrel Cage – no winding and slip ring Wound Rotor – the winding connected to slip rings Induction Motor - Basic Concept Induced Torque: - Speed of magnetic field - Induced Voltage - Induced Torque Induction Motor - Basic Concept Rotor Slip: Induction Motor - Basic Concept Rotor Frequency Example A 208V, 10hp, 4 ploe, 50 Hz, Y-connected induction motor has a full-load slip of 5%. a) What is the synchronous speed of this motor? b) What is the rotor speed of this motor at the rated load? c) What is the rotor frequency of this motor at the rated load? d) What is the shaft torque of this motor at the rated load? Induction Motor - Equivalent Circuit - Rotor voltage – Lockedrotor voltage Rotor frequency – Frequency of the induced voltage Induction Motor – Equi. Circuit RR is constant but XR depend on S Rotor current flow Overall rotor impedance considered S Referring to stator side, considered turn ratio – new Rotor voltage Induction Motor – Basic Concept Rotor impedance Rotor current If, Final perphase equi circuit Induction Motor - Power and Torque - Induction Motor - Power and Torque • No output voltage instead of mechanical force. • Higher speed, higher friction, windage and stray losses. • But component losses change inverse. Example/Tutorial A 480V, 60Hz, 50hp, 3 phase induction motor is drawing 60A at 0.85 Pf lagging. The stator copper losses are 2 kW, and the rotor losses are 700W. The friction and windage losses re 600W, the core losses are 180W, and the stray losses are negligible. Find: a) The air gap power PAG b) The power converted Pconv c) The output power Pout d) The efficiency of the motor Induction Motor - Power and Torque - Induction Motor - Power and Torque - Induction Motor - Power and Torque - Induction Motor - Power and Torque - Induction Motor - Power and Torque - Induction Motor - Power and Torque Air gap needs R2/S and rotor copper loss needs R2 Example/Tutorial A 460V, 25hp, 50Hz, 4 pole, Y-connected induction motor has the following impedances in ohms per phase referred to stator circuit: The total rotational losses are 1100W and are assumed to be constant. The core loss is lumped in with the rotational losses. For a rotor slip of 2.2% at rated voltage and rated frequency, find the motor’s a) speed d) Pconv and Pout b) stator current e) ind and load c) power factor f ) efficiency Induction Motor - Speed Control • Two methods a) Varying stator and rotor magnetic field speed - electrical frequency or changing the number of poles b) Varying slip - varying rotor resistance or terminal voltage Induction Motor - Speed Control (pole changing)Created two poles, N and S Created an extra poles by changing current polarity (only 2 speeds) Induction Motor - Speed Control (Frequency) With or without adjustment to the terminal voltage: a) Vary frequency, stator voltage adjusted – generally vary speed and maintain operating torque. b) Vary frequency, stator voltage maintained – able to achieve higher speeds but a reduction of torque. Could be combined together, easy way by introducing power electronic fields. Induction Motor - Speed Control (terminal voltage) - Induction Motor - Speed Control (rotor resistance) - Induction Motor - Testing There are basically 3 types of test in modeling induction motor parameters: 1. No-load test 2. DC test 3. Locked rotor test or blocked rotor test Equivalent circuit element – R1, R2, X1, X2 and XM Induction Motor - Testing (no load) - Measuring rotational losses and providing info about its magnetizing current (X1 + XM) Due to the rotor speed close to sync speed, S small and higher rotational losses Induction Motor - Testing (no load) - Induction Motor - Testing (DC test) - To find R1 (stator resistance). a) Dc voltage is applied and adjusted to rated condition. b) Voltage and current flow are recorded. Induction Motor - Testing (Locked rotor)- Steps: 1- locked 2- AC voltage is applied and currents flow is adjusted to full load condition 3- Measure voltage, current and power flow Induction Motor - Testing (Locked rotor)- Induction Motor - Testing (Locked rotor)- Example/Tutorial The following test data were taken on a 7.5 hp, 4-pole, 208V, 50Hz, design delta-star connected induction motor having a rated current of 28A (a) Sketch the per-phase equivalent circuit for this motor (b) Find the slip at the pullout torque, and find the value of the pullout torque