EMT462 Electrical System Technology LECTURE VI mohd hafiz ismail hafizism@gmail.com 04-9798330 level II jejawi EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology INDUCTION MACHINE EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Machine The machines are called induction machines because of the rotor voltage which produces the rotor current and the rotor magnetic field is induced in the rotor windings. Induction generator has many disadvantages and low efficiency. Therefore induction machines are usually referred to as induction motors. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Machine There are two different types of induction motor rotors that can be placed inside the stator: 1. Squirrel cage – the conductors would look like one of the exercise wheels that squirrel or hamsters run on. 2. Wound rotor – have a brushes and slip ring at the end of rotor The magnetic field's rotation of induction motors is given by EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Machine 1. Squirrel cage – the conductors would look like one of the exercise wheels that squirrel or hamsters run on. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Machine 2. Wound rotor – have a brushes and slip ring at the end of rotor EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Machine - Operation The stator’s rotating field cuts the rotors conductors thereby inducing voltages in the rotor circuit. The induced (Faraday) voltages cause currents to flow in the rotor. The rotor’s currents produce a rotating (rotor) field which is always aligned (travels with) the stator’s rotating field. The whole process is essentially that of a transformer. The induction motor is sometimes referred as a rotating transformer . EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Machine - Operation Speed of rotation (synchronous speed) P is the number of magnetic poles designed into the machine, fe is the power line frequency. EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology The Concept of Rotor Slip The voltage induced in a rotor bar of an induction motor depends on the speed of the rotor relative to the magnetic fields 1. Slip speed – defined as the difference between synchronous speed (magnetic field's speed) and rotor speed. nslip = nsync – nm nslip = slip speed of the machine nsync = speed of the magnetic fields nm = mechanical shaft speed of motor EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology The Concept of Rotor Slip 2. Slip – defined as the relative speed expressed on a per-unit (or sometimes as percentage) basis If the rotor turns at synchronous speed, s = 0, while if the rotor is stationary (standstill), s = 1. Mechanical speed (rotor's speed) can be expressed in term of synchronous speed and slip as follow: EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology The Electrical Frequency on the Rotor The rotor frequency can be expressed as fr = sfe where fr = rotor frequency s = slip fe = electrical frequency Alternative to find fr is defined as below EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Motor – Equivalent Circuit Same as a transformer Stator is connected to the ac source, and the rotor’s voltage and current are produced by induction. The primary of the transformer corresponds to the stator of the machine, whereas the secondary corresponds to the rotor Stator and Rotor as Coupled Circuits EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Motor – Equivalent Circuit EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Motor – Power and Torque The power flow diagram EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Motor – Power and Torque Example A 480-V, 50Hz, 50hp, three phase induction motor is drawing 60A at 0.80 PF lagging. The stator copper losses are 2 kW, and the rotor copper losses are 700W. The friction and windage losses are 600W, the core losses are 1800 W, and the stray losses are negligible. Fine the following quantities: a. The air gap power PAG b. The power converted Pconv c. The output power Pout d. The efficiency of the motor EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Motor – Equivalent Circuit EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Motor – Equivalent Circuit EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Motor – Power and Torque The output power can be found as Pout = Pconv – PF&W – Pmisc The induced torque or developed torque: EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology Induction Motor – Power and Torque Exercise A 460 V, 25-hp, 60Hz, four pole, Y-connected induction motor has the following impedances in ohms per phase referred to the stator circuit: R1 =0.641Ω R2 =0.332Ω X1 =1.106Ω X2 =0.464Ω Xm =26.3Ω The total rotational losses = 110 W, Rotor slip = 2.2% at rated voltage and frequency. Find the motor's i) Speed, ii) Stator Current, iii) Power factor, iv) Pconv, v) Pout vi)  ind, vii) load and viii) Efficiency EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology hafizism february 2007

EMT462 Electrical System Technology hafizism february 2007