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ELECTRICAL MACHINES DET 204/3 JIMIRAFIZI BIN JAMIL CHAPTER 3 Direct Current (DC) Motor Electrical Machine & Applications 1 DC Motor There are four major types of DC motor in general use: • Separately excited DC motor • Shunt DC Motor • Series DC Motor • Compounded DC Motor Electrical Machine & Applications 1 Speed Regulations • DC motors are often compared by their speed regulations. • Speed Regulations (SR) of the a motor is defined by nl fl SR x 100% fl n nl n fl SR x 100% n fl Electrical Machine & Applications 1 The Equivalent Circuit of a DC Motor Armature circuit represented by voltage source, EA and a resistor RA. The brush voltage drop represented by battery, Vbrush opposing the direction of current flow in the machine. The field coils, which produce the magnetic flux are represented by inductor LF and resistor RF. The separate resistor, Radj represents an external variable resistor used to control the amount of current in the field circuit. Electrical Machine & Applications 1 The Equivalent Circuit of a DC Motor The brush voltage drop is often only a very tiny fraction of the generated voltage in a machine so the voltage drop may be left out or approximately included in the value of RA. The internal resistor in the field coils is sometimes lump together with the variable resistor, and the total is called RF. Electrical Machine & Applications 1 The Equivalent Circuit of a DC Motor The internal generated voltage in DC motor is E A K The induce torque developed by DC motor is ind KI A Electrical Machine & Applications 1 Separately Excited and Shunt DC motors VF IF RF VT EA IA R A IL IA Separately excited DC motor IF = VT RF VT EA IA R A IL IA IF Shunt DC motor Electrical Machine & Applications 1 Example A 50hp, 250V, 1200rpm DC shunt motor with compensating windings has an armature resistance of 0.06Ω. Its filed circuit has a total resistance of 50Ω, which produces a no load speed of 1200rpm. There are 1200 turns per pole on the shunt field winding. Find: 1. 2. 3. 4. the speed of this motor when its input current is 100A the speed of this motor when its input current is 200A the speed of this motor when its input current is 300A. the induced torque of this motor for above conditions. Electrical Machine & Applications 1 Solution VT 250V n 1200rpm 2n 2 x 1200 125.67rads 1 60 60 E A K The relationship between the speeds and internal generated voltages of the motor at two different load conditions is E A1 K1 (1) EA2 K2 (2) The flux is constant and no armature reaction Electrical Machine & Applications 1 Solution E A1 K1 E A 2 K2 At no load, n 1200rpm IA = 0A EA1 VT 250V 1) VT 250 IA IL IF IL 100 95A RF 50 Electrical Machine & Applications 1 Solution EA at this load will be EA VT IA R A 250 95(0.06) 244.3V The resulting speed of this motor is E A1 K1 E A 2 K2 E A 2 1 244.3x125.67 2 122.8rads 1 E A1 250 60 60 x122.8 n2 1173rpm 2 2 Electrical Machine & Applications 1 Answer 2) IA = 195A, EA = 238.3V, n2 = 1144rpm 3) IA = 295A, EA = 232.3V, n2 = 1115rpm Electrical Machine & Applications 1 Solution 4) Pconv E A I A ind E A IA ind At IL = 100A ind 244.3x95 189 Nm 122.8 At IL = 200A ind 388Nm At IL = 300A ind 587Nm Electrical Machine & Applications 1 Series DC Motor VT E A I A (R A R s ) I L I A IS Electrical Machine & Applications 1 Compounded DC Motor A compounded DC motor is a motor with both a shunt and a series field. Current flowing into magnetomotive force. a dot produces a positive If current flows into the dots on both field coils, the resulting magnetomotive forces add to produces a larger total magnetomotive force. It is called cumulative compounding. Electrical Machine & Applications 1 Equivalent circuit of compounded DC motor VT E A I A (R A R s ) IA IL IF VF IF RF Long shunt connection Fnet F F FSE - FAR N SE FAR I IF IA NF NF * F Short shunt connection Electrical Machine & Applications 1 DC Generator There are five major types of DC generators: 1. Separately excited generator. In a separately excited generator, the field flux is derived from a separately power source independent of the generator itself. 2. Shunt generator. In a shunt generator, the field flux is derived by connecting the field circuit directly across the terminals of the generator. 3. Series generator. In a series generator, the field flux is produced by connecting the field circuit in series with the armature of the generator. 4. Cumulatively compounded generator. In a cumulatively compounded generator, both a shunt and a series field are present, and their effects are additive. 5. Differentially compounded generator. In a differentially compounded generator, both a shunt and a series field are present, but their effects are subtractive. Electrical Machine & Applications 1 DC Generator DC generators are compared by their voltages, power ratings, efficiencies, and voltage regulations. Voltage regulation, VR is defined by Vnl Vfl VR x 100% Vfl Electrical Machine & Applications 1 Separately Excited Generator IL IA IF VF RF VT EA IA R A Electrical Machine & Applications 1 Shunt DC Generator IA IF IL VT EA IA R A Electrical Machine & Applications 1 Series DC Generator I A IS I L VT E A I A (R A R S ) Electrical Machine & Applications 1 The Cumulatively Compounded DC Generator Total Magnetomotive force Fnet F F FSE - FAR N F I*F N F I F N SE I A FAR I*F I F Cumulatively compounded dc generator with a long shunt connection N SE I A FAR NF NF IA IF IL VT E A I A (R A R S ) VT IF RF Cumulatively compounded dc generator with a short shunt connection Electrical Machine & Applications 1 Differentially Compounded DC Generator With a long shunt connection Fnet F F FSE - FAR N I N F I F N SE I A FAR * F F Equivalent shunt field current, I eq N SE I A FAR NF NF N SE I A FAR I IF NF NF * F Electrical Machine & Applications 1 Assignment 4 A 12kW, 240V, 1200 rpm, separately excited DC generator has armature and field winding resistances of 0.20Ω and 200Ω, respectively. At no load, the terminal voltage is 240V, the field current is 1.2A, and the machine runs at 1200rpm. When the generator delivers rated current to a load at 240V, calculate a) The generated voltage, EA b) The field circuit voltage, VT c) The developed torque, τder