ERT 105 ELECTRICAL ENGINEERING THREE PHASE SYSTEM (week 8-9) THREE PHASE SYSTEM (week 8-9)

Ability to : describe a single-phase supply describe a three-phase supply understand a star and a delta connection calculate power in three-phase systems compare star and delta connections appreciate the advantages of three-phase systems EXPECTED OUTCOME

BASIC UNDERSTANDING

ALTERNATING CURRENT SINGLE PHASE THREE PHASE

Motion of a current-carrying loop in a magnetic field N S L R I F F Rotation Commutator (rotates with coil) brushes

Vertical position of the loop: N S Rotation

ONE PHASE SYSTEM

THREE PHASE SYSTEM

STAR CONNECTION OF THREE PHASE SYSTEM The voltages, V R, V Y and V B are called phase voltages or line to neutral voltages. The voltages, V RY, V YB and V BR are called line voltages.

STAR CONNECTION : Voltage calculation

The standard electricity supply to consumers is 415/240 V, 50 Hz, 3-phase, 4-wire alternating current

Problem example : EXAMPLE 1. Calculate the Line voltage for for a three- phase star system if it is known that the Phase voltage = 120 Volts

For the three phase star connection,

EXAMPLE 2. Three loads, each of resistance 30, are connected in star to a 415 V, 3-phase supply. Determine : (a)the system phase voltage, (a)the phase current (c) the line current.

ANSWER : A ‘415 V, 3-phase supply’ means that line voltage, V L = 415 V (a)For a star connection, V L = V  Phase voltage =~~~ 240 V (b) Phase current, = Phase voltage /30 --  Ip= 8 A (c) For a star connection, Line current = Phase current = 8 A

Three loads, each of resistance 50 are connected in delta to a 400 V, 3-phase supply. Determine (a)the phase voltage, (b)the phase current and (c)the line current. EXAMPLE 3.

Line voltage VL = 400 V Phase voltage = line voltage = 400 V Phase current = V/R  = 400/50= 8 A Line current = 1,7 x phase current = 13.4 A

Three coils each having resistance 5 Ohm are connected (i) in star and (ii) in delta to a 415 V, 3-phase supply. Calculate for each connection (a) the line and phase voltages (b) the phase and line currents. EXAMPLE 4.

the total power P is given by: The power dissipated in a three-phase load is given by the sum of the power dissipated in each phase. Is a power factor

Three 12 Ohm resistors are connected in star to a 415 V, 3-phase supply. Determine the total power dissipated by the resistors. EXAMPLE 4.

(Since for a star connection) Power = watt Or = 14.4 kW ANSWER

The input power to a 3-phase a.c. motor is measured as 5 kW. If the voltage and current to the motor are 400 V and 8.6 A respectively, determine the power factor of the system. EXAMPLE 4.

ANSWER