ThreePhase AC machines Resource 8 ThreePhase Synchronous Machine

Three-Phase AC machines Resource 8 Three-Phase Synchronous Machine Starters and Control Gear

Three-Phase AC Machines Resource 7 Three-Phase Synchronous Machines Aims • To understand the methods of starting a three-phase synchronous machine • To understand the control gear requirements for a three-phase synchronous machine

Three-Phase AC Machines Resource 7 Three-Phase Synchronous Machines Objectives • • To be able to explain why a synchronous machine is not self starting To be able to describe how to start a three phase synchronous machine with a prime mover To be able to describe how to synchronise a three phase synchronous machine to the mains supply To be able to describe how to start a synchronous machine with a pony motor

Starting Problems A three-phase synchronous machine cannot self start as a motor • Stator field moving at 50 Hz • Rotor field energised with DC via slip rings • Stator field moving too quickly for rotor to lock on

Prime Mover Method Start the machine as a generator using a prime mover and the three-lamp method

Prime Mover Method Start the machine as a generator using a prime mover and the three-lamp method Generator output must match the supply in 3 ways • The frequencies must be equal • The line voltages must be equal • The phase-relationships be the same

Prime Mover Method Main switch can be closed when L 1 is off and L 2 and L 3 are equally bright

Pony Motor Method Start the machine as a generator using a pony motor

Pony Motor Series Method • Start the machine using a pony motor (Q, M and C 3 closed) • When near synchronous speed switch on dc supply to rotor (C 2 closed, C 3 open) • Rotor synchronous motor catches up with the rotating field • When at synchronous speed short out pony motor stator windings (C 1 closed)

Pony Motor Parallel Method • Start the machine using a pony motor (Q, M, C 1 and C 3 closed) • When near synchronous speed switch on dc supply to rotor (C 2 closed, C 3 open) • Rotor synchronous motor catches up with the rotating field • When at synchronous speed short out pony motor stator windings (C 1 open)

Operation as a Synchronous Motor The V-curve The rotor current can be adjusted to vary the power factor of the stator Unity power factor is achieved when stator current is at its minimum This machine can be used to correct power factor of induction motors when connected in parallel