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

2. 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

3. Objectives 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

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

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

6. 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

7. Prime Mover Method
Main switch can be closed when L1 is off and L2 and L3 are equally bright

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

9. Pony Motor Series Method
Start the machine using a pony motor (Q, M and C3 closed)
When near synchronous speed switch on dc supply to rotor (C2 closed, C3 open)
Rotor synchronous motor catches up with the rotating field
When at synchronous speed short out pony motor stator windings (C1 closed)

10. Pony Motor Parallel Method
Start the machine using a pony motor (Q, M, C1 and C3 closed)
When near synchronous speed switch on dc supply to rotor (C2 closed, C3 open)
Rotor synchronous motor catches up with the rotating field
When at synchronous speed short out pony motor stator windings (C1 open)

11. 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