ThreePhase AC machines Resource 7 ThreePhase Synchronous Machines

Three-Phase AC machines Resource 7 Three-Phase Synchronous Machines

Three-Phase AC Machines Resource 7 Three-Phase Synchronous Machines Aim • To understand the construction and operation of a three-phase synchronous machine

Three-Phase AC Machines Resource 7 Three-Phase Synchronous Machines Objectives • • • To To To be be be able able to to to describe the construction of the stator describe the construction of a salient pole rotor describe the construction of a cylindrical rotor describe the operation of a synchronous machine as a generator calculate synchronous speed and terminals voltage describe the operation of a synchronous machine as a motor

Stator Construction • Stator is identical to the induction motor • Laminated low silicon steel rings joined together • Slots insulated with Mylar • Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase

Stator Construction • Stator is identical to the induction motor • Laminated low silicon steel rings joined together • Slots insulated with Mylar • Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase • Slot insulator inserted by hand Stator frame Stator slots with insulator

Stator Construction • Stator is identical to the induction motor • Laminated low silicon steel rings joined together Stator frame Stator slots with insulator • Slots insulated with Mylar • Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase • Slot insulator inserted by hand • Coils inserted by hand Coil

Stator Construction • Coils can be placed in single or double layers Stator slot

Stator Construction Single layer Coil 1 coil arm per slot Stator Slots

Stator Construction Stator Slots Double layer Coil 2 coil arms in each slot

Stator Construction Stators can be very large

Rotor Construction Two types of rotor • Salient Pole • Cylindrical

Rotor Construction Salient Pole Difference between pole face curvature and stator creates non-linear variation in flux across pole face Non-linear variation in flux across pole face produces sinusoidal change in the induced EMF

Rotor Construction Cylindrical Difference in coil spacing creates non-linear variation in flux around the rotor surface Non-linear variation in flux around rotor surface produces sinusoidal change in the induced EMF

Rotor Construction Cylindrical Difference in coil spacing creates non-linear variation in flux around the rotor surface Non-linear variation in flux around rotor surface produces sinusoidal change in the induced EMF

Operation as a Synchronous Generator Two pole cylindrical rotor example • Field produced on rotor by dc current through slip rings • Rotor field is turned at 3000 rpm by a prime mover A’ A’ BB N • EMFs induced in stator coils with frequency of 50 Hz • Magnetic Flux distributed around rotor produces sinusoidal variation in induced EMF • Phase coils separated by delay between phase EMFs 120 o causes C C S C’ B’ A A

Operation as a Synchronous Generator Two pole cylindrical rotor example • Field produced on rotor by dc current through slip rings • Rotor field is turned at 3000 rpm by a prime mover Period = 20 ms C A • EMFs induced in stator coils with frequency of 50 Hz • Magnetic Flux distributed around rotor produces sinusoidal variation in induced EMF • Phase coils separated by 120 o causes delay between phase EMFs • Delay between phases = 20/3 = 6. 667 ms B

Calculations Synchronous speed f. S = supply frequency required RPM p = pole pairs Induced EMF Volts per phase Φ = flux per pole set by rotor current z = conductor in series per phase

Operation as a Synchronous Generator Generated EMF relationship The open circuit EMF generated depends upon Open circuit stator EMF saturation • Rotor speed • Rotor current Relationship between open circuit stator EMF and rotor current is a straight line until the steel begins to saturate when it becomes non-linear. linear Rotor current

Operation as a Synchronous Motor Two pole cylindrical rotor example NS A’ • Stator field rotates at 3000 rpm from 50 Hz supply • Rotor field must be locked on to stator field speed • Motor runs a synchronous speed whatever the mechanical load provided rotor field is strong enough NR B N S C’ B’ NR = N S • This is impossible within an induction motor as there wound be no induced currents to cause rotation • This motor runs at synchronous speed hence the name – SYNCHRONOUS MOTOR A

Operation as a Synchronous Motor Two pole cylindrical rotor example • Stator field rotates at 3000 rpm from 50 Hz supply • Rotor field must be locked on to stator field speed • Motor runs a synchronous speed whatever the mechanical load provided rotor field is strong enough Rotor Speed (NR) NR = N S • This is impossible within an induction motor as there wound be no induced currents to cause rotation • This motor runs at synchronous speed hence the name – SYNCHRONOUS MOTOR NS Load Torque

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