AC Machines

3 Phase Induction Motor Principles At the end of this section you will be able to: List the characteristics of the magnetic fields produced by a 3Ø windings Calculate the speed of rotation of a rotating magnetic field Describe the basic principles of operation of an induction motor Reverse the direction of rotation of a 3 phase induction motor

So What Happens in the Stator?

A N N N C B

B = 87% C = – 87%

B = 87% = 150% 60 C = – 87%

C = – 87% A = 87%

Rotating Magnetic Field Synchronous Speed The speed of the RMF is dependant on two factors:- minutes Frequency of the supply Number of poles in the stator seconds There are always a multiple of 2 poles (North & South) per phase

Rotating Magnetic Field Converts Seconds to minutes Divides the poles by 2 Nsync = f= P= Synchronous Speed (RPM) Frequency of supply Number of poles per phase

Rotating Magnetic Field What happens if we increase the frequency of supply?  Nsync What happens if we increase the number of poles? Nsync 

So What Happens in the Rotor?

A north & south pole are near a conductor

Flemings Left Hand Rule Motion Current Field

Attraction Repulsion

Magnetic field cuts conductor Current is induced in conductor A force acts on conductor Conductor moves

What happens if the conductor is the same speed as the RMF? Slip What happens if the conductor is the same speed as the RMF?

Magnetic field does not cut conductor No current is induced in conductor Friction slows conductor Magnetic field then cuts the conductor

DIFFERENCE between SYNCHRONOUS speed and ROTOR speed Slip Slip Speed DIFFERENCE between SYNCHRONOUS speed and ROTOR speed Normally expressed as a percentage s% = n= nsync= Slip in % Rotor speed (RPM) Synchronous Speed (RPM)

Slip Slip Speed Slip At start up Running Highest Lowest

SLIP is the DIFFERENCE between SYNCHRONOUS speed and ROTOR Speed Stator’s Magnetic Field Rotor SLIP is the DIFFERENCE between SYNCHRONOUS speed and ROTOR Speed

Rotor Frequency Rotor Stator RMF Stationary Locked Cuts rotor a fast rate Frequency in rotor highest

Rotor Frequency Rotor Stator RMF At normal speed With some slip Cuts rotor a slow rate Frequency in rotor lowest

Rotor Frequency fr = s = f = Frequency in rotor % slip Supply frequency Frequency in Rotor % Slip Rotor RPM

By changing the “firing order “ of the poles Can we reverse a 3 Ø Motor? By changing the “firing order “ of the poles

A N N N C B B C Previous

Can we reverse a 3 Ø Motor? By changing the “firing order “ of the poles By reversing any two phase connections

U1 V1 W1 V2 W2 U2

U1 V1 W1 V2 W2 U2