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AC Machines

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**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

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**So What Happens in the Stator?**

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A N N N C B

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B = 87% C = – 87%

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B = 87% = 150% 60 C = – 87%

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C = – 87% A = 87%

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**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

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**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

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**Rotating Magnetic Field**

What happens if we increase the frequency of supply? Nsync What happens if we increase the number of poles? Nsync

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**So What Happens in the Rotor?**

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**A north & south pole are near a conductor**

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**Flemings Left Hand Rule**

Motion Current Field

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Attraction Repulsion

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**Magnetic field cuts conductor**

Current is induced in conductor A force acts on conductor Conductor moves

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**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?

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**Magnetic field does not cut conductor**

No current is induced in conductor Friction slows conductor Magnetic field then cuts the conductor

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**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)

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Slip Slip Speed Slip At start up Running Highest Lowest

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**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

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**Rotor Frequency Rotor Stator RMF Stationary Locked**

Cuts rotor a fast rate Frequency in rotor highest

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**Rotor Frequency Rotor Stator RMF At normal speed With some slip**

Cuts rotor a slow rate Frequency in rotor lowest

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**Rotor Frequency fr = s = f = Frequency in rotor % slip**

Supply frequency Frequency in Rotor % Slip Rotor RPM

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**By changing the “firing order “ of the poles**

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

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A N N N C B B C Previous

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

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U V W1 V W U2

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U V W1 V W U2

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