AC Machines 6077SA NUE 046

3 Phase Induction Motor Construction At the end of this section you will be able to: Identify the basic components of a 3Ø induction motor List the types of rotors used Test motor windings for suitability to connect to the supply Connect in both star and delta List the steps for dismantling it

3Ø Induction Motors Why are they so common in industry? Simple in construction Only one moving part Don’t have brush gear and commutators Now with VSD technology, Variable speed Speed of machine fixed to supply frequency

Rotor (The bit that Rotates) Construction Fixed Stator (stationary) Rotor (The bit that Rotates)

Construction Frame can be made from cast Iron Steel Aluminium End caps/shields bolted to each end of frame

Drip Proof Designed for air flow into motor Not waterproof Not suitable for flame proof atmospheres

Totally Enclosed No airflow between motor inside and outside atmosphere Waterproof Easier to adapt to for flame proof atmospheres

Open Designed for air flow into motor Not waterproof Very rare due to OH&S considerations

Core Laminations Carries the stator windings Made of silicone steel 0.3 to 0.5mm thick Insulated from one another to reduce eddy current loss

Stator windings 3 identical phase windings that are displaced by 120° electrical around the stator Each winding consists of series, parallel connected coils to achieve the number of poles required. 2, 4, 6, & 8 poles per phase are common Windings are generally connected in delta but can be connected in star

Rotors Two Main Types Wound rotor (not common) Squirrel cage (most common)

Squirrel Cage Rotor

Squirrel Cage Rotor Aluminium Copper Copper alloy Can be Cast Copper/Copper alloy bars inserted into the laminations and welded/brazed to shorting ring

Wound Rotor Slip ring Same number of poles as stator. One end of coil is connected to a star point Other end is terminated on a slip ring Resistance is added to change the mechanical and electrical characteristics of motor.

Motor Terminals

Motor Terminals

Motor Terminals U1 V1 W1 V2 W2 U2

Motor Terminals U1 V1 W1 V2 W2 U2

To Make Star Connection Motor Terminals U1 V1 W1 V2 W2 U2 To Make Star Connection

To Make Delta Connection Motor Terminals U1 V1 W1 V2 W2 U2 To Make Delta Connection

Electrical Testing Each winding should have the same resistance ie 1-10 Voltage between windings =400V Insulation test level =1000V IR reading no less than 1M