Induction Machines: Construction and Operation
Understand the construction and working principle of induction machines.
Drafted with Aria, reviewed by the AiCanCode.org team. Spotted an error? Use Give Feedback at the bottom of the page.
Why it matters
Induction motors produce torque without an external electrical connection to a squirrel-cage rotor. Relative motion between the rotor and the stator field induces rotor current.
Construction
A laminated stator carries a distributed three-phase winding. Balanced currents displaced by 120 electrical degrees create a rotating field. A squirrel-cage rotor contains conducting bars joined by end rings. A wound rotor has insulated windings connected to slip rings, allowing external resistance during suitable starting or control arrangements. Bearings support the shaft; the air gap permits rotation while coupling the magnetic field.
Operation and formulas
For frequency f in hertz and p poles, Ns = 120f/p rpm. Fractional slip s = (Ns − Nr)/Ns, and rotor electrical frequency fr = |s|f. In normal motoring 0 < s < 1. At exactly synchronous speed the ideal rotor sees no changing fundamental field, so no rotor current or induction torque is produced. Above synchronous speed, negative slip corresponds to generating operation when excitation and system conditions are suitable.
Worked example
A six-pole motor supplied at 50 Hz runs at 960 rpm. Ns = 120 × 50/6 = 1000 rpm. s = (1000 − 960)/1000 = 0.04 = 4%. fr = 0.04 × 50 = 2 Hz. At standstill, s = 1 and rotor frequency equals supply frequency.
Power flow
Air-gap power Pag divides into rotor copper loss sPag and converted mechanical power (1 − s)Pag for normal motoring. Subtract mechanical losses to obtain shaft output. Stator losses occur before the air gap. Slip is not the overall efficiency loss fraction.
Common mistakes
- Using pole pairs in the 120f/p formula instead of total poles.
- Assuming the rotor runs at synchronous speed under load.
- Treating all motors as self-starting: a balanced three-phase induction motor is self-starting, whereas an ideal single main-winding single-phase induction motor needs a starting arrangement.
Quick check
- Why is slip needed? To induce rotor current and torque.
- What connects cage bars? End rings.
- For s = 0.03 at 60 Hz, what is rotor frequency? 1.8 Hz.
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