Induction Machine

An induction machine is an AC electrical machine in which the rotor current is not supplied by wires or brushes but is induced by the rotating magnetic field of the stator, which makes the induction motor the most rugged, self-starting and economical way to turn electricity into motion. This guide explains what an induction motor is, the meaning of induction motor terminology, every major induction motor type, how ac induction motors are controlled and protected, how linear induction motors differ, and how modern engineering teams test an induction drive motor and its inverter without a spinning dynamometer.

What Is an Induction Machine? The Meaning of Induction Motor

The meaning of induction motor is right in the name: the stator winding acts like a transformer primary, and the rotor acts like a short-circuited secondary separated from it by an air gap. Alternating current in the stator creates a magnetic field that sweeps past the rotor conductors. That relative motion induces a voltage in the rotor, the voltage drives current around the shorted rotor circuit, and the current reacts with the field to produce torque.

Because energy crosses the air gap magnetically, a cage rotor needs no brushes, commutator or slip rings. Nothing electrical touches the rotating part, so there is nothing to wear out except the bearings.

Three properties made the induction machine the industrial default:

• Self-starting on a polyphase supply, with no auxiliary starting machine.
• Rugged, because the cage rotor is essentially a solid casting of metal bars in a laminated core.
• Cheap to build and maintain compared with DC or wound-field synchronous machines.

It has two built-in weaknesses that the rest of this article returns to:

• It always draws magnetizing (lagging) current, because it must set up its own field from the stator supply.
• Its speed depends on load, because the rotor must slip behind the field to produce torque.

A quick example shows the second point. A 4-pole machine on a 50 Hz supply has a field rotating at 1,500 rpm, yet the shaft at full load turns at about 1,460 rpm. On 60 Hz the field turns at 1,800 rpm and the shaft at about 1,750 rpm.

Scale matters too. The International Energy Agency describes electric motor-driven systems as the largest single end use of electricity, accounting for more than 40% of global electricity consumption, and IEA 4E Electric Motor Systems Platform puts the 2023 share at 53%, including 72% of electricity use in industry, with induction machines forming the workhorse of that fleet.

Induction Motor Types

Every induction motor type works on the same principle. A rotating or travelling magnetic field induces current in the rotor, and that current produces torque. They differ in how the rotor is built and in how many supply phases feed the stator. Engineers usually group induction machines in these two ways. A few special forms, such as linear induction motors and doubly-fed machines, are built for particular jobs.

Based on Rotor Type

Squirrel-cage rotor. This is the most common design by a wide margin. Aluminium or copper bars sit in the rotor slots, and end rings short them together at both ends. There are no brushes, slip rings or rotor connections, so the motor is rugged, cheap to build and needs very little maintenance. The shape of the rotor bars largely sets how the motor behaves. Shallow bars give low slip and high running efficiency. Deep bars or a double cage give stronger starting torque, because at standstill the rotor current crowds toward the surface of each bar and the rotor’s effective resistance rises.

The NEMA MG 1 standard uses this bar-shape trade-off to define design letters for cage motors:

• Design A allows higher starting current in exchange for high running efficiency.
• Design B is the general-purpose design for fans, pumps and compressors.
• Design C gives higher starting torque for loaded conveyors and crushers.
• Design D gives very high starting torque and more slip, for punch presses, hoists and high-inertia loads.

The IEC 60034-12 standard groups motors the same way, with design N for normal starting torque and design H for high starting torque.

Wound rotor (slip-ring). In this design the rotor carries a three-phase winding instead of bars. The winding is brought out through slip rings and brushes to external resistors. Adding resistance at start-up gives high starting torque and lowers inrush current. Once the motor is up to speed, the resistors are shorted out. Wound-rotor machines are used for very high-inertia loads. They are also the basis of the doubly-fed induction generator, where a power converter on the rotor circuit replaces the resistors. The cost of this flexibility is regular brush and slip-ring maintenance.

Based on the Number of Phases

Polyphase machines. A three phase induction motor produces a rotating field of constant strength directly from its stator windings. That makes it self-starting, smooth-running and efficient. It is the standard choice for industrial drives, pumps, compressors, conveyors, HVAC plant and inverter-fed EV and rail traction, and most of this article applies to it.

Single-phase machines. A single phase induction motor’s main winding produces only a pulsating field, which on its own cannot start the rotor. Each variant adds a different way to create a starting field:

• Split-phase: an auxiliary winding with higher resistance, which a centrifugal switch cuts out once the motor is running. Used in small fans, blowers and light pumps.
• Capacitor-start: a capacitor in series with the auxiliary winding gives more starting torque. Suits compressors and pumps.
• Capacitor-start, capacitor-run: a large start capacitor plus a smaller run capacitor that stays in circuit, for better running efficiency at higher power.
• Permanent split capacitor (PSC): a single capacitor that stays in circuit all the time. Common in HVAC blowers and fans.
• Shaded-pole: a copper ring on part of each pole creates a weak rotating effect. This is the simplest and least efficient design, used in very small fans and appliances.

Single-phase machine dominate household appliances and light commercial equipment, where only a single-phase supply is available.

Where Induction Machines Are Used

• Industrial: pumps, fans, compressors and conveyors, because the induction machine runs for decades on fixed-speed duty with almost no maintenance.
• HVAC and buildings: air-handling fans and chillers, where VFD-driven induction motors capture affinity-law savings.
• Automotive: EV traction inverter test and auxiliaries such as pumps, valued for magnet-free cost and high-speed robustness.
• Rail traction: inverter-fed cage motors in locomotives and metros, chosen for ruggedness and tolerance of heavy overloads.
• Aerospace: 400 Hz aircraft fuel pumps and cooling fans, where the higher frequency allows small, light machines, plus ground support equipment.
• Grid studies: induction-motor load models capture stall and fault-induced delayed voltage recovery (FIDVR), which NERC and DOE work traced to stalled single-phase air-conditioner compressor motors.
• Renewables: DFIG wind turbines and small hydro induction generators.
• Marine: propulsion and thruster drives, where vector-controlled induction machines offer robust, serviceable power.
• Domestic: single-phase motors in refrigerators, washing machines and fans.
• Academia: machines laboratories and drive-control research, because the induction machine is the classic platform for teaching FOC and parameter identification.

Induction Machine vs Induction Motor vs Asynchronous Machine

The vocabulary is confusing, so here is the short version:

• Induction machine is the general term for the device, whether it is motoring, generating or braking.
• Induction motor is the same machine used to drive a load, which is how most people meet it.
• Asynchronous machine refers to the fact that the rotor never turns in step with the field.
• AC induction motor, ac induction machine and induction drive motor are common search and catalog variants of the same thing.

One machine, three operating modes, separated by the sign of slip (the fractional speed difference between field and rotor):

The torque-speed section below shows all three regions on one curve.

How Does an Induction Motor Work?

The working principle is a four-step chain, and each step is quantified in the subsections that follow:

1. Three-phase stator currents create a rotating magnetic field.
2. The field sweeps across the rotor bars and induces an EMF and current in them (Faraday’s law).
3. Current-carrying bars inside the field experience a force (the Lorentz force), producing torque.
4. The rotor turns in the direction of the field, chasing it to reduce the relative motion that caused the current (Lenz’s law), but it never quite catches up.

Rotating Magnetic Field From a Three-Phase Stator

The stator carries three windings displaced 120 electrical degrees in space, fed by currents displaced 120 degrees in time. Each winding on its own produces a field that only pulses back and forth along its axis. Added together, the three pulsating fields combine into a single field of constant strength that rotates smoothly at a fixed speed. Its magnitude is 1.5 times the peak field of any one phase.

Snapshots make this concrete. At the instant phase A current is at its positive peak (0 degrees), the resultant field points along the axis of winding A. One third of a cycle later (120 degrees), phase B is at its peak and the field points along winding B. After another third (240 degrees), it points along winding C. The field has rotated one full pole pair in one electrical cycle without changing size.

Swap any two supply leads and the phase sequence reverses, so the field, and the motor, runs the other way. This is how reversing contactors work.

A single-phase winding cannot do this. It produces a pulsating field that is mathematically equivalent to two equal fields rotating in opposite directions. At standstill their torques cancel, so a single-phase induction motor has no starting torque. Designers add an auxiliary winding with a phase shift (from resistance or a capacitor) or a shading coil to create a weak rotating component that gets the rotor moving.

Synchronous Speed of an AC Induction Motor

The speed of the rotating field, called synchronous speed, depends on only two things: supply frequency and the number of poles. It rises in direct proportion to frequency and falls as the pole count rises. In rpm, it equals 120 times the frequency divided by the number of poles; if you count pole pairs instead, it is 60 times the frequency divided by the pole pairs. This article counts total poles throughout.

A 4-pole machine therefore runs its field at 1,500 rpm on 50 Hz and 1,800 rpm on 60 Hz.

Because pole count is fixed when the machine is wound, the practical way to change speed is to change frequency. That is exactly what a variable-frequency drive (VFD) does.

Slip: Why the Rotor Never Reaches Synchronous Speed

Slip is the difference between synchronous speed and rotor speed, expressed as a fraction (or percentage) of synchronous speed.

The physics is simple. If the rotor ever reached synchronous speed, the bars would move in lockstep with the field. With no relative motion there is no change of flux, so no induced EMF, no rotor current and no torque. The rotor must lag the field just enough to generate the torque the load demands.

The frequency of the current in the rotor equals slip multiplied by supply frequency. Worked example: a 1,500 rpm field and a 1,460 rpm shaft give a speed difference of 40 rpm, a slip of 0.0267 (2.67%), and a rotor current frequency of only 1.33 Hz. That very low rotor frequency is why rotor iron losses are small in normal running.

In the normal operating region, slip rises roughly in proportion to load. Maintenance engineers use this: comparing a tachometer reading against the nameplate full-load speed gives a quick estimate of how heavily a motor is loaded.

How Torque Is Produced in an Induction Drive Motor

Torque depends on how much rotor current flows and how well it lines up in time with the air-gap flux. Both the rotor’s induced voltage and its frequency scale with slip. The rotor circuit has resistance, which does not change with slip, and leakage reactance, which grows with rotor frequency and therefore with slip.

• At small slip, reactance is negligible, the rotor looks purely resistive, current stays in phase with the induced voltage, and torque rises almost linearly with slip. This is the stable running region.
• At large slip, reactance dominates. Current is high but lags badly, so much of it produces little torque. Torque falls even as current climbs. This is why a stalled motor draws huge current but delivers only moderate torque.

The classic per-phase torque relationship combines these effects: torque grows with slip, with the square of the rotor induced voltage, and with rotor resistance, and is divided down by the rotor’s total impedance at that slip. One consequence is important for design: raising rotor resistance moves the peak (breakdown) torque to a higher slip, but does not change the size of that peak. That single fact explains wound-rotor starting resistors and the NEMA design letters described later.