English
Menu

What is Induction motor?

An AC motor in which three-phase stator windings create a rotating magnetic field and the rotor turns by the force between that field and the currents induced in the rotor conductors. The rotor turns slightly slower than the field (the synchronous speed), and the fractional lag is called the slip. Simple and rugged, it is widely used in pumps, fans, machine tools and railway vehicles. Changing the frequency with an inverter adjusts its speed over a wide range.

Read the beginner explainer: What Is an Inverter? DC to AC for Air Conditioners and EVs

Japanese name: 誘導電動機

Induction motor appears in 5 practice questions across 1 topic, including Power Electronics Circuits.

Field: Power Electronics Circuits

Synchronous speed and slip

The synchronous speed is n_s = 120f/p [min⁻¹] (f: frequency, p: number of poles), so a four-pole motor runs its field at 1500 min⁻¹ at 50 Hz and 1800 min⁻¹ at 60 Hz. The slip is s = (n_s − n)/n_s; a rotor turning at 1440 min⁻¹ against a synchronous speed of 1500 min⁻¹ has s = 0.04. The frequency of the rotor current is s × f, only 2 Hz in this example.

Torque and slip

At synchronous speed the rotor conductors do not cut the field, so neither current nor torque arises. As the load increases, the speed drops and the slip grows, the rotor current and torque increase, and the motor settles at the speed where torque balances the load. Near the rating the torque is roughly proportional to the slip, and if a load beyond the maximum torque pushes the slip too far, the motor stalls.

Inverter drives

Changing the frequency with an inverter changes the synchronous speed and so adjusts the rotor speed. V/f control keeps the flux constant by holding the voltage-to-frequency ratio (V/f) roughly constant. For more precise control, vector control splits the current into a flux-producing and a torque-producing component and controls them separately, giving torque control as fast and accurate as with a DC motor.

Try it: questions about Induction motor

Which of the following is the synchronous speed of a four-pole induction motor operated from three-phase AC at 50 Hz?

  1. A750 min⁻¹
  2. B1500 min⁻¹
  3. C3000 min⁻¹
Show answer and explanation

Answer: B. 1500 min⁻¹

The synchronous speed is the speed of the rotating magnetic field created by the three-phase stator windings, given by n_s = 120f/p [min⁻¹] (f: frequency, p: number of poles). At 50 Hz with four poles it is 120 × 50/4 = 1500 min⁻¹. In regions with 60 Hz, the same motor has a synchronous speed of 1800 min⁻¹. The rotor of an induction motor turns slightly slower than the synchronous speed.

A four-pole induction motor operating at 60 Hz is running with a slip of 0.05. Which of the following is the rotor speed?

  1. A1710 min⁻¹
  2. B1800 min⁻¹
  3. C1890 min⁻¹
Show answer and explanation

Answer: A. 1710 min⁻¹

The synchronous speed is 120 × 60/4 = 1800 min⁻¹. The slip is s = (n_s − n)/n_s, so the rotor speed is n = n_s × (1 − s) = 1800 × 0.95 = 1710 min⁻¹. The rate at which the rotor conductors cut the rotating field is proportional to the difference from the synchronous speed, so the frequency of the rotor current is s × f = 0.05 × 60 = 3 Hz. The rated slip of a typical induction motor is a few percent.

FAQ

How does an induction motor differ from a synchronous motor?

A synchronous motor has permanent magnets or field windings on its rotor and turns at exactly the speed of the rotating field (the synchronous speed). An induction motor produces torque from the currents induced in its rotor, so it turns a little slower than the synchronous speed. Permanent-magnet synchronous motors are highly efficient and widely used in electric vehicles and home appliances.

Where does the energy go when a motor decelerates?

When the frequency is lowered and the rotor turns faster than the synchronous speed, the slip becomes negative and the induction motor acts as a generator. The energy returns to the DC side of the inverter, where a braking resistor turns it into heat or a regenerative converter sends it back to the supply.

Induction motor in the explanations

Questions that mention Induction motor (5)

  1. Which of the following is the synchronous speed of a four-pole induction motor operated from three-phase AC at 50 Hz?
  2. A four-pole induction motor operating at 60 Hz is running with a slip of 0.05. Which of the following is the rotor speed?
  3. Which of the following correctly explains why an induction motor cannot run at exactly the synchronous speed while operating as a motor?
  4. When the speed of an induction motor is varied with an inverter, which of the following is the main reason the voltage-to-frequency ratio (V/f) is kept roughly constant?
  5. Which of the following correctly combines what can happen when an induction motor driven by an inverter is decelerated quickly by lowering the frequency with the countermeasure?

Books on Power Electronics Circuits Ad

As an Amazon Associate I earn from qualifying purchases.

Comparisons that include this term

Related terms

Inverter Three-phase AC Smoothing capacitor Back EMF Boost converter Buck converter Buck-boost converter Continuous conduction mode Electrical transformer Power factor

Back to the glossary