An Induction motor is the most widely used AC motor in industrial and domestic applications. Now you might think, why is it so? It is because of the low cost, simple and rugged construction of the motor. Moreover, it has good operating characteristics with an efficiency as high as 90%. An induction motor does not have any commutator, like the one we saw in a DC motor. Hence it provides a good speed regulation without any sparking.
So, with that many advantages, it becomes essential to meet the mechanical power demand using an induction motor. But doesn’t it makes you wonder that how does this motor work and gives that many advantages? If yes, then stick with this article to grasp every detail on the induction motor.
Construction of an Induction Motor
An Induction motor consists of a frame, stator core, rotor, shafts, and bearings.
Frame
It is the outer body of the motor. It supports the stator core and protects the inner parts of the machine from the surroundings.
Stator core
A stator consists of a stack of laminations of silicon steel in the form of a ring. It fits inside the stator frame and contains slots on its inner periphery. These slots carry the three-phase winding, separated by 120 degrees in space. Right here, the figure shows the distribution of three-phase winding in a stator.
Rotor
A rotor consists of a stack of laminations in the form of a cylinder. It has slots punched on its outer periphery, which contains rotor windings. Based on the winding type, there are two categories of rotors.
Squirrel cage rotor
**Image courtesy:Wikipedia
This rotor uses copper or aluminum bars as the rotor conductors. Each slot of the rotor carries a conductor without any insulation from the core. All the conductors are shorted by annular rings, aka end rings.
Wound rotor
It uses wires or straps for the rotor windings. The distribution of the three-phase winding on the rotor is similar to that of the stator. The winding connects to an external resistance through slip rings and brushes. The wound rotor provides a higher starting torque as compared to the squirrel cage rotor.
Shafts and bearings
The induction motor uses ball-and-roller bearings. These bearings support the rotor shaft and make it rotate smoothly.
Induction motor working principle
The induction motor follows these two laws to generate a unidirectional torque.
Faraday’s law of electromagnetic induction
It states that a conductor placed in a varying magnetic field induces an Electromagnetic force (EMF). The closed conductor leads to the flow of current, known as induced current.
Lorentz force law
It states that a current-carrying conductor placed in a magnetic field experiences a force. The force on the conductor is orthogonal to the direction of the current and magnetic field.
Before understanding how these laws govern the rotation of an induction motor, let us see how the stator creates a rotating magnetic field.
The concept of rotating magnetic field
The creation of a rotating magnetic field requires two essential conditions:
- A three-phase distributed winding. The axis of the windings must make a space angle of 120 degrees.
- A three-phase AC source. The magnitude of current in the three phases is equal but is displaced in time by 120 degrees. The figure here shows a sinusoidal wave of a three-phase source. See how each phase current achieves its peak value at different times.
**Image courtesy: Wikipedia
When we give this three-phase supply to the stator windings, the windings start creating a magnetic flux. The figure below shows the orientation of magnetic flux by the three phases after giving a three-phase supply.
But as the current from the three-phase source achieves its peak value at different instant of time, the magnetic flux will follow the same behavior. Let us see how. Consider these time instants X, Y, and Z in the waveform.
Magnetic field vector orientation at selected instants
At point X
At point X, the current magnitude in phase A is more compared to phases B and C. Moreover, the Phase A current is positive, while the current in Phases B and C is negative. So, if we represent the magnetic field vectors at point X, it looks like this. Note the direction of the resultant magnetic field.
At point Y
At point Y, the current magnitude in phase B is more as well as positive. The figure below shows the orientation of the magnetic field vectors at point Y. In this case, the direction of the resultant magnetic field has changed.
At point Z
At point Z, the current magnitude in phase C is more and positive. The vector representation of the magnetic field looks like this. Note that the direction of the resultant magnetic field has again changed.
From the above three time instants, we conclude that the resultant magnitude of the magnetic field always remains uniform, but its direction changes periodically. On tracing the trajectory of the magnetic field vector, we obtain a circle.
Hence, the application of three-phase current to the three-phase distributed winding creates a rotating magnetic field. The field rotates with a constant speed known as synchronous speed.
How does the rotor rotate in an Induction motor?
So, after the generation of the rotating magnetic field, the rotor conductors start interacting with the magnetic field. Assume a rotor conductor interacting with the magnetic field as shown in the figure.
This interaction induces a current in the conductor (according to Faraday’s law of electromagnetic induction). Now, according to Lorentz law, a force starts acting on the conductor. This force tends to displace the rotor in a direction, as shown in the figure.
And the rotor slowly begins to accelerate and try to attain the stator’s synchronous speed.
From this, we conclude that the induction motor is self-starting and requires only a single excitation source.
What happens if the rotor attains the synchronous speed?
So, when the rotor starts rotating, a wide relative speed lies between the stator’s magnetic field and the rotor conductors. As the rotor accelerates, this relative speed starts decreasing. Hence, the current in the rotor conductor starts reducing.
Now, assume that the relative speed between them becomes zero, and the rotor starts rotating with synchronous speed. As this happens, the magnetic field no longer interacts with the rotor conductors. And hence, no current flows in the rotor conductors. As a consequence, no force acts on the rotor conductors. So, the rotor speed will get reduced. The rotor always tries to catch the synchronous speed but never attains it.
Thus we conclude that an induction motor always runs at a lesser Speed than synchronous speed. Hence it is also known as the Asynchronous motor.
What is Slip?
So, we saw that there always lies a speed difference between the rotating magnetic field and the rotor conductors. This speed difference is known as Slip. If we denote the synchronous speed with (Ns) and the rotor speed with (Nr), then the Slip (s) is given by
s = (Ns – Nr)/Ns.
The value of the Slip always ranges between 0 and 1.
Advantages of Induction Motor
- The induction motors are robust. Hence, its working is independent of the external environment conditions.
- They have high starting torque as compared to synchronous motors.
- The induction motor is self-starting. Hence no starting methods are required, unlike synchronous motors.
- The squirrel cage induction motor does not require a commutator, brushes, and slip rings. It eliminates the sparking issues and reduces the overall motor cost.
Disadvantages of Induction motor
- The high-capacity induction motors require a starter for the motor’s smooth operation. The starter reduces the motor’s input current to a safe value which would otherwise have created a voltage drop in the system.
- The induction motor operates at a lagging power factor under lightly loaded conditions. Hence it requires power factor correction devices.
- The speed control of the Induction motor is a bit challenging. Instead, for a wide range of speed control, DC motors are preferred.
**To know the differences between an Induction and a Synchronous motor, read this article: Difference between Induction and Synchronous motor
**To know about other types of AC motor, read this article: Types of AC Motors
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FAQs
What is the working principle of induction motor construction? ›
Working Principle of Induction Motor
The motor which works on the principle of electromagnetic induction is known as the induction motor. The electromagnetic induction is the phenomenon in which the electromotive force induces across the electrical conductor when it is placed in a rotating magnetic field.
Advantages of Three Phase Induction Motor
It requires less maintenance. It has high efficiency and good power factor. It is less expensive. It has self-starting torque.
The working of the three-phase induction motor is based on the principle of electromagnetic induction. When three-phase stator winding of an induction motor is energized from a 3 phase supply, a rotating magnetic field is set up which rotates around the stator at synchronous speed (Ns).
What are the advantages and disadvantages of induction machines? ›Starting torque in the case of slip-ring induction motor is comparitively better because of the presence of external resistor in the rotor circuit during starting. Other important disadvantage of Induction motor is that it draws high inrush currents causing large momentary voltage dip during starting of the machine.
What 3 things are needed for an induction motor to work? ›A 3-phase induction motor includes two essential components namely the stator & the rotor. In this motor, the stationary part is the stator whereas the rotating part is the rotor. In this motor, the load is connected to the shaft. Three-phase armature winding can be wound over the stator.
What are the three common methods that are used in starting an induction motor? ›- Direct On-Line (D.O.L.) Starting.
- Stator-Resistance Starting.
- Autotransformer Starting.
- Rotor-Resistance Starting.
One of the main disadvantages of induction motor is that speed control of induction motor is difficult. Efficiency drops at low loads. This is because, the low power factor causes a higher current to be drawn. This results in higher copper losses.
What is the difference between induction motor and normal motor? ›The fundamental difference between these two motors is that the speed of the rotor relative to the speed of the stator is equal for synchronous motors, while the rotor speed in induction motors is less than its synchronous speed. This is why induction motors are also known as asynchronous motors.
Why induction motor is mostly used? ›Unlike DC motors, induction motors are robust i.e. their mechanical construction is stronger, which enables them to be used in a tough industrial environment that other motors cannot withstand.
What is the efficiency of induction motor? ›Efficiency of three-phase induction motors can vary from 80% to 90%.
What are the applications of induction motor? ›
- Pumps.
- Compressors.
- Small fans.
- Mixers.
- Toys.
- High-speed vacuums.
- Electric shavers.
- Drilling machines.
The induction motor draws a large starting current, but produces very low starting torque about 1.5 to 2.5 times the starting torque. The reason for low starting torque is the poor power factor of the rotor circuit. At the start, the rotor inductance is very high as compared to the rotor resistance.
Is induction motor AC or DC? ›Induction motors, linear motors, and synchronous motors, for example, are all types of AC motors. AC motors can also include variable-frequency drives to control the motor's speed and torque, while DC motors are available in self-excited and separately excited-type models.
What are the two most important parts of induction motor? ›An induction or asynchronous motor is composed of two main components: the stator and rotor. The stator consists of the outer windings or magnets and is stationary. The stator is stationary. The rotor is the inner core and is what actually rotates in the motor.
What is induction motor in simple words? ›: an alternating-current motor in which torque is produced by the reaction between a varying magnetic field generated in the stator and the current induced in the coils of the rotor.
Which starter is best for induction motor? ›Explanation: A rotor resistance starter is used in a high rating slip ring induction motor. It uses an external resistance/phase in the rotor circuit so that the rotor will develop a high value of torque. High torque is produced at low speeds when the external resistance is at its higher value.
Is induction motor self starting? ›Single-phase induction motors are not self-starting without an auxiliary stator winding driven by an out of phase current of near 90°. Once started the auxiliary winding is optional.
Why do induction motors have high starting current? ›What causes high start-up or inrush current? An induction motor with a stationary rotor behaves like a transformer with shorted secondary windings; there is no back-emf to counter the emf in the stator windings, so a very high current flows.
What is the most common cause of induction motor failure? ›Low Resistance. The most common cause of motor failure, and arguably the most difficult to overcome, is low resistance. Low resistance is caused by the degradation of the insulation of the windings due to conditions such as overheating, corrosion, or physical damage.
What causes induction motors to fail? ›Physical damage, corrosion, improper installation and excessive loads are often the root cause.
How long do induction motors last? ›
...
Motor Starting.
| HP Range | Average Motor Life (years) | Life Range (years) |
|---|---|---|
| <1 | 12 | 10-15 |
| 1-5 | 17.1 | 13-19 |
| 5.1-20 | 19.4 | 16-20 |
| 21-50 | 21.8 | 18-26 |
Alternating Current (AC) Motors. AC motors include 3-phase and single phase types. 3-phase AC induction motors are the most widely used motors in industrial and commercial applications.
Which is better DC motor or induction motor? ›Due to induction current loss and motor slip, the efficiency of the AC motor is less. The efficiency of the DC motor is high as there is no slip and induction current loss. AC motors require less maintenance as brushes and commutators are absent.
How does induction motor change speed? ›(1) that the speed of the induction motor can be changed by varying the frequency (f), number of poles (P) or slip (s). By changing the stator poles, the speed of the induction motor can be changed. The number of stator poles can be changed by, Multiple Stator Windings.
Is induction motor speed constant? ›In this case, you can call an induction motor, which is defined as a constant speed motor. The rotor flux rate lags slightly behind the stator flux and there is a relative velocity between the stator and the rotor. The difference in engine speed changes slightly with increasing engine load.
What is the minimum speed of induction motor? ›Minimum Speed
Generally speaking, TEFC motors are not designed to operate at less than a 4:1 speed range, while some manufacturers make a 10:1 or 20:1 range motor. Most often, operating the motor slower than this requires an auxiliary cooling system.
- By increasing the supply voltage.
- By increasing the rotor resistance.
- By increasing the rotor reactance.
- By increasing rotor EMF.
Therefore, a starter is necessary to limit the current drawn by the motor at the start. Starters not only limit the starting current but also provide protection to the motor against overloading and low voltage conditions.
What creates torque in an induction motor? ›An induction motor develops torque by inducing current to the rotor, which is proportional to the differential speed of the rotor and the rotating magnetic field in the stator.
Is induction motor better than universal motor? ›Induction motors are quieter, far quieter than universal motors. Universal motors are widely known for their shriek but with an induction motor you can hear the quality. Induction motors are highly efficient motors that require less amperage per HP. Universal motors require more amperage per HP than induction motors.
What is the advantage of induction motors over permanent magnet motors? ›
AC induction motors can be used without a VFD to drive a pump or fan, but are often installed with variable frequency drives (VFD) in pump systems or fan systems in an effort to improve system efficiency. Permanent magnet synchronous motors require a drive to operate. PMSMs cannot run without a drive.
Why do induction motors fail? ›The most common cause of motor failure, and arguably the most difficult to overcome, is low resistance. Low resistance is caused by the degradation of the insulation of the windings due to conditions such as overheating, corrosion, or physical damage.
What is the highest efficiency of induction motor? ›= Output / (Output + losses)
Large induction motor efficiency can be as high as 95% at full load, however 90% is more common.
Reversing the polarity of the supply voltage will produce a force in the opposite direction, helping the motor to stop quickly.
Why does Tesla use induction motor and permanent magnet? ›Model 3 uses permanent magnet motors to improve energy efficiency and extend battery life. Due to the smaller body of the Model 3, even if a higher energy density 21700 lithium battery is used, the total energy is still lower than that of the Model S, and the battery life must be increased by improving efficiency.
What is brushless vs induction motor? ›The primary difference is the absence of magnets within the motor. Both have Rotors, both have Stator Cores and both have Distributed Windings. The magnets within the rotor of the DC brushless motor are replaced with stacked laminations of steel and the induction motor utilizes a squirrel cage type rotor.