Induction Electric Motor (Squirrel Cage)

Introduction

Induction motors – also known as asynchronous motors – are the most common type of electric motor used today. Due to their simple design, low cost, and high reliability, induction motors are used for a wide range of applications in all engineering industries.

Induction Motor

Induction Motor

Types/Designs

There are two main types of induction motor: single phase (1~) and three-phase (3~).

Single phase induction motor designs include:

  • Split phase induction motors - used in machines where the starting frequency is limited and where the drive does not exceed 1 kW.
  • Capacitor start (cap start) induction motors - used in machines with higher inertia loads and those that require frequent starts e.g. conveyors.
  • Capacitor start capacitor run induction motors - similar applications to capacitor start induction motors; an additional capacitor is installed for when the motor is in service (when the motor is running).
  • Shaded pole induction motors – the original AC induction motor. It is still used in small devices and those that require low starting torque e.g. record players, projector fans, photocopying machine fans, hair dryers etc.

Three-phase induction motor designs include:

  • Squirrel cage induction motors - chosen for their longevity and low maintenance. This induction motor design is the most common.  
  • Slip ring induction motors - provide a high torque and low starting current; used for e.g. elevators, cranes, and hoists.

While induction motors exist in numerous forms, this article focuses on the 3-phase squirrel cage induction motor design because it is by far the most common type of induction motor.

Tip - the term ‘induction’ refers to the fact that electrical current is induced in the rotor cage when the motor is in operation; this differs to other motors where the rotor current is supplied from an external source.

Induction Motor Components

 

The two primary assemblies within an induction motor are the stator and rotor; the stator and rotor are however built from smaller individual components.

Stator

The stator is the stationary part of the motor and consists of a housing with slots and a series of windings.

Stator Windings

The windings receive three-phase AC power, which causes a magnetic field around each of the windings to expand and contract when current is flowing. Each winding is energised in pairs and in sequence to produce a rotating magnetic field. Stator windings are usually manufactured from copper, although other materials are available e.g. aluminium.

Stator Housing

The stator is constructed by stacking very thin, highly permeable steel laminations inside a steel or cast-iron frame. The frame is bolted to the floor and it is the exterior of the frame that is painted and visible to outside viewers. Common motor frame construction materials include various grades of steel and cast iron.  

Rotor

Within the stator is a solid metal shaft, laminations, and a squirrel cage; this assembly is known as the rotor and is the rotating part of the motor.

The rotor shaft usually has a long and thin cylindrical shape, but this is design dependent. The steel laminations, squirrel cage, and bearings, are all mounted onto the rotor shaft. Rotor shafts are usually manufactured from stainless steel, as it is durable, mechanically strong, and has good corrosion and erosion resistance properties.

Induction Motor Rotor

Induction Motor Rotor

The squirrel cage is a cylindrical shaped cage that fits around the shaft with bars extending between its two ends. At either end of the squirrel cage, end rings are attached to create a short-circuit that induced current will flow through. Squirrel cages are typically made from copper or aluminium.

Induction Motor Squirrel Cage

Induction Motor Squirrel Cage

Thin steel laminations are slid onto the squirrel cage bars and compressed between the end rings; the rotor lamination materials involved are similar to those used for the stator laminations. The laminations do not follow a perfectly straight orientation, but are slightly skewed in order to increase the torque produced. There is a maximum degree of ‘skewedness’ these laminations can adopt, and this is dependent on the design of the motor. Skewing of the laminations also reduces the risk of the motor rotor ‘locking’ in a position between magnetic fields; this scenario causes the rotor to remain stationary and not rotate even when current is supplied to the stator windings.

End Shields and Bearings

End shields (end bells) are mounted at opposite ends of a motor’s frame; the rotor shaft passes through both end shields. One end of the shaft is the drive end (connected to the load) whilst the other is the non-drive end (usually connected to a cooling fan); both shaft ends have shaft keys for transferring mechanical motion from the shaft to its connections.

Anti-Friction Bearing

Anti-Friction Bearing

Dust shields may be installed between the shaft and end shields to prevent foreign particles from entering the motor’s interior. Foreign particles may lead to deterioration of a motor’s windings or other parts; moisture ingress is one of the most common forms of motor failure.

At either end of the rotor shaft, anti-friction bearings are installed. Each bearing is installed onto the rotor shaft and housed in a recess within each end bell. One bearing is usually retained using a c-clip (retaining clip) whilst a spring washer is used for the opposing bearing (motor design dependent). The use of bearings ensures that the shaft rotates smoothly and generates minimum friction, which is especially important as the shaft may rotate at high speeds.

Info – larger motors do not use anti-friction bearings, they use plain bearings. Changing the type of bearing used also changes the design considerably. For example, plain bearings require more space, a form of lubrication (oil usually), and do not use spring washers or c-clip retaining rings. Plain bearings also cater for higher loads.

Fan and Fan Guard

Attached to the non-drive end of the rotor shaft is an axial fan; when the rotor rotates, so too does the fan. The fan forces air across the exterior of the motor frame to cool it during operation. The fins of the motor frame serve as heat exchangers and have a large contact surface area, this increases the heat transfer rate from the motor to the air and thus increases the motor’s self-cooling capacity.

A fan guard protects the fan from large foreign bodies and protects persons or objects nearby from the moving fan blades.

Motor Fan Guard

Motor Fan Guard

Info - an overheated motor may melt the insulation around the windings and cause the motor to short circuit; this failure mode is unfortunately not uncommon but can easily be avoided providing adequate cooling is always provided.

Advantages and Disadvantages

Induction Motor Advantages

  • Wide use - induction motors are used for a wide range of applications in both domestic and industrial settings. It is estimated that around 70% of machines used in industry today are driven by three-phase induction motors.
  • Cheap and easy to install - squirrel cage induction motors do not contain brushes, slip rings, commutators, permanent magnets, position sensors, or other components that increase their overall cost. Their simple construction ensures they are generally easy to install and maintain. The absence of brushes within squirrel cage induction motors means no electrical discharges (sparks) are created within the motor (theoretically). Induction motors can therefore be operated in more hazardous environmental conditions providing they are modified to do so (Ex rated motors etc.).
  • Low maintenance - induction motors require relatively low levels of maintenance, especially in comparison to DC motors which contain carbon brushes that easily succumb to deterioration.  
  • Long service life - induction motors are considered to have a comparatively long service life because their parts are mechanically strong, resistant to corrosion and erosion, and have low wear rates.
  • High efficiency - induction motors have high efficiencies.
  • Self-starting - three-phase induction motors are inherently self-starting i.e. providing electrical current is supplied to the stator windings, the motor will rotate, no other external force is required.

Induction Motor Disadvantages

  • Poor starting torque - based on their design, induction motors typically have a low starting torque, a side-cost of their high efficiency. For this reason, induction motors are unsuitable for use in applications that require high starting torque. However, connecting the motor indirectly to the load i.e., via gears, pulleys etc. can circumvent this problem.
  • Low power factor during light load conditions – during start-up and low load, the motor requires a large magnetising current to overcome the resistance produced by the air gap between the stator and the rotor. The vector sum of the load and magnetising currents causes the voltage to lag, consequently yielding a low power factor. Due to the high magnetising current, the motor will also experience an increase in copper losses, which reduces its overall efficiency. 
  • Difficulty achieving speed control - as a three-phase induction motor is a constant speed motor, it naturally undergoes very little in the way of speed variation, making manipulation of its speed difficult. However, this problem has been mitigated significantly in recent years due to the advancement and application of variable speed frequency drive technology. 

Model Annotations

Fan Cover

A cover prevents accidental damage occurring to the fan and personnel.

Axial Fan

A fan is used to force cool the motor. Air is drawn through the fan cover grills due to the negative pressure created by the fan, the air is then directed across the motor housing. Flowing air cools the motor and reduces the risk of overheating.

Nut

Nuts and bolts are used for securing parts of the motor together. Chosen nuts should have suitable tensile strength and corrosion resistance characteristics.

Nuts are the ‘female’ part of a nut and bolt assembly.

Locking Washer

Locking washers are used to apply a continual tensile (stretching force) to the bolt and nut assembly. The tensile force reduces the possibility of the nut loosening due to vibration.

Plain Washer

The plain washer distributes the compressor force exerted by the nut and bolt assembly when tightened. The washer also prevents the nut and bolt from ‘digging’ into the metal surfaces when being tightened.

Motor End Cover

The end cover houses the bearing, c-clip and sometimes a dust seal. The two end covers support the weight of the shaft.

Bearing Housing

The bearing is housed in this space.

C-clip Housing

The c-clip is installed with c-clip pliers. After opening the pliers, the c-clip expands due to residual tensile forces. The residual force keeps the c-clip firmly within the groove and prevents axial movement of the bearing.

Sealed Ball Bearing

A sealed ball bearing allows the rotor to rotate without transferring the rotary motion to other stationary parts i.e. the motor housing.

C-Clip / Retaining Ring

A retaining ring is used to retain the bearing within the motor end cover housing. The ring prevents axial movement of the bearing.

Bolt

Nuts and bolts are used for securing parts of the motor together. Chosen bolts should have suitable tensile strength and corrosion resistance characteristics.

Bolts are the ‘male’ part of a nut and bolt assembly.

Motor Casing / Housing

The motor casing houses the stator and rotor assembly. The casing must be strong enough to withstand the electrical and mechanical stresses generated by the motor as well as the physical demands of its working environment e.g. severe weather.

Stator

The stator contains the insulated windings for the three phases of the motor.

The electrical current flowing through these windings is what causes the rotor to rotate.

The stator core is usually constructed of iron to reduce load losses.

Heat Exchanger

Radiator fins increase the motor casing surface area. A larger surface area allows heat to be removed more quickly by the forced air flow from the fan.

Connection Terminal

The three phase supply and earth cable are connected to the terminal board. Each of the three phases of the motor must be correctly wired to the incoming supply. Motors are connected in either a star or delta wiring configuration.

Terminal Housing

The terminal housing shields the connection board and electrical connections from foreign object damage such as water.

Lifting Eye

The lifting eye allows moving of the motor using a strop, rope, crane, chain block or cable etc. It is a requirement if the motor is too large to be moved using only manual labour. Multiple lifting eyes may be used for large motors.

Gasket

Usually constructed of rubber or card. The gasket is ‘squeezed’ between the two metal surfaces in order to create a sealed space. The gasket prevents water or contamination from passing between the metal surfaces and into the terminal casing.

Feet / Base

The complete weight of the motor is transferred to the structure or ground through the feet. The base has holes or channels drilled into it to allow alignment and fixture of the motor.

Rotor Shaft

The rotor shaft connects the rotor to the bearings, fan and load. When installing, the rotor is sometimes cooled and the bearings heated in order to allow easy assembly (only for small motors).

Rotor

The rotor core is constructed of steel laminations. The magnetic field created by the stator windings acts upon the rotor and causes it to turn. The type of rotor used in this example is a cage rotor (squirrel cage motor).

Shaft Key

The shaft key is the only connection between the rotor shaft and the load being driven, it is thus imperative the key can withstand the full load characteristics of the motor without failing.

End Ring

The end ring is used to compress the steel laminations together.

Shaft Key Groove

The shaft key sits within this groove.

Dust Seal

Depending upon the design, a rubber dust seal may sit in this space. The seal reduces the risk of contamination entering the motor housing. The seal is pressed between the rotor shaft and motor end cover.

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