Electric Motor - Components

Induction motor rotor - construction

The rotor of the induction motor has a core which is made of electrical steel.

The bars which constitute the squirrel cage are typically made of aluminium or copper. The bars are placed in slots on the rotor core. There is no need for insulation between the bars and the core as the voltage developed in the squirrel cage is very low.induction motor rotor

The rotor bars are skewed in order to prevent magnetic locking. Magnetic locking is also known as cogging.

Magnetic Locking can also be prevented by ensuring that the number of rotor slots is not equal to the number of stator slots.

Motor Bearings – Functions and Types
The bearings can be one of the two types, a plain bearing (sliding contact) or an anti-friction bearing (rolling bearing), depending on the design parameters of the machine element, each of the two types of bearings, plain and anti-friction, is available for design with linear motion, radial loads and axial loads.

Bearings may be classified into three general classes
Guide or flat bearings, which support linear motion in machine tables and slides.

Thrust bearings

Thrust bearings, which support rotational motion in machine elements that have axial loads i.e., the load is applied along the central axis of the rotating shaft

Radial bearings

Radial bearings, which support rotational motion in shafts with radial loads i.e., the load is applied along the radius of the rotating shaft.

Anti Friction or Roller Element bearings

Anti – friction bearings or roller – element bearings, as they are often called, use a rolling element (ball or roller) between the loaded surfaces.
Anti-friction bearings are divided into two categories,
a) ball bearings
b) Roller bearings.

Ball bearings have five general types:
Guide, Radial, Thrust, Self – aligning and Angular contact.

Roller bearings have four general types: Cylindrical, Thrust, Spherical and Taper.

Roller and Ball bearing types
Guide bearing: The ball guide bearing is used for linear motion where very low co-efficient of friction and extreme smoothness in operation are desired.
Radial bearing: The first radial bearing is the single – row, deep – groove ball bearing, most widely used anti – friction bearing. Second radial bearing is the cylindrical roller bearing is capable of carrying larger radial loads at moderate speeds than those carries by radial ball bearings using the same size bearing.
Thrust bearing: First the ball thrust bearing is designed for axial (thrust) loads only – no radial loads. Second spherical roller thrust bearing is capable of very heavy axial loads as well as moderate radial loads.
Angular contact ball bearing: The shoulders in this provides for thrust (in one direction only) that is larger than the single row, deep radial ball bearing can handle.
Taper roller bearing: A pair of taper roller bearing is capable of handling both very large axial and radial loads.

Shell bearings

In case of very large motors, shell bearings are used. Grease is used as the lubricant in case of roller and ball bearings.

For Shell bearings, lube oil is used as the lubricant.

The bearings are usually designed to withstand radial loads. However, in some applications such as in the use of gears and belts, the motor may also be subjected to axial loads. In such roller bearingscases, bearings such as angular ball bearings which can withstand axial loads can be used.

Laminations in Transformer Core and Motor Stator

 Electric machines, especially AC machines such as transformers and alternators are exposed to alternating magnetic fields during operation.
This alternating magnetic field causes the induction of eddy currents in the core of transformers and the stator of motors. The eddy current creates a loss of energy in the form of heat loss and hysteresis loss.

In order to avoid this, the core of transformers and the stator of motors and generators are made of a set of laminated steel sheets. Silicon Steel is used. This steel is cold rolled and has special grain orientation. Each steel sheet is around .3 mm thick.

The sheets are insulated on both sides and laid of top of one another. This arrangement ensures that the eddy current is reduced as it cannot flow over a wide area of cross section. The laminated surfaces need to be very clean. Presence of foreign particles can cause laminar faults which lead to core damage.

Eye Bolts in Motors-An Overview
An Eye bolt is an important component of the motor. It is used in lifting the machine. The eye bolt consists of a loop at one end and a threaded end at the other. The threaded end is screwed into the motor body. eye bolt

When the motor or alternator is to be lifted, a sling is connected to the eye bolt and the machine is lifted using a crane. It is important to note the capacity of the eye bolt. Every Eye bolt has a WLL ( Working Load Limit). If this is exceeded the eye bolt will fail.

This can result in injury to crew members or even death. Check the capacity of the eye bolt before lifting.

The eye bolt is intended only for vertical lifting. Angular lifting (lifting in an angle) will cause the bolt to fail quickly. shouldered eye boltAngular eye bolts will have the safe Working load limit. As the angle of lifting increases, the WLL decreases. Thus, while the WLL will be maximum at vertical, it decreases as the angle increases. The angle of lift should be calculated and the safe Working Load Limit determined.

There are special types of eyebolts which can withstand angular load (up to a certain degree, usually 45 degrees. Check the angle with the manual).

These are known as shouldered eye bolts. Check that the machine has shouldered eye bolts before lifting the machine.

Grease in Electric Motors
Grease is the most widely used lubricant in electric motors. Grease is used in motors with ball and roller bearings.
The function of grease is to minimize friction and wear,to prevent corrosion and to prevent the entry of foreign objects which can contaminate the bearing.

Grease, thus, has a sealing effect. Grease is a semi-solid lubricant. It is composed of a base oil, additives and a thickener. The base oil can be synthetic or natural. Synthetic oils are used in applications with high temperatures and longer regreasing intervals.

The function of the thickener is to prevent the base oil from leaking. Thickeners are usually metallic soaps. Additives include oxidation and corrosion inhibitors, anti-wear agents.

The grease in motors will have to be replaced over time. The regreasing intervals are based on the bearing manufacturers' recommendations.

Deep Bar Rotors in Induction Motors

Deep Bar Rotors are used in induction motors to increase the torque during starting. Deep bars indicate that the bars which comprise the cage in the rotor are deeper than those in normal rotors.

When an induction motor is started, the slip between the rotor and the stator is high. Thus the frequency of the rotor current is high.

This high frequency results in high reactance in the lower layers of the deep bar. Hence, most of the current flows in the surface of the rotor bars. This results in high current density and increased resistance. This resistance produces high torque during starting.

When the motor reaches its rated speed, the slip frequency drops and the reactance reduces. The current now, flows uniformly across the entire cross section of the rotor bar. The resistance in the rotor drops and the motor runs normally.