Motors

Introduction

  • Electric motors defined as electromechanical devices that convert electrical energy to mechanical energy; they are the interface between the electrical and mechanical systems of a facility.
  • Electric motors are an important part of any electrical system. They used throughout every manufacturing plant, office, and home consuming about 64% of all electricity generated.
  • There are numerous ways to design a motor, thus there are many different types of motors and each type possess different operating characteristics (that will be listed later). Based on these characteristics the motor can be chosen for a specified application.

Principle of How Motors Work:

  1. Electrical current flowing in a loop of wire will produce a magnetic field across the loop.
  2. When this loop is surrounded by the field of another magnet, the loop will turn, producing a force (called torque) that results in mechanical motion

Motor basic parts:

Electric machines are classified into two categories D.C. and A.C. motors, the basic parts for each type will be different for each type as follows:

1- AC Motor Basic Parts:

The basic parts for AC motors are as follows:

  1. Enclosure
  2. Stator
  3. Rotor
  4. Bearings
  5. Conduit Box
  6. Eye Bolt

1- Enclosure 

The enclosure consists of a frame (or yoke) and two end brackets (or bearing housings).

A motor’s enclosure not only holds the motor’s components together, it also protects the internal components from moisture and containments. The degree of protection depends on the enclosure type. In addition, the type of enclosure affects the motor’s cooling. There are two categories of enclosures as follows:

  • Open Enclosure.
  • Totally enclosed Enclosure.

A- Open Enclosure 

Open enclosures permit cooling air to flow through the motor. One type of open enclosure is the open drip proof (ODP) enclosure. This enclosure has vents that allow for air flow. Fan blades attached to the rotor move air through the motor when the rotor is turning. The vents are positioned so that liquids and solids falling from above at angles up to 15° from vertical cannot enter the interior of the motor when the motor is mounted on a horizontal surface. When the motor is mounted on a vertical surface, such as a wall or panel, a special cover may be needed. ODP enclosures should be used in environments free from contaminates.

B- Totally enclosed Enclosure

This category will include the following three types:

  • Totally Enclosed Non-Ventilated Enclosure.
  • Totally Enclosed Fan-Cooled Enclosure.
  • Explosion-Proof Enclosure.
a- Totally Enclosed Non-Ventilated Enclosure (TENV) 

In some applications, the air surrounding the motor contains corrosive or harmful elements which can damage the internal parts of a motor. A totally enclosed non-ventilated (TENV) motor enclosure limits the flow of air into the motor, but is not airtight. However, a seal at the point where the shaft passes through the housing prevents water, dust, and other foreign matter from entering the motor along the shaft.

Most TENV motors are fractional horsepower. However, integral horsepower TENV motors are used for special applications. The absence of ventilating openings means that all the heat from inside the motor must dissipate through the enclosure by conduction. These larger horsepower TENV motors have an enclosure that is heavily ribbed to help dissipate heat more quickly. TENV motors can be used indoors or outdoors.

b- Totally Enclosed Fan-Cooled Enclosure (TEFC) 

A totally enclosed fan-cooled (TEFC) motor is similar to a TENV motor, but has an external fan mounted opposite the drive end of the motor. The fan blows air over the motor’s exterior for additional cooling. The fan is covered by a shroud to prevent anyone from touching it. TEFC motors can be used in dirty, moist, or mildly corrosive environments.

c- Explosion-Proof Enclosure (XP) 
  • Hazardous duty applications are commonly found in chemical processing, mining, foundry, pulp and paper, waste management, and petrochemical industries. In these applications, motors have to comply with the strictest safety standards for the protection of life, machines and the environment. This often requires use of explosion proof (XP) motors.
  • An XP motor is similar in appearance to a TEFC motor, however, most XP enclosures are cast iron.
  • Division I locations normally have hazardous materials present in the atmosphere.
  • Division II locations may have hazardous material present in the atmosphere under abnormal conditions.
  • Locations defined as hazardous, are further defined by the class and group of hazard. For example,

– Class I, Groups A through D have gases or vapors present.

– Class II, Groups E, F, and G have flammable dust, such as coke or grain dust.

– Class III is not divided into groups. This class involves ignitable fibers and lint.

2- Stator 

The motor stator consists of two main parts:

A- Stator Core 

The stator is the stationary part of the motor’s electromagnetic circuit. The stator is electrical circuit that performs as electromagnet. The stator core is made up of many thin metal sheets, called laminations. Laminations are used to reduce energy losses that would result if a solid core were used.

B- Stator (Windings) 

Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core.

When the assembled motor is in operation, the stator windings are connected directly to the power source. Each grouping of coils, together with the steel core it surrounds, becomes an electromagnet when current is applied. Electromagnetism is the basic principle behind motor operation.

3- Rotor 

The rotor is the rotating part of the motor’s electromagnetic circuit. Magnetic field from the stator induces an opposing magnetic field onto the rotor causing the rotor to “push” away from the stator field.

There are a lot of rotor types like Squirrel cage rotor and wound rotor, they will be explained later.

4- Bearings 

Bearings, mounted on the shaft, support the rotor and allow it to turn. Not all bearings are suitable for every application; a universal, all-purpose bearing does not exist. The choice of bearing arrangement is based on the following qualities:

  • Load carrying capacity in the axial and radial direction.
  • Overspeed and duration.
  • Rotating speed.
  • Bearing life.

The size of the bearing to be used is initially selected on the basis of its load carrying capacity, in relation to the load to be carried, and the requirements regarding its life and reliability.

Other factors must also be taken into consideration, such as operating temperature, dirty and dusty environmental conditions, and vibration and shocks affecting bearings in running and resting conditions.