ELECTRICAL MACHINES

electrical machine

An electrical machine is a device that can convert either mechanical energy to

electrical energy or electrical energy to mechanical energy. When such a device is

used to convert mechanical energy to electrical energy, it is called a generator.

When it converts electrical energy to mechanical energy, it is called a motor. Since

any given electrical machine can convert power in either direction, any machine

can be used as either a generator or a motor. Almost all practical motors and generators convert energy from one form to another through the action of a magnetic

field.

The transformer is an electrical device that is closely related to electrical

machines. It converts ac electrical energy at one voltage level to ac electrical energy

at another voltage level. Since transformers operate on the same principles as

generators and motors, depending on the action of a magnetic field to accomplish

the change in voltage level, they are usually studied together with generators and

motors.

These three types of electric devices are ubiquitous in modern daily life.

Electric motors in the home run refrigerators, freezers, vacuum cleaners, blenders,

air conditioners, fans, and many similar appliances. In the workplace, motors provide

the motive power for almost all tools. Of course, generators are necessary to

supply the power used by all these motors.

Why are electric motors and generators so common? The answer is very simple: Electric power is a clean and efficient energy source that is easy to transmit

over long distances, and easy to control. An electric motor does not require

constant ventilation and fuel the way that an internal-combustion engine does, so

the motor is very well suited for use in environments where the pollutants associated

with combustion are not desirable. Instead, heat or mechanical energy can be

converted to electrical from at a distant location, the energy can be transmitted

over long distances to the place where it is to be used, and it can be used cleanly

in any home, office, or factory. Transformers aid this process by reducing the energy

loss between the point of electric power generation and the point of its use.

Almost all electric machines rotate about an axis, called the shaft of the machine.

Because of the rotational nature of machinery, it is important to have a basic understanding of rotational motion. This section contains a brief review of the concepts of distance, acceleration, and power as they apply to

rotating machinery. For a more detailed discussion of the concepts of rotational

dynamics.

AC machines are generators that convert mechanical energy to ac electrical

energy and motors that convert ac electrical energy to mechanical energy.

The fundamental principles of ac machines are very simple, but unfortunately,

they are somewhat obscured by the complicated construction of real machines.

There are two major classes of ac machines-synchronous machines and induction

machines. Synchronous machines are motors and generators whose magnetic

field current is supplied by a separate dc power source, while induction machines

are motors and generators whose field current is supplied by magnetic

induction (transformer action) into their field windings. The field circuits of most

synchronous and induction machines are located on their rotors.

WINDING INSULATION IN AN

AC MACHINE

One of the most critical parts of an ac machine design is the insulation of its windings.

If the insulation of a motor or generator breaks down, the machine shorts

out. The repair of a machine with shorted insulation is quite expensive, if it is even

possible. To prevent the winding insulation from breaking down as a result of

overheating, it is necessary to limit the temperature of the windings. This can be

partially done by providing a cooling air circulation over them, but ultimately the

maximum winding temperature limits the maximum power that can be supplied

continuously by the machine.

Insulation rarely fails from immediate breakdown at some critical temperature.

Instead, the increase in temperature produces a gradual degradation of the insulation,

making it subject to failure from another cause such as shock, vibration,

or electrical stress. there was an old rule of thumb that said that the life expectancy

of a motor with a given type of insulation is halved for each 10 percent

rise in temperature above the rated temperature of the winding. This rule still applies

to some extent today.

To standardize the temperature limits of machine insulation, the National

Electrical Manufacturers Association (NEMA) in the United States has defined a

series of insulation system classes. Each insulation system class specifies the

maximum temperature rise permissible for that class of insulation. there are three

common NEMA insulation classes for integral-horsepower ac motors: B, F, and

H. Each class represents a higher permissible winding temperature than the one

before it. For example, the armature winding temperature rise above ambient temperature

in one type of continuously operating ac induction motor must be limited

to 80°C for class B, 105°C for class F, and 125 °C for class H insulation.

The effect of operating temperature on insulation life for a typical machine

can be quite dramatic. A typical curve is shown in Figure. This curve shows

the mean life of a machine in thousands of hours versus the temperature of the

windings, for several different insulation classes.

The specific temperature specifications for each type of ac motor and generator

are set out in great detail in NEMA Standard MG 1-1993, Motors and Generators.

Similar standards have been defined by the International Electrotechnical

Commission (IEC) and by various national standards organizations in other

countries.