Nature of Electric Supply:

The nature of electric supply available may be 3-phase ac, single-phase ac or dc.

In case three-phase ac supply is available, polyphase induction motor, squirrel cage type for small ratings and slip-ring type for higher ratings may be used, provided this suits the requirements of the load. In the cases where speed variation is required these cannot be conveniently used, so pole changing motors or motors with stepped pulleys may be used. Where accurate control of speed is required, Schrage motors may be used. Use of single-phase motors is limited to small loads only owing to their limited outputs.

DC motors are not used so widely as ac ones. There are several reasons for this, the most important of which are given below :

1. At present a large percentage (in fact, the whole) of the electrical energy used for domestic and commercial purposes is generated in ac form because of economic and technical reasons, i.e., nature of electric supply available is usually ac. Additional equipment is, therefore, required for converting existing ac supply into dc supply.
2. DC motors have commutators that are subject to trouble resulting from sparking, brush wear, arc-over and the presence of moisture and destructive fumes in the surrounding air.
3. DC motors are generally more expensive than ac motors for similar working conditions.

On contrary, ac motors have the advantages of possessing few working parts, requiring less maintenance and replacement of spares and providing uninterrupted long service life. Three-phase induction motors have high efficiency and provide high starting and uniform operating torques and balanced loading of nature of electric supply.

DC motors are nevertheless used in a large number of applications, specially when their excellent torque and speed operating characteristics cannot be duplicated by ac motors. Moreover, the speed control that can be provided for dc motors is far more flexible and satisfactory than that which can be provided for ac motors. These requirements are particularly significant in connection with services such as operation of traction equipment, hoists and elevators. In some cases, such as in electric excavators, steel mills and cranes, the speed control is so important that existing ac supply is converted into dc supply in order to employ dc motors.

With electrical utilities transmitting energy on ac, the usual arrangement used to be a motor-generator set (a dc generator driven by an ac motor) supplying power to a dc motor. However, the advent of solid state diodes and silicon controlled rectifiers (SCRs) eliminated the need for the motor-generator set and made dc motor drives more versatile and more popular than even before.

Nature of dc supply available, to some extent affects the performance, and therefore, size of the motor required. DC supply available from a dc generator will be more or less smooth but the output voltage from thyristor converter consists of a dc component and ac harmonic components. Torque or mechanical power is developed by the dc component of the current where as armature heating is developed by the effective or rms value of current. The form factor for half-wave three-phase thyristors may be taken as 1.2 while for full-wave three-phase thyristors it is 1.1. This increases electrical losses, and, therefore, heating is 5 to 7% more for three-phase full-bridge converters while for three-phase half-bridge convertors it is from 15 to 20%. Thus when a dc motor is driven from thyristor convertor, slightly large size motor will be required for a given kW power requirement.

The commercial ac power is generated at a frequency of 50 Hz but this frequency is not capable of meeting the requirement of certain industries, defence services and railway signaling. As such, there has been recent trend towards development of high frequency power generation. Relative merits and demerits of high-frequency and power-frequency equipment are enumerated below.

Merits of High Frequency:

1. The output of the equipment increases with the increase in frequency or speed, as obvious from the following equations

The output of an alternator per phase,

where

• K_f, K_p and K_d are the form factor, pitch factor and distribution factor,
• Φ is the useful flux per pole in webers,
• f is the frequency in hertz,
• T is the number of turns in series per phase,
• I is the output current per phase and
• cos Φ is the power factor.

The output of a motor is given as

where

• D is diameter,
• L is the length and
• N is the speed of motor in rpm.

For a given power output, high frequency machine may be of one-eighth of the weight of the 50 Hz machine. Because of high power-weight ratio, relative cost of the equipment is reduced with the increase in frequency. Reduction in weight of equipment makes also transportation and handling of equipment easy and convenient at high altitude for defence services.

1. For the generation of high frequency power, along with the small sized generator, the prime mover will also be of smaller size. This will result in reduction of high noise problem.
2. High frequency equipment requires less maintenance and has low operating cost, higher efficiency and long life span requiring minimum repairing cost.
3. More reliable service is provided, in arduous conditions of high altitude, low temperature and high humidity, by high frequency equipment.

Disadvantages of High Frequency Equipment:

The drawback of high frequency equipment is that it has a problem of increased power losses due to hysteresis and eddy currents. However, this problem of large power losses can be overcome by using low loss materials, proper flux density and adequate cooling arrangements.