Motors:Asynchronous induction motors

Asynchronous induction motors

Main types and operating principles

The cage induction motor is by far the most widely used type of electric motor in industry. It is both rugged and reliable; it is also the preferred choice for most variable speed drive applications. Simplicity, low cost, high reliability, fairly high efficiency, coupled with its ease of manufacture, make it widely used in most parts of the world.

Figure 6.3 shows the typical arrangement of a squirrel-cage induction

motor, which is built with three sets of stator windings arranged around the stator core. The rotor has conductors, which are a cage of copper or aluminium bars and short-circuiting end rings. There are no electrical connections to the rotor.

The wound-rotor induction motor, as the name suggests, has insulated copper windings in the rotor similar to those in the stator. The rotor wind­ ings are connected to starting equipment using slip rings and brushes, and therefore this design is substantially more costly, and gives more mainten­ ance problems than a squirrel-cage rotor. This type of induction motor was formerly used in many industrial applications where the starting current and torque needed to be controlled. There are also a number of methods of achieving speed control with a wound rotor machine. However, in new applications, squirrel cage motors are by far the most widely used solution.

For all induction motors the basic principle of operation is that a three­ phase voltage supply applied to the stator windings results in the creation of a magnetic field that moves around the stator- a rotating magnetic field. The moving magnetic field induces currents in the rotor conductors, in turn creating the rotor magnetic field, see Figure 6.4.

Magnetic forces in the rotor follow the stator magnetic field, producing a motor torque. The speed of an induction motor is determined by the frequency of the power supply, by the number of poles in the stator winding and to a smaller extent by the motor load.

The standard construction of a cage rotor motor is designed for use with a symmetrical three-phase electricity supply. The three phases are dis­ placed by 120° electrically, which provides an inherent rotation of the internal magnetic field. It is also possible to make alternative constructions, the most common being a single phase motor. In this case a phase dis­ placement is usually achieved by means of a capacitor which provides a 90° basic phase shift, and the winding displacement is based on this value.

An asynchronous motor powered from the mains, has essentially a constant output speed, which decreases only a few percent, typically 1 to 3% (known as slip), when the motor goes from no-load to full load operation. Therefore, to control the motor speed without the use of external mechanical devices, it is necessary to control the supply voltage and frequency.

The no-load, or synchronous speed for a motor is defined by the

formula:

Motor efficiency

The majority of induction motors are designed and manufactured to meet design standards, which specify output power, synchronous speed and critical dimensions.

Table 6.1: No load (synchronous) speeds for typical motors with different pole numbers and supply frequencies

There are two basic sets of standards, one issued by the IEC and defining metric dimensioned, kW rated motors. These are "Harmonised" within the EU by CENELEC and become "EN" standards.1 NEMA has developed the other set of standards in North America, which define inch/horsepower designs.

Neither basic standard specifies the efficiency or power factor of the motor in question. However, in order to achieve the physical dimensional constraints laid down, there is a practical minimum efficiency inherent in each standard. This is relatively low and a wide range of efficiency and other characteristics can be offered. Within the EU efficiency bands have been agreed. In North America, minimum efficiency standards are man­ dated by the EPACT legislation, for more information refer to Appendix A 1.

No motor is 100% efficient, and therefore its design must take into

account the removal of the losses, which are predominantly in the form of heat. Heat from the stator can be dissipated through the motor carcass, however heat from the rotor is generally passed down the shaft, and can increase the bearing heating.

As the windings of motors generate losses, the insulation materials used will also have to cope with the resulting temperature rise. The specifi­ cations for winding insulation are based on the temperature rise capacity of the materials. In general the specification is based on a 40oc ambient, with Class B materials being capable of a rise of 80 Kelvin, Class F of 105 Kelvin and Class H of 125 Kelvin for continuous duty (NEMA service factor 1.0). Motor outputs are quoted on the basis of a specific temperature rise

i.e. 75 kW with Class B rise, while the motor may well be wound with Class F materials. Class H insulation is more costly and less usual in AC motors.

Output torque

It is conventional to designate motor output in terms of power, but it must be remembered that the motor is in fact a source of torque.

For a variable speed system it is always necessary to consider the required torque/speed characteristic. Many motors, both fixed and variable speed, are designed specifically to match the load speed/torque demand.

The physical size of a motor is directly related to the torque it can deliver.

Catalogue values for two typical IEC motor frames fed at 50 Hz and for two typical NEMA motor frames fed at 60 Hz, are shown in Table 6.2. This shows how similar torque levels are generated by the frame, irrespective of the number of poles.

Induction motors operated at variable speed

Induction motors, which are the most widely used drivers for pumps in the industrial and in the residential market sectors, can in most cases be operated at variable speed, by an electronic frequency converter, without modification.

By applying the appropriate variable frequency and variable voltage waveforms to the stator windings, the motor speed will vary proportionally to the applied frequency.

Some specific guidelines do apply to selecting a suitable motor, or to retrofitting a drive to an existing motor (see Chapters 10 and 11).