Page 139 - Handbook of Electrical Engineering

INDUCTION MOTORS 121

500 to 516 give recommendations for the use of motors in hazardous areas and different types of
environment, see also Chapter 10. Air is arranged to pass along the air gap to absorb the rotor heat
and along and between the stator windings to absorb the stator heat. The heat is radiated from the
outer surface of the stator frame. The design of the fans and the air paths is a complicated subject
and has to be optimised for each type of motor and its rated speed.

As the motors become larger the removal of heat becomes more difficult to achieve and hence
more elaborate means need to be employed. To rely solely on simple surface radiation from the stator
would not be a sufficient means for motors above about 50 kW. A second air circuit is created by
mounting an external fan on the non-drive end of the rotor. This fan draws in cool air from the
non-drive end face, under a cowling, and blows it over the stator surface. The stator surface may
be ribbed to increase the surface area or be fitted with longitudinal air tubes. These methods are
satisfactory for motors up to about 500 kW. Beyond 500 kW the methods of fan cooling can become
very elaborate, involving large air-to-air heat exchangers or even air-to-water exchangers.
Ingress of water and particles is defined in various international standards as outlined in
Chapters 3 and 10.

5.4 DERATING FACTORS
In common with other power system equipment, motors need to be derated to suit a high ambient
temperature. Equipment that is manufactured in America, UK and Europe is usually based on a
◦
◦
maximum design temperature of 40 C. For higher ambient temperatures, e.g. 50 C as found in the
Middle East and Far East, the continuous duty output power and supply current would need to
be reduced. The continuous duty is that as defined as type S1 in IEC60034 part 1. International
◦
standards recommend performance and design criteria suitable for 40 C. Although most of these
◦
standard requirements will apply to ambient temperatures above 40 C there may be some addition
restrictions to apply. In particular aspects of full-load current, duty, radiation of heat loss and outer
surface temperature will need to be considered, see for example IEC60034 part 1 clauses 11 and
16.3. Some countries that experience high ambient temperatures and who enjoy a substantial ‘home
market’ for their own products, such as India, use national standards that set the ambient temperature
◦
to a higher value such as 45 C, which is more practical in their circumstances. When a purchase
specification is being prepared it is recommended that this aspect of operating a motor continuously
at or near its full-load rating in a high ambient temperature is highlighted.
IEC60085 and IEC60034 part 1 describe the limitations placed on materials used inside motors
(and other electrical equipment). Most electrical machines with air or gas as the cooling medium use
Class B or F solid insulation material. Where the environment is harsh, and high ambient temperatures
occur, then it is advisable to specify Class F insulation materials but with a restriction of Class B
temperature rise. Such a specification will inherently increase the mean time to failure of the materials
since they will be less stressed.

5.5 MATCHING THE MOTOR RATING TO THE DRIVEN MACHINE
RATING

The importance of having sufficient motor torque for all speeds has been described earlier. For general
guidance it is possible to choose the kW rating of the motor on a ‘rule-of-thumb’ basis by using
Table 5.11 below.