In any case, the basic application conditions will have to be defined, whereby the
following factors are significant:
• power transmission: As a single drive the motor can be coupled to the load
directly or via a transmission, or it can be used as a central motor connected
to intermediate shafts, belt and chain drives, etc.
• operating conditions such as overload capacity, frequency of starting, operating mode, peak torques, ambient temperature, etc., affect not only the motor
size requirement, but also the selection of motor accessories.
• space conditions and the layout possibilities of the entire system affect mainly the choice of motor accessories.
4.1 Motor Capacity
The three-phase induction motor is most widely used in drive technologies
because of its simple mechanical and electrical structure and due to its high reliability. Its application is limited only by its torque and speed characteristics.
In the stator winding as well as in the rotor the current passage generates heat;
this heat may not exceed the temperatures specified for insulation materials IP
class. The temperatures which develop depend on the level of the motor load, its
variation over time, and cooling conditions. Motors should be sized such that at
constant load with rated power and rated cooling conditions they do not exceed
maximum temperatures.
• The torque required for accelerating the centrifugal mass increases motor
acceleration time. The starting current flowing during this time heats up the
winding dramatically.
• The maximum change-over frequency, i.e., the number of consecutive starts,
is limited. During frequent starting processes the motor reaches its allowable
temperature limit even without load torque and without an additional centrifugal mass.
• The duty cycle is another important factor for selection. The cooling time at
switching intervals must be long enough to ensure that the temperature limit
is not exceeded during subsequent starting. If the duty cycle is short, the
motor can accept a higher load since it cannot heat up to the temperature limit
during this short time and cools down again during the intervals.
• Undersized motors can be thermally overloaded because of an overly long
starting time, whereas oversized motors would overload the transmission and
the driven machine during the starting process.
Three-phase Induction
Motors
4.2