ABB
6
ABB | Motors and Generators
| Synchronous motors
The rotor is key to excellent overall performance
The rotor plays a crucial part in achieving the best
possible electrical and mechanical performance.
Higher speed motors
(four and six poles) have solid rotors with integrated poles
made from a one-piece steel forging. A solid pole plate is
attached to the pole, and this design has good overload
capability and low harmonics. At the same time this rotor
configuration contributes to the motors’ excellent starting
characteristics, high starting torque and low starting current.
The large cooling surfaces and effective flow of cooling air
mean that the rotor temperature remains low and uniform,
helping to ensure reliability and a long operating life.
The coils are class H insulated for extra thermal margins.
The rigid rotor construction and minimum distance
between bearings ensure that the motor operates below
the first critical speed, keeping vibration levels low.
Lower speed motors
(eight or more poles) typically have rotors with the poles
manufactured from 2 mm laminated steel sheet. The sheets
are pressed together with inserted steel bars which are
welded to the end plates. The poles are secured to the shaft
or rotor center by bolts from above or below, or by means of
dovetails. A copper or brass damper winding is often fitted.
For lower speed motors the technically preferred option is to
vacuum pressure impregnate (VPI) the rotor assembly after
it is wound to achieve excellent insulation and mechanical
strength. After impregnation, the complete rotor assembly
is dynamically checked for balance.
Permanent magnet poles are used in certain variable speed
applications. Permanent magnet rotors are straightforward
constructions and do not need an excitation system.
When necessary, high pole numbers can be used in low
speed applications.