ABB的同步电机样本里有一段话，也许可以说明什么情况用有刷，什么情况用无刷。我的理解是，按照ABB的样本，有刷无刷跟定速和调速有关。如果认为这个理解是正确的，那我就想知道为什么？这也是我发这个帖子的目的之一。

'For fixed speed applications and variable speed applications with less demanding dynamic control requirements, a brushless exciter and automatic voltage regulator are generally provided. The brushless system has no wearing parts, and the external excitation power requirement is low.
The brushless exciter is a separate AC generator mounted on the motor shaft at the non-drive end. In most fixed speed motors the field winding is DC fed; in variable speed motors (and fixed speed motors with LCI starting) the field winding is AC fed.'

'For variable speed applications where very fast and accurate speed or torque control are required, the motor is generally equipped with brushes and a slip ring unit to allow excitation and control of the motor from the frequency converter. The slip rings are mounted on the motor shaft...'