Genuine Dyson V8 motorhead assembly

When would this part need replacing?
When it is damaged
When the brushbar stops spinning
When there is a burning smell coming from the motorhead

Fits The following Dyson V8 models:

Dyson V8 Total Clean vacuum
Dyson V8 Absolute Pro
Dyson V8 Absolute vacuum

Genuine Dyson V8 Motorhead assembly 967483-01