Many people do not realize that it is necessary to clean out the bucket of a central vac as soon as it is half full, not only to keep it working properly but to avoid extra stress on the motor.
Watch the suction level at every outlet. If it is weak, there is a clog someplace, which must be cleared out, both for the machine to work properly and to reduce pressure on the motor.
Even with a portable vacuum cleaner, you need to keep the bag less than half full, and occasionally clean all the filters, which are designed to protect the motor from dust. If they are clogged, no dust will get to the motor, but it will over-heat from a lack of cooling air, which is just as bad.
Motors are probably the weakest part of any vacuum and are expensive to replace if you burn them out. If you keep everything clean and the air flowing freely, the motor should last for a very long time.