Regarding those cheap Ametek motors, I've had about 3 of them, the first one got very hot and stopped working first time out the box, the other two were smelling of plastic and over heating also. By swapping brushes around I realised this was due to the new brushes which are ribbed and only have around half the contact area on the commutator, until they are sufficiently worn in.
However I have had other similar motors that haven't had this problem but, once you get the brushes right on these motors I think they can last just as long as others.
I've got a temperature gun, but if you don't have one, I would switch the motor on, wait maybe 5 mins maximum and if you're getting a strong melting plastic smell turn the motor off. I would then try and remove the ridges on the end of the brushes by some suitable method, so that the full area of the brush is on contact with the commutator.
It could be that the motors are designed to run hotter for the first few hours of their life but I'm not sure.
I believe the brushes are the main cause of nearly all vac motor problems, with them getting either too short, over heating etc. I've had many motors that have taken in plenty of water but after new brushes and a sanding down of the commutator they are fine, plus after taking the motors apart you find the water never reaches any electrical parts of the motor or any bearings.
Hope this possibly helps someone. If you learn how to regularly change the brushes then I don't see why these cheap Ametek motors shouldn't be alot more economical than paying over £100 for a motor.
