Spindle motor temperature and cooling

I have recently been renovating a Denford Orac lathe, mainly replacing all the electronics, and which is now almost finished to the point that I can start using the machine. It has the original ASEA TEFC 1/2HP spindle motor driven from a VFD. The motor is nominally 1425rpm at 50Hz but driven at up to 100Hz to get the full range of spindle speeds.

I was doing some testing where the machine was running for an hour or so at about 600rpm which means that the motor was being driven at about 30Hz and running quite a bit slower than its rated speed. The motor case reached around 60C or maybe a touch more, judging with a hand on the case. Seems a bit hot to me. I have taken the fan cowl off to clean dust out of the ducts and brushed out the motor fins. My concern is that the fan loses effectiveness as the speed drops.

Is this temperature normal or should I be concerned? Should I be looking at a separate fan to provide consistent cooling? This is what the variable speed motor on my mill uses and the airflow there is much greater - although it is a 2HP motor with 200-10000rpm range.

Pain threshold for most people is 50-55C, so that sounds like a pretty modest temp rise. The motor label will tell you the "insulation class" which is most likely "B" (130C) or possibly "F" (155C) or even "H" (180C). If you can touch it with your hand you have nothing to worry about. Motors that are intended for continuous use at low speeds often have a cooling fan attached but I wouldn't worry about it myself.

I've always taken the "just about able to hold it" temperature to be about 60C, based on something I read years ago, but I must admit that I have never actually tested my personal temperature calibration!

My concern is that I'm not that certain that the motor is fully rated for slow-speed operation, but as it is the one that Denford fitted together with their original speed control board, I guess it's OK. Have to say that it does seem like a fairly big motor for only 1/2HP but it's presumably the one that came with the lathe and that dates back to the 70s. It originally came with a cassette recorder for toolpath storage!

Thanks for the comment - I'll check the label and just keep an eye/hand on the motor when using it to make sure it doesn't get stupidly hot.

60C seems modest to me but a computer fan is cheap and easy to add.