Spindle motor temperature and cooling

[Neale]

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.