If its a variable speed motor a likely technical reason is that the motor controller has an internal "run-up to the set speed" process when first turned on to avoid any potential overloading issues during acceleration. If the system is effectively just a simple motor on switch set-up coupled to an already running control board there is no intrinsic control of start up loads as the motor tries to demand all the power it needs to get going.
There are various ways of implementing this sort of load control with the motor board still powered up. Forcing a full power down sequence on the control side each time the motor stops is probably the simplest, near bullet proof, method. If the control board stays powered up there are also, arguably, potential issues should the user change their mind and restart things during the run down period after the stop button has been pressed. Another potential problem area is if the motor fails to start or properly run-up to speed when the start button is pressed and the user simply presses the stop button before instantly hitting the start button for another try. Inadvertently trying to run up a large cutter with the spindle speed set close to maximum or trying to start one too close to the job so it immediately goes into cut sound like the sort of thing that could cause such issues. The ease of making such errors is one reason why I'm not particularly in favour of wide range varispeed motors.
As ever the practical validity and impact of such concerns depends on the engineering of the machine in question. As always things done right will be fine but in a practical word the price performance / ratio required of product that folk can actually afford means some compromise between always works whatever, works but don't be stupid and usually works. Sometimes bit of tweaking on how you can use something keeps you out of the region where, presumably less costly, usually works doesn't.
Clive.
Edited By Clive Foster on 04/10/2017 13:17:42
Edited By Clive Foster on 04/10/2017 13:28:26