The main board in my old rechargeable philoshave failed. Not wanting to give up easily, I soldered direct onto the DC motor (as pictured). The motor is rated for 3.6V (pictured), but I get a much better cut if I “overclock” the motor by about 20% in terms of voltage - i.e. 4.5V rather than 3.6V. Is this safe to do ? The motor doesn’t seem to be running hot after a few minutes under load (in this case a shave). What are the general guidelines / tolerances allowed for this sort of thing ?
It will probably last for ever. Thats not much over and for the short time of a shave. would not worry it the least.
The voltage for a given dc motor is the nominal or average voltage for it based on n number of hours of operation, number of wire loops in the coil, construction, etc. The operating voltage is actually a range from minimum to turn on, minimum to stay on, and maximum voltage, just like any inductor. You can see the specs if you can find the data sheet from the motor's part number. There is no general guideline for it because there's no "general" type of combination of motor parameters.
That said, +20% (more like 25%) is likely the most you want to run through it or it might overheat and burn itself out, over time since you are just running it for short periods.
I had a 2.4V shaver that when the batteries failed, rather than replace them, I built a 12V to 3V converter to power it. ( I had also discovered that it worked better at the higher voltage. ) I kept both in the car for several years until the blades got too dull to use. So it should be safe and last as long for you.