I'd like to improve the very slow startup of a bench grinder motor. I have linked a video which shows its current behavior:
The video shows the grinder being switched on from 'cold' and taking a few seconds to accelerate up to full speed. Note that the (LED) lamp doesn't light until its close to full speed.
In my experience with other motors, this is pretty strange. Generally they go to full speed extremely quickly, even under load.
Its a single-phase 120V motor. The bearings seem fine - it rotates normally.
It has a run capacitor but no start capacitor.
What I have done so far:
I thought it might be a bad capacitor - when I replaced it with one having the same specification as the original (25 uf), there was no change. So I assume the original was OK. When I tried a much larger capacitor (300-360 uf) it accelerated instantly; but that cap was clearly much too large as it seemed to trigger the thermal overload.
Its possible the original capacitor was not OEM (the grinder is second-hand). But according to Electric Motor Starting Capacitor Selection table "Estimated Small Motor Capacitor "Ballpark" Sizes Based on Motor Type" on it seems roughly OK. Its also a bit larger than the recommended info in table "Estimated Small Motor Capacitor "Ballpark" Sizes Based on Motor Horsepower".
Can I select a larger capacitor to improve startup, but without causing other problems? Or is there some other (electrical) update I could make?
Same-sized replacement capacitor:
Oversized replacement cap: