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I'd like to improve the very slow startup of a bench grinder motor. I have linked a video which shows its current behavior:

https://youtu.be/KPIgmcJJi5E

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?

Original capacitor:

enter image description here

Same-sized replacement capacitor:

enter image description here

Oversized replacement cap:

enter image description here

Nameplate:

enter image description here

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  • \$\begingroup\$ You do know the new capacitor is (a) 12-15x the value of the original and (b) not rated for continuous operation, right? Why didn't you replace the original with another 25uF 250V run capacitor? \$\endgroup\$
    – user16324
    Commented Sep 6, 2020 at 21:48
  • \$\begingroup\$ @BrianDrummond given that the motor was not running correctly with the original capacitor, IIRC I wasn't sure that was the right thing to do. Also I had read (maybe incorrectly?) that a larger than necessary capacitor would be OK. Possibly something smaller than the current on e was not readily available at the time (this is a project from a few years ago I am "resurrecting") \$\endgroup\$
    – StayOnTarget
    Commented Sep 6, 2020 at 21:57
  • \$\begingroup\$ Was slow time to accelerate the only problem originally? How long does it take now? I suspect it is now drawing too much current because of wrong capacitor. Internal protection may be shutting it down then resetting after a time that you neglected to mention. \$\endgroup\$
    – user80875
    Commented Sep 6, 2020 at 23:12
  • \$\begingroup\$ @CharlesCowie yes, slow acceleration + the dimming lamp (voltage drop?). As it is now, it starts up instantly when switched on. Do you mean like a thermal switch is opening? \$\endgroup\$
    – StayOnTarget
    Commented Sep 6, 2020 at 23:19
  • \$\begingroup\$ Yes, I mean like a thermal switch. What is the time before it restarts. \$\endgroup\$
    – user80875
    Commented Sep 6, 2020 at 23:22

1 Answer 1

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That time to start up seems fairly normal for a bench grinder. But the behavior of the lamp seems to indicate a lot of voltage drop, which could be a weak supply, or excessive current draw.

You might be able to add a start pushbutton or relay to briefly connect a larger capacitor to get quicker start-up, but you might risk burning the run winding.

I know this is an old question, but since there has been another (not very helpful) answer, I thought I should give a better answer that might help the OP or others who might see this post.

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