Induction motor starting capacitor - Page 1
Forum section: electronics / repair

author: Emotor
author_rank: Newbie
timestamp: January 04, 2022, 01:51:45 pm
content: I’m looking for advice regarding the replacement of a START capacitor in a single-phase 4-pole induction motor. This is a capacitor-start motor with a centrifugal clutch. There is is no run capacitor.SOME BACKGROUND INFO FIRST:From memory, the motor is plated as follows:230 V50 HZ0.75 HPThe motor was purchased secondhand in working order. When I checked the motor start capacitor I was surprised to find it rated 100 uF 125 WV. The working voltage appears to be under-spec as this motor is rated 230 V RMS (325 VAC peak). I presume the capacitor should ideally be replaced with one rated 450 V as this appears to be a typical value used for motor starting capacitors. Although the motor started and ran with the 100 uF 125 WV capacitor fitted, I do not know the recommended capacitance value that should be used.MY QUESTIONS:Am I correct in saying the existing capacitor should be replaced?Is there a calculation / measurement / rule of thumb I can use to choose a suitable capacitance value? The only “references” I can find are dubious YouTube videos that lack authority or sources.I have a new 100 uF 450 V motor starting capacitor that could be used on this motor. I would estimate it has 8 times the volume of the existing capacitor (it looks physically massive) and would need a new mounting bracket / terminal cover which I could easily fabricate. If I end up needing 100 uF 450V, are much smaller capacitors available with suitable electrical characteristics?

author: Ian.M
author_rank: Super Contributor
timestamp: January 04, 2022, 02:32:25 pm
content: 1. Set up to measure the voltage across the capacitor during a start. Its likely to be a lot less than the supply voltage, and due to its limited duty cycle, the capacitor can be operated very close to its voltage rating.2. Only the motor manufacturer knows the originally intended starting winding current and phase angle. Without that, you are reduced to dubious rules of thumb and comparison with similar motors.3. The volume is pretty much determined by the capacitance times the working voltage. Doubling the working voltage doubles the required dielectric insulation thickness, which halves the capacitance so twice as much plate area is needed. If that 450V rating is for AC, you'd do better to find a 250V rated cap instead which will be significantly more compact.

author: Emotor
author_rank: Newbie
timestamp: January 04, 2022, 10:56:11 pm
content: Ian, thank you for your reply.It now seems plausible the 125 VAC "working voltage" capacitor fitted may be original to the motor. I will follow your suggestion and measure voltage across the capacitor during a start. It certainly gives me a reasonable place to begin.Incidentally, now that I am home I checked the 450 V capacitor. It's actually labelled 450 VAC and therefore over-rated for this particular application.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: January 04, 2022, 11:05:00 pm
content: only see numbers, i.e will assume it full parallel to M ?try RE to get schematic that most likely not so, but C is series in V divider instead to surely have it far lower V