Clarke 0.50HP 2800 rpm motor behaving oddly.

[Andrew TinsleyParticipant @andrewtinsley63637]

The motor is a Clarke 71B/2 split phase device with a quoted speed of 2800 rpm and 0.5HP output. It has probably been used for less than 30 minutes from new. It was given to me by a friend who needed a higher HP motor for his project.

I fitted it to a linisher and all seemed OK, but it seemed to be running slow. Checked with an accurate mechanical rpm indicator and yes it was running at 1000 rpm! After a minute or so, it suddenly speeded up and was running at around 2600 rpm! This was quite repeatable, although the time to full rpm varied considerably

I assumed, naively, that being a split phase motor and hence lacking starting torque, this was the problem. So I removed the motor and decided to fit a capacitor start / capacitor run motor.

Just to make sure, I again checked the motor speed with nothing connected to the shaft. Exactly the same problem!! although the time between starting speed (1000rpm) and final speed (2600rpm) varied from approximately 1 second to a minute or so!

I then checked the wiring against the Clarke wiring diagram, no problem there. So I finally checked the 400 V capacitor. My capacitance meter gave 16.4 microfarad against a stated 17 microfarad on the capacitor. I very gingerly checked the insulation resistance with a hand cranked Megger (keeping below 400 V) all was fine.

Has anyone any idea what is causing this weird behaviour? I cannot figure out what is going on.

Andrew.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Andrew, no idea what’s going on, but your 16.4 mf capacitor is within a +/- 5% tolerance for a 17 mf one.

Regards Nick.

[Peter Cook 6Participant @petercook6]

Sticky centrifugal switch?

[Andrew TinsleyParticipant @andrewtinsley63637]

Hello Peter,

I did say that the motor is the split phase variety. They don’t have centrifugal switches. They act as a 2 phase motor with the capacitor permanently in circuit to give the approx 90 degree phase shift. By far the most vibration free single phase set up. But woefully poor on starting torque.

Nick I deliberately posted the measured value and the nominal value, to show that there was nothing wrong with the capacitor!

Andrew.

[noel shelleyParticipant @noelshelley55608]

Can we assume that the bearings are free ? Does it spin free ? Noel.

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