Clarke 0.50HP 2800 rpm motor behaving oddly.

This topic has 16 replies, 9 voices, and was last updated 6 August 2026 at 21:00 by Andrew Tinsley.

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5 August 2026 at 15:20 #859498
Andrew Tinsley
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@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.
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5 August 2026 at 16:12 #859501
Nicholas Farr
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@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.
5 August 2026 at 17:17 #859502
Peter Cook 6
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@petercook6
Sticky centrifugal switch?
5 August 2026 at 17:30 #859503
Andrew Tinsley
Participant
@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.
5 August 2026 at 18:54 #859515
noel shelley
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@noelshelley55608
Can we assume that the bearings are free ? Does it spin free ? Noel.
5 August 2026 at 19:30 #859517
duncan webster 1
Participant
@duncanwebster1
On 5 August 2026 at 17:30 Andrew Tinsley Said:

Hello Peter,

….

They don’t have centrifugal switches. They act as a 2 phase motor with the capacitor permanently in circuit to give ……

Andrew.

I don’t think that is correct, you can have capacitor start, which do have centrifugal switch, and capacitor run, which don’t, both are split phase
5 August 2026 at 19:48 #859519
Andrew Tinsley
Participant
@andrewtinsley63637
Hello Duncan,

Not going to argue on the definition. For what it is worth, my view of capacitor start motors, is that they are only split phase, until the start winding is disconnected via the centrifugal switch. The capacitor run motor is a true split phase motor. This Clarke motor does not have a centrifugal switch, it is a true split phase motor and is effectively a two phase system.

Always willing to be proved wrong!

Andrew.
5 August 2026 at 21:51 #859536
Macolm
Participant
@macolm
There are several alternative start winding switching arrangements other than a centrifugal switch. For example, domestic single-phase refrigerator compressors often use a timer to control the start winding. It is possible that the motor has some such device that is a bit out of spec.
5 August 2026 at 22:20 #859537
Andrew Tinsley
Participant
@andrewtinsley63637
Hello Malcolm,

There is no form of internal switch in this particular motor. I know that for certain, having just dismantled it. It is a standard capacitor run, split phase motor.

Having reassembled it, it now appears to be running correctly, it immediately it runs up to 2750 or so. I will reinstall it in the linisher and see if it behaves, The rotor still runs very freely as it did before I dismantled it, so nothing to do with alignment of endplates. Anyway, I know that the motor has never been dismantled since purchase from new.

Andrew.
6 August 2026 at 10:11 #859586
Andrew Tinsley
Participant
@andrewtinsley63637
The mystery deepens. Put the motor back on the linisher and the previous faults reappeared! Took it off and in standalone mode, the fault was still there. It almost appears that there is something that is preventing the motor from running up to full speed. Yet the rotor is completely free with no sign of any resistance to turning. I give up, A capacitor start motor, having the same frame, will be installed. Can’t waste any more time on this weird problem.

Andrew.
6 August 2026 at 10:41 #859601
Howard Lewis
Participant
@howardlewis46836
If the motor spins up normally when off the linisher, but not when installed, it suggests thst the linisher is overloading the motor until it frees up, (presumably bearings freeing off?) Have a look at the machine, now that you have examined the motor.

Howard
6 August 2026 at 11:21 #859602
Dave Halford
Participant
@davehalford22513
Howard,

Andrew says Took it off and in standalone mode, the fault was still there. I think he just ran out of patience with it.
6 August 2026 at 11:23 #859603
duncan webster 1
Participant
@duncanwebster1
According to the interweb, motors with a permanent single capacitor have low starting torque. Those with a cap and centrifugal switch are better, presumably because the start winding gets higher current which it can withstand for a short time without cooking
6 August 2026 at 11:37 #859608
Andrew Tinsley
Participant
@andrewtinsley63637
Hello Howard,

My initial thought, at the beginning of this saga, was that the linisher was overloading the motor. Being a true split phase motor, its starting torque is poor. However when I removed the motor and fired it up without load, it still turned over at a little above 1000rpm, sometimes taking a minute or so before it wound up to its correct 2750 rpm.

After dismantling and reassembling the motor, it behaved correctly, running up to 2750 straightaway. Back on the linisher and the problem reappeared. Removed from the linisher, it still ran at a little above 1000rpm and after a variable time (ranging from a second to a minute plus) it would then run up to its rated 2750 rpm. The rotor is totally free and turns very easily. So not a lack of torque problem!

I now have a 0.5 HP motor, with a start winding and centrifugal switch, installed on the linisher. This is behaving as expected. I think I will bin the Clarke motor. I have wasted too much time on it already.

Regards,

Andrew.
6 August 2026 at 12:50 #859617
Robert Atkinson 2
Participant
@robertatkinson2
I suspect an intermittent winding fault. The connection of one set of poles going open circuit would match the described fault. Dismantling it may have remade the connection until it was loaded on the linisher.

Robert.
6 August 2026 at 15:24 #859656
Andrew Tinsley
Participant
@andrewtinsley63637
Robert,

An interesting explanation, but not entirely convinced. In order for the motor to start at all, both windings would need to be intact. After starting, one of the windings would need to become open circuit. It would then at a later stage have to remake the connection in order to speed up to its rated 2,750 rpm. Now would the initial start (both coils energised) and then immediately one coil going open circuit produce the 1000 rpm (actually nearer 1100rpm) that is observed? Just to remind you it is a 2 pole motor. I need to think that one out.

Looking back, there was one instance of the motor not starting on switch on, but I am pretty sure that this was caused by the linisher belt being overtight. But it could possibly be caused by one winding being open circuit. Needless to say both windings show a constant (expected) resistance when checked.

Andrew.
6 August 2026 at 21:00 #859692
Andrew Tinsley
Participant
@andrewtinsley63637
I tried the motor out again (Stand alone) before binning it. It ran for 90 seconds at just over 1000 rpm before taking off and doing the expected 2750 rpm. Switched off and repeated. This time it ran for around a minute before doing 2750 rpm. As the motor warmed up, the time spent at low speed decreased continuously.

It is now in my scrap bin. Thanks everyone for your input.

Andrew.
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