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Clarkson T&C grinder motor stopped

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Oily Rag06/08/2020 18:03:11
123 forum posts
57 photos

Hi Guys,

Hope all are keeping safe and well.

I was using my Clarkson T&C grinder yesterday to sharpen a 6 tooth shell mill when, just as I had finished the last pass, the motor slowly 'died'. It was as if the speed controller on the VFD had been turned down gradually. I was at first concerned that the VFD had 'blown' - but as I interchange the VFD with one of my lathes by using a 3 Phase 5 pin socket I reconnected the lathe - turned on the power and the lathe ran as normal. This made me suspect the Clarkson motor had died - as an initial check today I Fluked the pin outs of the Clarkson connector and got the following Ohm readings - they were not stable readings though, as they tended to ramp up from zero to between the values of 11.5 Ohm to 12 Ohm. This I put down to flux effect within the motor.

The readings between the major 'earth' pin (at 12 o clock) and the remaining 4 pins was Infinity and Infinity between pin 1 (clockwise from the earth pin) and all other pins, then the 'ramping/ up to 11 / 12 Ohms between the pins 2, 3, & 4.

What have I got? a deceased motor? BTW reconnecting the VFD and trying to run the Clarkson motor did not result in any motion, humming, escaping smoke or otherwise.

Photo of 3 phase 5 pin socket:-



Edited By Oily Rag on 06/08/2020 18:04:58

John Haine06/08/2020 18:27:57
3254 forum posts
175 photos

I'm not sure what you mean by "ramping" - did they fluctuate around the 11 or 12 ohm level, or start from zero and increase ("ramp up" ) to 11 or 12 ohms, or start from infinity and "ramp down" to 11/12 ohms? As the motor is inductive you should in principle see some sort of "ramping" but at the current levels that typical multimeters use I wouldn't expect the time constant to be very long for you to notice a ramp. If the resistance across every pair of pins 2,3,and 4 is roughly the same that indicates that the windings are still balanced so you haven't lost a phase. One possibility is that pin 2, which seems disconnected, is actually a neutral that should go to the motor star point, assuming the motor is star not delta connected, but has come adrift and the VFD having been set up with a star connected motor has gone into a fault mode. See if there is a star point at the motor end and if the wire in the cable to it has continuity.

Oily Rag06/08/2020 18:47:42
123 forum posts
57 photos


It's a 220/240 Volt Delta wired motor so pin 1 being disconnected is normal in that configuration. The ramp up was from 0 > 12 Ohm not from Infinity down and was pretty quick in that the fluke was flashing 0.8 / 2.6 /4.9 / 7/8 / 10.4 /then stabilised at around 11.5 to 12 Ohm's (indicative readings in the ramp up) the ramp time was probably less than 2 seconds.

I,ve located another Clarkson nearby which the owner has offered to lend me his motor so I can swop them over to see if it has passed onto the EM home in the sky!

Thanks for your valuable input, always good to get a second opinion on things which seem to be baffling!

Emgee06/08/2020 19:29:06
1637 forum posts
224 photos

Best have a look inside the motor terminal box to check security of connections and confirm the resistance readings taken at the plug.
If the VFD has a fault changing the motor is clearly not going to help.


As you have a plug in the motor supply are you using the VFD to power more than 1 machine ? if so do the other machines function correctly.

Edited By Emgee on 06/08/2020 19:32:28

John Haine06/08/2020 19:40:17
3254 forum posts
175 photos

Those numbers sound from zero up not infinity down! Not what you would expect from an inductive device. Do the same measurements on your lathe.

Oldiron06/08/2020 21:58:17
481 forum posts
22 photos
Posted by Emgee on 06/08/2020 19:29:06:

As you have a plug in the motor supply are you using the VFD to power more than 1 machine ? if so do the other machines function correctly

I believe the OP stated that his lathe works ok with the same VFD


peak406/08/2020 22:09:29
1153 forum posts
135 photos

I guess the two obvious questions are;
Does the motor spin OK by hand?
Does the Clarkson spindle spin OK by hand?
That should eliminate duff bearings

As regards the motor wiring box; I presume it;'s a dual voltage motor.
If it has the little brass bars to select the voltage, try removing them and measure the resistance on each coil individually.
That should give you some idea if there's one coil down.

Oily Rag06/08/2020 22:43:48
123 forum posts
57 photos

Thanks Guys - I'll follow all those suggestions and report back.

Just to reconfirm - I have tried the lathe off the VFD (I use one VFD to power about three machines, all with similar rated motors) the VFD 3 phase output is terminated into a 3 phase 5 pin socket - I disconnect the plug with the remote control for the VFD power to off, swop over plugs and then use the remote low voltage controller to start / stop and speed control accordingly. There is also a jog function for my drill. Both the lathe and drill work as expected off the VFD so I am happy to believe the VFD is not at fault.

In reply to John Haine's second post - yes the initial value shown on the Fluke is 0, or at least a very low number (such as 0.7 ohm) and then ramps up to around 12 Ohms within 2 secs. I've tried the lathe motor resistances by the same method and get even numbers across all pins representing U,V, and W (motor connections) but a slightly different Ohm reading, 15.3 to 15.6 Ohms in the case of the lathe (slightly different power motor but still within the VFD rating). Just a note to add that the 'fat' plug pin is the motor earth, and pin 1 is the shielded cable earth connection connected at the VFD end but not at the motor end, this to ensure no earth loop to fool the RGB in the consumer unit!

The motor off my mates Clarkson is exactly the same type but his runs 440V star (Y) connections and runs OK - I'll change it over to Delta (|> before trying it on my machine.

Spindle and motor of the Clarkson both turn OK with no notchiness or excessive resistance, as you would expect. When the motor stopped - and after powering down! I checked the motor for any sign of overheating, it was cool to touch as was the spindle head on the grinder.

The Fluke meter response to the Ohm reading (the 'ramping' response) has been described to me by an electronics buff as a normal Fluke meter characteristic when checking a motor due to residual flux.

Thanks all for your responses!

Oily Rag12/08/2020 14:27:29
123 forum posts
57 photos

Just a quick update on this problem:-

Motor replaced with a Siemens 3phz delta connected unit - everything works as expected. The original motor, an ASEA which was the original fitment I believe, had a minor thermal event deep in its bowels (bit like a curry but more destructive!).

Thank you to all the respondents to my initial query. It is always good to talk a problem over with like minded folks and this is the enduring strength of this forum. Thanks again!

One last point was the flat drive belt had seen far better days and I was surprised to find some machinery dealers asking for £50+ pounds for a Clarkson replacement! So I went to a local Bearing Supplier who advertised 'transmission products' and he got me one next day for £12 + Vodka and Tonic. It is a Pirelli Hevaflex, so looks to be a quality item.

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