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Clarke CL300 Speed control

PC Board failure

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Pat Bravery04/11/2009 21:28:20
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96 forum posts
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I recently bought on Ebay a Clarke CL300 mini lathe and it worked fine, then after a week or so fuses began blowing, I traced the fault to the mosfet's and changing them cured the fault. Last week the lathe failed again for the same reason. Although the mosfets are cheap and easy to fit I would like to fit another motor and control system. I keep looking at my pillar drill ( also Clarke ) and wonder if the motor and pulleys could be utilised. Has any one out there had similar problems and could advise? Thanks Pat
Michael Cox 104/11/2009 22:30:38
555 forum posts
27 photos
I have had my CL300 lathe for 5 years and have only had one problem that was due to the switch attached to the speed control board failing.  The failure was a purely mechanical problem with the switch. Unfortunately the failure lead to the mosfets and diodes. I ended up buying a new controller board from Clarke as well as the replacement pot.
Other than that I have had a few jam ups causing the fuse to blow but this has lead to no damage to the circuit board.
Mike
John Baguley05/11/2009 00:12:22
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517 forum posts
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Hi Pat,
 
Earlier this year I repaired 2 of the Clarke speed controller boards for a member of our club (he was on his third!!). I fitted higher spec Mosfets - IRFN22N50A - from RS and so far one of the repaired boards he fitted has been working ok. The problem is that the output Mosfets originally fitted are only just up to the job, the slightest surge and they're gone! Repairing the two boards cost less than the cost of a replacement board from Clarke.
 
John

Edited By John Baguley on 05/11/2009 00:14:24

Pat Bravery05/11/2009 00:33:20
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96 forum posts
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Hi John, Yes I replaced the IRFP450 mosfets and it cured it but as you say they are only just up to the job. I have got hold of some IRFP460 ones which I have read that they are uprated ones. I got them from ebay at £3 each. I shall try them tomorrow. I also replaced the pot, Machine mart wanted about £12 for one, Maplins or ebay prices are a lot cheaper. The only thing is that the pot switch is 'make to break' I would still like to put another motor on though. Thanks for the info, Regards Pat
Anthony Knights12/04/2010 01:16:52
681 forum posts
260 photos
One suggestion I have come across is to remove the electronics completely and use a 500VA variac transformer and a suitably rated bridge rectifier to drive the motor.
Peter G. Shaw16/04/2010 20:56:35
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1531 forum posts
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One suggestion I have come across is to remove the electronics completely and use a 500VA variac transformer and a suitably rated bridge rectifier to drive the motor.
 
But wouldn't that mess up the torque and speed characteristicks?
 
Regards

Peter G. Shaw
Anthony Knights25/04/2010 00:01:11
681 forum posts
260 photos
probably
Martin W25/04/2010 01:12:18
940 forum posts
30 photos
Hi
 
Anthony & Peter you right and there is no probably about it. The speed control system forms a closed loop feedback system and as the motor speed falls under load the control system senses this and ups the voltage. hence current/power, to the motor in order to restore the speed.
 
Using just a variac and rectifier only controls the voltage, and pretty crudely at that, which means there is no feedback and the lower the speed is set the less torque the motor will develop. The result is that at low speeds the motor becomes increasingly sensitive to load and will slow with load and eventually stall.
 
The speed control boards are relatively sophisticated electronic units and are designed to minimise speed change with load variations. They are, or should be, tailored to the motor characteristics so that speed remains constant regardless of load up to a preset threshold at which point they should limit the power being applied to the motor.
 
Cheers
 
Martin

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