diy power on mill

This topic has 14 replies, 7 voices, and was last updated 27 November 2019 at 13:21 by John Haine.

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26 November 2019 at 13:49 #438863
Philip Sewell
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@philipsewell33761
I've had ideas to add a power feed to the z axis of my mill. I have a 12v dc motor off a golf trolley cart and rigged that up with a car battery to see if it would work and have the power to lift the apron and at what speed. All worked ok so I've made mounts for the motor and am ready to hook it up to permanent power.

I bought this from ebay

I temporarily wired it up this morning to the motor mounted on the mill but without the drive chain. The motor ran for a split 2nd then the RCD tripped.

I then took the motor off the machine and put it on the floor on a bit of carpet. It didn't trip the RCD this time, the motor ran for a moment then nothing happened. Obviously this isn't going to work, but I'd like to know why if anyone could let me know.

The unit is rated for 400w, the motor is 150w. Just done some calculations and the motor (I think) will be drawing 12.5A

As this unit is spec'd at 0 to 2.2 amps presumably that's why it's not working.

Ok, any ideas for running this motor, looks like I can forget about speed control.

Thanks, Phil,
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26 November 2019 at 13:49 #26976
Philip Sewell
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@philipsewell33761
26 November 2019 at 13:59 #438865
Martin Connelly
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@martinconnelly55370
It may need to be started at the low setting to restrict starting current. 12.5A is correct if the motor is outputting 150W but this should not be the case if the motor is unloaded.

Martin C

The current draw from the mains for 400W should be 400/220=1.81A plus a bit more for the electronics in the box so 2.2A rating is probably reasonable. 2.2 x 220 = 484W.

Martin C

Edited By Martin Connelly on 26/11/2019 14:03:53
26 November 2019 at 14:18 #438868
Philip Sewell
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@philipsewell33761
Thanks Martin. I had the dial set to the lowest, can you explain the initial split second of rotation and why it tripped the RCD when on the machine. I did wire it up to an old 12v lawn mower motor I've inherited, it again didn't start but made noises as if it was trying!

Philip.
26 November 2019 at 15:06 #438872
Jeff Dayman
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@jeffdayman43397
If you can find a model train speed controller with a max 5 A output @ 12 VDC, you could try that as a speed control. Failing that, you could try a variable bench power supply from Meanwell via one of the e stores on the web. You can get 30 VDC 10A ones quite cheaply.
26 November 2019 at 15:19 #438875
John Haine
Participant
@johnhaine32865
Given the size of the unit my guess is that it can't incorporate an isolating transformer, in which case the DC output is not isolated from the mains. Your motor, if designed for low voltage DC, may have its negative pole connected to the frame. When mounted on the mill, and presumably earthed, you then created a wonderful earth fault that tripped the breaker. It may also have blown a diode or triac in the controller. Can you post a link to the controller on eBay?
26 November 2019 at 15:51 #438880
Ian P
Participant
@ianp
The most suitable power supply for that motor would be the same as it used when it was on the golf cart. The motor armature will be quite low resistance and the start up current very high. Low voltage heavy current PSUs are available but one to drive that motor would be quite expensive. Controlling the speed is best done by PWM but finding one (unless off a golf cart or mobility trolley) might also be expensive.

Ian P
26 November 2019 at 19:54 #438903
Philip Sewell
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@philipsewell33761
ebay link

Thanks for the input from people.

I had the same thoughts re a short to earth and tripping the RCD which is why I tried it off the machine. If the negative pole is connected to the frame would it have 2 leads? I'll check for continuity between the frame and the leads tomorrow.

Just to be clear is this what we mean re a psu. Would this work and would I be able to control the speed with it or do I need a pwm as well which would work in conjunction with the psu.

As these units look bulky do they have a means of fitting an external potentiometer and 3 way switch so that I could mount the unit away from the machine.

Philip
26 November 2019 at 20:23 #438910
SillyOldDuffer
Moderator
@sillyoldduffer
It's the wrong sort of power supply for your motor. Can't see it in the ebay description but the maximum current available at any voltage from these is 2A. While it will do 400W at 200V, only 24W available at 12V. Not enough amps to run a golf cart motor.

Quick look on ebay failed to find an obvious alternative. Fixed voltage LED psu's galore, but speed control would have to be tackled. Plenty of lab style 10A/30V PSUs available, but they're pricey. Anyone know of a moderately priced 0-12Vdc variable PSU good for 10 – 20A?

Dave
26 November 2019 at 22:42 #438932
John Haine
Participant
@johnhaine32865
The second unit you linked to will supply 10A at 12V so should do the job, but it's designed as a lab supply so is overspecified and pricey. You would control the speed by controlling the voltage – I assume that your motor is a permanent magnet type for which the speed is basically proportional to the voltage. PWM wouldn't do much for you if using that. On the other hand you could get one of these which is much cheaper, gives 20A, and pair it with a simple PWM speed controller like this.
26 November 2019 at 22:49 #438933
AJAX
Participant
@ajax
You might be able to find an old ATX PSU that can supply the required current at 12V. I even picked up a couple of old server PSUs capable of supplying 12V at 80A. They cost me 50p each at a car boot sale. It's quite easy to design a PWM circuit to drive a DC motor like yours – I've done it and it works well.
27 November 2019 at 08:40 #438957
Philip Sewell
Participant
@philipsewell33761
There isn’t continuity between the leads and the frame so to my logic the frame isn’t part of the circuit (please correct me if this is the wrong assumption). There is continuity between the leads which I wasn’t expecting, I need to gen up on how a dc motor actually works. The motor is magnetic.

I think designing a pwm circuit is probably beyond me but thanks for the idea.

I’m going to follow your advice John, I’ll be able to mount the power supply out of the way and have the potentiometer and a 3 way switch in the operating zone.

Thanks everyone, another learning experience for me.

Philip.
27 November 2019 at 09:23 #438970
John Haine
Participant
@johnhaine32865
When you say there is "continuity" between the leads, I assume you mean a low resistance? What do you actually measure? For a DC permanent field motor (which this seems to be as you say it is magnetic), the armature windings are effectively connected to the input leads by the commutator and you should see just the winding DC resistance, which will be very low. When the motor is running it generates a back EMF which limits the current.
27 November 2019 at 12:40 #439009
Philip Sewell
Participant
@philipsewell33761
I'm get a reading of 1.7 ohms across the leads John.
27 November 2019 at 13:21 #439021
John Haine
Participant
@johnhaine32865
That sounds about right to me. Starting current would be 12V/1.7 = 7 amps.
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