Wondering where the power went.....

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
146
I thought that I needed a bit more power for the cutter on my CNC router. So I ordered a 775 Spindle Motor
  • Rated Voltage: 12V-36V
  • 12V Load Current: 1.7A (starting current 2.2A)
  • Stall Current: 4.7A
  • Load Power: 20.4

That needed a speed controller, and ebay had a flash PWM Motor Speed Controller with Digital Display and soft start https://www.ebay.co.uk/itm/DC-9-60V...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649
  • DC 9-60V
  • 12A
  • 500W

All this needed a decent power supply - around 30V and at least 5A - probably 10A. But I didn't have one handy, so while waiting for a delivery I used a laptop power supply. Probably switch mode, claimed output 18.5V, 2.7A. I put it all together and turned it all on.

It started perfectly. The only problem was that as soon as I put a medium heavy load on the motor, the PWM cut out. I measured the amps - the motor was taking around 0.1A at no load, then as I loaded it up the speed controller cut out about 0.3A, measured at the motor. Immediately afterwards it reset, and I could start the motor again quite readily. But I could not operate at more than about 0.3A without the motor cutting out again.

At first I thought that the speed controller was faulty, or specified in Chinese amps. But then I noticed that when the motor cut out, the indicator light on the laptop supply blinked as well. Which made me think that the power supply ran out of juice, which is why the controller died.

Now I had thought that the laptop power supply would be good for at least 2A at 18.5V. That is well within the range of the PWM speed controller, and well below what the motor was drawing. But is there some kind of interaction between a Switch Mode suply and a PWM that I am unaware of? Or is something else happening here? Should I be measuring some other parameter to discover what is happening?
 

MaxHeadRoom

Joined Jul 18, 2013
22,372
Just to eliminate the power supply problem, try it with an automotive battery maybe!
For this application you should be looking at a linear DC supply, Not a SMPS.
Max.
 

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
146
Suppose you are right - I'll try a battery in the morning. But it would be nice to know what was happening..... Thanks for the idea!

I don't suppose there's any way to use a switch mode - all the cheap heavy power supplies seem to be of that type...?
 

Wolframore

Joined Jan 21, 2019
2,234
switch mode is fine, they're efficient and less expensive. You're stall current may be tripping an over current protection on your small supply.
 

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
146
switch mode is fine, they're efficient and less expensive. You're stall current may be tripping an over current protection on your small supply.
That's what I couldn't understand. If I were pulling 2A out of it that would make sense - but 0.3A should be fine...
 

MaxHeadRoom

Joined Jul 18, 2013
22,372
Suppose you are right - I'll try a battery in the morning. But it would be nice to know what was happening..... Thanks for the idea!

I don't suppose there's any way to use a switch mode - all the cheap heavy power supplies seem to be of that type...?
Yes they are cheaper, but if they crap out, it is usually game over, the linear supply 'just keeps on going' !
Especially for motor control, I would never use SMPS .
Max.
 

Ylli

Joined Nov 13, 2015
1,020
That's what I couldn't understand. If I were pulling 2A out of it that would make sense - but 0.3A should be fine...
Did you use a scope and look at peak currents? You could be drawing 0.3A average, but peak currents well in excess of the supplies rated output. SMPSs don't like even short overloads, and will commonly shut down.
 

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
146
A battery is the simplest way to isolate the problem, at least initially.

A brushed (and unsuppressed) motor is going to fling a lot of spikes back down the line. I will do a bit of suppression with 0.1ufs before getting the scope out. But it would be illuminating to see what is in there.

The rest of the CNC machine used SMPS for the stepper motors and such. But they don't have brushes...
 

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
146
Is that a typo. 20.4 Watts isn't much.
I just copied that from the motor spec. It has no units, and I don't know what it means. I suspect poor Chinese-English translation....



Just noticed that it DOES have units in the original - 20.4W, as KISS said. But I'm not at all sure that I believe any of this data without checking. Have just added some caps to the motor for suppression and ran it again - no-load current 0.13A, supply cuts out around indicated 0.35A. I am waiting for a battery to charge. 20Watts seems reasonable for 12V at 1.76A, which is the motor spec under load, though I am hoping to run it at 30V, about 4A, making 120W...
 
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Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
146
While waiting for the battery, I found another laptop SMPS, which was specced at 20V 3.25A. A bit more poke than the 18V 2.7A one I was using before. Ran things up again, and this time the motor cut out at about 0.55A, which is a distinct improvement.

So it looks as if the issue really is the SMPS, as MaxHeadRoom suggested initially. I am using a laptop supply for the smaller motor I have installed at the moment, and had thought that the SMPS would be putting out reliable power of a couple of amps, no matter what the load was.

It would be a matter of concern if the cutting motor stopped during a cut. The carriage would continue to move against the workpiece, and the end-mill would probably snap in two and fly across the room at speed. On the other hand, smooth linear supplies are expensive bits of kit. Could I simply source a transformer and rectifier to provide the power? How much smoothing do I really need?
 

MaxHeadRoom

Joined Jul 18, 2013
22,372
Generally for motor power you do not require a great deal of smoothing, if any, for e.g. 2hp-3hp treadmill motors were ran on the out put of a SCR bridge, you cannot get much more ripple than that!
Max.
 

shortbus

Joined Sep 30, 2009
8,330
Another one of these threads where someone has no problem spending money on the mechanical system of their CNC or what ever motorized machine and then won't supply it with the correct power supply. Because it costs too much!!!
 

Thread Starter

Dodgy Geezer

Joined Nov 30, 2009
146
@shortbus - actually, I was very unwilling to spend ANY money on my CNC, and it's a lash-up made from kit channels, some ball bearings, flat aluminium plates and an arduino running GRBL. The gullible may rarely belive (sic) that they are gullible and the closed-minded don't believe they are closed-minded, while those who know it all are also probably mistaken. Which is why I always start by asking questions....

@MaxHeadRoom - That sounds helpful! But of course I am running a PWM unit between the power supply and the motor, and I don't know what impact that will have. I have several possible transformers - an RS one at 50VA (25 + 25 at 15V) seems a likely candidate. That, an FL406 bridge and a 3A fuse would let me run at 15V, perhaps?
 
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