DC MOTOR SET UP PROBLEM

[KEITH BEAUMONTParticipant @keithbeaumont45476]

A few years ago I inherited a Unimat 3 lathe and Mill, with one motor between them.I have,on occasion,looked for another motor so that I can sell the Mill as a workable machine. The recent postings by Grayham Meek, of his refurbish and fitment of a DC Electric scooter motor to his Unimat 3 lathe, inspired me to do the same and I contacted Gray for information on parts used. The three main parts are a 12 volt DC 150Watt Brushed motor. A DC 6-60 Volt PWM Speed Control and a 12 volt 30 amp DC 360 Watt Power Supply.. All available from many places on Amazon. I have copied Grays cabinet mounting to accomodate these parts. When all wired up I was very disapointed in its performance. Whatever pulley combination, or speed selected the motor stalled at the slightest cut. An exchange of mail with Gray confirmed that he was able to take impressive cuts with his set up.. I then returned the Power Supply for replacement and received another promptly. When this was wired in and tested,I got the same result,stalling the motor at the slightest load. This time I thought the problem had to be the Speed Control, so ordered another. This was wired in and made no change at all,still stalling the motor at the slightest load. At this point I wondered if the Motor was the problem, so I removed all belts, put on a heavy leather glove and tried to stop the motor by hand. I found I could stop it using just finger and thumb! I did this several times, then I heard soome sounds coming from the Power Supply and the power kicked in,such that I could not even slow it by hand. Wonderful, I thought , I have Power. I then found that the Speed Control was not working. The LED figures were lit but all zeros. No on/off or speed control. Now very frustrated I packed up and made a cup of coffee. I then decided to replace the dead Speed Control with the previous item I thought was faulty. When this was wired in I found everything was now working the same as Grays. I have spent an hour making various test cuts and am impressed with its performance.

We now come to the reason for this post. I would like an explanation from the experts in electronics why, when I had stalled the motor so many times by trial cutting, did it not detect the load in an instant with the same increase in power on all my previous set ups? What was different with my stopping the motor by hand? Also, why did it knock out the Speed Control when it did detect a load? My understanding of PWM is that changes are made in real time. Grahams set up worked straight out of the box. Answers in simple Englisfh please.

[KEITH BEAUMONTParticipant @keithbeaumont45476]

[Robert Atkinson 2Participant @robertatkinson2]

Most likely is a stuck brush in the motor or something under a brush. You may have applied some axial load to he spindle which unstuck it.
PWM failure could have been related or just random.

Robert G8RPI.

[SillyOldDufferModerator @sillyoldduffer]

Don't get that at all. If I had to guess, it would be a mismatch between the regulator in the power supply and the load.

The common type of switched mode power supplies are designed to deliver stabilised DC into a load that doesn't vary much – such as a computer, or a string of LED lights. The regulator may not start unless a certain minimum current is drawn by the load, and if too much current is drawn the power supply throttles back to prevent damage.

In this case the load is Pulse Width Modulator, varying it's output between 0 and 100%, and it's load is a DC motor requiring a brief burst of extra current to start. Very variable, probably not what the power supply was designed to do.

Possibly the control circuitry in the power supply decided the load was out-of-range, which usually indicates a fault, and went into survival mode by limiting output. A loose wire or poor contact can have the same effect.

There's nothing simple about a modern switch mode power supply: they achieve high efficiency and good regulation with complex circuitry and maybe a microprocessor. Unfortunately, their intelligent detection of fault conditions means they can go into a sulk.

In theory, there's no particular reason to assume a switched mode power supply will tolerate any abuse whatsoever. In practice, I've used many well outside their comfort zone with no problems at all – they don't seem that fussy, which is why I mention the possibility of a poor contact.

Hard to debug electronics without test equipment and some knowledge of the circuits used.

Dave

[Robert Atkinson 2Participant @robertatkinson2]

I think you are over thinking it Dave. As I undersatnd it there was low motor torque, both PWM controller and Power supply were replaced with no effect. Hand "stalling" the motor caused the lack of torque to resolve. At some point the second PWM controller went short circuit (full on). refitting the first PWM made all good.

So the original fault was in the motor with all the symptoms of brush gear issues. Not uncommon an a new motor that has not been bedded in.

The failure of the second PWM controller is a separate issue.

Robert G8RPI.

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Dave, I checked,re-checked and even got a pal to check all terminals,so I do not think the problem was a loose connection.

Robert, The motor sounds like it might be the cause as it was the only part not changed, but would a brush problem not show itself when run at max revs.? There is absolutely no play in the motor spindle. The question still remains that previous stalling did not remove any problem at the brushes. Also ,what were the sounds from the Power Supply when I hand stalled it? Thinking about it, The duration of stall by hand was probably a fraction longer than winding the tool back when it stalled. Could that be an influence ?

What is the sequence of events after power on, max revs and then takeing a cut ?

Keith

[Paul LousickParticipant @paullousick59116]

I'm not an expert on power supplies but if it is a 12v DC motor why not connect it to a 12v battery to check if it runs with full speed and torque. If OK, then the problem is with the controller or power supply.

Paul.

[Joseph Noci 1Participant @josephnoci1]

Posted by KEITH BEAUMONT on 27/09/2022 21:02:33:

The question still remains that previous stalling did not remove any problem at the brushes. Also ,what were the sounds from the Power Supply when I hand stalled it? Thinking about it,

Keith

I believe the proposed brush problems the most likely – If the brush was stuck or had some guff between it and commutator, then contact might be 'OK' for low current, ie, low torque, and fail with load. Repeated stalls dislodged the guff or stuck brush.

The odd sound from the PSU – this could easily be due to the contact tween brush and comm coming and going as the brush made good contact and broke that a few times during the stall – the PSU would then PWM itself from high current to low current mode and back, in sync with the brush contact rate, and that is reflected into a more and less intense magnetic field in the PSU switchmode transformer, the resulting magnetostriction resulting in audible clicking.

I presume the PSU and motor were in close proximity – are you sure the audible sounds were from the PSU and not from the motor as the brushes made/broke contact?

PWM can change in real time, but only if the control loop commands it – this is inherent in the PSU – the loop is a closed one with the pwm dependant on output voltage.

The speed controller is another issue though – Basic PWM speed controllers are not closed loop and do not control torque or speed – simply set speed with a specific PWM width, and then any load will cause speed to drop, etc.

Better PWM speed controllers use motor back-EMF to measure motor speed and close the loop, but most cheapies don't. Even better controller measure motor current and back EMF and control the speed/torque curve.

What is the speed controller part number?

[SillyOldDufferModerator @sillyoldduffer]

Posted by Robert Atkinson 2 on 27/09/2022 18:14:23:

I think you are over thinking it Dave. …

I agree for 3 reasons:

• I assumed the motor was OK and didn't think about brushes at all – a big gap
• A brush problem is a closer match to Keith's observations than my suggestion
• The brush answer is simpler than my suggestion, hence Occam's Razor applies

Your post went up whilst I was typing. If I'd read it first, I wouldn't have pressed Send!

Dave

[KEITH BEAUMONTParticipant @keithbeaumont45476]

Joseph,

No Part No, But Google -DC 6-60V 12V 24V 36V 48V 30A PWM DC MOTOR SPEED CONTROLER (PWM) and you get loads of the same item from different suppliers. The sounds could have come from the motor,as they are adjacent, but my impression was it came from PS

The Motor was the first part to arrive and I did connect it to a car battery to check its direction of rotation, but did not try to stall it as it had a ribbed pulley on it.

I appreciate all these opinions from you all.

Keith

[Joseph Noci 1Participant @josephnoci1]

Posted by KEITH BEAUMONT on 28/09/2022 09:26:00:

Joseph,

No Part No, But Google -DC 6-60V 12V 24V 36V 48V 30A PWM DC MOTOR SPEED CONTROLER (PWM) and you get loads of the same item from different suppliers.

Oddly, Google says…

No results containing all your search terms were found.

Your search – -DC 6-60V 12V 24V 36V 48V 30A PWM DC MOTOR SPEED CONTROLLER (PWM) – did not match any documents.

Suggestions:

• Make sure that all words are spelled correctly.
• Try different keywords.
• Try more general keywords.
• Try fewer keywords.

Anyway, dug some more and as you say, lots of hits. That PWM module is just that – only chops the input DC into variable width pulse, so no closed loop control, so no 'real-time' update of the PWM pulse width to keep motor speed constant. This will be most noticeable at low motor speeds. It will probably not be noticed on that small lathe – torque requirements are low, so it should be just fine.

Nice little lathe…

[KEITH BEAUMONTParticipant @keithbeaumont45476]

It looks like the consensus is that the Motor was the problem, Would the hiatus created when it finally cleared what ever it was, be responsible for knocking out the PWM Control?

Thans again for all your expert help

Keith.

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