Author Topic: 180W+ DC motor power supply from mains  (Read 2632 times)

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Offline doub

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180W+ DC motor power supply from mains
« on: June 02, 2016, 02:50:45 pm »
I have an existing setup with a moderately not-small DC motor to which a spool is attached, and the spool pulls a wire against a spring to move an arm up to a stop. The goal is to be able to switch the arm between two positions from some controller. So the motor is spending most of its time either off, or energized but stalled (against the stop). The arm only has to stay in the pulled position (and the motor stalled) for 20 seconds at a time. But we need to turn it on and off a couple hundred times a day, 2 days a week.

The motor is a brushed DC motor salvaged from a disabled person electric cart. It is rated 24V 30A. I measured the coil resistance and it is approximately 0.8 ohm. From this and my basic understanding of motors I deducted that the stall current is Vsupply / 0.8 (because stalled it's a resistor), and that should also be the maximum current ever running through it (because the inductance goes against current change).

The existing working system uses a 12V car battery, which provides enough starting torque to pull the arm against the spring and keep it (stalled) against the stop. The motor is switched on and off using relays on an Arduino shield (3 relays share the load).

So what I tried to do next is replace the battery with a mains AC to 12V DC power supply rated for 15A (= 12V / 0.8ohm). This is cheap chinese crap from ebay. It worked at first, but after a few evenings of use, it failed. The PSU kind of works under low loads (just the Arduino), but when I energize the motor the voltage drops to 8-9V. I can't see anything blown inside, so I assumed it might have been slightly under-rated for the motor (if it outputs slightly more than 12V the motor would have pulled slightly more than 15A).

So I purchased another similar 12V PSU rated for 30A. It also worked fine at first, but failed after a short while.

So my first question is: Is my math OK? Is the maximum current ever going through a brushed DC motor equal to Vsupply / Rcoil?

If so, should a cheap switching power supply be able to provide that kind of power, or is there some kind of technology incompatibility? Or is it just a quality problem? If so where to get a decent one and what would be the cost?

If my idea just can't work (I'd love to know why), can you suggest alternative solutions?

The car battery solution is really annoying us because we need to constantly attach and detach the battery charger we borrowed (which also can't be returned). So any solution using mains power would help immensely. The only constraint is it would have to be relatively cheap, and not involve too much DIY (I can modify an off-the-shelf PSU to add some output filtering, I don't have time or skills to design and build one from scratch).
 

Offline Seekonk

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Re: 180W+ DC motor power supply from mains
« Reply #1 on: June 02, 2016, 03:23:12 pm »
Your motor should be driven by current not voltage.  You have an arduino, use it in PWM mode to create a linear current booster. Current doesn't even need to be measured, just set a low PWM rate.
 

Offline metzjtm

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Re: 180W+ DC motor power supply from mains
« Reply #2 on: June 02, 2016, 04:56:12 pm »
I had a 1998 Chryler Town & Country that killed motor fan relays(sealed aluminium can 4 pin socket). At the time $85.00 each and somewhat of a pain to change. I used a 555 and MOSFET power stage. Now you can get the same thing only better off the shelf. There is a link below to what basically I came up with if you want to build it. I also put a couple links if you want to buy it. These things are all over the internet. Just Google ac to dc 30 amp PWM motor controller. It will give you the basic solution. Just post or pm me if you want the 14 year old design i came up with. I'll a little embarrass to look at it now.

The theory and drawings:
https://www.picotech.com/library/application-note/some-power-pwm-drivers-for-electric-dc-motors

The kit on E-bay:
http://www.ebay.com/itm/like/281058239802?lpid=82&chn=ps&ul_noapp=true

The complete PWM 30 amp. MOSFET motor driver.
http://www.ebay.com/itm/like/141064642177?lpid=82&chn=ps&ul_noapp=true

PS: By the way the EMF kick from the motor is probably killing your power supplies.  A big fat diode and maybe a cap could just fix that. You also may want to find some other way to hold the arm in position like a solenoid or electrical clamp or a screw and nut instead of a cable and pulley. Stall torque can generate a lot of heat and current.
« Last Edit: June 02, 2016, 05:49:56 pm by metzjtm »
 

Offline metzjtm

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Re: 180W+ DC motor power supply from mains
« Reply #3 on: June 02, 2016, 06:05:55 pm »
 

Offline doub

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Re: 180W+ DC motor power supply from mains
« Reply #4 on: June 02, 2016, 06:36:11 pm »
Just Google ac to dc 30 amp PWM motor controller. It will give you the basic solution.

I don't really understand how a PWM controller is going to fix the problem. Isn't that to vary the speed of the motor when it's turning? I want my motor to go as fast as possible for the 0.5sec it takes to turn the spool, and then just hold the position.

Quote
PS: By the way the EMF kick from the motor is probably killing your power supplies.  A big fat diode and maybe a cap could just fix that.

Can you explain in more details when and how the EMF might affect the power supply? Is that when the motor hits the stop? How would I install that diode and cap?
 

Offline metzjtm

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Re: 180W+ DC motor power supply from mains
« Reply #5 on: June 02, 2016, 07:17:05 pm »
Just Google ac to dc 30 amp PWM motor controller. It will give you the basic solution.

I don't really understand how a PWM controller is going to fix the problem. Isn't that to vary the speed of the motor when it's turning? I want my motor to go as fast as possible for the 0.5sec it takes to turn the spool, and then just hold the position.

Quote
PS: By the way the EMF kick from the motor is probably killing your power supplies.  A big fat diode and maybe a cap could just fix that.

Can you explain in more details when and how the EMF might affect the power supply? Is that when the motor hits the stop? How would I install that diode and cap?

The great thing about PWM is you can adjust the accel and decel of the motor with an inexpensive  drive. The 12 dc supply you have now just tries to supply 12 vdc to it's rated current output. At that point if it is current limited the voltage starts to drop if not it melts. PWM allows you to adjust the the torque cruve and there by control the current. The motor gearing (the size of the pulley has a lot to do with the current necessary). Think of it this way. If you dump 30 amp into the motor and get the accel rate way up and then slam it into a hard stop(decel way up). How long do you think the output transistors on the power supply will survive the EMF from the rapid field collapse ? That is not saying anything about the mechanics of the machine. With PWM you could put 100 amperes into the motor to get it turning and accelerating. Then cut back on the duty cycle so as not to over heat the motor and steep ramp down to stop at contact. If you had the right screw arrangement moving your lever you could just turn off the motor then reverse to cycle.

I could find some magnetic coil and impedance effect theory sites and leave some links if you like. That would explain it in much more detail.

Adding a diode across the output of the power supply would give the EMF spike some way to get to ground other than through your output MOSFET. A cap would slow the drive responce but as a last resort could be necessary.
« Last Edit: June 02, 2016, 07:43:24 pm by metzjtm »
 

Offline doub

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Re: 180W+ DC motor power supply from mains
« Reply #6 on: June 02, 2016, 09:29:53 pm »
The coil of the motor has a resistance of 0.8 ohm. How coud there ever be more than 15A going through it with a 12V supply?

Also can you explain or give links about the effect of the sudden deceleration of the motor? When I google for EMF and rapid field collapse all I find is related to switching off the supply.
 

Offline Circlotron

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Re: 180W+ DC motor power supply from mains
« Reply #7 on: June 02, 2016, 09:40:57 pm »
Why not keep the car battery and have the power supply set to 14.2 volts and feed the battery with it? You would only need a very small supply too, several amps would probably be heaps.
 


Offline doub

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Re: 180W+ DC motor power supply from mains
« Reply #9 on: June 02, 2016, 09:58:02 pm »
Why not keep the car battery and have the power supply set to 14.2 volts and feed the battery with it? You would only need a very small supply too, several amps would probably be heaps.

You mean keep the battery permanently attached to the power supply in parallel? How would that protect the power supply?
 

Offline Seekonk

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Re: 180W+ DC motor power supply from mains
« Reply #10 on: June 02, 2016, 10:46:08 pm »
This isn't rocket science, although your methodology is amateurish.  Use the PWM port to drive the motor. The motor will act like an inductor in a buck converter if you place a diode across it.  This will boost current and save power. The PWM rate can easily be changed dynamically in software to get the response you need.  Do a little research and read up on linear current boosters, buck converters, and the LM1949 application notes. You can not accomplish anything without an understanding of the basics.  You will never learn anything by going on a message board and asking for help.  Read, experiment and try things.  Message boards are filled with bad advice.
 
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Offline Circlotron

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Re: 180W+ DC motor power supply from mains
« Reply #11 on: June 03, 2016, 12:06:12 am »
Why not keep the car battery and have the power supply set to 14.2 volts and feed the battery with it? You would only need a very small supply too, several amps would probably be heaps.

You mean keep the battery permanently attached to the power supply in parallel? How would that protect the power supply?
Yes.
Use a smaller but better quality power supply that has proper current limiting. It will be very much harder for the motor to yank the battery voltage around than it would be to overload the power supply by itself. The battery acts as a stabiliser for the voltage so that big currents can be supplied for short periods. But make sure if you use a *car* battery you have it floating at 14.2 volts. Normal SLA battery would float at 13.65 volts. Car battery may be better in this application because they are specifically designed to give big currents for short periods.

Just reading the original post again - if you are going to keep the motor stalled against a stop with full voltage applied then it is going to burn out very soon! You MUST reduce the voltage once it is against the stop otherwise it *will* die.
 

Offline metzjtm

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Re: 180W+ DC motor power supply from mains
« Reply #12 on: June 03, 2016, 12:40:30 am »
This isn't rocket science, although your methodology is amateurish.  Use the PWM port to drive the motor. The motor will act like an inductor in a buck converter if you place a diode across it.  This will boost current and save power. The PWM rate can easily be changed dynamically in software to get the response you need.  Do a little research and read up on linear current boosters, buck converters, and the LM1949 application notes. You can not accomplish anything without an understanding of the basics.  You will never learn anything by going on a message board and asking for help.  Read, experiment and try things.  Message boards are filled with bad advice.

You don't expect it to sink more than 15 A @ 12 vdc. A PWM controller that you would find on a machine tool servo drive normal start at 80 vdc 20 khz 50 %, as the acceleration increases the voltage is lowered in order to lower current to lessen motor heating and wear. That will "break the inertia" and get the armature and ball screw turning or what ever is attached too it. Weight, distance to travel and speed have a lot to do with the traverse voltage. As the end point is neared the pulse width and voltage are lower until the end point are reached. I don't know what you are doing but you problaly don't need the voltage amplitude modulation(AM) and Pulse Width Modulation(PWM). But PWM should do the trick nicely.

You can learn more if you google "servo motor control theory".

Thanks for the hand SeekonK
« Last Edit: June 03, 2016, 01:07:49 am by metzjtm »
 

Offline max_torque

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Re: 180W+ DC motor power supply from mains
« Reply #13 on: June 03, 2016, 10:44:11 am »
Here are two rather important (simplified) facts about motors you may have missed:

1) the TORQUE of a motor is proportional to the CURRENT flowing through it

2) the SPEED of a motor is proportional to the VOLTAGE applied to it




Consider a motor stalled at zero rpm.  The only voltage you need to apply is the voltage required to drive a suitable current through the motor to generate the torque you need to hold it stalled.  As the motor isn't turning, then it isn't generating any Back Emf.

For most motors, the maximum continuous current (and hence max continuous torque) will be WAY lower than the DC "stalled" current (ie the current that flows when the full supply voltage is applied across the DC resistance of the windings).  Applying the full Vsupply to a stalled motor will produce a large current and magnetically saturate the motor, resulting in very poor efficiency (Torque per unit current) and lots of heat (due to resistive losses ie I2R)


The Back EMF of a motor is linearly proportional to it's rotational speed. zero at zero rpm.  Lets say your  motor turns at 1200rpm at 12v when completely unloaded (free spinning) so it's KE is 100rpm/volt.

Hence, you need a way to regulate the supply voltage to you motor, so you can apply a high voltage to enable the motor to spin fast when it needs to move from position to position, and then apply a low voltage, just enough to drive sufficient current to make enough torque to hold it stalled, at the end stops.

The best way to do this is with a PWM half bridge driver, and if you need to drive the motor in both directions, then you need a full "H" bridge driver. 
 


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