DC Motor RPMs?

They are very cheap...buy one open it up and have a look!

I havn't done this myself but my guess would be it's a peristaltic pump or a piston pump driven from a flywheel on the motor shaft with an eccentric pin driving a piston...I'd also guess at much lower than 1750 rpm .

General... a DC motor has a commutator which switches the DC to the coils in the correct sequence to keep the motor turning... easiest way to understand it is get a dc motor out of an old toy and take it to bits.

Have fun

Del

PS. I'm assuming you mean the sort of compressor you keep in the car...not a commercial one...

Just general, how is DC power chopped up to turn a motor, or how is it done?

Using a high-frequency chopper circuit. The speed of the motor is partly determined by the mark-space ratio of the incoming supply.

What determines the rpm?

The torque applied, and the peak voltage and mark-space ratio of the incoming supply. Oh, just one more thing. If the power supply has a feedback control circuit in it, then the motor's own back-EMF can be monitored in the space between each pulse, and the width of the next few pulses adjusted to maintain constant speed under varying load, provided the mark-space ratio is <100%.

"Just general, how is DC power chopped up to turn a motor, or how is it done? What determines the rpm?"

Can you say "Commutator?" And the number of field coils in the motor.

You could learn a great deal by building a very simple dc motor.

Have Fun & Learn by doing.

Dear Guest

An automotive DC motor will most likely be a permanent magnet type motor, i.e the field is of fixed strength. As the motor starts to turn it is fed in 'resistance mode', i.e. the current is limited only by the resistance of the armature windings. It speeds up and immediately also starts acting like a generator, which produces a 'back emf' trying to limit the current flowing. (emf = electro motoric force acting opposite to the applied voltage)at a certain RPM the back emf will have reduced the applied voltage to such a degree that only friction is overcome. This is the 'no-load' speed.

This no-load speed depends on the field strength, the number of armature windings, the winding wire diameter, friction and some other factors of less significance.

Any load applied will slow the motor down to a new equiliblium, where the lesser back emf will allow more current to flow to produce more torque at the output shaft.

A permanent magnet DC motor will produce the largest torque at a standstill! But - it will also get very hot and then either melt or burn out. Usually these motors are only loaded to 10% speed reduction from thier no-load speed. This data can be found in the manufacturers specifications.

As for 'chopping' the supply, the commutator takes over this job and switches the current to the correct coils. No external switching necessary. This is however the cheap way of switching and also usually determines the life of the motor. The picture is different for brushless motors which have an electronic control external to the mechanicals of the motor - these are used on items like fans and last until the bearings are so loose that the armature touches something else - but they are more expensive making the permanent magnet motor the predominant type for the time being.

Compressors for car use are simple piston driven ones with 'flutter' valves - spring steel plates that bend open when the air lifts them and close by air back pressure and steel elasticity. The capacities of the motor and the compressor need to be matched for good operation - lots of trial and error there!

Another consideration is the service life of an automotive DC motor: It is usually not possible to repair it. It is easier just to replace it. These motors can be expected to last just 2500 hours - the same as an automobile!

For more details look at websites like www.howstuffworks.com and www.innovatechsolutions.com.au under the motors link.

Have fun

Rolf

Also, the speed of the motor is not constant across the air pressure load while inflating the tank/tubes.

Max RPM is no load, and gets slower as the pressure increases,

but its also 180W (12V) with the car engine not running, with it running, it could be up to 207W (13.8V) or if running boost charge about 216W (14.4V) but this is not a concern.

Thanks for all the answers, all very helpful.

Come to think of it, that was already covered in Public school way back. I had just forgotten about it.

I am answering to the last post, just to reply to all.

Del the Cat,

Yes, I meant the sort of small compressor to keep in the car. (Since the gas stations over this side of the Atlantic stopped supplying free air)

I still wonder though what the rpm might be for such a small unit.

Horrible Old Bat,

Thanks for the info on what determines the rpm. Torque applied and peak voltage make sense, where the torque, within limits (within the operating range), should have a minor input. The mark-space ratio is an interesting concept.

When I searched for 'mark-space' I first came across this one. But that is not it. :)

http://www.euro.net/mark-space/MarkSpaceFAQ.html

Stirling Stan,

Thanks for the great links. Are you pursuing the Stirling engine?

I think it would make the best power plant for hybrid cars. Relative small battery and E-motor, also used to recover braking energy. When you start driving off, you start the Stirling as well, when it is warm in two minutes or so, you have full power.

Overall efficiency guaranteed to be above 30% also at part load and in the parking garage.

Guest,

Very good background information of what is happening. Thank you kindly.

If load is applied, speed drops which in turn reduces back emf allowing more amps to flow increasing torque, very nice. Would there be a maximum torque at some predetermined rpm? Or is this curve going down linearly with increase in torque load until locked rotor current is reached and the motor burns out?

Would there be a certain speed that is predetermined by the motor layout and design at which the current does no longer increase which would represent max torque of the motor and which torque would be known by calculation? I would guess the rpm curve is reducing its slope going down reaching a plateau at a certain torque and then suddenly falling off when more torque is applied?

I am getting at to find out what max torque could I expect for 12V DC Motor on a 15 amp fuse. Well, since torque in Nm is power in kW *9550 / rpm, it is the rpm that will determine the torque available based on the rpm which seems to be determined by the motor design. Typically, a small 12 V DC motor for such purposes as a tire inflater would be running on what rpm when its maximum torque is reached, you would think?

You have stated that the maximum load is usually at a 10% speed reduction. As you said, I will have to look at manufacturers data. Perhaps that torque curve is linear from minimum all the way up to maximum at standstill?

Thank you for the links.

One thing puzzles me. How did you know my name? Or is that your signature? If not, and if you know me, you may as well give me a shout. ropex@rogers.com

Snaketail,

I agree with you. As pressure goes up in the tire, compressor torque will also increase, slowing the motor down assuming constant power. Does power produced by the E-Motor vary with speed as well? But that is a questions for the Electrical experts.

Regards

Rolf

"Are you pursuing the Stirling engine?"

For about 15 years more or less. See

<notscar.wik.is> for links to both Stirling and Hot Air Engines of all kinds. There is an oddity on the page with images at bottom. A Stirling rwy locomotive. Also an email address to contact, SS.

Hello Stirling Stan,

The email address at the Stirling site is not a valid one. it bounced. Thanks for that most simple E-motor.

Rolf

GUEST: TRY A "NET" on the end of the email address. IT WORKS. Made correction.

Another thing to look at is wire gauge for 15A operation, most of the compressors I have seen have about 0.5 or 0.75^2 mm wire, thus it heats up during the compressor operation,

This will suck away some of the "supply" voltage appearing on the terminals of the motor.

you could have a no load voltage on the motor with a 12V supply of just about 12V, but at almost full load could be 8 or 10V, ;o)

forgot to add... This is why you shouldn't cable tie extension leads or leave the wires for the compressor bound up ...

The torque speed curve is a linear one, from no-load speed ie max speed / 0 torque to 0 speed max torque. See http://www.innovatechsolutions.com.au/Curves.jpg, and http://www.innovatechsolutions.com.au/New%20Adds.htm.

This also shows the relationship to current, which is offset linear, the offset being friction based.

If you are looking for a commercial compressor there are models available in 4x4 accessory shops here in Australia - it might pay to ask your local motor accessory shop.

Good luck

Rolf

Thanks Rolf,

That graph explains it. I get it: DL = Dauerlast und KB = Kurzbetrieb.

Looks like we have the same first names. However, I am up over as opposed to down under.

Regards

Rolf

Here is the simplest electric motor.

Only Three Parts for Unipolar Motor !

Can't be made much simpler.

7.2V DC motor for RC car has RPM pass 10000 RPM. Of course higher RPM = less torque. It depend on the wire size and how many turns if wire is there.

Since you can feel the air pulse on the tire compressor, you can just count it and find out RPM of the motor. You can listen to it and count also.

BTW it work if the tire is low. If you got a flat, it'll be faster to push the car to a gas station with air pump.

Since you can feel the air pulse on the tire compressor, you can just count it and find out RPM of the motor. You can listen to it and count also.

It ain't necessarilly so ... there may be a gear box between motor and pump head (certainly will be if it's a peristaltic)

Del

Hi Del,

That peristaltic pump sure is a neat invention. I trust is it good for fluids and for low volumes.

Pumping a gas might be a bit dicey? do you have experience with it pumping air or gases in general?

As you said it will have to be geared down. May be it can take 2 revs per sec or 120 rpm, would be my guess. That sure would tale a long time to fill a flat tire. The pressure is another worry, could it build up 35 psi when used for air?

As far as I know it was developed in the medical field pumping blood or other fluids, is that right? It sure is a simple design.

Rolf

I don't want to be dogmatic () but I reckon the one in the back of my car is a peristaltic.. however I'm not going to dismantle it to assuage someone else's curiosity.

They are fine pumping air ... I design peristaltic pump control systems for a living and pumps of that size go upto 3bar easy peasy.

One needs to be cautious trying to work out rpm vs load vs gear ratio (assuming a gear box) even the motor gear box manufaturers with their graphs and spreadsheets don't get it spot on. Increase the gear ratio by 15% the output rpm will shift by some other percentage due to the changed load on the motor and possibly a different gearbox efficiency as the gear arrangement may be completely different.

Heck but whadda I know..I'm just a cat.

Del

I love cats, I grew up with them, one sleeping with my every night.

Good to know they are good for gases and can muster some pressure.

Perhaps they need a more serious consideration? The tube material likely makes all the difference.

"In the back of my car ... "? if you mean the gas tank I doubt it. The most common are 'roller' pumps. Like a radial vane pump that has rollers instead of vanes. (I thought you might be curious too :) ) Don't worry about it, please don't 'disable' it.

Come to think of it, that system is actually very similar to peristaltic pumps. The "tube" is formed by the outer cylinder wall, by the rotor wall (eccentric mounted) and by the two end plates. Sure, there is more leakage, but for fuel pumps they are great generating sufficient pressure as they are only used for fuel transportation to the injector pump. Motor and pump are all one unit, all submerged in the fuel.

Rolf

I don't think you've read the whole thread... I'm talking car tyre compressor pump which is in the boot(trunk) of my car.

Yes tube type is very important. 3 most common types Silicone, EPDM and thermoplastic (santoprene is a common trade name) .

A thick wall EPDM tube say 6.3 bore 3.2 wall can pump a decent volume say 220mls a minute at 2 to 3 bar easilly form a small DC motor gearbox.

"I love cats, I grew up with them, one sleeping with my every night.

Good to know they are good for gases and can muster some pressure."

Huh???

It was a new paragraph.!!

Not the cats, I meant the peristaltic pumps. :)

I can see your point, Sorry for misleading...

Next time I'll name the subject when starting a new paragraph.

Regards.

So, its a bit like on a transformer, more turns provide higher speed. Since I have to buy one I have little input on the number of turns and wire diameter. But it is good to know that.

That counting business I find a bit hard to do. Its not that easy to count that fast. And as Del has said, any gear box in between would screw it up.

Regarding the time needed you may have a point. I look at it this way. Based on the Machinery's Handbook you need for a single stage compression .063 hp to compress 1 cuft of air per minute from 14.7 psi to 35 psi. 180 watts ( 12 V x 15 A), say 160 W is .2 hp which gives you roughly 3 cfm. (.2 / .063).

Of course that is free air, compressed at 35 psi one needs about 2.4 times more free air. How much is the volume of a tire? Say we need to pump from flat. As a rough guess its about 1 cuft where you need about 2.5 or say 3 cuft free air which can be pumped in ~ 1 minute.

From experience I agree with you that it takes longer than that. What gives? This calculation is really independent of pump rpm as long as the pump and motor are tuned.

Rolf