speed control of PMDC motors

Interpreting this post as two questions:

<Q1: How we can change the rating of permanent magnet motor?>

A1: By changing the winding of the rotor. Frame size is also a consideration, for heat dissipation, so re-winding is not the only consideration. Replacing the motor with one of a different size will be a more practical proposition.

<Q2: How we can increase its speed?>

A2: By increasing the applied voltage, or by adding a gearbox.

We have a dc motor of 1100 rpm for 24v supply.We think that we can increase the speed of the motor by decreasing the parllel paths of the rotor winding,if there are many numbers of the commutator segments.Is it possible? Refer me some site that desscribe about the winding of rotor of the PMDC motor.

For most applications a reducing gearbox would be employed.

It is not unknown for PMDC motors to go beyond 33,000rpm (the Tenshodo MkIII is an excellent example) while the Mashima TA12 tops out at 12,000rpm. Escap do a range of coreless 12VDC instrument motors that have numerous applications; the Escap 1219 and its factory-made gearbox remains a personal favourite (usual disclaimers).

We have a dc motor of 1100 rpm for 24v supply.We think that we can increase the speed of the motor by decreasing the parallel paths of the rotor winding,if there are many numbers of the commutator segments.Is it possible? Refer me some site that desscribe about the winding of rotor of the PMDC motor.

Basically if you want to increase the rotor speed say to 2200rpm rewind the rotor with 1/2 the number of turns and double the cross sectional area of the wire .If the wire is to heavy to nest in the rotor neatly use two parallel wires of equivalent CSA

You can interpolate the above numbers within the physical capability of the motors construction to give a range of speeds but there will be some loss of efficiency if taken to extremes

Quickest way to 'tune up' a PM motor....You can add thin paper or card shims between the magnets and the housing to minimise the gap between rotor and magnets thus increasing the magnetic field.

This is ok for radio controlled cars etc...I wouldn't do it on anything serious or safety related...it will effect reliability as the bearings wear the clearance may disappear

You can also try varying the timing...e.g rotate the backplate which hold the brushes slightly, they often have a series of notches for slightly different positions. The best position depends on the load/speed of your application.

Rewinding is a pain.

Del (Now where did I put my old scratch built off road RC car?)

One gap is reduced but another gap is build up the magnet is between the 2 its field will not be changed.

If it is SaCo or similar magnet it is possible without losses to take it from its recess but if it is an AlNico it was magnetized in place and will lose a lot if taken away and put back.

One gap is reduced but another gap is build up

Nope...

Both magnets will move in towards the rotor slightly this will.

a) Decrease the gap between magnets and rotor.
b) Decrease the gap between the magnets, as measured across the diameter of the motor.
c) Very slightly decrease the gap between the adjacent magnets

What you say implies we could take the magnets out to infinity and they would still work...I think not.

Unless you think a paper/card shim will upset the magnetic circuit through the motor can???

Discuss....

Del

Hi Del,

any nonmagnetic material in a magnetised airgap will do nothing.

So if you move the magnets nearly nothing will happen.

If you can move the magnets you can add some soft magnetic material as a shim.

This will bring up the flux density as the magnetic resistance of the airgap is going down.

Same situation as in an electric circuit by lowering one of two different series connected (distributed) resistors.

But normally there is no space available to move the magnets else its a pretty bad motor. Or the tolerances of the windings are bad.

RHABE

So doing this on my Radio Controlled off roader about 20 years ago was all a fiment of my imagination

Hi Del,

lets try to find some explanation:

your motor you did change with shims beneath the magnets:

was it a cylindrical armature or a pancake like?

What was the shape of the magnets?

Were the magnets mounted with gaps in-between that narrowed at placing the shims below the magnets?

I did build some motors - mostly planar ones - example-coils here:

.

Maybe there is something to learn how to optimise the magnetic parts??

RHABE

Maybe we are just at cross purposes?

I'm talking about a small PM DC motor made by Mabuchi, like you find in zillions of everyday objects, this sort of thing.

Bottom line is the closer a magnet is to another magnet the more force they will exert on eachother... good old inverse square law and all that.
I think we will all agree on that!

Del

Hi Del,

this Mabuchi (there are many others, I took apart those from Escap and Maxon and Faulhaber) has very likely a magnet-assembly made of two halves of 120°tube-segments that are magnetised radially one inward one outward. (Some of these motors have a solid cylindrical magnet, some of the cylindrical magnets have a bore to accommodate the axis).

If you did change the distance then there must have been two halves.

If the coil is inside the magnets (often butt not always) then the magnets have at the outside a cylindrical housing (most often the motor housing) that is made from steel not only for rigidity but to provide a magnetic flux path. The magnetic flux entering the magnets from outside (one flux in at the other magnet out) is conducted by this motor housing with a very low magnetic resistance (high permeability) half way around the motor so that the flux path is closed:

Flux entering one magnet is going through the winding radially then may be catched by a second steel inner part then going through the other side winding and entering the outer steel tube or housing.

As copper and paper and air are all "magnetically equal" to air this flux circuit sees "air" at the coil and its safety margins of real air and its isolation, it sees the steel flux-conducting steel or iron parts and it sees the magnets (on its closed loop). The magnets are the pumps or the generators in this circuit, the iron or steel has very low resistance the air (anything near permeability 1) has high magnetic resistance.

So in good motors there is no possibility to move the magnets more in or out.

But in not so good motors the safety margins between magnets and coils are too wide so you may adjust the magnets. But if you do this you do not gain anything as you decrease the airgap at the inside and increase the airgap at the outside!

So from which source??? is your improvement of the motor originating?

If you used a soft magnetic shim then I agree that this will boost the motors magnetic flux density giving more torque per current.

Regards

RHABE

Yup I hear what you are saying, and it all makes sense.
However in practice does the flux path around the outside actually make any difference?

I havn't any figures to back up my assertions I am only recounting what all the RC Car guys did at the time .

If you replace the steel can with a wooden one the motor will still work...I'm just not sure how relevant the flux path round the back is?
Del

"If you replace the steel can with a wooden one the motor will still work...I'm just not sure how relevant the flux path round the back is?"

But how? The magnetic field will be sooo week that if it turns (which is most improbable if there are any frictions) it will turn only at low speed. The torque is proportional to the product: field intensity x Current.

The magnetic field depends on the length of the PM in the magnetisation direction and the gap. The bigger the gap the lower the field intensity. If you do as you wrote replace iron with wood then the loop closes through air and the field is almost nil.

Why not look in a wikipedia or other source for it you will see immediately where is the problem.

If you dissmantle one of these motors, and bring 'em slowly together (or slowly towards the rotor, they will still act like magnets, with a strong flux between them.

They don't suddenly stop being magnets beacuse there is no flux path round the back!

He is a simplified picture....

http://teamster.usc.edu/~fixture/Robotics/Course_files/image046.jpg

I know the flux path around the back will make some difference, but I don't reckon its much.

Are you guys really going to make me take apart a motor just to prove what I think I know already? I'll be a cross kitty if you make me . I'll bet you a six pack...(tuna or beer?)

This could have the makings of a good thread on it's own.

Del

Hang on...I've just realised I have actually done this.

I have shimmed a motor with paper/card whatever nut deffinitely not steel/soft iron.
The moter deffinitely still worked just as well or better.

The theory must agree with practice... NOT the other way around.

I've done the experiment...maybe one of you guys should repeat it?

Del

Just look at this link guys...

A PM wind generator ... all made of wood except manets and wire!

Del

Hi,

the ampere x turns in these motors are generating the torque by interacting with the flux density in the magnetic field.

So if you put more turns into the windings you will need a higher voltage to get the same amps x turns. As the lower current cancels the effect of smaller cross section of the wire and the longer length of the wire the dissipated heat will be the same if the same torque is needed. (Not ideally as the thinner wire needs in effect more space for isolation.)

So by changing the windings you will get only a matching to your existing voltage of the power-supply.

If you can do better cooling this will enable more torque. But not very effective as twice the current will produce twice the torque but 4times the resistive loss.

If you change the magnets to stronger ones (or allow more volume for the magnets or a magnetic configuration with less stray flux) this will proportionally improve your torque/current ratio. This torque/current ratio is often named k = motor-constant.

This motor-constant equals the ratio of emf-voltage/rotation-speed, rotational speed to be expressed in rad/s. (If friction is neglected.)

So back to your questions:

You can increase the speed by applying a higher voltage.

This will cause higher resistive loss in the windings: how to improve cooling?

Or get a winding made from wire class H or C (temperature limit up to 200/ 230°C).

The limit will be the lifetime of the commutator as there will be arcing at too high speed. If this is limiting the speed then a lower resistance of the windings is useful as the inductance will be lower and the voltage will be lower.

If you take a motor with rotating magnet (electronically commutated DC or synchronous) then the centrifugal limit of the rotating parts will limit your allowed speed - not many motors are specified for this limit.

Magnets are not really high strength materials - so often these have to get a winding of glass or carbon fibers with epoxy.

Very high speed motors (driving grinding- or milling- or drilling- spindles) are usually asynchronous as these can be made from materials of higher strength.

Typical limit in these spindles is today at or near 2000 to 2500 rev per second!

We made small experimental airbearing-spindles up to 6000 rev/sec but these need air-turbines as drives.

RHABE