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Name: Jun
Status: Student
Grade: 9-12
Location: N/A
Country: United States
Date: October 2006


Question:
Will a motor function the same way if the diameter of a stator motor was increased to 5 times the original size, everything in the original motor was scaled up 5 times, would it work the same way? I am not sure if the magnetic flux density is more dense if it is scaled up. I am using a basic stator motor, one of those hobbyist motors with only one pair of magnets inside.

Also if two magnets of different polarity were placed so that the ields were concentric, would the fields cancel one >another rendering it neutral?



Replies:
Hi Jun,

The motor you describe as a "stator motor" is actually called a Perminent Magnet ("PM") DC motor. There is no motor type called a "stator motor".

Scaling up the diameter of the motor 5 times will certainly not increase the magnetic flux density 5 times, In fact it will not likely increase it significantly at all, since the armature will have the same number of turns of wire. But the real limit is the magnetic saturation limit of the iron core. The iron in the motor has a limit to how high it can be magnetized. The limit is called magnetic saturation.

As to how the motor would work when scaled up, this would require a much more detailed analysis. Motor design is relatively complex, with many more variables to consider than just those you have mentioned.

Unfortunately I am puzzled about your question about the motor magnets. The field magnets in a PM motor do not have a "concentric field". In a simple PM motor such as you are working with, the cylindrical magnet is magnetized so North is on one side of the cylinder, and South is on the other. This motor is therefore said to have a 2-pole field. Some motors have more poles, for example, a 4-pole motor has its poles located at 90 degrees around the outside instead of 180 degrees.

Here are some basics to consider about PM motors. A stronger field magnet actually results in a slower rotating speed (but higher torque). This is so, because the Back EMF (or "reverse voltage") generated by any motor as it spins, opposes (or blocks) the applied operating voltage. The faster a motor spins, the more Back EMF it generates, and the more this tends to block and reduce the current being fed to the motor. This sets a limit on how fast the motor can run. A stronger field strength means that the motor will develop the required Back EMF that limits its speed, at a slower rotational speed. Conversely, a weaker magnetic field results in a higher rotational speed (but less torque). Most people think (incorrectly) that a stronger field magnet will make the motor go faster.

Hope this helps clarify your question.

Regards,

Bob Wilson


Yes, Jun, I think it will work mostly the same way.

There are many scaling differences which you can figure out.

A difference: I think it will accelerate slower because the angular inertia of the rotor will increase as a higher power of the size than the torque.

If the wire gets both thicker and longer in exact proportion to the size, then the wire's DC resistance goes down, and for a given applied voltage the motor's stall-current goes up. It gets to be a better motor, more efficient at low current and capable of higher powers and perhaps a wider range of powers.

Things you might need to figure out:

Each turn of wire cuts more magnetic-flux lines, because you bought larger magnets and the flux density stays the same, and the face of the magnet got bigger. (Total flux is the product of area and density. ) So the torque produced by a fixed current would increase. Likewise the back-EMF (voltage induced in the coil by motion) produced by a certain rotation speed. (Permanent-magnet motors are all generators too, although small motors are not very good generators. They actually get to be better generators, as they get bigger.)

Think about our chosen operating voltage and current: do they stay the same, or do they increase, as the motor gets bigger? I am not sure how the voltage and current should be assumed to change as the motor's size increases. If you stick to the same voltage and current, the motor might well make the same mechanical output power, but that amount of power will start looking pretty weak compared to the inertia of the rotor.

In fact, the commutator (if your motor has one) will have a larger friction torque, and at some large size the original current will not produce enough torque to turn the rotor. You might presume the "right" voltage and current are the largest values that do not burn out the motor. That means doing estimates of resistive losses in the wire, and thermal resistance to ambient air, with the rotor stirring it. Big motors often have fans built in somewhere, to make sure the warmed-up air gets pushed out rapidly.

Permanent magnets have a pretty high cost per unit volume. Your bigger motor must have magnets and iron that are proportionately bigger in all dimensions, and the magnets could get expensive. Maybe that is why most big motors use 2 electromagnets (one in stator, one in rotor) instead of one electromagnet and one permanent magnet.

I guess passively-magnetic iron is a lot cheaper than good metal permanent magnet. It also can be smaller, because an iron electromagnet can be powered up to almost 20,000 Gauss of flux density, whereas permanent magnets all have less than 5,000 Gauss within their own volume and emerging from each face.

Yes, concentric fields can be externally neutral. I am thinking primarily of flux-line in the radial direction. If tangential, how do they interact with the external world, even if they are not opposite.

hope that helps-
Jim Swenson



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