Permanent magnet DC motors

[klims]

From what i undertand this is one of the fundamental equations for permanent magnet dc motors.

F=nBIL

where F=force, n=number of turns, B=flux desity, I=current, L=length of the wire.

The equation

t = F r

where t=torque, F=force, r= radius

relates this force to a torque output, but what i have never understood is how does this all relates to rpm? i understand that as F increases, be it by increasing n or I or B, torque increases and rpm decreases but are these directly realated? are they proportional?

I understand that this is all theory but how well does this theory relate to practice? how much difference does say a longer wire/more turns make? does this greatly impact I as a result of increased resistance? How much does friction impact all of this?????

My main aim was to be able to look at two motors and say, well today i need higher torque so i’ll go for the ……… setup bla bla bla

Thanks for your help guys

[BludyYank]

klims wrote:

The equation

t = F r

where t=torque, F=force, r= radius

relates this force to a torque output, but what i have never understood is how does this all relates to rpm? i understand that as F increases, be it by increasing n or I or B, torque increases and rpm decreases but are these directly realated? are they proportional?

Relationship between torque and power

If a force is allowed to act through a distance, it is doing mechanical work. Similarly, if torque is allowed to act through a rotational distance, it is doing work. Power is the work per unit time. However, time and rotational distance are related by the angular speed where each revolution results in the circumference of the circle being travelled by the force that is generating the torque. This means that torque that is causing the angular speed to increase is doing work and the generated power may be calculated as:

Power=torque X angular speed

Mathematically, the equation may be rearranged to compute torque for a given power output. However in practice there is no direct way to measure power whereas torque and angular speed can be measured directly.

Consistent units must be used. For metric SI units power is watts, torque is newton-metres and angular speed is radians per second (not rpm and not even revolutions per second).

Conversion to other units

For different units of power, torque, or angular speed, a conversion factor must be inserted into the equation. For example, if the angular speed is measured in revolutions instead of radians, a conversion factor of 2π must be added because there are 2π radians in a revolution:

Power=torque X 2 pi X rotational speed, where rotational speed is in revolutions per unit time.

The rotational speed of a DC motor is proportional to the voltage applied to it, and the torque is proportional to the current. By varying the field current it is possible to alter the speed/torque ratio of the motor. Typically the field winding will be placed in series (series wound) with the armature winding to get a high torque low speed motor, in parallel (shunt wound) with the armature to get a high speed low torque motor, or to have a winding partly in parallel, and partly in series (compound wound) for a balance that gives steady speed over a range of loads. Further reductions in field current (less winds) gain higher speed but correspondingly lower torque, called “weak field” operation.

Hope that helps some.

[klims]

wow. who ever thought uni would be useful! thanks for the responce BludyYank. i guess i really should have been paying attention at uni last year. that all sounds like stuff i should know.

Typically the field winding will be placed in series (series wound) with the armature winding to get a high torque low speed motor, in parallel (shunt wound) with the armature to get a high speed low torque motor, or to have a winding partly in parallel, and partly in series (compound wound) for a balance that gives steady speed over a range of loads. Further reductions in field current (less winds) gain higher speed but correspondingly lower torque, called “weak field” operation.

this is just general information right? from what i remember this is not how the 130 class motors(mini z) are set up. what configuration(s) are they?

[BludyYank]

As far as I know, there is only one individual that produces what is called double and triple wind patterns in 130 sized motors. There is also drill vs. epoxy balancing, crossover vs. hemi winding, single vs. multiple winds, etc.
A single wind pattern is what you see on ALL current micro motors, where one single strand of wire is wound continuosly around the armature. In doubles and triples, two or three strands of a smaller gauge wire are wound simultaneously around the armature. This creates smoother power and a broader power band, as it creates a more dense magnetic field. As far as crossover and hemi winds, the difference isn’t something that necessarily would be very noticable in driving. If you were to take the armature out of a stock 130 and look at the comm you would see the wires cross each other going from the comm to the armature. Now if you were to look at a pattern wound armature (like the 21 turn at http://www.minizworkshop.com) you would notice that the wires come straight down from the comm to the armature. This essentially makes a 28 turn motor a 27 1/2 turn motor, as the final wind doesn’t create a full magnetic loop. The idea behind this is that it basically generates more top end speed. If you look at 10th scale racing motors, you would note that the stock motors all have crossover winds and that the vast majority of modified motors have a hemi wind.
(I must give full credit to a mate that goes by the name of Flashsp-2 for the above information, he makes custom handwound 130 armatures.)

[klims]

thats very informative. thanks for that bludyyank.

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