Permanent magnet brushed electric motors. Main features. Basic elements of distinction. How to evaluate an electric motor of this type at first glance.
Hi Jumpugly I hope this can be useful to you.
If something is not clear or is badly written please tell me, I will not be offended but I will be grateful.
I will also reread it several times and discover various errors.
I will only refer to this type: Permanent magnet brushed electric motors.
For simplicity I will write only electric motor but I am referring only to this category.
So, if we have the factory data available (in Italy we say the motor plate data) we can choose the best but we do not always have this information, especially when they are motors given as gifts or taken from other devices.
1) Physical characteristics and their impact.
1a) Armature size and power:
Generally speaking, a larger armature corresponds to a higher power, because a larger size allows for more magnetic and conductive material, therefore a greater ability to generate torque and power.
However, sizing is not the only factor: the quality of the materials, cooling and efficiency of the design also play an important role. There is no simple formula to correlate size to power, but a useful analogy is with transformers, where the power is proportional to the volume of the magnetic core (with some margin).
For motors, this correlation can be more complex.
I would add that: since the magnetic moment (in physics the moment is given by the Force times the arm) that is generated on a (rectangular) coil is proportional to its surface, as well as to the intensity of the current and the magnetic field (in fact [M] = I x S x [B]), I would say that a larger motor has larger coil surfaces.
Small digression:
I remember that for transformers (I mean the classic ones, not the switching ones) there is a formula to size the ferromagnetic core based on the power.
In this formula the section appears and it is therefore clear that there is a correlation between power and size of the ferromagnetic core. Until a few years ago (or decade) there was no escape, for greater power you had to resort to increasingly larger devices (in power plants they are enormous).
For common uses this problem has been overcome with the "switching" technology (in which the alternating voltage is not lowered first and then rectified and stabilized subsequently, but we proceed differently).
Now a power supply for a cell phone or a PC can be very small, which was unthinkable years ago.
However, classic power supplies even for everyday use and small power applications have not been completely abandoned in favor of the so-called "switching" ones because the latter (precisely because of their high-frequency operating principle) introduce electromagnetic noise.
Therefore, where a certain cleanliness of the signal is required and where electromagnetic interference is harmful, old-fashioned power supplies (AC/AC step-down transformer and then stabilizer-filter rectifier) โโare still very popular.
For brushed DC motors this argument is not applicable, there is no other technology.
Dear Jumpugly, there is no escape, if you see a small electric motor you cannot expect it to deliver a higher power than a much larger one, so without having in-depth knowledge of electronics and electricity always consider the size of the motor which is an obvious and easily visible parameter.
If we compare a small motor with a nominal voltage of 12 volts with an electric motor of the same voltage but much larger than the first, you can be sure that the latter is more powerful.
But how big should it be for an RC ship model?
I am of this opinion:
I start from the size of the propeller (which is established by the project or chosen based on the experience of the modeler who evaluates various factors, such as the size and type of model, its use and destination).
Once the diameter of the propeller is known, I would use a motor with a larger diameter (of the casing).This in very empirical terms.
Consider that I prefer never to exploit the electric motor to its maximum capacity.
1b) Number of poles:
In all DC machines, the number of poles does not affect the speed. The speed is governed by voltage, magnetic flux and load. The number of poles can affect other aspects of the design (such as torque and stability), but not the speed as in AC machines.
The issue of voltage is easy to understand.
We know that the force (and therefore the rotation torque) is directly proportional to the current that flows in the coil.
If I want to increase the intensity of the current I can do nothing but increase the electrical voltage.
You can easily verify that a 12 volt electric motor if powered by 6 volts will be slower and if you try to block the axis of the motor you will need much less effort.
The magnetic flux (in this case we are talking about permanent magnets) does not depend on the current but on the type and quality of the permanent magnets (rare earth rather than ferrite ones for example).
By number of poles I mean the number of cavities in the rotor (โNumber of armature poleโ in the MabuchiMotors document that Roy attached).
Only two poles, in theory, would work the same but in fits and starts, in short not smoothly.
The more they are, the more stable and regular the rotation is.
The dynamo, invented by the Italian Pacinotti, would help to make this last concept even better understood but I don't want to broaden the discussion too much and get lost in too many digressions.
The manufacturers start from a minimum of three pole armatures.
I have seen up to twelve, I don't know if there are any with higher numbers but I doubt it.
2) Influence of the number of turns and rotation speed.
2a) Number of turns:
All other things being equal, the number of turns (in the MabuchiMotors document it is indicated as โNumber of turns of armature winding per slotโ) affects both the torque (because more turns means more conductors generating magnetic force) and the maximum speed (which tends to decrease as the number of turns increases, because the resistance and inductance of the winding increase).
2b) Rotation speed (RPM):
The nominal speed of a motor depends on the supply voltage (the higher the voltage, the higher the speed) and the applied load (with a greater load, the speed tends to decrease).
The torque is proportional to the current, so increasing the voltage (within the limits of the design), increases both the speed and the torque.
As already mentioned in point 1a), in a single rectangular coil the magnetic moment (in physics the moment is given by the force for the arm) is directly proportional to the current and to increase the current we must increase the voltage.
3) Stall current:
The stall current is the current absorbed by the motor when the axis is blocked (the motor does not turn).
It is the maximum current that the motor can absorb, since the impedance is minimum.
This value is critical because it represents the condition of maximum electrical and thermal stress for the motor.
4) Current absorption with load:
When the mechanical load on the axis increases, the motor requires more energy to maintain the speed. Consequently, it increases the current absorbed to generate the necessary torque. This phenomenon is related to the law of conservation of energy: more mechanical energy required implies more electrical energy supplied.
5) Determination of the nominal voltage of the motor:
To estimate the nominal voltage of a motor without data you could start by supplying it with a very low voltage and gradually increase it, monitoring the speed and the current absorbed.
When the motor reaches a constant speed without drawing too much current, you are close to the nominal voltage. Be careful not to exceed the thermal or electrical limits (check the heating and noise of the motor).
Also, look at the physical characteristics of the motor: motors designed for low voltages (6-24V) tend to have thicker wires and a compact design.
5a) I add a comment from Roycv regarding this last point:
"I did learn years ago with an unknown motor that assuming you have an amp meter with a high end reading, then put your selected voltage to the terminals and measure the stall current of the motor.
This should be done for the shortest time to get a reading. Too long and it can affect the permanent magnet. Take 20% of this reading and it will give a guide to the operating and most efficient current to operate the motor under load."
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If I have taken some concepts for granted, here are some explanations (without formulas).
The main logical and comprehension steps are these. Understanding how a coil rotates, understanding how a motor is built from a coil, understanding why a single coil is not used in practical applications.
Operating principle:
What makes the rotor move is the force (the push) that the electrons provide as they pass through a conductor immersed in a magnetic field. This force is called Lorentz force after the Dutch physicist Hendrik Lorentz.
If current passes through an electric wire immersed in a magnetic field, that wire will tend to move. This is the physical principle underlying the operation of the electric motor.
But in what direction and in what direction will this wire move?
To understand it, just think that the force is perpendicular to both the lines of the magnetic field and to the (conventional) direction of the current.
To understand it and immediately identify this direction, apply the right-hand rule.
It's very simple: if the thumb indicates the direction of the current and the index finger indicates the direction of the magnetic field, the middle finger will indicate the direction of the force.
Once you understand this, you can assimilate the wire where current passes to the coil (i.e. an electric wire wrapped in a closed ring).
The two opposing forces applied to the two useful arms of the coil create a torque.
The torque creates a rotation. Here's the trick.
It's much easier to acquire the concept by observing drawings or animations. In this regard, look at the links I put at the end of this message.
On the two vertical sections of the coil (the horizontal ones are irrelevant) the Lorentz force has a vector perpendicular to both the magnetic field and the direction of the current. In this way the two forces create a torque that makes the coil rotate.
The coil tends to align with the magnetic flux. When this condition occurs (i.e. the coil is perpendicular to the magnetic field and the magnetic moment is aligned with the magnetic field, the Lorentz force no longer generates torque (due to a vectorial issue the forces cancel each other out).
Therefore the rotation should stop. In reality, due to inertia, the coil continues to rotate, recreating the torque. It is clear that this rotation would be very not very fluid; this is why the motors have a number of armature poles greater than two (minimum three).
Constructively speaking:
So our motor is made up of a stator in which the permanent magnets are housed, which have the sole function of generating the magnetic field in which the coils will be immersed.
The rotor is made up of several coils on which the rotary force described up to now is generated. The rotation is transmitted to the motor axis (integral to the coils and the ferromagnetic armature in which the coils are located) by the movement of the coils themselves.
The ferromagnetic armature of the rotor simply serves to facilitate the magnetic flux created by the permanent magnets. In short, we affect the modulus of B (in physics B is the magnetic flux).
Ok, but how do we supply current to the rotor coils?
The trick is in the commutator (or collector) and the brushes.
The commutator is electrically connected to the coils and has as many segments as there are poles.
The brushes (there are two, one positive and one negative, corresponding to the electrical supply voltage) rub on the commutator (which turns in solidarity with the coils) and supply current to the rotor pole (coil and group of coils that are in the best condition to generate torque, i.e. perpendicular to the flow).
To understand this, use the videos at the end of the message.
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Other parameters covered in the MabuchiMotors document, attached by Roy:
1. Diameter of the wires that form the coils (Diameter of magnet wire).
Why is it important?
According to Ohm's second law. The greater the diameter of the conductor (of the coil), the lower the resistance, the greater the current (other parameters such as voltage being equal, of course), the greater the Lorentz force, and the greater the torque developed.
But a larger conductor diameter means larger and more expensive motors.
2. housing length
It affects the Lorentz force because it increases the length of the useful side of the coil but (constructively speaking) it must be balanced with other parameters such as width. Width is important for the rotational moment arm for example, as well as for the general size of the ferromagnetic armature.
3. Type of magnet
It is quite obvious that in a permanent magnet motor, one way to increase the magnetic field is to choose quality (strong) magnets.
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Careful.
In the third image the loop generates the Lorentz forces. However, these forces are aligned and do not generate any torque.
It's the stalemate position.
In the fourth image the two forces acting on the coil generate a torque on it.
This is the maximum torque position.
To overcome the stall phases of the coils, the rotary movement is guaranteed by the other coils.
https://www.youtube.com/watch?v=k4gDVlCsMzg&list=LL&index=13 |
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https://youtu.be/dVCBhBkTWko?si=VpsWdr2nojcBtFuU |
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https://www.youtube.com/watch?v=RRDOAgqwsWc&list=LL&index=11 |
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https://www.youtube.com/watch?v=bP7kpYpCcYY&list=LL&index=16 |
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https://www.youtube.com/watch?v=j_F4limaHYI&t=2s |
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https://www.youtube.com/watch?v=LAtPHANEfQo&list=LL&index=5 |
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https://www.youtube.com/watch?v=CWulQ1ZSE3c&t=362s |
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https://www.youtube.com/shorts/KxudWvuNGA0 |
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