Hacksaw – I think you are saying that it’s obvious that the force in a motor doesn’t act on the conductor because the conductor is shielded from flux by the slot.
I agree 100%. But the misconception is very widespread and sometimes it is difficult to convince people otherwise. You will find many books that show the picture of a current loop in a magnetic field and tell you that’s how a motor works... with forces acting ON the conductor.
I had quite an interesting experience on another forum where I raised this discussion. A PhD in engineering who claims to be an expert on motors (has written a few books, gives training, writes articles) jumped into the thread and quickly proclaimed I was wrong (the force acted on the conductors), that I didn’t know what I was talking about, and that all my proofs were wrong. He also posted his credentials in the thread as if that were supposed to make him right. The discussion went on for 3 months and 95 pages, not including many detailed attachments that I posted. The thread was eventually deleted by the forum administrators due to unprofessionalism and I was also asked to remove the copy of the thread from my website.
So, you can say it, explain the physics, and talk to you’re blue in the face, but some people won’t listen. I did do the video which I think convinced a lot of people that didn’t exactly understand the theory.
There are a number of reasons people grab onto the idea of force on the condcutors. It is very widely taught in textbooks. The F=qVxB is very commonly shown along with the right-hand rule showing the direction of force. (the force is sometimes called the motor effect and rule often called right hand rule for motors... to distinguish from gnenerators). Some authors are careful to mention that this gives the magnitude of the force but the force doesn’t actually act on the conductors. Others clearly don’t even understand the reality and tell us the the force acts on the conductors.
Here is a sampling of what you will see in the literature on the subject:
“The fundamental principle upon which electromagnetic motors are based is that there is a mechanical force ON any current-carrying wire contained within a magnetic field”
Modeling and High Performance Control Of Electric Machines”
“1.1 – Motors work on the basic principle that magnetic fields produce forces on wires carrying currents”
(this is the third sentence of the book.... he does clarify much later in chapter 4 that this force on conductors applies only under the assumption that the conductors are assumed to be located in the airgap)
“Modern Electric, Hybrid Electric, and Fuel Cell Vehicles”
6.1.1 Principle of Operation and Performance
The operation principle of a DC motor is straightforward. When a wire carrying electric current is placed in a magnetic field, a magnetic force acting ON THE WIRE is produced. The force is perpendicular to the wire and the magnetic field as shown in Figure 6.3. The magnetic force is proportional to the wire length, magnitude of the electric current, and the density of the magnetic field; that is, F = BIL.
Shows the picture of the wires sitting between two magnets
Electric Circuit Theory and Technology by Byrd
In an electric motor, conductors rotate in a uniform magnetic field. A single-loop conductor mounted between permanent magnets is shown in Figure 21.1. A voltage is applied at points A and B in Figure 21.1(a). A force, F, acts ON THE LOOP due to the interaction of the magnetic field of the permanent magnets and the magnetic field created by the current flowing in the loop. This force is proportional to the flux density, B, the current flowing, I, and the effective length of the conductor, l, i.e. F D BIl. The force is made up of two parts, one acting vertically downwards due to the current flowing from C to D and the other acting vertically upwards due to the current flowing from E to F (from Fleming’s left hand rule). If the loop is free to rotate, then when it has rotated through 180°, the conductors are as shown in Figure 21.1(b). For rotation to continue in the same direction, it is necessary for the current flow to be as shown in Figure 21.1(b), i.e. from D to C and from F to E.
ac – “Similar forces are applied to all the conductors on the rotor, so that a” torque is produced causing the rotor to rotate.”
Practical Troubleshooting of Electrical Equipment
“The rotating magnetic field induces emf in the rotor by the transformer action. Since the rotor is a closed set of conductors, current flows in the rotor. The rotating fields due to stator currents react with the rotor currents, to produce forces ON the rotor conductors and torques.”
First Course in Power Electronics and Drives
11-3 – Introduction to Electrical Machines and the Basic Principles of Operations: 11-3-1: Electromagnetic Force: Consider a condutor of length L in figure 11-6a. The conductor is carrying a current i and subjected to an externally-established magnetic field of a uniform flux-density B perpendicular to the conductor length. A force Fem is exterted
on the conductor due to the electromagnetic interaction between the external magnetic field and the conductor current. The magnitude of this force Fem is given by F= B I L”. (There is no more clarifying discussion of force in all of section 11-3, so a reader would naturally assume that this principle is explaining the basic principle of operation of electrical machines.
EE Ref Book 16e Newnes Laughton (
20.3 The elements discussed [above] indicate that there are two methods of developing a mechanical force in an electromagnetic machine
Interaction - The force fe ON A CONDUCTOR carrying a current
i and lying in a magnetic field of density B is fe = Bxi
per unit length, provided that the directions of B and i are
at right angles; the direction of fe is then at right angles to
both B and i. This is the most common arrangement.
(2) Alignment. Use is made of the force of alignment between
two ferromagnetic parts, either or both of which may be
magnetically excited. The principle is LESS OFTEN APPLIED,
but appears in certain cases, e.g. in salient-pole synchronous
machines and in reluctance motors.
"Standard Handbook for Electrical Engineers"
(S.H.E.E.), 13th ed, edited by Fink and Beaty. Page 20-10 section 21 gives theory of operation of a synchronous motor and states "The electromagnetic torque acting between the rotor and the stator is produced by the interaction of the main field Bd and the stator current density Ja, as a J x B force on each unit volume of stator conductor".
Schaums Electromagnetics – has a whole chapter on torque and force. Talks only about F=qVxB!
Operation and Maintenance of Large Turbo Generators
This basic law is attributed to the French physicists Andre Marie Ampere (1775– 1836), Jean Baptiste Biot (1774–1862), and Victor Savart (1803–1862). In its simplest form this law can be seen as the “reverse” of Faraday’s law. While Faraday predicts a voltage induced in a conductor moving across a magnetic field, the Ampere-Biot-Savart law establishes that a force is generated on a currentcarrying conductor located in a magnetic field. Figure 1.14 presents the basic elements of the Ampere-Biot-Savart’s law as applicable to electric machines. The figure also shows the existing numerical relationships, and a simple hand-rule to determine the direction of the resultant force.
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