First created in , the homopolar motor was the first electrical motor to be built. A homopolar motor is a direct current DC electric motor which produces constant circular motion.
The great thing is that the homopolar motor is the simplest example of a motor possible. Plus it is really easy ti experiment with.
The homopolar motor causes a continuous circular motion that was engineered by the circular magnetic force around a wire that extended in a pool of mercury.
Although not the configuration first used, a homopolar motor can be made using a single AA battery, a single neodymium disc magnet and a piece of copper wire! Homopolar motors have two magnetic poles that are provided by the permanent magnet that is used to create the magnetic field.
For a suffciently short connecting wire such a placement leads to the wire being swung through, in dramatic fashion, from one side of the roller If a rechargeable AA Nickel-Cadmium NiCd battery is used instead of a normal non-rechargeable alkaline cell battery, much higher currents can be delivered into a short circuit of the order of 10 A for the former compared to 5 A, and then only briefly, for the latter.
Higher sustained currents lead to greater sustained torques. With such a substitution in battery the homopolar roller tends to roll faster over a flat surface, though the connecting wire was found to dislodge far more readily. More interestingly, the roller could be made to roll up an incline.
Guala-Valverde has suggested that closing the circuit in the homopolar roller using a moulded U-shaped connecting wire is more than a trivial modification it may at first appear to be [20]. A complete analysis leading to a calculation for the torques which act on each disc magnet in the roller configuration is no easy task. In particular, the path followed by the current as it flows through an extended conductor such as the disc magnet is not well defined. In what follows a simplified analysis for the dynamics of the homopolar roller whose dominant contribution to the torques arise from radial current flows over the surface of the disc magnets is presented.
Consider a disc magnet of mass M and radius r. Assume the path followed by the current as it passes through the disc magnet is over its surface. Furthermore, all current will be assumed to flow along straight line paths.
The total torque which acts on the disc magnets is therefore taken to be dominated by a surface radial flow of current inwards towards the centre of the disc. For a steady current i flowing along an element of conductor d l directed along the direction of current flow and in an external magnetic field B , the total force acting on each element is [21]. The total force on the conductor is obtained by integrating along the path C followed by the current. Since the current follows along a straight line path given by the positive y -axis, from Eqs.
Once the disc is rotating a back emf in the motor resulting from a changing magnetic flux through the rotating disc will be set up.
The size of the current flowing in the closed circuit therefore reduces over time and approaches some fixed value whose size depends on the speed of rotation of the disc. The size of the induced emf set up between the centre and rim of a disc rotating with angular speed w in a constant external magnetic field can be found from Faraday's law. Its value is [22]. The current flowing in the closed series circuit formed by the double disc magnet arrangement can therefore be expressed as.
Here V is the voltage across the terminals of the battery while R is the total resistance in the circuit. Proceeding with an analysis of the dynamics for the roller one recognises the homopolar roller forms a driven wheel system.
Here each wheel formed by the disc magnets is driven by a magnetically produced torque such that the two wheels and axle battery roll off together in unison about the system's axis of symmetry. Approximating the homopolar roller as a rigid system consisting of two uniform discs the magnets connected concentrically to a third, slightly smaller but longer uniform cylinder the battery , since the arrangement rotates about its axis of symmetry the moment of inertia for the system will be. For a driven wheel which rolls without slipping, the surface exerts a frictional force on the wheel in the same direction as the direction the wheel rolls.
If the effects of rolling friction are ignored, from Newton's second law, for translational centre of mass motion along the horizontal one has. Here f is the frictional force exerted by the surface on each of the wheels, a is the acceleration of the centre of mass of the roller while is the frictional force exerted by the connecting wire on the upper rim of each wheel and is assumed to act in the horizontal direction.
From the rotational form of Newton's second law, for rotational motion about the axis of symmetry of the roller one has. Here a is the angular acceleration of the roller. Finally, for rolling without slipping the linear and angular accelerations are related by.
Initially the roller is taken to be at rest. Equation 16 imposes an upper bound on the size of the frictional force that can be applied to each rim of the roller if it is to move.
Doing so yields. Equation 19 can be used to estimate the distance travelled by the roller in a given time. A single D84 magnet would be the same, though we found it would work even with a single D82 magnet. In this configuration, we hold the current-carrying wire still, along with the battery, and the magnet and screw spin. This is fairly easy to reproduce, though we had some problems with the screw slowly migrating off to the side.
We solved this by adding a piece of tape with a hole in it. We have seen other videos where the battery was hit with a chisel or other sharp object to make a small indentation for the screw-point to sit in. In this second setup, we hang the battery and magnet beneath a stationary steel bolt.
The magnet-battery combination sticks to the bolt because a bit of the magnetic field goes up through the battery. We used the same AA battery, the same piece of wire, and a single DC6 magnet. This produced some obvious results, with the battery spinning out of control.
This is a good way to demonstrate the motor in front of a large group, where you need motion that's more obvious from a distance.
It can be challenging to come up with a stable configuration of the wire that allows it to spin well without falling off. Be prepared to fiddle with the wire a bit to get it right. Use a piece of solid not stranded wire. We used 18 gauge wire, though other sizes would work as well. We removed all the insulation from the wire before experimenting with it. Technically, you only have to remove the insulation where it contacts the battery and the magnet, but removing it all seemed easier.
What is an electric motor and for what purpose it is used. Electric motors are devices that can transform electrical energy into mechanical energy by means of electromagnetic fields. There are some motors that can do the reverse, they can transform mechanical energy into electrical energy to operate as generators. The two major part of an electrical motor is stator and rotor, the stator is the fixed part and the rotor is the mobile part. Stator would act as a base that allows performing the rotation of the motor, and the rotation is not done mechanically it is done magnetically and the rotor is the rotating element that allows the transmission of mechanical energy.
A homopolar motor is a direct current electric motor, which has two magnetic poles providing a static magnetic field. The homopolar electric generation process is done by using a moving electric conductor and this conductor will be enclosed by a unidirectional and constant magnetic field.
In this process, there is a strict relationship between the electric field, magnetic field, and inertia. The electric power which is generated is determined by their magnitude. If there is a flowing electric current then there will be a magnetic field too and reverse will happen too.
Inertia is formed because of the moving mass in the surrounding space.
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