Fırçalı DC Elektrik Motorlar 51053
A brushed DC motor is an electric motor designed to be run from a direct current power source. Brushed motors were the first commercially important application of electric power to driving mechanical loads, and DC distribution systems were used for more than 100 years to operate motors in commercial and industrial buildings.
Brushed DC motors can be varied in speed by changing the operating voltage or the strength of the magnetic field. Depending on the connections of the field to the power supply, the speed and torque characteristics of a brushed motor can be altered to provide steady speed or speed inversely proportional to the mechanical load.
Brushed motors continue to be used for electrical propulsion, crans, paper machines and steel rolling mills. Since the brushes wear down and require replacement, brushless motors using power electronic devices have displaced brushed motors from many applications.
Brushed DC Motors
Simple Two-pole DC Motor
The simplest form of a Brushed DC motor is a two pole motor design. This design is the best to understand the forces and rotation.
When a current passes through the coil wound around a soft iron core, the side of the positive pole is acted upon by an upwards force, while the other side is acted upon by a downward force. According to Fleming’s left hand rule, the forces cause a turning effect on the coil, making it rotate. To make the motor rotate in a constant direction, “direct current” commutators make the current reverse in direction every half a cycle (in a two-pole motor) thus causing the motor to continue to rotate in the same direction.
Drawbacks of a 2 Pole DC Motor
A problem with the motor shown above is that when the plane of the coil is parallel to the magnetic field—i.e. when the rotor poles are 90 degrees from the stator poles—the torque is zero. In the pictures above, this occurs when the core of the coil is horizontal—the position it is just about to reach in the last picture on the right. The motor would not be able to start in this position. However, once it was started, it would continue to rotate through this position by momentum.
There is a second problem with this simple pole design. At the zero-torque position, both commutator brushes are touching (bridging) both commutator plates, resulting in a short-circuit. The power leads are shorted together through the commutator plates, and the coil is also short-circuited through both brushes (the coil is shorted twice, once through each brush independently).
The problem here is that this short uselessly consumes power without producing any motion (nor even any coil current.) In a low-current battery-powered demonstration this short-circuiting is generally not considered harmful. However, if a two-pole motor were designed to do actual work with several hundred watts of power output, this shorting could result in severe commutator overheating, brush damage, and potential welding of the brushes—if they were metallic—to the commutator. Carbon brushes, which are often used, would not weld. In any case, a short like this is very wasteful, drains batteries rapidly and, at a minimum, requires power supply components to be designed to much higher standards than would be needed just to run the motor without the shorting.
Basics of Brushed DC Motors
Electromotive Force (EMF) – An Important Point About DC Motors
If the shaft of a DC motor is turned by an external force, the motor will act like a generator and produce an Electromotive force (EMF). During normal operation, the spinning of the motor produces a voltage, known as the counter-EMF (CEMF) or back EMF, because it opposes the applied voltage on the motor.
The back EMF is the reason that the motor when free-running does not appear to have the same low electrical resistance as the wire contained in its winding. This is the same EMF that is produced when the motor is used as a generator (for example when an electrical load, such as a light bulb, is placed across the terminals of the motor and the motor shaft is driven with an external torque). Therefore, the total voltage drop across a motor consists of the CEMF voltage drop, and the parasitic voltage drop resulting from the internal resistance of the armature’s windings.
The current through a motor is given by the following equation:
The mechanical power produced by the motor is given by:
As an unloaded DC motor spins, it generates a backwards-flowing electromotive force that resists the current being applied to the motor. The current through the motor drops as the rotational speed increases, and a free-spinning motor has very little current. It is only when a load is applied to the motor that slows the rotor that the current draw through the motor increases.
TERMS used to Explain Brushed and Brushless Motors
Armature: The armature (or arm) is what spins in the motor and makes your rc helicopter move. It is made up of the commutator, laminations, shaft and winds. Electricity flows through the wires from the ESC to the end bell. It then travels through the brushes to the commutator, and into the windings on the arm. Since the windings are wrapped into a coil, they create a magnetic field when current is passed through them. This magnetic field is repelled and attracted to the magnets in the can causing the arm to turn.
Brush: Made of a silver, copper, or graphite compound and at the end of the shunt wire. They are what makes contact with the commutator and transfer the electric current to the commutator.
Commutator: Typically referred to as the Comm. The comm takes current from your brushes, which ride on this part of the arm, and sends it to the windings. The comm is not one solid piece, but is actually made up of 3 separate pieces. This allows the current to be switched to the different windings of the arm as it spins. Because it rubs against the brushes as the arm spins, the comm needs to be cleaned and shaped regularly.
Continuous / Burst Current: Continuous current measures how much current a motor can handle continuously, for an extended period of time. Burst current measures how much current a motor can handle for a short amount of time, about a few seconds.
Current Rating: This is the maximum current that a given motor can handle, measured in amps.
Endbell: The part of the motor that consists of the brush hoods and the tabs. The endbell holds the bearing that supports the short end of the shaft.
Kv Rating: The Kv number is the RPM per volt supplied to the motor. The KV number’s useful because it let’s you figure out how many volts you need to achieve a certain RPM, or vice versa. For example, a 1200 Kv motor, supplied with 3 volts, will run at a nominal 3600 rpm. The Kv rating always assumes no load on the motor, so the actual RPM that your achieve will be less than the one you calculate. Note that Kv is the voltage constant (capital-K, subscript v), not to be confused with the kilovolt, whose symbol is kV (lower-case k, capital V).
Laminations: The part of the armature the winds are wrapped around. These are usually about half a millimeter thick, and are stacked on top of each other. The laminations are sometimes shaped to provide a stronger field. They are usually made of iron ferrite.
Magnets: Provides the opposing force that the armature’s magnetic force pushes against.
RPM: This is a measure of angular speed, or how fast something is rotating. A motor’s RPM is simply how fast it can rotate per minute.
Torque: Torque is a measure of angular force, or how much “push” a rotating shaft has.
Volt: This measures electric potential, or how much “push” the electrons from a battery have. A greater voltage means that more energy is being applied to a given amount of charge.
Watt: This is a measure of power, or how fast energy is used.