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Motor is an electromagnetic device which realises the conversion or transmission of electric energy according to the law of electromagnetic induction. Electric motors can be classified as electric motors and generators. An electric motor is represented by the letter M in the circuit. Its main function is to produce driving torque, as the power source of electrical appliances or various machinery. A generator is represented by the letter G in the circuit. Its main function is to convert mechanical energy into electrical energy.
Components of motors
An electric motor has two mechanical parts: a rotor and a stator. It also has two electrical parts: a magnet and an armature, one of which is connected to the rotor and the other to the stator. The magnets produce a magnetic field that passes through the armature. These magnets can be electromagnets or permanent magnets. The magnetic field magnets are usually located on the stator, while the armature is located on the rotor. However, the two may also be reversed.
Bearings
The rotor is supported by bearings. The bearings transmit the force of the axial and radial loads from the shaft to the motor housing, thus turning the rotor on the shaft.
Rotor
The rotor is the moving part that provides mechanical power. The rotor is usually fitted with conductors that carry an electric current. The magnetic field of the stator exerts a force on the conductor, causing the shaft to rotate. Some rotors carry permanent magnets. Permanent magnets have high efficiency over a wide range of operating speeds and power.
Air Gap
The air gap between the stator and rotor allows for rotation. The width of the air gap has a significant effect on the electrical characteristics of the motor. In general, the smaller the air gap, the better the motor performance. This is because an excessive air gap reduces performance. Conversely, an air gap that is too small can create friction in addition to noise.
The motor shaft extends outside the motor to meet the load requirements. Since the force of the load extends beyond the outermost bearing, it is referred to as a suspended load.
Stator
TThe stator is positioned around the rotor and typically incorporates field magnets, which can be permanent magnets or electromagnets (comprising windings around a ferromagnetic core). These magnets create a magnetic field that permeates the rotor armature and generates a force on the rotor windings. The stator core contains multiple thin, insulated metal sheets called laminations, made from electrical steel which possesses specific properties such as permeability, hysteresis, and saturation. If a solid core were used, eddy currents would be produced, but this effect is minimized by stacking the sheets. For AC motors powered by mains electricity, the conductors in the windings are impregnated with varnish in a vacuum to prevent wire vibration, which can wear away insulation and reduce motor life. In contrast, resin-encapsulated motors found in deep-well submersible pumps, washing machines, and air conditioners have their stators encased in plastic resin, which helps to prevent corrosion and minimize conducted noise.
Armature
An armature consists of a wire wound around a ferromagnetic core. As current passes through the wires, a magnetic field exerts a force (Lorentz force) on them which causes the rotor to turn. The windings are coils wound around a laminated soft iron ferromagnetic core, which when energised form magnetic poles.
Motors are classified into two configurations, with and without magnetic poles. In salt pole motors, the rotor and stator ferromagnetic cores have projections called poles facing each other. Wires are wrapped around each pole below the pole face. As current flows through the wires, these poles become north and south poles. In a non-skewed pole (distributed field or circular rotor) motor, the ferromagnetic core is a smooth cylinder. The windings are evenly distributed in slots around the circumference. The alternating current in the windings creates continuously rotating magnetic poles in the core. Shaded pole motors have a winding around some of the poles that delays the phase of the magnetic field at that pole.
Commutator
A commutator is a rotary electrical switch that supplies current to the rotor. The commutator periodically reverses the current in the rotor windings as the shaft rotates. The commutator consists of a cylinder consisting of a plurality of metal contact segments on an armature. Pressed against the commutator are two or more electrical contacts, called “brushes,” made of a soft conductive material such as carbon. When the rotor rotates, the brushes are in sliding contact with successive commutator segments, supplying current to the rotor. The windings on the rotor are connected to the commutator blades. Each half-turn (180°) of the commutator reverses the direction of the current in the rotor windings. Thus the direction of the torque applied to the rotor always remains the same. Without this reversal, the direction of the torque on the rotor winding is reversed every half-turn, thus stopping the rotor. Commutated motors have mostly been replaced by brushless motors, permanent magnet motors and induction motors.
Motor Supply & Control
Motor Supply
As mentioned above, DC motors are usually supplied via an open-close ring commutator. An AC motor can be commutated using a slip ring commutator or an external commutator. It can be a fixed or variable speed control type and can be synchronous or asynchronous. General purpose motors can run on AC or DC.
Motor Control
DC motors can run at variable speeds by adjusting the voltage applied to the terminals or by using pulse width modulation (PWM).
AC motors running at fixed speeds are usually powered directly from the grid or through a motor soft starter; AC motors running at variable speeds are powered by a variety of power inverters, variable frequency drives or electronic commutator technologies.
The term electronically commutated is commonly associated with self-commutated brushless DC motors and switched reluctance motor applications.
Principals
Electric motors rely on magnetic fields to operate. Magnetic fields can be generated by magnets or by windings around a magnetic core. The theory begins with an explanation of the magnetic force on a current carrying wire exposed to a magnetic field. A magnet produces a magnetic field between the N and S poles. The magnetic field lines come out of the N-pole and enter the S-pole. This magnetic field is constant, there are no fluctuations in the magnetic field and it looks like a DC magnetic field.
When a current-carrying wire enters a magnetic field, the wire is subjected to a magnetic force and thus moves. The magnitude of the magnetic force depends on a number of parameters that will be discussed in this paper. The first parameter that affects the magnetic force is the current through the wire. If the current through the wire is zero, no force will be applied to the wire and the force is directly related to the current. Therefore, the following equation can be written :
(1). F ∝ I
where F is the magnetic force and I is the current in the wire. The other parameter is the length of the wire that sees the magnetic field. The relationship between the magnetic force and the length of the exposed wire is also simple and can be written as :
(2). F ∝ l
where l is the length of wire. The last parameter is magnetic field strength which has a direct relationship with magnetic force as :
(3). F ∝ B
These three parameters determine the maximum value of the magnetic force when the field is perpendicular to the wire. Therefore, any deviation from the perpendicular position reduces the force on the wire. This means that the magnetic force does not reach its maximum value. This is because there is an angle between the magnetic field and the current in the conductor.
Considering all the parameters, the magnetic force can be calculated from the given equations :
(4). F=B·I·l·sinθ
Now, instead of a single conductor between the poles, there is a loop. The loop can be any shape. But for better understanding, assume that the loop is rectangular. In this case, each side of the loop carries current and is subject to a magnetic force. The direction of the force can be obtained by the left-hand rule.
In this rule, the thumb is aligned with the magnetic force, the index finger indicates the magnetic field, and the middle digit indicates the direction of the current. All these fingers are perpendicular to each other. According to Equation 4, the magnetic force is zero when the current carried is parallel to the magnetic field. Therefore, the magnetic force on BC and AD is zero.
In this case, only AB and CD are magnetised. Applying the left hand rule to the AB and CD paths, the direction of the magnetic force will be up for the AB path and down for the CD path. These two opposite forces cause the loop to rotate. However, the rotation cannot be accomplished because the direction of the current in the loop remains the same. This means that when the loop is perpendicular to the magnetic field, it is the stable position of the loop. In this position, the upward and downward forces neutralise each other and the wire loop cannot move. To solve this problem, the direction of the current in the loop must be redirected at each half-turn of the rotation to allow the wire loop to rotate. In addition, inertia will help the wire loop continue to rotate and pass through the stable position.
