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Electric motors
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- 2. Introduction WorkingPrinciple Construction Types DC Motors AC Motors Motor Efficiency Factors Affecting Motor Performance Applications
- 3. Definition :- Anelectric motor is an electrical machine that converts electric energy into mechanical energy Working :- Electric motors operate through the interaction between an electric motor's magnetic field and winding current to generate force
- 4. In a basicmotor, an armature is placed in between magnetic poles. If the armature winding is supplied by an external source, current starts flowing through the armature conductors. As the conductors are carrying current inside a magnetic field, they will experience a force which tends to rotate the armature.
- 6. ROTOR: • In anelectric motor, the moving part is the rotor, which turns the shaft to deliver the mechanical power. • The rotor usually has conductors laid into it that carry currents, which interact with the magnetic field of the stator to generate the forces that turn the shaft. STATOR: • The stator is the stationary part of the motor’s electromagnetic circuit and usually consists of either windings or permanent magnets. • The stator core is made up of many thin metal sheets, called laminations. • Laminations are used to reduce energy losses that would result if a solid core were used. WINDINGS: • Windings are wires that are laid in coils, usually wrapped around a laminated soft iron magnetic core so as to form magnetic poles when energized with current. • Electric machines come in two basic magnet field pole configurations: salient-pole machine & non salient-pole machine. • In the salient-pole machine the pole's magnetic field is produced by a winding wound around the pole below the pole face. • In the non salient-pole, or distributed field, or round-rotor, machine, the winding is distributed in pole face slots.
- 7. COMMUTATOR: •A commutator isa mechanism used to switch the input of most DC machines and certain AC machines consisting of slip ring segments insulated from each other and from the electric motor's shaft. • The motor's armature current is supplied through the stationary brushes in contact with the revolving commutator, which causes required current reversal and applies power to the machine in an optimal manner as the rotor rotates from pole to pole. • In absence of such current reversal, the motor would brake to a stop. BEARINGS: • The rotor is supported by bearings, which allow the rotor to turn on its axis. The bearings are in turn supported by the motor housing. • The motor shaft extends through the bearings to the outside of the motor, where the load is applied. Because the forces of the load are exerted beyond the outermost bearing, the load is said to be overhung.
- 10. Voltage Equation ofthe DC Motor : Input Voltage provided to the motor armature can perform the following two tasks: Obtain control on Back E.M.F Eb of the Motor. Provide supply to the Ohmic IaRa drop. V = Eb + IaRa ………………….(1) Where, Eb = Back E.M.F IaRa = Armature Current X Armature Resistance Power equation of a DC Motor : Multiply both sides of Equation (1) by Ia , we get, VIa= EbIa +Ia 2Ra ………. (2) where, VIa = Input Power supply (Armature Input) EbIa = Mechanical Power developed in Armature (Armature Output) Ia 2Ra= Power loss in armature (Armature Cupper (Cu) Loss)
- 13. SQUIRREL CAGE MOTORWORKING • The rotor consists of bars which are shorted at both ends by end rings • External resistance cannot be added in the rotor circuit • Hence starting torque is less
- 14. SLIP RING MOTORWORKING • Rotor consists of three phase winding similar to the stator winding • External resistance can be added in the rotor circuit • Hence high starting torque can be achieved
- 17. Electrical Motor Efficiencywhen Shaft Output is measured in Watt if power output is measured in Watt(W), efficiency can be expressed as: ηm = Pout / Pin …………..(1) where ηm = motor efficiency Pout = shaft power out (Watt, W) Pin = electric power in to the motor (Watt, W) but ηm = Pout / Pout – losses ………(2)
- 18. Primary and SecondaryResistance Losses : The electrical power lost in the primary rotor and secondary stator winding resistance are also called copper losses. The copper loss varies with the load in proportion to the current squared and can be expressed as Pcl = R I2 where Pcl = stator winding - copper loss (W, watts) R = resistance (Ω) I = current (A, amps) Iron Losses : These losses are the result of magnetic energy dissipated when the motors magnetic field is applied to the stator core.
- 19. Stray Losses : •Straylosses are the losses that remains after primary copper and secondary losses, iron losses and mechanical losses. • The largest contribution to the stray losses is harmonic energies generated when the motor operates under load. • These energies are dissipated as currents in the copper winding, harmonic flux components in the iron parts, leakage in the laminate core. Mechanical Losses : • Mechanical losses includes friction in the motor bearings and the fan for air cooling. Variable losses: • These include copper losses in stator and rotor winding due to current flowing in the winding.
- 21. Supply voltage If onlythe voltage of power supply changes, the change in motor performance is in direct proportion to that change in voltage. Type of power supply Unlike the constant-voltage power supply, the power supply with resistance will have a voltage drop due to its internal resistance, causing the stall current and, therefore, the stall torque to drop with the speed line bottomed to the left
- 22. Number of TurnsPer Slot Increased number of turns per slot results in a drop in speed in direct proportion. Diameter of Magnet Wire The increased diameter of magnet wire results in increasing stall torque and stall current in inverse proportion of the wire diameter raised to the second power. (Note: however, that these inversely proportional changes are not obtainable any more beyond a certain limit range. )
- 23. Environmental Temperature Environmental temperature affectsthe magnetic forces of magnets and the winding resistance, so indicates changes in motor performance. Type of Magnet Magnets are available in a wide variety of types, but discussed here taking them as changes in magnetic force. Changing the anisotropic magnets from wet to dry results in decreasing stall torque (Ts) and increasing no-load speed (N0) due to magnetic force weakened.
- 24. Flux Yoke Magnets areheld in a housing for their full capability, while using a thin-wall housing will result in magnetic force leaking through the housing wall. That is where magnet yokes are used to prevent magnetic leakage thus maintaining the full capacity of the magnets. Phase Motors, assembled initially with neutral phase, runs under load lagging momentarily in the electrical phase due to its phenomenal armature reaction. Lagging in phase, therefore, is not preferable, since it causes efficiency, electrical noise, and life to become even worse. To avoid these situations, improved motors are assembled with forward brush- shifting that offsets the armature reaction during load operation to keep them on neutral electrically.
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