2 Index Classification Laws of electromagnetism Rotating Magnetic Field AC MotorInduction MotorSynchronous MotorMaintenance Practices
3 Laws of Electromagnetism Faraday’s LawLenz’s LawFleming’s Right Hand ruleFleming’s Left Hand ruleInteraction of two magnetic fields
4 Faraday’s Law of Electromagnetic Induction When the magnetic flux through a circuit is changing an induced EMF is setup in that circuit and its magnitude is proportional to the rate of change of flux”Simulation
5 Lenz’s Law“ The direction of an induced EMF is such that its effect tends to oppose the change producing it”Simulation
6 Fleming’s Right Hand rule Used to measure the direction of induced current in a conductor when cut by a magnetic field.
7 Fleming’s Left Hand rule Used to measure the direction of motion of a current carrying conductor when placed in magnetic field.
8 Interaction of two magnetic fields MotionCurrentField
9 Production of Three Phase Rotating Magnetic Field RMF may be set up in two-phase or three-phase machines.The number of pole pairs must be the same as the number of phases in the applied voltage.The poles are displaced from each other by an angle equal to the phase angle between the individual phases of the applied voltage.
10 + When Current positive and going into · When Current negative and coming from
11 Speed of RMFThe magnetic field established rotates at a speed given byN = 60* f / Pwhere f = frequency of stator currentP = Number of pair of poles
12 A machine which converts AC electrical energy into Mechanical energy AC MotorsA machine which converts AC electrical energy into Mechanical energy
13 Induction Motor Introduction Construction Principle of Induction Motor SlipStarting CurrentTorqueTorque Speed characteristicsTwo Phase Induction MotorSingle phase Induction MotorSummaryUses in Aircraft
14 Induction Motor-Intro. The induction motor is the most commonly used type of ac motor. It is simple, low cost and rugged in construction.The induction motor derives its name from the fact that ac voltages are induced in the rotor circuit by the rotating magnetic field of the stator.The induction motor, rotor is not connected to an external source of voltage.
15 Induction Motor Main Parts Stator Wound Rotor Start Resistance
20 Slip in Induction Motor slip speed = synchronous speed - rotor speedmeasured in RPMSlip = (synchronous speed - rotor speed ) /synchronous speedexpressed as a percentageThe greater the slip speed, the greater is the force oneach conductor and the torque exerted by the whole.The squirrel cage rotor of the induction motor set in the rotating field of the stator should accelerate until it is running steadily at a speed which is slightly less than the synchronous speed at which the magnetic field rotates.
21 Starting CurrentThe starting current is very high which may damage the stator winding.To reduce this heavy starting current, star-delta starting switch is used.For starting, the stator winding are connected up in star via the switch to the supply so that the phase voltage is 1/√3 of the normal voltage. This reduced voltage limits the starting current.When the stator winding is energised with the rotor stopped, the slip is 100% and maximum emf is induced in the rotor. A heavy current is thus established into the rotor and this produces a flux which opposes and weakens the stator flux.Since rotor is stationary or until at low speed the emf induced into the stator due to rotor is very low and therefore the stator draws a large amount of current during starting which may damage the stator winding.To reduce this heavy starting current, the voltage applied to the stator winding is reduced by the use of star-delta starting switch. For normal running, the motor is designed to operate with the stator phases mesh or delta connected to the supply via the switch so that the phase voltage is equal to the line voltage.For starting, the stator winding are connected up in star via the switch to the supply so that the phase voltage is 1/√3 of the normal voltage. This reduced voltage limits the starting current.
22 phase voltage is 1/√3 of the normal voltage phase voltage is equal to the line voltage.
23 Torque- Starting The resistance of the squirrel cage rotor is small and inductance high.Thus on starting rotor current and the rotor emf are nearly 90 degrees out of phase.The lagging rotor current interacts little with stator current and therefore the starting torque is poor.The frequency of the current induced in the rotor is the frequency With which the stator field rotates relative to each conductor. When the rotor is at rest, this equals the supply frequency. When the rotor is running lightly loaded the slip is small, and the frequency of induced rotor current may be only a few cycles per second. Now the resistance of the squirrel cage rotor is small and inductance high. Its reactance will therefore be large at the frequency of supply when the rotor is stationary and much less when it is running. Thus on starting rotor current and the rotor emf are nearly 90 degrees out of phase. The flux produced by this lagging rotor current is such that there is little interaction between it and the stator flux, and the starting torque is poor. As the rotor current come into phase with the rotor emf with increased rotor speed (decreased slip and inductive reactance) the rotor and the stator flux comes more into phase and the torque increases
24 Torque- RunningAs the rotor current come into phase with the rotor emf with increased rotor speed (decreased slip and inductive reactance) the rotor and the stator flux comes more into phase and the torque increases.
25 Methods Of Improving Starting Torque In creasing the resistance of the rotor conductorsUsing a combination of high and low resistance conductorsUsing a wire wound rotor connected to variable resistor
31 Single Phase Induction Motor The single phase induction motor produces a pulsating field.However, if the rotor is rotated forward at a bit less than the synchronous speed, It will develop some torque.If the rotor is started in the reverse direction, it will develop a same torque in other direction
32 Split Phase Induction Motor Two phases are produced by splitting a single phase.A capacitor is inserted in one of the windings and is called a permanent-split capacitor motor.The direction of the motor is easily reversed by switching the capacitor in series with the other winding.This motor configuration works well at low horsepower. Though, usually applied to smaller motors
33 Summary The three phase induction motor Is very robust in construction No need for slip rings and therefore less maintenance.Has a high starting current reduced by star-delta switch.Has a poor starting torque.Runs at a speed less than synchronous speed.
34 Direction of rotation can be reversed by interchanging any two stator phases. Is of two types depending on motor construction: Squirrel Cage or Slip Ring
35 Uses in AircraftConstant speed with varying loads and require smoother torque e.g. fuel booster pumps, hydraulic system’s Electric Motor Driven pumps.Systems which need high torque and reversing e.g. Flap Power units (for alternate flap drives), Stabilizer Trim Actuator.Two phase induction motors also used in aircraft such as aileron trim actuators and in reversible valve actuators in Fuel, hydraulic, oil, and pneumatic systems etc.
36 Induction Motor Example A 100 hp, 8 pole, 60 Hz, 3 phase induction motor runs at 891 rpm under full load. Determine the synchronous speed in rpm, slip speed and Slip.Solution on white board
37 Synchronous Motor Synchronous Motor-Intro Synchronous Motor-principle Changing the LoadStarting TorqueImprovement of starting torqueSynchronous Machine ConstructionV curvesTorque versus SpeedSummary
38 Synchronous Motor- Intro The synchronous motor rotates at the synchronous speed i.e. the speed of the RMF.Stator is similar in construction to that of an induction motor, so same principle is applied to the synchronous motor rotor.Field excitation is provided on the rotor by either permanent or electromagnets with number of poles equal to the poles of the RMF caused by stator
39 Synchronous Motor-Principle The rotor acting as a bar magnet will turn to line up with the rotating magnet field. The rotor gets locked to the RMF and rotates unlike induction motor at synchronous speed under all load condition
40 Changing The LoadAn increase in the load will cause the rotor to lag the stator field but still maintain synchronous speed. Increase in load has increased the torque component, but the field strength has decreased due to the increase in length of the air gap between the rotor and the stator.If the synchronous motor is overloaded it pulls out of synchronism and comes to rest. The minimum amount of torque which causes this is called the “ pull out torque”.Lightly loaded motorHeavily loaded motor
41 Starting TorqueIt cannot be started from a standstill by applying ac to the stator. When ac is applied to the stator a high speed RMF appears around the stator. This RMF rushes past the rotor poles so quickly that the rotor is unable to get started. It is attracted first in one direction and then in the other and hence no starting torque.
42 Improvement of starting torque It is started by using a squirrel cage within a rotor construction and therefore starts as an induction motor.At synchronous speed the squirrel cage has no part to play.
48 Summary The synchronous motor: requires to be started by an external prime mover.Runs only at synchronous speed, this is an advantage where continuous speed is required but a disadvantage where a variable speed is required.Can be used to adjust the power factor of a system at the same time it is driving a mechanical load.
49 Maintenance Practices-A.C. Motors Clean, but don’t forget to inspect before and after cleaningCheck electrical connections for security, the insulation to be in satisfactory condition.Examine for signs of over heatingCheck that the motor is secureDo an audible checkEnsure that the motor is not over heating when operating, a rule of thumb is that if it is too hot for the hand, it is too high.When replacing a motor always ensure that the load, valve has not seized.Also ensure that the motor operates in the correct directionEnd of Motors