PHY 152 (ELECTRICITY AND MAGNETISM) ELECTRIC MOTORS (AC & DC) ELECTRIC GENERATORS (AC & DC) AIMS Students should be able to Describe the principle of magnetic induction as it applies to DC and AC generators. Describe the differences between the two basic types of ac generators. INTRODUCTION Regardless of size, all electrical generators, whether dc or ac, depend upon the principle of magnetic induction. An emf is induced in a coil as a result of (1) a coil cutting through a magnetic field, or (2) a magnetic field cutting through a coil. As long as there is relative motion between a conductor and a magnetic field, a voltage will be induced in the conductor. That part of a generator that produces the magnetic field is called the field. That part in which the voltage is induced is called the armature. For relative motion to take place between the conductor and the magnetic field, all generators must have two mechanical parts - a rotor and a stator. The ROTor is the part that ROTates; the STATor is the part that remains STATionary. In a dc generator, the armature is always the rotor. In alternators, the armature may be either the rotor or stator. QUESTION: Magnetic induction occurs when there is relative motion between what two elements? 1
DC Motor A simple DC motor has a coil of wire that can rotate in a magnetic field. The current in the coil is supplied via two brushes that make moving contact with a split ring (commutator). The coil lies in a steady magnetic field. The forces exerted on the current-carrying wires create a torque on the coil. The force F on a wire of length L carrying a current I in a magnetic field B is (i.e ILB times the sine of the angle between B and I, which would be 90 if the field were uniformly vertical. The direction of F comes from the right hand rule. Fig 1: DC Motor 2
The two forces shown above are equal and opposite, but they are displaced vertically, so they exert a torque. The commutator is that device/component that periodically reverses the direction of an electric current. NOTE A DC motor requires often an external resistor or rheostat to limit the current. The value, in Ohms, of that resistor is reduced in steps as the speed of the motor increases, until finally that resistor is removed from the circuit as the motor reaches close to its final speed. DC Generator A DC generator is also a DC motor (with the same components of a motor), but the coil is turned to generate an e.m.f. If you use mechanical energy to rotate the coil (N turns, area A) at uniform angular velocity ω in the magnetic field B, it will produce a sinusoidal e.m.f in the coil. Let θ be the angle between B and the normal to the coil, so the magnetic flux φ is NAB.cos θ. Faraday's law gives: Fig 2: DC Generator 3
As in the DC motor, the ends of the coil connect to a split ring, whose two halves are contacted by the brushes. Note that the brushes and split ring 'rectify' the e.m.f produced: the contacts are organized so that the current will always flow in the same direction, because when the coil turns past the dead spot, where the brushes meet the gap in the ring, the connections between the ends of the coil and external terminals are reversed. AC Motor As in the DC motor case, a current is passed through the coil, generating a torque on the coil. Since the current is alternating, the motor will run smoothly only at the frequency of the sine wave. It is called a synchronous motor. More common is the induction motor, where electric current is induced in the rotating coils rather than supplied to them directly. Fig 3: AC Motor One of the drawbacks of the induced AC motor is the high current which must flow through the rotating contacts. In common AC motors the magnetic field is produced by an electromagnet powered by the same AC voltage as the motor coil. The coils which produce the magnetic field are sometimes referred to as the "stator", while the coils and the solid core which rotates is called the "armature". In an AC motor the magnetic field is sinusoidally varying, just as the current in the coil varies. 4
AC motors are generally divided into two categories, induction and synchronous motors. The most common AC motor is the "Squirrel cage motor", a type of induction motor. Most AC motors require a starter, or method of limiting the inrush current to a reasonable level. AC Generator The turning of a coil in a magnetic field produces motional emfs in both sides of the coil which add. Since the component of the velocity perpendicular to the magnetic field changes sinusoidally with the rotation, the generated voltage is sinusoidal or AC. This process can be described in terms of Faraday's law when you see that the rotation of the coil continually changes the magnetic flux through the coil and therefore generates a voltage. AC generators are generally called an alternators. AC GENERATOR AND MOTOR Fig 4: AC Generator Fig 5: AC motor and generator 5
A hand-cranked generator can be used to generate voltage to turn a motor. This is an example of energy conversion from mechanical to electrical energy and then back to mechanical energy. As the motor is turning, it also acts as a generator and generates a "back emf". By Lenz's law, the emf generated by the motor coil will oppose the change that created it. If the motor is not driving a load, then the generated back e.m.f will almost balance the input voltage and very little current will flow in the coil of the motor. But if the motor is driving a heavy load, the back e.m.f will be less and more current will flow in the motor coil and that electric power being used is converted to the mechanical power to drive the load. Types of Alternator 1. Rotating amarture alternators: the rotating-armature alternator is similar in construction to the dc generator in that the armature rotates in a stationary magnetic field. 2. Rotating field alternators: the rotating-field alternator has a stationary armature winding and a rotating-field winding. The advantage of having a stationary armature winding is that the generated voltage can be connected directly to the load. Alternators 6
QUESTIONS What is the part of an alternator in which the output voltage is generated? What are the two basic types of alternators? What is the main advantage of the rotating field alternator? 7