Chapter 11 Electrical Generators A generator is a device that converts mechanical energy (motion) into electrical energy (current – voltage).

Basic Principle of Generators Faraday’s Law: “When a piece of wire moves within a magnetic field, it causes current to be induced in the conductor.” See figures on page 101 and 102

Magnitude of Induced Current I is proportional to the speed of the conductor (v) I is proportional to the angle of travel of the conductor with respect to the magnetic field (B). I is max when v is perpendicular to B. I is min (I=0) when v is parallel to B. I = K B V (sin of angle between v and B) See figures on page 101 and 102

Basic Generator The basic generator consists of a loop of wire wound on an armature drum residing within a magnetic field (B) produced by a permanent magnet. Each end of the loop is connected to a slip ring which conducts electricity. Attached to each slip ring are electrical contactors called “brushes”. See figures on page 103.

See figures on pages 104, 105, and 106 Working of a Generator As the loop (armature) turns, both ends of the loop start to rotate a circular path. At 0 degrees the loop v is parallel to B and I = 0 At 90 degrees, loop v is perpendicular to B and I reaches its maximum value. At 180 degrees, loop v is parallel to B and I = 0 At 270 degrees, loop v is perpendicular to B and I reaches its minimum value. At 360 degrees, loop is parallel to B and I = 0 At any other angle I = K v B (sin angle v,B) See figures on pages 104, 105, and 106

Methods of Generating Electricity Hydroelectric (Water) Plants. Nuclear power plants. Coal driven power plants. Wind power plants. Geothermal power plants. Solar power plants. The first five operate with a generator moved by water, air or steam. The last one (solar), the electricity is produced by the action of the sun given energy to electrons to move on semiconductor materials.

To Increase the current Remember that I = K v B (sin of angle v,B) Then we can increase I by: Increasing v. Increasing B Using many loops in parallel

To Increase the Frequency (f) Increase the angular velocity of the loop (rotation of the loop) However, by doing this, the amplitude of current and voltage also increase.

Single-Phase Generator A generators with a single source or AC voltage is called a Single-Phase generator. See figure on bottom of page 107

Two-Phase Generator Two-Phase Generator is a generator built with two loops at 90 degrees as shown in Figure on page 108. See figure on page 108

Three-Phase Generator Three-Phase Generator is a generator built with three loops at 60 degrees as shown in figure on page 109. See figure on page 109

To Increase the Generator Output Increase the strength of the electromagnetic field. Increase the number of wires which make up the loop. Increase the rotation speed of the loop, however, remember that by doing this, the output frequency is also changed. See figure on page 110 Website

Chapter 12: DC Motors Motor operation is dependent on the interaction of magnetic fields To understand how a motor operates, we need to review: The rules of magnetism. The relationship between I and B.

Magnetism A permanent magnet has two poles, N and S, and has an electromagnetic field (B), with flux lines traveling from N to S external to the magnet.

Effect of Like / Unlike Poles Like poles of a magnet repel. Unlike poles attract each other.

Current Flow and Magnetic Field A current flow in a conductor produces a magnetic field. Perpendicular to the current flow. Left-hand rule gives direction of magnetic field Thumb = direction of conductor motion Index finger = magnetic lines of force Middle finger = induced current in conductor. See figure on top of page 105.

See figure on top of page 114 Electromagnets Electromagnets are built with a conductor formed into a coil around an iron core. See figure on top of page 114

Direct Current Motors A simple DC motor looks just like the DC generator. . . . . . but, in the generator the input is the motion and the output is the current . . . . . . and, in the DC motor the input is the current and the output is the motion. See figure on page 112.

Operation of a DC Motor When voltage is applied to the loop of wire a current flows, and a magnetic field is created that will interact with the field of the magnet. Repulsion and attraction of the fields will cause the loop to turn. The loop moves away from the strong field toward the weak field. The direction of the rotation can be determined by “the right-hand rule”. See figure on page 113.

Motor / Generator notation Field Windings / Armature Stator / Rotor See figure on top of page 107

Types of DC Motors 1. Field windings (Electromagnets) DC motors have two types of windings: 1. Field windings (Electromagnets) 2. Armature winding (Loop) Depending on how these windings are connected to the voltage supply, motors are classified in to three types Series DC Motor Shunt DC Motor Compound DC Motor

See figure on top of page 114 Series DC Motor Field and armature windings in series. Use left-hand rule to find “N” and “S” of magnets. Use right-hand rule to find if motor is turning CW or CCW. See figure on top of page 114

Use of DC Series Motors To operate small electrical appliances Portable electric tools cranes, winches, hoists

Load Concerns of DC Motors Some load must ALWAYS be connected to a Series DC Motor. Otherwise its speed will increase and may damage the bearings or windings. Small motors, such as the ones used in electric hand drills, have enough internal resistance to load themselves

See figures on bottom of page 114, and top of page 115 Shunt DC Motor Field windings and armature windings are connected in parallel. Use the left-hand rule to draw the electromagnetic fields, and right-hand rule to show that the motor turns CW. See figures on bottom of page 114, and top of page 115

Speed of DC Shunt Motors Shunt DC motors provide constant speed, even if the load requirements change during operation. Therefore, the shunt DC Motors show excellent speed regulation.

See figure on middle of page 115 Reversing a DC Motor Will the rotation of the motor change if we switch the connections at the voltage source? NO! Because if reverse the polarity, the current will flow in opposite directions in both armature and field windings. Verify this by inspection in the figure of page 115 (shown below). Remember to use the left-hand rule for the fields and the right-hand rule for the motor motion. See figure on middle of page 115

Reversing a DC Motor (2) To change the rotation of a DC motor we need to ensure that the current flowing in only one of the windings (Field or loop) changes its flow. Using left-hand rule for the field and right-hand rule for the rotation, verify that the motor below turns CCW. See figure on page 116

Generator Action in DC Motors In the figure below, the battery voltage (emf ) makes the current flow from its negative to its positive terminal. Since the armature is a loop turning in a magnetic field, it induces a current in opposite direction to the battery current producing a voltage in opposite direction ( cemf ). Therefore, the total voltage (EMFT) is emfTotal = emf – cemf See figure on page 117

Effect of cemf on DC Motor Speed emfTotal = emf - cemf cemf strength depends of loop rotation speed Therefore, a motor at rest has no cemf. As motor starts to turn, cemf increases. Thus, emfTotal decreases, and… cemf is a self speed regulation in a DC motor

Speed Regulation in DC Motor REVIEW Speed of a DC motor depends on ILOOP ILOOP is proportional to emfTotal cemf is directly proportional to motor speed SPEED REGULATION If load increases, then motor speed decreases, cemf decreases, emfTotal increases , & ILOOP increases Since ILOOP increases, motor generates more Force Loop turns faster compensating for any reduction in speed due to mechanical load

Measure of Motor Output Output is measure in horsepower (hp). 1 hp = power to lift 550 pounds one foot in one second. 1 hp = 746 Watts For residential uses motors < 1 hp Motors < 1hp are called Fractional Horsepower Motors * For large industrial applications motors with multiple horsepower ratings are used