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OBJECTIVESAfter studying Chapter 6, the reader should be able to:1. Describe the operation of DC and AC electric motors.2. Explain how a brushless DC motor works.3. Discuss the advantages and disadvantages of using electric motors in hybrid electric vehicles.4. Explain how electric power steering works.5. Describe how a DC-to-DC converter works. 6. Discuss how a DC-to-AC inverter works.
FUNDAMENTALS OF MAGNETISMMagnetism is a form of energy that is generated by the motion of electrons and alignment of atoms in some materials. A type of iron ore, called lodestone, exists as a magnet in nature.
Lines of ForceIn a bar magnet, the lines are concentrated at both ends of the bar and form closed, parallel loops in three dimensions around the magnet. They come out of one end, or pole, of the magnet and enter at the other end; they NEVER intersect.
The opposite ends of a magnet are called its north and south poles. In reality, they should be called the "north seeking" and "south seeking" poles, because they seek the earth's North Pole and South Pole, respectively. The stronger the magnet, the more magnetic lines that are formed. The magnetic lines of force, also called magnetic flux or flux lines, form a magnetic field. The terms magnetic field, lines of force, flux, and flux lines are used interchangeably. Flux density refers to the number of flux lines per unit of area.
A magnetic field can also be observed by using a compass. A compass is simply a thin magnet or magnetized iron needle balanced on a pivot. The needle will rotate to point toward the opposite pole of a magnet. It can be very sensitive to small magnetic fields. Since it is a small magnet, a compass usually has one end marked N and the other marked S.
If like poles are placed close together, the curving flux lines meet head-on, forcing the magnets apart. Therefore, like poles of a magnet repel and the unlike poles attract.
PermeabilityMagnetic flux lines cannot be insulated. There is no known material through which magnetic force does not pass, if the force is strong enough. However, some materials allow the force to pass though more easily than others. This degree of passage is called permeability.
ReluctanceWhile there is no absolute insulation for magnetism, certain materials resist the passage of magnetic force. Air does not allow easy passage, so air has a high reluctance.
ELECTROMAGNETISMScientists discovered that current-carrying conductors are also surrounded by a magnetic field about 1820. The creation of a magnetic field by the use of an electrical current is called electromagnetism. As current increases, more flux lines are created and the magnetic field expands. As current decreases, the magnetic field contracts, or collapses.
The magnetic field surrounding a straight, current-carrying conductor exists along the entire length of the wire. The strength of the current determines how many flux lines there will be and how far out they extend from the surface of the wire.
Right-Hand RuleMagnetic flux cylinders have direction, just as the flux lines surrounding a bar magnet have direction. Most automotive circuits use the conventional theory of current flow (+ to -), and therefore the right-hand rule is used to determine the direction of the magnetic flux lines.
Current-carrying conductors tend to move out of a strong field into a weak field, so the conductors move away from each other.
Electric motors, such as automobile starter motors, use this field interaction to change electrical energy into mechanical energy. These forces cause the center of the motor, where the conductors are mounted, to turn clockwise.
Coil ConductorIf several loops of wire are made into a coil, the magnetic flux density is strengthened. Flux lines around a coil are the same as the flux lines around a bar magnet.
ElectromagnetsThe magnetic field surrounding a current-carrying conductor can be strengthened by using a soft iron core. The concentration of force greatly increases the strength of the magnetic field inside the coil. Coils with an iron core are called electromagnets.
ELECTROMAGNETIC INDUCTIONMagnetic flux lines can create an electromotive force, or voltage, in a conductor if either the flux lines or the conductor is moving. This creation of a voltage in a conductor by a moving magnetic field is called electromagnetic induction.
The highest voltage is generated when the motion is at right angles.
Voltage StrengthThere are four ways to increase induced voltage:Increase the strength of the magnetic field, so there are more flux lines.Increase the number of conductors that are breaking the flux lines.Increase the speed of the relative motion.Increase the angle between the flux lines and the conductor to a maximum of 90 degrees.
Maximum voltage is induced if the conductors break flux lines at 90 degrees and the voltage decreases when the flux lines are cut at angles between 0 and 90 degrees.
An induced current moves so that its magnetic field opposes the motion that induced the current. This principle is called Lenz’s law.
ELECTRIC MOTORSElectric motor power is expressed in kilowatts (kW).A kilowatt is equal to 1,000 watts.One hp is equal to 746 watts.
Electric Motor OperationThe torque of a starter is determined by the strength of the magnetic fields. Magnetic field strength is measured in ampere-turns. If the current or the number of turns of wire is increased, the magnetic field strength is increased.
The following are the basic principles of a typical DC motor:The speed of the motor is proportional to the applied voltage. The torque is proportional to the applied current. The speed is typically controlled by altering the voltage or current flow by using taps in the motor windings or by using a variable voltage supply.
The use of brushes in an electric motor has many disadvantges including: 1. Any arcing of the brushes also causes electrical noise.2. The brushes eventually wear out and require replacement.
What Is a “Traction Motor” and an “Electric Machine”?A traction motor is a motor used to propel a vehicle. The term traction comes from the engineering term that describes what must occur for a drive wheel to rotate and to transfer torque from the propulsion unit to the drive wheels.The term machine is commonly used to describe an electric motor because it is often more than a simple motor and requires controllers and other components to function.
BRUSHLESS MOTORSThere are two types of electric brushless motors: the AC induction motor and the AC synchronous motor.
AC Induction MotorAn AC induction motor, as is used in the General Motors parallel hybrid truck (PHT).An AC induction motor is also known as an AC asynchronous motor, or AC induction motor (ACIM).The term asynchronous means that the speed of the motor is not necessarily related to the frequency of the current flowing through the stator windings. ACIMs include squirrel-cage and wound-rotor induction designs.
AC Synchronous MotorThe AC synchronous motor rotates exactly at the supply frequency or a submultiple of the supply frequency. The speed is controlled by varying the frequency of the AC supply and the number of poles in the stator winding, according to the relation:RPM = 120F ÷ pwhereRPM = Synchronous speedF = AC power frequency p = Number of poles, usually an even number but always a multiple of the number of phases
Permanent magnet rotors.Brushless permanent magnet motors use two designs of rotors:The permanent magnets are part of the rotor in the core of the rotor. These are called a surface permanent magnets (SPMs).
In the other type, the permanent magnets are housed inside the outer shell of the rotor and are called interior permanent magnets (IPMs). The Honda Accord, Ford hybrids, and Toyota hybrids use an IPM-type rotor assembly.
In both types of motors, the stator coils are stationary and the permanent magnet assembly rotates. Alternating current (AC) is fed to the varous phases in the stator in order to get the permanent magnets in the rotor to “chase” the changing magnetic field.
There are ten electric motors, sometimes more, in most hybrid vehicles. These include: 1. One or two drive motor(s) 2. Motor to circulate the coolant during idle stop to keep the passengers warm (Toyotas/Honda Accord/Honda Civic)3. Motor for electric power steering (EPS) (all) 4. Motor for A/C compressor (New Prius and Accord)5. Motor for cooling CVT (Ford/Toyota/Lexus) 6. Motor for AT trans (hybrid Accord, GM PHT, and Saturn VUE) 7. Motor(s) for air cooling the HV batteries (all) 8. Motor for hydroelectric power steering (GM 42/36v) 9. Motor in CVT to engage park (2004+ Prius) 10. Rear differential (Lexus RX400h/Highlander 4WD)
Electric Motors Are Perfect for VehiclesAn electric motor produces maximum torque at low speeds, making it the perfect power source to get a vehicle moving from a stop. Then, when the torque of an electric motor starts to drop off, the torque multiplied by the speed (RPM) results in power. Therefore, a typical electric motor used on a hybrid vehicle has the following characteristics:
Delivers constant torque at low speed, typically from zero to 1,500 RPMDelivers constant power above 1,500 RPM
MOTOR CONTROLAn example of traction motor control is the motor control module (MCM) usedon Honda hybrid electric vehicles. The MCM has three inputs from three rotor position sensors, A, B, and C.
The MCM has three outputs: U, V, and W. Each winding sends control information (digital high-low) to the power drive unit (PDU).
The current flow through the PDU is controlled by six insulated gate bipolar transistors (IGBTs). The IGBTs are current drivers that send current from the battery pack through the stator windings to energize the stator coils and move the rotor to power the drive wheels. Most motor controllers include Hall-effect current sensors.
The IGBTs process drive current to the electric drive motor. The diodes form a rectifier bridge to change the AC generated in the electric drive motor to pulsating DC to charge the battery pack.
Toyota and Lexus use a speed sensor called a resolver to detect the rotor position.
Cooling the ElectronicsThe current flow and the electronic devices in hybrid electric control units generate a lot of heat. Toyota, Ford, and GM hybrids use a liquid cooling method to control the temperature of the electronics.
CAPACITORS IN HYBRID CONTROLLERSA capacitor consists of two conductive plates with an insulating material between them. The insulating material is commonly called a dielectric. It may be air, mica, ceramic, glass, paper, plastic, or any similar nonconductive material. The higher the dielectric constant number of a material, the better it is as an insulator.
