A Cracked Magnet Becomes Two Magnets

Hybrid and Alternative Fuel VehiclesBy James D Halderman and Tony Martin

Upper Saddle River, NJ 07458

OBJECTIVES

After 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 MAGNETISM

Magnetism 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.

A Cracked Magnet Becomes Two Magnets

Lines of Force

In 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.

Permeability

Magnetic 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.

Reluctance

While 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.

ELECTROMAGNETISM

Scientists 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 Rule

Magnetic 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 Conductor

If 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.

Electromagnets

The 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 INDUCTION

Magnetic 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 Strength

There 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 MOTORS

Electric motor power is expressed in kilowatts (kW).A kilowatt is equal to 1,000 watts.

One hp is equal to 746 watts.

Electric Motor Operation

The 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.

A typical DC motor today uses four poles.

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 MOTORS

There are two types of electric brushless motors: the AC induction motor and the AC synchronous motor.

AC Induction Motor

An 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.

A squirrel-cage rotor.

An alternate design is called the wound rotor.

AC Synchronous Motor

The 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 ÷ p

whereRPM = 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 Vehicles

An 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 RPM

Delivers constant power above 1,500 RPM

MOTOR CONTROL

An example of traction motor control is the motor control module (MCM) used on Honda hybrid electric vehicles. The MCM has three inputs from three rotor position sensors, A, B, and

The MCM has three outputs: U, V, and W. Each winding sends control information (digital high-low) to the

power drive unit (PDU).

A typical Honda PDU schematic is shown.

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 Electronics

The 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 CONTROLLERS

A 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.

When a capacitor is placed in a closed circuit, the voltage source, such as a battery, forces electrons around the circuit. Because electrons cannot flow through the dielectric of the capacitor, excess electrons collect on what becomes the negatively

charged plate. At the same time the other plate loses electrons, and therefore becomes positively charged.

Current continues until the voltage charge across the capacitor plates becomes the same as the source voltage. At that time, the negative plate of the

capacitor and the negative terminal of the battery are at the same negative potential.

An electrostatic field now exists between the capacitor plates because of their opposite charges. It is this

field that stores energy.

If the circuit is opened, the capacitor will hold its charge until it is connected into an external circuit through which it can discharge. When the charged capacitor is connected to an external circuit, it discharges. After discharging, both plates of the capacitor are neutral because all the energy from a circuit stored in a capacitor is returned when it is

discharged.

When the ignition of an HEV is turned off, vehicle manufacturers warn that you must wait 5 to 10 minutes for the capacitors to discharge before servicing the high-voltage system. While these capacitors often

discharge in less than five minutes, it is wise to wait the amount of time specified by the vehicle

manufacturer.

To avoid an electrical shock, any capacitor should be treated as if it were charged until it is proven to be discharged.

Capacitors are also called condensers. This term developed because electric charges collect, or condense, on the plates of a capacitor much like water vapor collects and condenses on

a cold bottle or glass.

Capacitance is measured in farads, which is named after Michael Faraday (1791-1867). The symbol for farads is F. If a charge of 1 coulomb is placed on the plates of a capacitor and

the potential difference between them is 1 volt, the capacitance is then defined to be 1 farad.

Snubbers

Snubbers are capacitors and resistors arranged in a circuit to control the high-voltages surges that can occur when circuits containing coils are switched on

and off.

CONVERTERS AND INVERTERS

Converters

DC-to-DC converters (usually written DC-DC converter) are electronic devices used to transform DC voltage from one level

of DC voltage to another higher or lower level.

One example of a DC-DC converter circuit is the circuit the PCM uses to convert 14 V to 5 V. The 5 volts is

called the reference voltage, abbreviated V-ref, and is used to power many sensors in a computer-controlled

engine management system.

Hybrid electric vehicles use DC-DC converters to provide higher or lower DC voltage levels and current

requirements.

DC-DC Converter Circuit Testing

1. Always follow the manufacturer’s safety precautions when working with high-voltage circuits. These circuits are

usually indicated by orange wiring.2. Never tap into wires in a DC-DC converter circuit to

access power for another circuit.3. Never tap into wires in a DC-DC converter circuit to

access a ground for another circuit.4. Never block airflow to a DC-DC converter heat sink.

5. Never use a heat sink for a ground connection for a meter, scope, or accessory connection.

6. Never connect or disconnect a DC-to-DC converter while the converter is powered up.

7. Never connect a DC-to-DC converter to a larger-voltage source than specified.

Inverters

An inverter is an electronic circuit that changes direct current (DC) into alternating current (AC). In most DC-AC inverters, the switching transistors - usually metal oxide semiconductor field-effect

transistors (MOSFETs) are turned on alternately for short pulses.

The waveform produced by an inverter is not the perfect sine wave of household AC current, but is rather more like a pulsing DC current that reacts similar to sine wave AC in transformers and in induction motors.

AC motors are powered by inverters. An inverter converts DC power to AC power at the required frequency and amplitude. The inverter consists of three half-bridge units and the

output voltage is mostly created by a pulse width modulation (PWM) technique. The three-phase voltage waves are shifted

120° to each other to power each of the three phases.

ELECTRIC POWER STEERING

The electric power steering (EPS), also called electric power-assisted steering (EPAS), system includes the

following components and inputs/outputs:A DC motor

Reduction gearTorque sensor

The electric power steering (EPS) is controlled by the EPS ECU, which calculates the amount of needed assist based on the input from the steering torque sensor. The steering torque sensor is a noncontact sensor that detects the movement and

torque applied to the torsion bar. The torsion bar twists when the driver exerts torque to the steering wheel, and the more

torque applied causes the bar to twist further. This generates a higher-voltage signal to the EPS ECU.

The Toyota Highlander and Lexus RX 400h use a different electric power steering unit due to the larger size of the vehicles. This unit uses a concentric brushless DC

motor on the steering rack.

The Honda electric power steering uses an electric motor to provide steering assist and replaces the need for a hydraulic pump, hoses, and gear. A torque sensor is used to measure road resistance and the direction that the driver is turning the steering wheel. The torque sensor input and the vehicle speed is used by the EPS controller to supply the EPS motor

with the specified current to help assist the steering effort.

SUMMARY

1. Magnetic lines of force leave the north pole and enter the south pole of a magnet.

2. Magnetic lines of force are called flux lines.3. Any conductor carrying an electrical current generates a magnetic field around the conductor, and a moving magnetic

field across a conductor creates electricity.4. Like poles repel and unlike poles attract.

5. A brushless DC motor is also known as an AC synchronous motor.

6. Powerful permanent magnets are used in the rotors of both DC and AC brushless motors.

7. The operation of motors is performed by the controller, which is capable of switching the voltage and/or the frequency of the current flowing through the stationary windings of the

motor.8. DC-DC converters are used in hybrid electric vehicles to convert the high-voltage battery current into a lower voltage

used by the accessories and lighting systems.

REVIEW QUESTIONS

1. How is an electrical current induced in a wire?2. How does an AC synchronous motor work?3. How does an AC induction motor work?

4. How is the operation of a brushless DC motor controlled?5. What is a DC-DC converter, and why is it needed in a

hybrid electric vehicle?

A Cracked Magnet Becomes Two Magnets

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