Automotive battery basics

Automotive battery basics

The battery stores electricity in the form of chemical energy. Through a chemical reaction process the battery creates and releases electricity as needed by the electrical system or devices. Since the battery loses its chemical energy in this process, the battery must be recharged by the alternator. By reversing electrical current flow through the battery the chemical process is reversed, thus charging the battery. The cycle of discharging and charging is repeated continuously and is called "battery cycling".

The purpose of the battery

The battery supplies electricity when the:

Engine is off:
electricity from the battery is used to operate lighting, accessories, or other electrical systems when the engine is not running.

Engine is starting:
electricity from the battery is used to operate the starter motor and to provide current for the ignition system during engine cranking. Starting the car is the battery’s most important function.

Engine is running:
electricity from the battery may be needed to supplement the charging system when the vehicle’s electrical load requirements exceed the charging system’s ability to produce electricity. Both the battery and the alternator supply electricity when demand is high.

Batteries - primary or secondary

Batteries can either be a primary cell, such as a flashlight battery once used, throw it away, or a secondary cell, such as a car battery (when the charge is gone, it can be recharged).

Primary cell:
because the chemical reaction totally destroys one of the metals after a period of time, primary cells cannot be recharged. Small batteries such as flashlight and radio batteries are primary cells.

Secondary cell:
the metal plates and acid mixture change as the battery supplies voltage. As the battery drains the metal plates become similar and the acid strength weakens. This process is called discharging. By applying current to the battery in the reverse direction, the battery materials can be restored, thus recharging the battery. This process is called charging. Automotive lead-acid batteries are secondary cells and can be recharged.

Batteries - wet or dry charged

Batteries can be produced as wet-charged, such as current automotive batteries are today, or they can be dry-charged, such as a motorcycle battery where an electrolyte solution is added when put into service.

Wet-charged:
the lead-acid battery is filled with electrolyte and charged when it is built. During storage, a slow chemical reaction will cause self-discharge. Periodic charging is required. Most batteries sold today are wet charged.

Dry-charged: the battery is built, charged, washed and dried, sealed, and shipped without electrolyte. It can be stored for up to 18 months. When put into use, electrolyte and charging are required. Batteries of this type have a long shelf life. Motorcycle batteries are typically dry charged batteries.

Battery construction

An automobile battery contains a diluted sulfuric acid electrolyte, positive and negative electrodes, in the form of several plates. Since the plates are made of lead or lead-derived materials, this type of battery is often called a lead acid battery. A battery is separated into several cells (usually six in the case of automobile batteries), and in each cell there are several battery elements, all bathed in the electrolyte solution.

Cell voltage

Each cell element of the battery produces approximately 2.1 volts, regardless of the quantity or size of the plates. Automobile batteries have six cells that are connected in series, which produces a total voltage of 12.6 volts.

Specific gravity of electrolyte

Specific gravity means exact weight. A "hydrometer" or a "refractometer" compares the exact weight of electrolyte with that of water. Electrolyte in a charged battery is stronger and heavier than electrolyte in a discharged battery. By weight, the electrolyte in a fully charged battery is about 36% acid and 64% water. The specific gravity of water is 1.000, and the specific gravity of sulfuric acid is 1.835, which means the acid is 1.835 times heavier than the water. The battery electrolyte mixture of water and acid has a specific gravity of 1.270 and is usually stated as "twelve and seventy."

Battery terminal identification

Battery terminals are identified as either "positive" or "negative". Battery cases are marked with a "+" for the positive terminal, and a "-" on the negative terminal. The words "pos" or "neg" are often used instead of the + or -. On top post terminal batteries, the positive post is slightly wider than the negative terminal post. This allow for easy identification.

Battery capacity

Batteries are generally rated using two parameters. The first, and most obvious, is voltage. The second, and less intuitive, is amp-hours. Amp-hours indicate the maximum current that a battery can continuously deliver for a period of 1 hour. When a battery is discharged at this rate, usually its full charge will be expended. A battery that has a 250 amp-hour rating is capable of delivering 250 amps at the batteries full voltage for 1 hour. A battery that has a 500 ma-hour rating is capable of delivering 1/2 amp for 1 hour. It should also be noted that the amp-hour rating is an indication of the capacity of battery. If our 250 amp-hour battery is discharged at a rate of 2 amps, its charge life will be 125 hours. Similarly, if we discharge the battery at 375 amps, the charge life will be 0.66 hours or 39.6 minutes.

Amp-hours/discharge rate= charge life

Another figure we see on automotive batteries is “cold cranking amps.” This figure is generally higher than the amp- hour rating. This rating refers to the maximum current that the battery can deliver at full charge for a short period of time. This is a loose standard and shouldn’t be relied on when selecting a battery for peak demand applications. It should also be noted that when a battery is pressed into this type of service, it can get fairly hot and a long cool down period is required.

Battery service

Battery services are routinely performed. These services include:

1) Testing

2) Charging

3) Cleaning

4) Jumping a dead battery.

5) Adding water

Battery testing

Battery testing has changed in recent years; although the three areas are basically the same, the equipment has improved

· Visual inspection

· State of charge

a. Specific gravity

b. Open circuit voltage

· Capacity or heavy load test

Visual inspection

Battery service should begin with a thorough visual inspection. This inspection may reveal simple, easily corrected problems.

1. Check for cracks in the battery case and broken terminals. Either may allow electrolyte leakage, which requires battery replacement.

2. Check for cracked or broken cables or connections. Replace, as needed.

3. Check for corrosion on terminals and dirt or acid on the case top. Clean the terminals and case top with a mixture of water and baking soda. A battery wire brush tool is needed for heavy corrosion on the terminals.

4. Check for a loose battery hold-down or loose cable connections. Clean and tighten, as needed.

5. Check the electrolyte fluid level. The level can be viewed through the translucent plastic case or by removing the vent caps and looking directly into each cell. The proper level is 1/2" above the separators (about 1/8" below the fill ring shown below). Add distilled water if necessary. Do not overfill.

6. Check for cloudy or discolored electrolyte caused by overcharging or vibration. This could cause high self discharge. Correct the cause and replace the battery.

State of charge

The state of charge of a battery can be easily check in one of two ways:

· Specific gravity test

·Open circuit voltage test

Specific gravity readings By measuring the specific gravity of the electrolyte, you can tell if the battery is fully charged, requires charging, or must be replaced. It can tell you if the battery is sufficiently charged for a capacity (heavy-load) test. The battery must be at least 75% charged to perform a heavy load test. In other words, each cell must have a specific gravity of 1.230 or higher to proceed.

Cell readings percent charged 1.270 = 100 %

1.230 = 75%

1.190 = 50%

1.145 = 25%

1.100 = 0%

Open circuit voltage

A digital voltmeter must be used to check the battery’s open-circuit voltage. Analog meters are not accurate and cannot be used.

1. Turn on the headlamps’ high beam for several minutes to remove any surface charge.

2. Turn headlamps off, and connect the digital voltmeter across the battery terminals.

3. Read the voltmeter. A fully charged battery will have an open-circuit voltage of 12.6 volts.

On the other hand, a totally dead battery will have an open-circuit voltage of less than 12.0 volts.

Battery terminal cleaning Over a period of time, sulfuric acid will corrode battery terminals, clamps, and hold-down. This corrosion adds resistance and lowers current flow to and from the battery. Corrosion can be easily cleaned with a mild solution of baking soda and water. Battery terminals and cables are routinely removed, cleaned, and reinstalled.

Battery jumping with booster cables

Jump starting a dead battery with a booster battery or battery in a car can be dangerous, so the proper sequence of connections will prevent sparks. First, connect the two positive terminals, one from the good battery and the other to the dead battery. Next connect one end of the jumper cable to the negative terminal of the booster (good) battery. Finally connect the other end to a good ground on the engine away from the dead battery. If a spark occurs, it won’t be near the battery, thus reducing the chance for explosion. If the jump starting from another vehicle, start the vehicle, running the engine at 1500 rpm for a few minutes. While the engine is running, start the dead vehicle. Never jump start a frozen battery.

Adding water

Under the rare occurrence of adding water to a battery, use only distilled water. Minerals and chemicals that are commonly found in regular drinking water will react with the plate material and shorten battery life. Under normal conditions the addition of water should not be required. However, the addition of water may be necessary when the battery has been overcharged, for overcharging results in excessive evaporation of water from the electrolyte. The water level should be no higher than 1/8 inch below the bottom of the vent well. To avoid permanent damage, make sure the electrolyte level never drops below the top of the plates. Also, avoid over filling, this may result in electrolyte overflow from the battery.

Introduction to Jet Engines

Introduction to Jet Engines

An aircraft engine, or power plant, produces thrust to propel an aircraft. Reciprocating engines and turboprop engines work in combination with a propeller to produce thrust. Turbojet and turbofan engines produce thrust by increasing the velocity of air flowing through the engine. All of these power plants also drive the various systems that support the operation of an aircraft.

Turbine Engines

An aircraft turbine engine consists of an air inlet, compressor, combustion chambers, a turbine section, and exhaust. Thrust is produced by increasing the velocity of the air flowing through the engine. Turbine engines are highly desirable aircraft power plants. They are characterized by

• Smooth operation
  

• High power-to-weight ratio
  

• Readily available jet fuel.
  

Types of Turbine EnginesTurbine engines are classified according to the type of compressors they use. There are three types of compressors—centrifugal flow, axial flow, and centrifugal-axial flow. Compression of inlet air is achieved in a centrifugal flow engine by accelerating air outward perpendicular to the longitudinal axis of the machine. The axial-flow engine compresses air by a series of rotating and stationary airfoils moving the air parallel to the longitudinal axis. The centrifugal-axial flow design uses both kinds of compressors to achieve the desired compression.
The path the air takes through the engine and how power is produced determines the type of engine. There are four types of aircraft turbine engines—turbojet, turboprop, turbofan, and turboshaft.

Turbojet

The turbojet engine consists of four sections: compressor, combustion chamber, turbine section, and exhaust. The compressor section passes inlet air at a high rate of speed to the combustion chamber. The combustion chamber contains the fuel inlet and igniter for combustion. The expanding air drives a turbine, which is connected by a shaft to the compressor, sustaining engine operation. The accelerated exhaust gases from the engine provide thrust. This is a basic application of compressing air, igniting the fuel-air mixture, producing power to self-sustain the engine operation, and exhaust for propulsion.

Turboprop

A turboprop engine is a turbine engine that drives a propeller through a reduction gear. The exhaust gases drive a power turbine connected by a shaft that drives the reduction gear assembly. Reduction gearing is necessary in turboprop engines because optimum propeller performance is achieved at much slower speeds than the engine’s operating rpm. Turboprop engines are a compromise between turbojet engines and reciprocating power plants. Turboprop engines are most efficient at speeds between 250 and 400 mph and altitudes between 18,000 and 30,000 feet. They also perform well at the slow airspeeds required for takeoff and landing, and are fuel efficient.

Turbofan

Turbofans were developed to combine some of the best features of the turbojet and the turboprop. Turbofan engines are designed to create additional thrust by diverting a secondary airflow around the combustion chamber. The turbofan bypass air generates increased thrust, cools the engine, and aids in exhaust noise suppression. This provides turbojet-type cruise speed and lower fuel consumption.

The inlet air that passes through a turbofan engine is usually divided into two separate streams of air. One stream passes through the engine core, while a second stream bypasses the engine core. It is this bypass stream of air that is responsible for the term “bypass engine.” A turbofan’s bypass ratio refers to the ratio of the mass airflow that passes through the fan divided by the mass airflow that passes through the engine core.

Turboshaft

The fourth common type of jet engine is the turboshaft. It delivers power to a shaft that drives something other than a propeller. The biggest difference between a turbojet and turboshaft engine is that on a turboshaft engine, most of the energy produced by the expanding gases is used to drive a turbine rather than produce thrust. Many helicopters use a turboshaft gas turbine engine. In addition, turboshaft engines are widely used as auxiliary power units on large aircraft.

PLASMA SPRAY PROCESS

PLASMA SPRAY PROCESS

An inert gas such as Argon, when excited by an electric arc, becomes partially ionized and in this state is able to carry an electric current for the generation of a hot gas stream having temperatures approaching 12,000°C. Powder material is injected into the flame at optimum conditions and projected in a semi-molten (plastic) state on to a suitably prepared work piece to form high integrity coatings of typically 0.05mm. to 3mm.thickness. The gun is manipulated by hand or by using a robot, enabling a wide range of component configurations to be coated. The Plasma Spray process is probably the most versatile of all the major Thermal Spray processes. This is predominantly due to the extremely high heat source temperatures available.The plasma generated for plasma spraying usually incorporates one or a mixture of the following gases:

• Argon
• Helium
• Nitrogen
• Hydrogen

Plasma flames for thermal spraying can produce temperatures around 7,000 to 20,000K far above the melting temperature (and vapour temperature) of any known material. The extreme temperature of the plasma is not the only reason for the effective heating properties. If for example helium gas is heated to around 13,000K without a plasma forming, it would have insufficient energy for normal plasma spraying. Nitrogen on the other hand heated to 10,000K going through dissociation and ionisation forming a plasma is an effective heating media for thermal spraying, being able to supply about six times more energy than an equal volume of helium at 13,000K. The plasma is able to supply large amounts of energy due to the energy changes associated with dissociating molecular gases to atomic gases and ionisation which occur with little change in temperature.

The Plasma Spray Process is basically the spraying of molten or heat softened material onto a surface to provide a coating. Material in the form of powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. The hot material impacts on the substrate surface and rapidly cools forming a coating. This plasma spray process carried out correctly is called a "cold process" (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material.

The processes gases used, in combination with the current applied to the electrode controls the amount of energy produced by the process. Since the flow of each of the gases and the applied current can be accurately regulated, repeatable and predictable coating results can be obtained. In addition, the point and angle that the material is injected into the plume, as well as the distance of the gun to the target, component can also be controlled. This provides a high degree of flexibility to develop appropriate spray parameters for materials with melting temperatures across a very large range.

The distance of the plasma gun from the target components, gun and component speeds relative to each other, and part cooling (usually with the help of air jets focused on the target substrate) keep the part at a controlled spray temperature that is usually in the range of 38 °C to 260 °C (100 °F to 500 °F).

The plasma spray gun comprises a copper anode and tungsten cathode, both of which are water cooled. Plasma gas (argon, nitrogen, hydrogen, helium) flows around the cathode and through the anode which is shaped as a constricting nozzle. The plasma is initiated by a high voltage discharge which causes localised ionisation and a conductive path for a DC arc to form between cathode and anode. The resistance heating from the arc causes the gas to reach extreme temperatures, dissociate and ionise to form a plasma. The plasma exits the anode nozzle as a free or neutral plasma flame (plasma which does not carry electric current) which is quite different to the Plasma Transferred Arc coating process where the arc extends to the surface to be coated. When the plasma is stabilised ready for spraying the electric arc extends down the nozzle, instead of shorting out to the nearest edge of the anode nozzle. This stretching of the arc is due to a thermal pinch effect. Cold gas around the surface of the water cooled anode nozzle being electrically non-conductive constricts the plasma arc, raising its temperature and velocity. Powder is fed into the plasma flame most commonly via an external powder port mounted near the anode nozzle exit. The powder is so rapidly heated and accelerated that spray distances can be in the order of 25 to 150 mm.

Plasma Spray Process advantage
Plasma spraying has the advantage that it can spray very high melting point materials such as refractory metals like tungsten and ceramics like zirconia unlike combustion processes, Plasma sprayed coatings are generally much denser, stronger and cleaner than the other thermal spray processes. Plasma spray coatings probably account for the widest range of thermal spray coatings and applications and make this process the most versatile. Disadvantages of the plasma spray process are relative high cost and complexity of process.

Features of the Atmospheric Plasma Spray Process:

• Large choice of coating materials, including metals, alloys, ceramics, cermets, carbides and others.


• Coating systems are possible, using layers of different materials.


• Produces surfaces for a wide variety of applications, including resistance to many different types or wear and corrosion mechanisms, desirable thermal or electrical characteristics, and surface restoration and dimensional control.


• Excellent control of coating thickness and surface characteristics, such as porosity and hardness .


• No heat affected zone or component distortion.


• High deposition rate.


• High bond of the coating to the substrate.


• Coating of complex geometries.


• Easy masking of areas that should not be coated.


• Process can be fully automated.


• Coating of internal geometries possible.

AC MOTOR INTRODUCTION

AC MOTOR INTRODUCTION

AC MOTOR INTRODUCTION

AC motors are used worldwide in many applications to transform electrical energy into mechanical energy. There are many types of AC motors, but three phase AC induction motors, is the most common type of motor used in industrial applications. An AC motor of this type may be part of a pump or fan or connected to some other form of mechanical equipment such as a winder, conveyor, or mixer. The electric motor in its simplest terms is a converter of electrical energy to useful mechanical energy. The electric motor has played a leading role in the high productivity of modern industry, and it is therefore directly responsible for the high standard of living being enjoyed throughout the industrialized world.

AC motors provide the motive power to lift, shift, pump, drive, blow, drill, and perform a variety of other tasks in industrial, domestic, and commercial applications. The induction motor, the most versatile of the AC motors, has truly emerged as the prime mover in industry, powering machine tools, pumps, fans, compressors, and a variety of industrial equipments.

Fundamentals of three-phase AC motors
Three-phase AC motors are known as the ‘workhorses of industry’ because of their wide use and acceptance. They are popular because they are low in cost, compact in size, require less maintenance, withstand harsh industrial environments, etc. Three-phase AC motors are a class of motors that convert the three-phase electric power supplied at the input terminals, to mechanical power at the rotating shaft, through the action of a rotating magnetic field, produced by a distributed winding on the stator.

Three-phase AC motors are broadly classified as:
1. Induction motor
2. Synchronous motor
3. Wound rotor induction motor.

1. Induction motor
As the name implies, no voltage is applied to the rotor. The voltage is applied to the stator winding and when the current flows in the stator winding, a current is induced in the rotor by transformer action. The resulting rotor magnetic field will interact with the stator magnetic field, causing torque to exert on the rotor.

2. Synchronous motor
As the name suggests, rotor speed remains in synchronism with that of the stator magnetic field. The motor runs at the same speed. Unlike induction motors, synchronous motors are not self-starting. They have to be brought up to synchronous speed. Once they are locked then the rotor will continuously rotate.

3. Wound rotor induction motor
This motor has a ‘wire wound rotor’ from which three leads are brought out to the slip rings. It is possible to vary the rotor resistance. Introducing different resistances in the rotor circuit through the slip rings does this. The speed and the starting torque will now be variable.

Principle of operation of a induction motor
An electric motor’s principle of operation is based on the fact that a current-carrying conductor, when placed in a magnetic field, will have a force exerted on the conductor proportional to the current flowing in the conductor and to the strength of the magnetic field. In alternating current induction motors, the windings placed in the laminated stator core produce the magnetic field. The aluminum bars in the laminated rotor core are the current-carrying conductors upon which the force acts. The resultant action is the rotary motion of the rotor and shaft, which can then be coupled to various devices to be driven and produce the output.

FORCE & MOTION

Before discussing AC motors it is necessary to understand some of the basic terminology associated with motor operation. Force In simple terms, a force is a push or a pull. Force may be caused by electromagnetism, gravity, or a combination of physical means. Net Force Net force is the vector sum of all forces that act on an object, including friction and gravity. When forces are applied in the same direction, they are added. For example, if two 10 pound forces are applied in the same direction the net force would be 20 pounds.

TORQUE
Torque is a twisting or turning force that causes an object to rotate. For example, a force applied to the end of a lever causes a turning effect or torque at the pivot point. Torque (τ) is the product of force and radius (lever distance). τ = Force x Radius In the English system of measurements, torque is measured in pound-feet (lb-ft) or pound inches (lb-in). For example, if 10 lbs of force is applied to a lever 1 foot long, the resulting torque is 10 lb-ft.

SPEED
An object in motion takes time to travel any distance. Speed is the ratio of the distance traveled and the time it takes to travel the distance.

ANGULAR SPEEDThe angular speed of a rotating object determines how long it takes for an object to rotate a specified angular distance. Angular speed is often expressed in revolutions per minute (RPM). For example, an object that makes ten complete revolutions in one minute, has a speed of 10 RPM.

INERTIA

Mechanical systems are subject to the law of inertia. The law of inertia states that an object will tend to remain in its current state of rest or motion unless acted upon by an external force. This property of resistance to acceleration/deceleration is referred to as the moment of inertia. The English system unit of measurement for inertia is pound-feet squared (lb-ft2).

FRICTION

Friction occurs when objects contact one another. As we all know, when we try to move one object across the surface of another object, friction increases the force we must apply. Friction is one of the most significant causes of energy loss in a machine.

WORK
Whenever a force causes motion, work is accomplished. Work can be calculated simply by multiplying the force that causes the motion times the distance the force is applied. Work = Force x Distance Since work is the product of force times the distance applied, work can be expressed in any compound unit of force times distance. For example, in physics, work is commonly expressed in joules. 1 joule is equal to 1 newton-meter, a force of 1 newton for a distance of 1 meter. In the English system of measurements, work is often expressed in foot-pounds (ft-lb), where 1 ft-lb equals 1 foot times 1 pound

POWER
Another often used quantity is power. Power is the rate of doing work or the amount of work done in a period of time.

HORSEPOWER
Power can be expressed in foot-pounds per second, but is often expressed in horsepower. This unit was defined in the 18 th century by James Watt. Watt sold steam engines and was asked how many horses one steam engine would replace. He had horses walk around a wheel that would lift a weight. He found that a horse would average about 550 foot-pounds of work per second. Therefore, one horsepower is equal to 550 foot-pounds per second or 33,000 foot-pounds per minute.

KILOWATTS

AC motors manufactured in the United States are generally rated in horsepower, but motors manufactured in many other countries are generally rated in kilowatts (kW). Fortunately it is easy to convert between these units
Horsepower=1.341 * kilowatts

CONSTRUCTION
Three-phase AC induction motors are commonly used in industrial applications. This type of motor has three main parts, rotor, stator, and enclosure. The stator and rotor do the work, and the enclosure protects the stator and rotor.The stator is the stationary part of the motor’s electromagnetic circuit. The stator core is made up of many thin metal sheets, called laminations. Laminations are used to reduce energy loses that would result if a solid core were used. Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core,

The rotor is the rotating part of the motor’s electromagnetic circuit. The most common type of rotor used in a three phase induction motor is a squirrel cage rotor. Other types of rotor construction is discussed later in the course. The squirrel cage rotor is so called because its construction is reminiscent of the rotating exercise wheels found in some pet cages

ENCLOSUREThe enclosure consists of a frame (or yoke) and two end brackets (or bearing housings). The stator is mounted inside the frame. The rotor fits inside the stator with a slight air gap separating it from the stator. There is no direct physical connection between the rotor and the stator.

BEARING
Bearings, mounted on the shaft, support the rotor and allow it to turn. Some motors also use a fan mounted on the rotor shaft, to cool the motor when the shaft is rotating.

MOTOR SPECIFICATIONS (Important Nameplate Data)
• Catalog number.
• Motor model number.
• Frame.
• Type (classification varies from manufacturer to manufacturer).
• Phase - single, three or direct current.
• HP - horsepower at rated full load speed.
• HZ - frequency in cycles per second. Usually 60 hz in United States,
50 hz overseas.
• RPM - revolutions per minute.

• Voltage.
• Amperage (F.L.A.) - full load motor current.
• Maximum ambient temperature in centigrade - usually +40°C (104°F).
• Duty - most motors are rated continuous. Some applications,
however, may use motors designed for intermittent, special, 15, 30 or
60 minute duty.
• NEMA electrical design - B, C and D are most common. Design letter
represents the torque characteristics of the motor.
• Insulation class - standard insulation classes are B, F, and H. NEMA has
established safe maximum operating temperatures for motors. This
maximum temperature is the sum of the maximum ambient and
maximum rise at maximum ambient.
• Code - indicates locked rotor kVA per horsepower.
• Service factor - a measure of continuous overload capacity.

Electrical Characteristics and Connections
Voltage, frequency and phase of power supply should be consistent with the motor nameplate rating. A motor will operate satisfactorily on voltage within 10% of nameplate value, or frequency within 5%, or combined voltage and frequency variation not to exceed 10%.

Voltage
Common 60 hz voltages for single-phase motors are 115 volt, 230 volt, and
115/230 volt. Common 60 hz voltage for three-phase motors are 230 volt, 460 volt and 230/460 volt.

PhaseSingle-phase motors account for up to 80% of the motors used in the United States but are used mostly in homes and in auxiliary low-horsepower industrial applications such as fans and on farms.

Three-phase motors are generally used on larger commercial and industrial equipment.

Speeds
The approximate RPM at rated load for small and medium motors operating at 60 hz and 50 hz at rated volts are as follows:
60 hz 50 hz Synch. Speed
2 Pole 3450 2850 3600
4 Pole 1725 1425 1800
6 Pole 1140 950 1200
8 Pole 850 700 900

Synchronous speed (no-load) can be determined by this formula:
Frequency (Hertz) x 120 / Number of Poles

Insulation Class
Insulation systems are rated by standard NEMA classifications according to maximum allowable operating temperatures. They are as follows:
Class Maximum Allowed Temperature*
A 105°C (221°F)
B 130°C (266°F)
F 155°C (311°F)
H 180°C (356°F)
* Motor temperature rise plus maximum ambient

Generally, replace a motor with one having an equal or higher insulation class. Replacement with one of lower temperature rating could result in premature failure of the motor. Each 10°C rise above these ratings can reduce the motor’s service life by one half.

Service Factor
The service factor (SF) is a measure of continuous overload capacity at which a motor can operate without overload or damage, provided the other design parameters such as rated voltage, frequency and ambient temperature are within norms. Example: a 3/4 HP motor with a 1.15 SF can operate at .86 HP, (.75 HP x 1.15 = .862 HP) without overheating or otherwise damaging the motor if rated voltage and frequency are supplied at the motor’s leads.

GEARS

GEARS

Gears are toothed wheels which transmit motion and power between rotating shafts by means of successively engaging teeth. They give a constant velocity ratio and different types are available to suit different relative positions of the axes of the shafts

The slipping of a belt or rope is a common phenomenon, in the transmission of motion or power between two shafts. The effect of slipping is to reduce the velocity ratio of the system. In precision machines, in which a definite velocity ratio is of importance (as in watch mechanism) , the only positive drive is by gears or toothed wheels. A gear drive is also provided, when the distance between the driver and the follower is very small.

Gears are used in tons of mechanical devices. They do several important jobs, but most important, they provide a gear reduction in motorized equipment. This is key because, often, a small motor spinning very fast can provide enough power for a device, but not enough torque. For instance, an electric screw driver has a very large gear reduction because it needs lots of torque to turn screws, but the motor only produces a small amount of torque at a high speed. With a gear reduction, the output speed can be reduced while the torque is increased.

Advantages and Disadvantages of Gear Drives
The following are the advantages and disadvantages of the gear drive as compared to other drives, i.e. belt, rope and chain drives :

Advantages
1. It transmits exact velocity ratio.
2. It may be used to transmit large power.
3. It may be used for small centre distances of shafts.
4. It has high efficiency.
5. It has reliable service.
6. It has compact layout.

Disadvantages
1. Since the manufacture of gears require special tools and equipment, therefore it is costlier than other drives.
2. The error in cutting teeth may cause vibrations and noise during operation.
3. It requires suitable lubricant and reliable method of applying it, for the proper operation of gear drives.
Classification of GearsThe gears or toothed wheels may be classified as follows :
1. According to the position of axes of the shafts. The axes of the two shafts between which he motion is to be transmitted, maybe
a) Parallel
b) Intersecting
c) Non-intersecting and non-parallel.
The two parallel and co-planar shafts connected by gears are usually spur gears and the arrangement is known as spur gearing. These gears have teeth parallel to the axis of the wheel as shown. Spur gears are one of the most used gears in the industry

Another name given to the spur gearing is helical gearing, in which the teeth are inclined to the axis. The tooth profile have an angle with rotation axis in a helical gear. Helical gears are more silent and can carry more load than spur gears. They are used in gearboxes of vehicles, gear reducers and machinery.

Another type is double helical gear.The object of the double helical gear is to balance out the end thrusts that are induced in single helical gears when transmitting load. The double helical gears are known as herringbone gears. Herringbone gears (also known as double helical gears) have two helical gears with opposite angles placed in the same body. They are generally used in rolling mills

The two non-parallel or intersecting, but coplaner shafts connected by gears is called bevel gears and the arrangement is known as bevel gearing. Bevel gears enable a change in the axes of rotation of the respective shafts, commonly 90

The bevel gears, like spur gears may also have their teeth inclined to the face of the bevel, in which case they are known as helical bevel gears. Spiral bevel gears have the same functionality with straight bevel gears. More than that, these gears are more silent and offer more load carrying capacity with higher rpm


Notes :

(i) When equal bevel gears (having equal teeth) connect two shafts whose axes are mutually perpendicular, then the bevel gears are known as mitres.

The gears may also be classified as

2. According to the peripheral velocity o fthe gears. The gears, according to the peripheral velocity of the gears, may be classified as :
(a) Low velocity, (b) Mediumvelocity, and (c) High velocity.
The gears having velocity less than 3 m/s are termed as low velocity gears and gears having velocity between 3 and 15 m/ s are known as medium velocity gears. If the velocity of gears is more than 15 m / s, then these are called high speed gears.

3. According to the type of gearing. The gears, according to the type of gearing, may be
classified as :
(a) External gearing, (b) Internal gearing, and (c) Rack and pinion.In external gearing, the gears of the two shafts mesh externally with each other. The larger of these two wheels is called spur wheel or gear and the smaller wheel is called pinion. In an external gearing, the motion of the two wheels is always unlike .In internal gearing, the gears of the two shafts mesh internally with each other. The larger of these two wheels is called annular wheel and the smaller wheel is called pinion. In an internal gearing, the motion of the wheels is always like.

Sometimes, the gear of a shaft meshes externally and internally with the gears in a straight line, as shown in Fig. Such a type of gear is called rack and pinion. The straight line gear is called rack and the circular wheel is called pinion. A little consideration will show that with the help of a rack and pinion, we can convert linear motion into rotary motion and vice-versa. They are generally used in vinches, routers and sliding gates. A perfect example of this is the steering system on many cars. The steering wheel rotates a gear which engages the rack. As the gear turns, it slides the rack either to the right or left, depending on which way you turn the wheel.

4. According to the position of teeth on the gear surface. The teeth on the gear surface may be
(a) Straight, (b) Inclined, and (c) Curved.

We have discussed earlier that the spur gears have straight teeth where as helical gears have their teeth inclined to the wheel rim. In case of spiral gears, the teeth are curved over the rim surface.

Worm Gears

The worm gear only has one tooth but it is like a screw thread. The wormwheel is like a normal gear wheel or spur gear. The worm always drives the worm wheel round, it is never the opposite way round as the system tends to lock and jam. Worm gears are used when large gear reductions are needed. It is common for worm gears to have reductions of 20:1, and even up to 300:1 or greater.

Gear Ratio

The ratio of the speed of rotation of the powered gear of a gear train to that of the final or driven gear is the gear ratio. A gear ratio is a numerical value that describes the relationship between two gears (a spur and pinion gear for our discussion). A gear ratio can also describe the relationship between the FIRST gear and the LAST gear in a gear train (or a transmission). You take the number of teeth on the DRIVEN (Spur) gear and divide it by the number of teeth on the DRIVE (Pinion) gear

GEAR RATIO (VELOCITY RATIO)

The reason bicycles are easier to cycle up a hill when the gears are changed is due to what is called Gear Ratio (velocity ratio). Gear ratio can be worked out in the form of numbers and examples are shown below. Basically, the ratio is determined by the number of teeth on each gear wheel, the chain is ignored and does no enter the equation.

EXAMPLE:

Pedal gear = 60 teeth

Sprocket = 30 teeth

If the pedal gear revolves once how many times will the sprocket gear revolve?

No of teeth on pedal gear/ No of teeth on sprocket = 60/30 = 2

The example above shows that every time the pedal gear revolves once the sprocket gear on the back wheel revolves twice making it easier to cycle up hill.

Gear Ratio = 1 : 2