THREE PHASE VOLTAGE

The relationship between magnetism and electrical current was discovered and documented by Oerstad in 1819. He found that if an electric current was caused to flow through a conductor that a magnetic field was produced around that conductor. In 1831, Michael Faraday discovered that if a conductor is moved through a magnetic field, an electrical voltage is induced in the conductor. The magnitude of this generated voltage is directly proportional to the strength of the magnetic field and the rate at which the conductor crosses the magnetic field. The induced voltage has a polarity that will oppose the change causing the induction – Lenz’s law. This natural phenomenon is known as Generator Action and is described today by Faraday’s Law of
Electro Magnetic Induction: (Vind = ∆Ø/∆t), where Vind = induced voltage, ∆Ø = change in flux density, ∆t = change in time All rotary generators built today use the basic principles of Generator Action.

THREE PHASE VOLTAGE

Three phase voltage is developed using the same principles as the development of single phase voltage. Three (3) coils are required positioned 120 electrical degrees apart. A rotating magnetic field induces voltage in the coils which when aggregated produce the familiar three phase voltage pattern.

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TYPES OF GENERATORS

Essentially, there are two basic types of generators:
• DC generators
• AC generators: Asynchronous (Induction) generators and Synchronous generators

INDUCTION GENERATORS

The induction generator is nothing more than an induction motor driven above its synchronous speed by an amount not exceeding the full load slip the unit would have as a motor. Assuming a full load slip of 3%, a motor with a synchronous speed of 1200 rpm would have a full load speed of 1164 rpm. This unit could also be driven by an external prime mover at 1236 rpm for use as an induction generator.

The induction generator requires one additional item before it can produce power – it requires a source of leading VAR’s for excitation. The VAR’s may be supplied by capacitors (this requires complex control) or from the utility grid. Induction generators are inexpensive and simple machines, however, they offer little control over their output. The induction generator requires no separate DC excitation, regulator controls, frequency control or governor.

SYNCHRONOUS GENERATORS

Synchronous generators are used because they offer precise control of voltage, frequency, VARs and WATTs. This control is achieved through the use of voltage
regulators and governors. A synchronous machine consists of a stationary armature winding (stator) with many wires connected in series or parallel to obtain the desired terminal voltage.

The armature winding is placed into a slotted laminated steel core. A synchronous machine also consists of a revolving DC field - the rotor. A mutual flux developed across the air gap between the rotor and stator causes the interaction necessary to produce an EMF. As the magnetic flux developed by the DC field poles crosses the air gap of the stator windings, a sinusoidal voltage is developed at the generator output terminals. This process is called electromagnetic induction.

The magnitude of the AC voltage generated is controlled by the amount of DC exciting current supplied to the field. if “FIXED” excitation were applied, the voltage magnitude would be controlled by the speed of the rotor (E=4.44fnBA), however, thismwould necessitate a changing frequency! Since the frequency component of the power system is to be held constant, solid state voltage regulators or static exciters are commonly used to control the field current and thereby accurately control generator terminal voltage.

The frequency of the voltage developed by the generator depends on the speed of the rotor and the number of field poles. For a 60 Hz system, Frequency = speed(rpm)*pole pairs/60.

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History Of Electric Power

Benjamin Franklin is known for his discovery of electricity. Born in 1706, he began studying electricity in the early 1750s. His observations, including his kite experiment, verified the nature of electricity. He knew that lightning was very powerful and dangerous. The famous 1752 kite experiment featured a pointed metal piece on the top of the kite and a metal key at the base end of the kite string. The string went through the key and attached to a Leyden jar. (A Leyden jar consists of two metal conductors separated by an insulator.) He held the string with a short section of dry silk as insulation from the lightning energy. He then flew the kite in a thunderstorm. He first noticed that some loose strands of the hemp string stood erect, avoiding one another. (Hemp is a perennial American plant used in rope making by the Indians.) He proceeded to touch the key with his knuckle and received a small electrical shock.

Between 1750 and 1850 there were many great discoveries in the principles of electricity and magnetism by Volta, Coulomb, Gauss, Henry, Faraday, and others. It was found that electric current produces a magnetic field and that a moving magnetic field produces electricity in a wire. This led to many inventions such as the battery (1800), generator (1831), electric motor (1831), telegraph (1837), and telephone (1876), plus many other intriguing inventions.

In 1879, Thomas Edison invented a more efficient lightbulb, similar to those in use today. In 1882, he placed into operation the historic Pearl Street steam–electric plant and the first direct current (dc) distribution system in New York City, powering over 10,000 electric lightbulbs. By the late 1880s, power demand for electric motors required 24-hour service and dramatically raised electricity demand for transportation and other industry needs. By the end of the 1880s, small, centralized areas of electrical power distribution were sprinkled across U.S. cities. Each distribution center was limited to a service range of a few blocks because of the inefficiencies of transmitting direct current. Voltage could not be increased or decreased using direct current systems, and a way to to transport power longer distances was needed.

To solve the problem of transporting electrical power over long distances, George Westinghouse developed a device called the “transformer.” The transformer allowed electrical energy to be transported over long distances efficiently. This made it possible to supply electric power to homes and businesses located far from the electric generating plants. The application of transformers required the distribution system to be of the alternating current (ac) type as opposed to direct current (dc) type.

The development of the Niagara Falls hydroelectric power plant in 1896 initiated the practice of placing electric power generating plants far from consumption areas. The Niagara plant provided electricity to Buffalo, New York, more than 20 miles away. With the Niagara plant, Westinghouse convincingly demonstrated the superiority of transporting electric power over long distances using alternating current (ac). Niagara was the first large power system to supply multiple large consumers with only one power line.

Since the early 1900s alternating current power systems began appearing throughout the United States. These power systems became interconnected to form what we know today as the three major power grids in the United States and Canada. The remainder of this chapter discusses the fundamental terms used in today’s electric power systems based on this history.

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How to select the correct motor starting method - Overview

The selection of a suitable starting method for a motor/load combination is becoming increasingly important as additional loads are added to existing utility and distribution systems. In the past, ample capacities were available in the utility networks to support the needs of industry. As well, the distribution systems of most North American industries were established during the industrial and technological boom years preceding the Second World War. In the 1990’s, the viability for the construction of new electrical generation plants became prohibitive. With lower North American utility generating capacity and increased risks involved in the de-regulation of this utility, stability and voltage drop concerns have become very real issues that face all electrical power consumers.

Problem
Full voltage starting of motors can produce objectionable voltage flicker, mechanical stress to gear boxes or belt drive systems and create pressure surges or water hammer in pumping applications. Starting a motor at reduced voltage can help reduce or overcome these problems. If the load cannot be accelerated to full speed using full voltage and current, it cannot be accelerated to full speed using reduced voltage and current.

Application Solution
There are several factors to be considered when selecting the starting equipment for any electric motor driven load. These include, but are not limited to:
1. The source of power and the effects the motor starting currents will have on the source and the stability of the system voltage
2. The starting and breakdown torque characteristics of the motor (motor speed torque characteristics)
3. The motor starting characteristics (torque) that correspond to the motor best suited to the load
characteristics at full load and speed
4. The starting characteristics of: motor torque load torque, accelerating load torque (load inertia) the motor starting, accelerating and running torque on the driven load
5. The available short circuit capacity of the distribution system
6. The operating speed range of the connected load.
7. Process considerations: shock, vibration, mechanical hammer, the control and maintenance of different starting methods.

The initial inrush currents, locked rotor currents and the resulting torque values produced, are the factors that determine whether the motor can be applied directly across the line, or whether the current has to be reduced to get the required performance to match the load requirements and/or utility line voltage flicker or voltage dip specifications.

Full voltage starting can be used whenever the driven load can withstand the shock of instantaneously applying full voltage to the motor and where line disturbances can be tolerated. Full voltage starting uses a main contactor to apply the motor stator windings directly across the main system voltage. This type of starting method provides the lowest cost, a basic and simple design of controller, resulting in low maintenance and the highest starting torque.

Reduced voltage starting may be required if full voltage starting creates objectionable line disturbances on the distribution system or where reduction of mechanical stress to gear boxes or belt drive systems is required. It must be noted that when the starting torque will decrease proportional to the percent squared of voltage applied (i.e. 50% voltage produces 25% torque =0.50 squared). This phenomenon also occurs in the opposite manner when the voltage is increased.
There are three main reasons to apply reduced voltage to medium voltage motors:
1. To reduce the mechanical effect of across the starting and stopping
2. To limit the inrush current inherent with full voltage starting
3. To reduce the effects of pressure surges and water hammer in pumping systems.

Mechanical Shock
This reason for applying reduced voltage has various different names; it can be called mechanical shock, mechanical stress, or various other names. The effect is the same. When a medium voltage motor is started at full voltage the torque being applied from the motor to the driven load rises to a very high value almost immediately. This can cause damage to the bearings in the motor or the load, the rotor of the motor or to the mechanical coupling method which connects the motor to the load. The load itself can sustain damage depending on what the application may be. In the case of conveyor application if the load is started to quickly the belts of the conveyor can be stretched or broken. If the motor is connected to the load via chains or belts these coupling means can be damaged as well by sudden starting techniques.

Utility Restrictions
As utility power systems continue to be run at maximum capacity the effect of starting medium voltage motors across the line can put stress on the factory’s power distribution system. The lights go dim, process control systems can fail or trip out or you may be restricted as to when and how often you are allowed to start the motor.

Load Related Reasons
By soft starting the load you may see improvements in the way the equipment performs. For example, when a soft starter is applied to an agitator by slowing ramping up the speed of the motor the material being agitated tends to splash less and causes fewer problems than when started across the line. In the case of mill applications the material will start to move slower than when started across the line and cause less wear on the driven load. The ways in which a soft start can improve system performance are only limited to the number of applications that the end user can think of. New uses are being thought of and applied all the time.

Torque Requirements
It is important to reiterate that when the voltage is reduced when starting a motor, so are the current and torque values. It should be apparent that a motor that will not start a load at full voltage, it will not start that same load under reduced voltage conditions. This conflict between torque and current requirements of induction motors is one typical dilemma facing the user of reduced voltage starting equipment. It may be only one of several problems but is the most common and most important.

Reduced voltage starting can be accomplished in several different ways.

Reactor Starting
This method also reduces the voltage, current and torque to the motor according to the reactor tap setting. It is possible to reduce the motor terminal voltage as required by placing a primary reactor in series with the motor windings, for a period during starting. The use of a reactor during starting results in an exceptionally low starting power factor. Reactors must be carefully designed and applied since any saturation in the reactor will produce in-rush currents close to those seen during full voltage starting. Reactor starting has one major advantage; the voltage to the motor is a function of the current taken from the line. It can therefore be assumed that during acceleration the motor voltage will rise as the line current drops. This relationship results in greater accelerating energy at higher speeds and less severe disturbances during the transition to full voltage.

Autotransformer Starting
Autotransformer starting automatically switching between taps of an autotransformer reduces the voltage, current and torque to the motor according to the tap setting used on the autotransformer. There are two very distinctive characteristics of an autotransformer starter.
1. The motor terminal voltage is not a function of load current and remains constant during the
acceleration time.
2. Due to the turn ratio advantages, the primary line current is less than the secondary motor currents. A three-coil autotransformer is connected in a wye configuration and connected to the motor in such a way as to supply reduced voltage to the motor when the line voltage is applied to the Autotransformer. Several sets of taps are usually available to the user to provide different values of reduced voltage (NEMA standards are 80%, 65% and 50% of the full line voltage).

Solid State Reduced Voltage Starting
The use of solid state Reduced Voltage Starting can provide a smooth stepless method of accelerating and smoothly decelerating a squirrel cage induction motor. This type of starting method, when properly applied can provide an efficient and reliable means of smoothly starting and stopping a motor and load. The use of solid-state reduced voltage starting will perform, in most cases, more efficiently than field coupling, eddy current drives and clutches. The stepless ramped acceleration and deceleration capabilities of these types of starter reduces the inrush currents to the motor, eliminating transitional shocks to the load and reducing voltage flicker on the distribution system.

Selection of Appropriate Starting System
The selection of an appropriate starting system requires the reviewer to compare or weigh the importance of several factors.

Cost and Economics:
When determining the starting method, the economics of the decision can also provide important tips to the selection of an appropriate controlling means.

Maintainability:
The capabilities of the mechanical and electrical support facilities and personnel can have an important impact on the starting method determination.

Remote Control Requirements:
As businesses become increasingly more competitive, the reductions in the area of personnel related to the control and operation of industrial processes are becoming dramatic. This required reduction of personnel has subsequently hastened the development of the remote control capabilities of modern motor control equipment. In many cases, production flow and efficiency rate adjustments can be critical to the quality of the end product. For example in the case of remote pumping stations, the ability to control the speed of a booster pump, based on the product in the line at the time, can allow a remote location to monitor and adjust flow rates to maximize the capacity of the pipeline.

Process Control Requirements:
In today’s competitive environment, industry is endeavoring to continually improve processes to produce high quality products, at accelerated periods, at the least possible cost. If the process requires variation of speeds, a controller that will vary the motor speed would be appropriate. If depressed distribution voltage, during motor start cycles, is an issue, a solid state or other reduced voltage starting method may suffice.

Physical size restraints:
The limitations of available physical floor space could be a major concern when retrofitting new equipment into an existing control area. The physical construction of equipment housing the newer technology may not be suitable incorporated within existing facilities. Careful consideration may be required for the removal of heat generated by semi-conducting devices, printed circuit cards, transformers and other electronic control devices. Cooling systems may be needed to aid in the maintenance of a suitable environment for these types of equipment.

Ease of use:
One consideration that is sometimes overlooked is the capabilities of the personnel required to service and maintain the equipment. Simpler more traditional starting means may be suitable where basic personnel training levels are maintained. The simpler relay control logic systems may be adequate for the system configuration required for the specific process.

source: http://www.eng-tips.com/

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Motor Starting Techniques

When large motors are started, noticeable voltage dips or flicker can occur on the consumers wiring system, the utility’s system, or both. Depending on the voltage sensitivity of other connected loads, these voltage dips can be unnoticeable, annoying, or harmful to the equipment. For example, lightbulbs can dim and be annoying to office personnel; however, voltage dips can cause other motor loads to slow down, overheat, and possibly fail. Reduced motor starting equipment is often used to minimize voltage dips and flicker.

The iron and copper wires in large motors need to become magnetized before running at full speed. The inrush current required to start the motor to create the necessary magnetic fields can be as high as 7–11 times the full load current of the motor. Therefore, when large motors start, they often cause low-voltage conditions from voltage drop on the conductors from high-current flows. Utilities normally adopt guidelines or policies for starting large motors. When starting a motor exceeds the utility requirement for voltage dip or flicker (usually set around 3–7%), then special motor starting techniques are usually required.

There are several methods for reducing voltage dip and flicker. Reduced voltage motor starting equipment (i.e., soft starting), such as capacitors, transformers, special winding connections, and other control devices, are commonly used in motor circuitry to reduce the inrush current requirements of large motors during start-up conditions.

The three most common means of providing soft starting or reduced voltage starters on large motors are the following:

1. Resistance is temporarily placed in series with the motor starter breaker contacts or contactor to cause reduced current to flow into the motor when started. This approach can reduce the inrush current to less than five times full load current. Once the motor comes up to full speed, the resistors are shorted out, leaving solid conductors serving the motor power requirements.

2. Wye–delta connection changeover in the motor windings is another very effective way to reduce inrush current. The motor windings are first connected in wye, where the applied voltage is only line to ground; then the motor windings are connected in delta for full voltage and output power.

3. Auto-transformers are sometimes used to apply a reduced voltage to the terminals when started and then switched out to full voltage after the motor reaches full speed. This scheme can be used with motors that do not have external access to the internal windings.

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