AC Motors 2

Introduction to motors cont.:
An object in motion travels a distance in a given time. Speed is the ratio of the distance traveled and the time it takes to travel the distance.

The linear speed of an object is a measure of how long it takes the object to get from point A to point B. Linear speed is usually given in a form such as meters per second (m/s).

The angular speed of a rotating object is a measurement of how long it takes a given point on the object to make one complete revolution from its starting point. Angular speed is generally given in revolutions per minute (RPM).

An object can change speed. An increase in speed is called acceleration. Acceleration occurs only when there is a change in the force acting upon the object. An object can also change from a higher to a lower speed. This is known as deceleration (negative acceleration).

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 of measurement is pound-feet squared (lb-ft2).



If we look at a continuous roll of paper, for example, we know that when the roll is stopped it would take a certain amount of force to overcome the inertia of the roll to get it rolling. The force required to overcome this inertia can come from a source of energy such as a motor. Once rolling, the paper will continue unwinding until another force acts on it to bring it to a stop.

Circuit Breakers Types

Instantaneous magnetic-trip-only circuit breakers do not provide overload protection and are used on motor circuits where overload protection is provided by a motor starter. The current level at which an instantaneous trip circuit breaker trips is adjustable. The name comes from the electromagnet used to sense short circuit current. The purpose of overload protection is to prevent the motor from operating beyond its full-load capability. In the schematic illustrated below, a motor is supplied through a 3-pole circuit breaker, motor starter contacts and separately supplied overload contacts. Heat generated from excessive current will cause the overload contacts to open, removing power from the motor.

Thermal-magnetic circuit breakers have both overload and instantaneous trip features. When an overload condition exists, the excess current will generate heat, which is detected in the circuit breaker. After a short period of time, dependent on the rating of the breaker and amount of overload, the breaker will trip, disconnecting the load from the voltage source. If a short circuit occurs, the breaker responds instantaneously to the fault current and disconnects the circuit.

The user does not have access to the trip unit on some circuit breakers. This means the trip unit cannot be changed with another. Interchangeable trip is actually a design feature that is available on some thermal-magnetic and some solid state breakers. The advantage of a breaker with an interchangeable trip unit is the user can change the continuous current rating of the breaker without replacing the breaker. This is done by replacing the trip unit with one of a different rating.

molded case circuit breakers are available as a molded case switch. Molded case switches employ the same operating mechanism as the thermal magnetic and magnetic only units.  A preset instantaneous function is factory installed to allow the switch to trip and protect itself at a high fault current, but the switch provides no thermal overload protection.

Many electrical distribution systems can deliver large short circuit currents to electrical equipment. This high current can cause extensive damage. Current limiting circuit breakers will reduce the current flowing in the faulted circuit to substantially less magnitude. This helps protect expensive equipment. One way to accomplish current limiting is with an additional set of contacts that feature two moveable arms. These are referred to as dual-pivot contacts, which separate even more quickly than the single-pivot contacts. The dual-pivot contacts are connected in series with the single-pivot contacts. As with the single-pivot design, current flows in opposite directions through the contact arms, creating a magnetic repulsion. As current increases, the magnetic repulsion force increases.

In an overload condition where current may only be one to six times normal current, the contacts remain closed until the breaker trips. In a short circuit condition fault current is extremely high, both sets of contact arms may open simultaneously, generating high impedance arcs. The contact gap of the dual-pivot contacts increases more rapidly, therefore generating arc impedance more rapidly. Once the arcs are extinguished, the dual-pivot contacts close on their own due to spring tension. The single-pivot contacts are held open by the breaker mechanism, which will have tripped during the fault and must be manually reset.

Solid state circuit breakers function similarly to thermal-magnetic breakers. The basic breaker mechanism is still mechanical. The tripping unit is solid state.  As with the thermal-magnetic tripping unit, the Sensitrip circuit breaker tripping unit performs the following three functions:
 Senses magnitude of current flow,Determines when current becomes excessive,Determines when to send a trip signal to the breaker.


 Mechanism of these circuit breakers use a microprocessor to execute numerous functions programmed in the unit. These units have a greater degree of accuracy and repeatability. Adjustments on the trip unit allow the user to select numerical values the microprocessor will use in performing protective functions. Current sensors mounted in the trip unit monitor the value of load current. The value of current is reduced to a low level and converted to a digital voltage, which is used by the microprocessor. The microprocessor continuously compares the line current with the value set by the user. When current exceeds a preset value for the selected time, the trip unit sends a signal to a magnetic latch. The magnetic latch opens the breaker’s contacts, disconnecting the protected circuit from the power source.

Power Supply Systems

Homes built prior to 1936, especially in rural areas, used a two wire supply system. This system provided 120 volts between a hot conductor and a grounded conductor. A two-wire system is usually inadequate for today’s residential electrical demands.

The most common supply system used in residential applications today is a three-wire supply system. There are 120 volts between any phase and neutral and 240 volts between phases.

Load centers can also be used in commercial applications. Electric power is brought into a building at one location and then is distributed through the building by means of separate circuits. Two distribution systems used in commercial applications that are suitable for load centers are three-wire, three-phase, 240 volts; and four-wire, three-phase, 208Y/120 volts.

Surge Protection

Today’s homes have many semiconductor-based devices such as televisions, stereos, computers and microwave ovens.These devices are highly susceptible to voltage spikes. Devices used in the home which generate voltage spikes include vacuum cleaners and other motor driven devices, and spark igniters on gas ranges, furnaces and water heaters. The most damaging voltage spikes are caused by lighting strikes. A lighting strike on a power line several miles away still has the potential to cause extensive electrical damage in a home.Lightning strikes on high voltage lines are generally dissipated by utility transmission and arresters. The average home, however, will experience eight to ten voltage surges of 1,000 to 10,000 volts annually. Damage to expensive electrical equipment can be instantaneous or cumulative.

A typical lightning strike consists of 25,000 amps at 30 million volts. The following map shows the approximate mean annual number of days with thunderstorms in the United States.

An electrical surge, whether it is caused by electrical equipment or lightning, always seeks ground. Any component between the source of the surge and ground can be damaged.

Installation is as simple as mounting a conventional circuit breaker. After power is switched off and the trim removed, the circuit breaker/surge arrestor plugs into place. A lead wire is provided to connect the ground side of the module to the load center’s neutral bus. It is best to position the circuit breaker/surge arrestor in the first position of the load center and connect the lead wire in the first neutral position.

One device provides protection for the electrical system. Two red LEDs indicate that the device is working. The device does not require a dedicated space and can be added on to existing load centers. The circuit breaker portion of the surge arrester can be used on noncritical lighting circuits to provide additional visual indication that the device is working. If the device trips due to a high voltage surge, it is reset like any other circuit breaker in the panel.


Clamping voltage is the amount of voltage allowed across a surge suppression device when it is conducting a specific current created by a surge.

Peak current rating specifies the maximum energy that can be dissipated from a single surge without causing the protecting device to sacrifice itself.

Arc Fault Protection

GFCI devices are designed to protect a person from getting a shock when touching an ungrounded appliance. Arc Fault Circuit Interrupters (AFCI), in comparison, protect against a fire being started from an unintended arc. An arc fault occurs when a current-carrying conductor has an arching condition to ground or another conductor. Damaged insulation, for example, can lead to an arc fault, which may not generate enough fault current to trip a circuit breaker. In the following example a staple has been driven through the insulation of a wire during installation.

An AFCI device is intended to provide protection from the effects of arc faults by recognizing the characteristics unique to arcing and de-energizing the circuit when an arc fault is detected. The arc generated will cause the AFCI to trip. Arcs normally generated from electric equipment such as a light switch or power drill will not cause the AFCI to trip.

Ground Fault Protection

A ground fault occurs when a current-carrying conductor comes in contact with ground. A faulty appliance or the presence of water in contact with a conductor are two possible ways a ground fault can occur. One way ground fault protection is accomplished is by the use of GFCI receptacles. These are installed in place of a normal receptacle.

A ground fault circuit interrupter (GFCI) compares current on the hot wire with current returning on the neutral wire. Under normal circumstances the current is equal.

When a ground fault occurs some of the current will return to the source through ground. In the following illustration, for example, a ground fault has occurred in a common household appliance. Anyone coming in contact with the appliance will become part of the circuit. The sensing and test circuit will detect that the amount of current returning on the neutral is less than the current on the hot wire. The sensing and test circuit will cause the trip coil to automatically open the circuit breaker, removing power from the appliance. GFCI devices trip between 4 to 6 milliamps. The amount of time it takes for a GFCI device to trip depends on the current. The higher the current the faster the device will trip.

Circuits providing power to certain areas of the home require ground fault circuit interrupters (GFCI).
Ground fault protection is required on the following circuits:
Bathroom receptacles,Residential garage receptacles,Outdoor receptacles,Receptacles in unfinished basements,Receptacles in crawl spaces,Receptacles within six feet of a kitchen or bar sink,Pools.

GFCI type circuit breakers have one white neutral lead which is connected to the neutral bus in the load center. The phase and load neutral are connected to lugs in the GFCI. They mount in the load center in the same way as a standard circuit breaker.

Electrical Power Residential Distribution

A distribution system is a system that distributes electrical power throughout a building. Distribution systems are used in every residential, commercial, and industrial building.

Power, purchased from a utility company, enters the house through a metering device.The incoming power then goes to a load center which provides circuit control and overcurrent protection. The power is distributed from the load center to various branch circuits for lighting, appliances and electrical outlets. Careful planning is required so that the distribution system safely and efficiently supplies adequate electric service for present and possible future needs.

The term “load center” is an industry term used to identify a panelboard used in certain applications. Load centers are typically rated 225 amps or less and 240 volts maximum and are intended for use in residential applications.

The load center (panel-board) as a single panel or group of panel units designed for assembly in the form of a single panel; including buses, automatic overcurrent devices, and equipped with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall, partition, or other support; and accessible only from the front

Circuit Breakers

Another device used for overcurrent protection is a circuit breaker.circuit breaker is a device designed to open and close a circuit by nonautomatic means, and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.

Circuit breakers provide a manual means of energizing and de-energizing a circuit. In addition, circuit breakers provide automatic overcurrent protection of a circuit. A circuit breaker allows a circuit to be reactivated quickly after a short circuit or overload is cleared. Unlike fuses which must be replaced when they open, a simple flip of the breaker’s handle restores the circuit.

Like fuses, every circuit breaker has a specific ampere, voltage, and fault current interruption rating. The ampere rating is the maximum continuous current a circuit breaker can carry without exceeding its rating. As a general rule, the circuit breaker ampere rating should match the conductor ampere rating.

Generally the ampere rating of a circuit breaker is selected at 125% of the continuous load current. This usually corresponds to the conductor ampacity which is also selected at 125% of continuous load current. For example, a 125 ampere circuit breaker would be selected for a load of 100 amperes.

The voltage rating of the circuit breaker must be at least equal to the circuit voltage. The voltage rating of a circuit breaker can be higher than the circuit voltage, but never lower.

Circuit breakers are also rated according to the level of fault current they can interrupt. When applying a circuit breaker, one must be selected which can sustain the largest potential short circuit current which can occur in the selected application.

Distribution Systems

A distribution system is a system that distributes electrical power throughout a building. Distribution systems are used in every residential, commercial, and industrial building.

Distribution systems used in commercial and industrial locations are complex. A distribution system consists of metering devices to measure power consumption, main and branch disconnects, protective devices, switching devices to start and stop power flow, conductors, and transformers. Power may be distributed through various switchboards, transformers, and panel boards. Good distribution systems don’t just happen. Careful engineering is required so that the distribution system safely and efficiently supplies adequate electric service to both present and possible future loads.

A feeder is a set of conductors that originate at a main distribution center and supplies one or more secondary, or one or more branch circuit distribution centers.

Commercial and industrial distribution systems use several methods to transport electrical energy. These methods may include heavy conductors run in trays or conduit. Once installed, cable and conduit assemblies are difficult to change. Power may also be distributed using bus bars in an enclosure. This is referred to as busway.

Fuses

Circuit protection would be unnecessary if overloads and short circuits could be eliminated. Unfortunately, they do occur. To protect a circuit against these destructive currents, a protective device must determine when a fault condition develops and automatically disconnect the electrical equipment from the power source. A fuse is the simplest device for interrupting a circuit experiencing an overload or a short circuit.

A typical fuse consists of an element electrically connected to ferrules. These ferrules may also have attached end blades.The element provides a current path through the fuse. It is enclosed in a tube, and surrounded
by a filler material.

Current flowing through the fuse element generates heat, which is absorbed and dissipated by the filler material. When an overcurrent occurs, temperature in the element rises. In the event of a transient overload condition the excess heat is absorbed by the filler material. However, if a sustained overload occurs, the heat will eventually melt open an element segment. This will stop the flow of current.

Fuses have an inverse time-current characteristic. The greater the overcurrent, the less time it takes for the fuse to open. This is referred to as the clearing time of the fuse.

Short-circuit current can be several thousand amperes, and generates extreme heat. When a short circuit occurs several element segments can melt simultaneously, which helps remove the load from the power source quickly. Short-circuit current is typically cut off in less than half a cycle, before it can reach its full value.

Non time-delay fuses provide excellent short circuit protection. However, short-term overloads, such as motor starting current, may cause nuisance openings of nontime-delay fuses. For this reason, they are best used in circuits not subject to large transient surge currents. Nontime-delay fuses usually hold 500% of their rating for approximately one-fourth of a second, after which the current-carrying element melts. This means that these fuses should not be used in motor circuits, which often have starting currents greater than 500%.

Time-delay fuses provide both overload and short-circuit protection. Time-delay fuses usually allow five times the rated current for up to ten seconds. This is normally sufficient time to allow a motor to start without nuisance opening of the fuse. However, if an overload condition occurs and persists, the fuse
will open.

Why need circuit protection?

Current flow in a conductor always generates heat. The greater the current flow in a given size conductor, the hotter the conductor. Excess heat is damaging to electrical components and conductor insulation. For this reason conductors have a rated continuous current carrying capacity, or ampacity. Overcurrent protection devices, such as fuses, are used to protect conductors from excessive current flow. Fuses are designed to keep the flow of current in a circuit at a safe level to prevent the circuit conductors from overheating.

Excessive current is referred to as overcurrent.overcurrent is any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault

An overload occurs when too many devices are operated on a single circuit, or if a piece of electrical equipment is made to work harder than it is designed to work.

Every circuit requires some form of protection against overcurrent and the heat it produces. For example, high levels of heat to insulated wire can cause the insulation to break down and flake off, exposing the conductors.

When exposed conductors touch, a short circuit occurs, and the circuit resistance drops to nearly zero. Because of this very low resistance, short circuit current can be thousands of times higher than normal operating current.

The heat generated by short-circuit current can rise to dangerous levels quickly, causing extensive damage to
conductors and connected equipment. This heat-generating current must be interrupted as soon as possible after a short circuit occurs. Slight overcurrents can be allowed to continue for some period of time, but as the overcurrent magnitude increases, the protection device must act more quickly. In order to minimize costly damage, outright short circuits must be interrupted almost instantaneously.

When a short circuit occurs in an unprotected circuit, current will continue to flow until the circuit is damaged, or until the power is removed manually. The peak short-circuit current of the first cycle is the greatest, and is referred to as peak let-through current . The electromagnetic force associated with this current can cause mechanical damage to electrical components.

A properly used overcurrent protecting device will open the circuit quickly, limiting peak let-through current and energy.

Safety Switches

A switch is generally used for two purposes:
1) A disconnecting means for a service entrance
2) A disconnecting means and fault protection for motors

A safety switch is simply a switch located in its own enclosure. The enclosure provides a degree of protection to personnel against incidental contact with live electrical equipment. It also provides protection to the enclosed equipment against specific environmental conditions. Safety switches may consist of a switch only, or may consist of a switch and fuses. There are two families of Siemens safety switches: general duty and heavy duty.

Safety switches can be used in any number of applications.for example, requires that a disconnecting means shall be located in sight from the motor location and the driven machinery location.Regardless of where the safety switch is used, the function is to provide a means to connect and disconnect the load from its source of
electrical power.

With power removed the operator can safely service the machinery without coming into contact with live electrical components or having the motor accidently start.

A switch with no associated fuses is referred to as a non-fusible safety switch. A non-fusible safety switch has no circuit protection capability. It simply provides a convenient means to open and close a circuit. Opening the circuit disconnects the load from its source of electrical power, while closing the circuit
connects the load. Circuit protection must be provided by external overcurrent devices such as circuit breakers or fuses.

A safety switch can be combined with fuses in a single enclosure. This is referred to as a fusible safety switch. The switch provides a convenient means to manually open and close the circuit, while the fuse provides overcurrent protection.

Circuit Breakers

 Circuit breakers supply a manual means of energizing and de-energizing a circuit. In addition, circuit breakers provide automatic over-current protection of a circuit. Siemens residential circuit breakers are available with current ratings from 15-125 amps and a voltage rating of 120/240 volts. In residential applications, single-pole breakers protect 120 volt circuits; two-pole breakers protect 240 volt circuits.

circuit breaker/surge arrester mounts in a load center similarly to a conventional circuit breaker. This
device protects (defend)electronic equipment, such as televisions or computers, from electrical surges on the system. Surges can come from electrical equipment, switching, or lightning.

The ground fault circuit interrupter (GFCI) is required on certain residential receptacles, such as bathroom receptacles, receptacles located within six feet of a kitchen sink, and outdoor receptacles. The GFCI is designed to interrupt a circuit when a ground fault occurs. Often the GFCI is mounted at the
receptacle. When this is not feasible, a Siemens GFCI circuit breaker is installed in the load center.

GFCI devices are designed to protect a person from getting a shock when touching an ungrounded appliance. Arc Fault Circuit Interrupters (AFCI), in comparison, protect against a fire being started from an unintended arc. An arc fault occurs when a current-carrying conductor has an arching condition to ground
or another conductor. An AFCI device is intended to provide protection from the effects of arc faults by recognizing the characteristics unique to arcing and de-energizing the circuit when an arc fault is detected. The arc generated will cause the AFCI to trip. Arcs normally generated from electric equipment such as a light switch or power drill will not cause the AFCI to trip.

The transformers

A transformer is a device made of two or more inductors, one of which is powered by AC,
inducing an AC voltage across the second inductor. If the second inductor is connected to a
load, power will be electromagnetically coupled from the first inductor's power source to that
load.

The powered inductor in a transformer is called the primary winding. The unpowered inductor
in a transformer is called the secondary winding.

Total primary current in an unloaded transformer is called the exciting current, and is comprised of magnetizing current plus any additional current necessary to overcome core losses. It
is never perfectly sinusoidal in a real transformer, but may be made more so if the transformer
is designed and operated so that magnetic flux density is kept to a minimum.

Core flux induces a voltage in any coil wrapped around the core. The induces voltage(s) are
ideally in phase with the primary winding source voltage and share the same wave-shape.


Any current drawn through the secondary winding by a load will be "reflected" to the primary
winding and drawn from the voltage source, as if the source were directly powering a similar
load.