L3
T3 Overload heater elements Contacts L2
T2 L1
T1
Motor
Figure 2-35 Manual motor starter.
Photo courtesy Rockwell Automation, www.rockwellautomation.com.
Coil 3
2
L3
T3 L2
T2 L1
T1
Figure 2-36 Typical three-phase, across-the-line (full- voltage) magnetic starter.
Photo courtesy Rockwell Automation, www.rockwellautomation.com.
PART 5 Manual and Magnetic Motor Starters
Manual Starter
Manual motor starters are a very basic way to supply power to a motor. A manual control circuit is a circuit that requires the operator to control the motor directly at the location of the starter. Figure 2-35 shows an example of a three-phase manual-start motor control circuit. The dot- ted line across the contacts designates a manual starter (as opposed to a magnetic starter). Incoming power supply wires (L1, L2, and L3) connect to the top of the contacts, and the opposite sides of the contacts are connected to the overload heater elements. The motor terminal connec- tions (T1, T2, and T3) connect to the 3ϕ motor.
Manual starters are operated by the manual start/stop mechanism located on the front of the starter enclosure.
The start/stop mechanism moves all three contacts at once to close (start) or open (stop) the circuit to the motor. The National Electrical Code requires that a starter not only turn a motor on and off but also protect it from overloads.
The three thermal overload protective devices are installed to mechanically trip open the starter contacts when an overload condition is sensed. Manual three-phase start- ers are used in low horsepower applications such as drill presses and table saws where remote pushbutton control is not required.
Magnetic Starter
Magnetic motor starters allow a motor to be controlled from any location. Figure 2-36 shows a typical three- phase across-the-line (full-voltage) magnetic starter. The
line terminals, load terminals, motor starter coil, overload relays, and auxiliary holding contact are shown. When the starter coil is energized, the three main contacts as well as the holding contact close. Should an overload condi- tion occur, the normally closed OL relay contact would open. In addition to the power circuit, the manufacturer provides some control circuit wiring. In this case the prewired control circuit wiring consists of two connec- tions to the starter coil. One side of the starter coil is fac- tory wired to the overload relay contact and the other side to the holding contact.
Magnetic motor control circuits are divided into two basic types: the two-wire control circuit, and the three- wire control circuit. Two-wire control circuits are designed to start or stop a motor when a remote control device such as a thermostat or pressure switch is activated or deacti- vated. Figure 2-37 shows a typical two-wire control cir- cuit. Notice that the circuit has only two wires leading from the control device to the magnetic starter. The starter operates automatically in response to the state of the con- trol device without the assistance of an operator. When the contacts of the control device close, power is supplied to the starter coil, causing it to energize. As a result, the motor is connected to the line through the power contacts.
The starter coil is deenergized when the contacts of the control device open, switching the motor off.
The two-wire control systems provide low-voltage release but not low-voltage protection . They use a main- tained rather than a momentary-contact type of control device. If the motor is stopped by a power interruption, the starter deenergizes (low-voltage release), but also reenergizes if the control device remains closed when the circuit has power restored. Low-voltage protection is not provided, as there is no way for the operator to be automatically protected from the circuit once power has
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38 Chapter 2 Understanding Electrical Drawings
Ladder control diagram Start
Stop OL
L1 L2
M
2 3
M (holding contact)
L3
T3 L2
Wiring diagram
Start
Stop T2
L1
T1 M
M 3
1
1
M
OL 2 Three wires
Motor
Figure 2-38 Three-wire control circuit.
This material and associated copyrights are proprietary to, and used with the permission of Schneider Electric.
Remote pressure control switch
L3
T3 L2
Wiring diagram Two wires
T2 L1
T1 M
M 3
1
M
OL 2
M 3 OL
1
L1 L2
Switch
Ladder control diagram Motor
Figure 2-37 Two-wire control circuit.
Photo courtesy Honeywell, www.honeywell.com.
been restored. Two-wire control circuits are used to auto- matically operate machinery where the automatic restart- ing characteristic is desirable and there is no danger of persons being injured if the equipment should suddenly restart after a power failure. Sump pumps and refrigera- tor compressor controls are two common applications for two-wire control systems.
Three-wire control provides low-voltage protection . The starter will drop out when there is a voltage failure, but it will not pick up automatically when voltage returns. Three- wire control uses a momentary-contact control device and a holding circuit to provide the power failure protection.
Figure 2-38 shows a typical three-wire control circuit. The operation of the circuit can be summarized as follows:
• Three-wires are run from the start/stop pushbutton station to the starter.
• The circuit uses a normally closed (NC) stop push button wired in series with the parallel combination consisting of normally open (NO) start push button and normally open holding contact (M).
• When the momentary-contact start button is closed, line voltage is applied to the starter coil to energize it.
• The three main M contacts close to apply voltage to the motor.
• The auxiliary M contact closes to establish a circuit around the start button.
• When the start button is released, the starter coil remains energized by the closed M auxiliary con- tact (also known as the holding, seal-in, or memory contact) and the motor will continue to operate.
• When the momentary-contact stop button is opened, all voltage to the starter coil is lost. The main con- tacts are opened along with the holding contact and the motor stops.
• The starter drops out at low or no voltage and can- not be reenergized unless line voltage returns and the start button is closed.
Basically, three-wire control uses a maintaining circuit consisting of a holding contact wired in parallel with a start button. When the starter drops out, the holding con- tact opens and breaks the circuit to the coil until the start button is pressed to restart the motor. In the event of a power failure, the maintaining circuit is designed to pro- tect against automatic restarting when the power returns.
This type of protection must be used where accidents or damage might result from unexpected starts. All devices that start the circuit are connected while those that stop the circuit are connected in series.
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Discussion Topics 39
PART 5 Review Questions 1. How are the contacts of a manual motor starter
closed and opened?
2. One advantage of the magnetic motor starter over manual types is that it allows a motor to be controlled from any location. What makes this possible?
3. Power is lost and then returned to a two-wire motor control circuit. What will happen? Why?
4. Trace the current path of the holding circuit found in a three-wire motor control circuit.
TROUBLESHOOTING SCENARIOS 1. Heat is the greatest enemy of a motor. Discuss in
what way nonadherence to each of the following motor nameplate parameters could cause a motor to overheat: ( a ) voltage rating; ( b ) current rating;
( c ) ambient temperature; ( d ) duty cycle.
2. Two identical control relay coils are incorrectly connected in series instead of parallel across a 230-V source. Discuss how this might affect the operation of the circuit.
3. A two-wire magnetic motor control circuit control- ling a furnace fan uses a thermostat to automatically operate the motor on and off. A single-pole switch is to be installed next to the remote thermostat and wired so that, when closed, it will override the auto- matic control and allow the fan to operate at all times regardless of the thermostat setting. Draw a ladder control diagram of a circuit that will accomplish this.
4. A three-wire magnetic motor control circuit uses a remote start/stop pushbutton station to operate the motor on and off. Assume the start button is pressed but the starter coil does not energize. List the possible causes of the problem.
5. How is the control voltage obtained in most motor control circuits?
6. Assume you have to purchase a motor to replace the one with the specifications shown below. Visit the website of a motor manufacturer and report on the specifications and price of a replacement motor.
1. Why are contacts from control devices placed only in series with loads?
2. Record all the nameplate data for a given motor and write a short description of what each item specifies.
3. Search the Internet for electric motor connection diagrams. Record all information given for the con- nection of the following types of motors:
a. DC compound motor
b. AC single-phase dual-voltage induction motor
c. AC three-phase two-speed induction motor 4. The AC squirrel-cage induction motor is the domi-
nant motor technology in use today. Why?
DISCUSSION TOPICS AND CRITICAL THINKING QUESTIONS
Horsepower 10
Voltage 200
Hertz 60
Phase 3
Full-load amperes 33
RPM 1725
Frame size 215T
Service factor 1.15
Rating 40C AMB-CONT
Locked rotor code J
NEMA design code B
Insulation class B
Full-load efficiency 85.5
Power factor 76
Enclosure OPEN
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40
3
Motor Transformers and Distribution Systems
Chapter Objectives
This chapter will help you to:
1. Understand the principles that are used to effi- ciently transmit power from the power generat- ing plant to the customer.
2. Recognize the different sections and functions of a unit substation.
3. Differentiate between the service entrance, feeders, and branch circuits of the electrical distribution system within a building.
4. Be familiar with the function and types of race- ways used in electrical distribution systems.
5. Explain the function of switchboards, panel- boards, and motor control centers.
6. Understand the theory of operation of a transformer.
7. Properly connect single-phase and three-phase transformers as part of a motor power and control circuit.
Transformers transfer electric energy from one electric circuit to another by means of elec- tromagnetic mutual induction. In its broadest sense, a distribution system refers to the man- ner in which electrical energy is transmitted from the generators to its many points of use.
In this chapter we will study the role that trans- formers play in motor power distribution and control systems.
PART 1 Power Distribution Systems
Transmission Systems
The central-station system of power generation and distribution enables power to be produced at one location for immediate use at another location many miles away. Transmitting large amounts of electric energy over fairly long dis- tances is accomplished most efficiently by using
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PART 1 Power Distribution Systems 41 current required for a given load. Their operation is sum- marized as follows:
• 10,000 W of power is to be transmitted.
• When transmitted at the 100-V level, the required transmission current would be 100 A:
P = V × I = 100 V × 100 A = 10,000 W • When the transmission voltage is stepped up to
10,000 V, a current flow of only 1 A is needed to transmit the same 10,000 W of power:
P = V × I = 10,000 V × 1 A = 10,000 W There are certain limitations to the use of high volt- age in power transmission and distribution systems. The higher the voltage, the more difficult and expensive it becomes to safely insulate between line wires, as well as from line wires to ground. The use of transformers in power systems allows generation of electricity at the most suitable voltage level for generation and at the same time allows this voltage to be changed to a higher and more economical voltage for transmission. At the load centers transformers allow the voltage to be lowered to a safer voltage and more suitable voltage for a particular load.
Power grid transformers, used to step up or step down voltage, make possible the conversion between high and low voltages and accordingly between low and high cur- rents (Figure 3-3). By use of transformers, each stage of the system can be operated at an appropriate voltage level. Single-phase three-wire power is normally supplied to residential customers, while three-phase power is sup- plied to commercial and industrial customers.
Unit Substations
Electric power comes off the transmission lines and is stepped down to the distribution lines. This may happen in several phases. The place where the conversion from trans- mission to distribution occurs is in a power substation . It has transformers that step transmission voltage levels down to distribution voltage levels. Basically a power substation consists of equipment installed for switching, changing, or regulating line voltages. Substations provide a safe point in the electricity grid system for disconnecting the power in high voltages. Figure 3-1 illustrates the typical transforma-
tion stages through which the distribution system must go in delivering power to a commercial or industrial user.
Without transformers the widespread distribution of electric power would be impractical. Transformers are electrical devices that transfer energy from one electrical circuit to another by magnetic coupling. Their purpose in a power distribution system is to convert AC power at one voltage level to AC power of the same frequency at another voltage level. High voltages are used in transmission lines to reduce the amount of current flow. The power transmit- ted in a system is proportional to the voltage multiplied by the current. If the voltage is raised, the current can be reduced to a smaller value, while still transmitting the same amount of power. Because of the reduction of current flow at high voltage, the size and cost of wiring are greatly reduced. Reducing the current also minimizes voltage drop ( IR ) and amount of power lost ( I 2 R ) in the lines.
The circuits of Figure 3-2 illustrate how the use of high voltage reduces the required amount of transmission Figure 3-1 Transformation stages of a power distribution system.
4,000 V 345,000-V High-voltage transmission grid
Power generating
station
Step-up transformer
Step-down transformer
Commercial/
industrial customer 20,000 V
Figure 3-2 High voltage reduces the required amount of transmission current required.
Step-up transformer
Step-down transformer Transmission at 100-V level
Transmission at 10,000-V level 1 A
100 A Power
generating plant
Customer
10,000 W 10,000 W
10,000 W 10,000 W
Figure 3-3 Power grid transformer.
Photo courtesy ABB, www.abb.com.
Low-voltage power lines High-voltage
power lines
Low-voltage, high-current winding High-voltage,
low-current winding
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42 Chapter 3 Motor Transformers and Distribution Systems
switches or molded-case circuit breakers in addition to metering for the measurement of voltage, cur- rent, power, power factor, and energy. The secondary switchgear is intended to be tripped out in the event of overload or faults in the secondary circuit fed from the transformer; the primary gear should trip if a short circuit or ground fault occurs in the transformer itself.
Before attempting to do any work on a unit substation, first the loads should be disconnected from the trans- former and locked open. Then the transformer primary should be disconnected, locked out, and grounded tempo- rarily if over 600 V.
Distribution Systems
Distribution systems used to distribute power throughout large commercial and industrial facilities can be complex.
Power must be distributed through various switchboards, transformers, and panelboards (Figure 3-6) without any component overheating or unacceptable voltage drops.
This power is used for such applications as lighting, heat- ing, cooling, and motor-driven machinery.
The single-line diagram for a typical electrical distribu- tion system within a large premise is shown in Figure 3-7.
Typically the distribution system is divided into the fol- lowing sections:
Service entrance —This section includes conductors for delivering energy from the electricity supply sys- tem to the premises being served.
Feeders —A feeder is a set of conductors that origi- nates at a main distribution center and supplies one or more secondary or branch circuit distribution centers.
This section includes conductors for delivering the energy from the service equipment location to the final branch circuit overcurrent device; this protects each the event of trouble, as well as a convenient place to take
measurements and check the operation of the system.
The power needs of some users are so great that they are fed through individual substations dedicated to them. These secondary unit substations form the heart of an industrial plant’s or commercial building’s electrical distribution.
They receive the electric power from the electric utility and step it down to the utilization voltage level of 600 V nom- inal or less for distribution throughout the building. Unit substations offer an integrated switchgear and transformer package. A typical unit substation is shown in Figure 3-4.
Substations are factory assembled and tested and therefore require a minimum amount of labor for installation at the site. The unit substation is completely enclosed on all sides with sheet metal (except for the required ventilating open- ings and viewing windows) so that no live parts are exposed.
Access within the enclosure is provided only through inter- locked doors or bolted-on removable panels.
The single-line diagram for a typical unit substation is illustrated in Figure 3-5. It consists of the following sections:
High-voltage primary switchgear —This section incorporates the terminations for the primary feeder cables and primary switchgear, all housed in one metal-clad enclosure.
Transformer section —This section houses the trans- former for stepping down the primary voltage to the low-voltage utilization level. Dry-type, air-cooled transformers are universally used because they do not require any special fireproof vault construction.
Low-voltage distribution section —This switchboard section provides the protection and control for the low-voltage feeder circuits. It may contain fusible Figure 3-4 Factory assembled unit substation.
This material and associated copyrights are proprietary to, and used with the permission of Schneider Electric.
Low-voltage main bus
Main secondary breaker Primary switchgear Transformer
Primary service feeder
Branch feeders
Feeder breakers
Figure 3-5 Single-line diagram for a typical unit substation.
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44 Chapter 3 Motor Transformers and Distribution Systems
properly supported and have sufficient access points to facilitate the installation of the conductors. Con- duits must be large enough to accommodate the num- ber of conductors, based generally, on a 40 percent fill ratio.
Cable trays— Cable trays are used to support feeder cables where a number of them are to be run from the same location. They consist of heavy feeder conduc- tors run in troughs or trays.
Low-impedance busways (bus duct)— The busways are used in buildings for high-current feeders. They consist of heavy bus bars enclosed in ventilated ducts.
Plug-in busways —These busways are used for over- head distribution systems. They provide convenient power tap-offs to the utilization equipment.
Switchboards and Panelboards
The Code defines a switchboard as a single panel or group of assembled panels with buses, overcurrent devices, and instruments. Figure 3-9 shows a typical combination service entrance and switchboard installed in a com- mercial building. The service entrance is the point where turn needs its own properly coordinated overcurrent
protection in the form of a circuit breaker or fused switch.
Correct selection of conductors for feeders and branch circuits must take into account ampacity, short-circuit, and voltage-drop requirements. Conductor ampacity refers to the maximum amount of current the conductor can safely carry without becoming overheated. The ampacity rat- ing of conductors in a raceway depends on the conductor material, gauge size, and temperature rating; the number of current-carrying conductors in the raceway; and the ambient temperature.
The National Electrical Code (NEC) contains tables that list the ampacity for approved types of conductor size, insulation, and operating conditions. NEC rules regarding specific motor installations will be covered throughout the text. Installers should always follow the NEC, applicable state and local codes, manufacturers’ instructions, and project specifications when installing motors and motor controllers.
All conductors installed in a building must be properly protected, usually by installing them in raceways. Race- ways provide space, support, and mechanical protection for conductors, and they minimize the hazards from elec- tric shocks and electric fires. Commonly used types of raceways found in motor installations are illustrated in Figure 3-8 and include:
Conduits— Conduits are available in rigid and flex- ible, metallic and nonmetallic types. They must be
Busway sections bolted together
Plug-in type busway Cable trays
Rigid conduit Flexible conduit
Rigid and f lexible conduit Motor
Figure 3-8 Common types of raceways.
Busway photos courtesy Siemens, www.siemens.com. Cable tray photo cour- tesy of Hyperline Systems (www.hyperline.com). The copyrights are owned by the Hyperline Systems or the original creator of the material.
Figure 3-9 Combination service entrance switchboard.
Photos courtesy Siemens, www.siemens.com.
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