Schematics Made Easy
SCHEMATICS MADE EASY CONTENTS POWER & CONTROL CIRCUITS............. ........................... ............................ ............................. ............................. ............................ ............................ ............................ ............................ ...................... ............. ....... .. CONTROL CIRCUITS.............. ............................ ............................ ............................ ............................ ............................. ............................. ............................ ............................ ............................ ....................... .............. .......... ..... TWO & THREE WIRE CIRCUITS.............. ............................ ............................ ............................ ............................. ............................. ............................ ............................ ............................ ........................... ............... SYMBOLS CHART.............. ............................ ............................ ............................ ............................ ............................ ............................. ............................. ............................ ............................ ............................ ..................... .......... ... EXAMPLES:............. ........................... ............................ ............................ ............................ ............................. ............................. ............................ ............................ ............................ ............................ ............................. ............... ...... ........
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Page 1 of 8
Schematics Made Easy
POWER & CONTROL CIRCUITS Circuit diagrams may seem complex when viewed in their entirety, but they can be simplified by breaking them into basic basic circuits. circuits. The overall overall diagram, diagram, and basic basic machine machine functions, are then easier to understand.
The control circuit is shown in light lines and consists of the stop-start stop-start pushbutto pushbuttons, ns, holding holding interlock, interlock, magnetic magnetic starter coil M, and overload relay contact X2.
Control systems are usually designed so that an individual circui circuitt contro controls ls only only one functi function on of a machin machine. e. For example, this could be the starting and stopping of an electric electric motor by means means of pushbutton pushbuttons, s, or controlli controlling ng solenoid valves through the use of limit switches. There are many variations in individual circuits. The main factor to remember is that a basic circuit is usually associated with a basic machine function. Electric circuits may be of two types, Power Circuits and Control Circuits. POWER CIRCUITS are usually shown in a diagram with heavy lines since they are the heavy conductors or wires carrying motor or load current. CONTROL CIRCUITS are usually associated with pilot or control of the power switching equipment, such as the coil circuit in a magnetic starter. These wires are shown using lighter lines in the diagram. Some graphic symbols and designations used in diagrams are shown on Page 6. These are used in the circuits described in this article. Wiring Wiring Diagram — Figu Figure re 1 show showss the the wiri wiring ng or connection diagram of a magnetic starter with a start-stop pushbutton station. The location of each wire and terminal identifications identifications are shown. L2
L1
2
L3
A 3
M V
Schematic Schematic Diagram Diagram — A sche schema mati ticc or elem elemen enta tary ry diagram of the starter shown (Fig. 2) is illustrated in Fig. 3. The schematic does not show the physical relationship of each wire location. It does indicate in straight line form the circuit functions of the various devices.
Note Note that that the same same termin terminal al identi identific ficati ation on lette letters rs and numb number erss are are used used in both both the the wiri wiring ng and and sche schema mati ticc diagrams to designate the control and power connections. The starter and pushbuttons can be wired directly from the schematic, if desired, since it does show how the devices are connected into the circuit. For troubleshooting, it is much easier to work from a schematic diagram rather than a wiring diagram. This is particularly true with a complex circuit.
1
C
Fig. 2. Pushbutton station and 3-phase magnetic starter with arc box cover removed to show contacts. Heater elements shown installed .
W
X2
HEATER ELEMENTS L1
T1
M
T3
T2
L2
T2 T1
T1 MOTOR
T2
M T3
MOTOR
T3
L3 M
Fig. 1. The wiring diagram for the 3-phase magnetic starter with start-stop pushbutton control pictured in Fig. 2.
Note the 3-phase power circuit is shown in heavy black lines. L1, L2, and L3 indicate the line or supply. T1, T2, and T3 are on the load side or motor terminals.
Page 2 of 8
START L1
1
2
3
STOP M
L2
M V MOTOR STARTER COIL
W OL RELAY CONTACTS
Fig. 3. Schematic of the magnetic starter pictured in Fig. 2.
Schematics Made Easy
Now let us consider some of the basic everyday circuits and the meaning of the more common terms. These circuits will be illustrated in schematic form showing only the control control portion portion and its variation variationss for different different machine machine functions.
CONTROL CIRCUITS The circ circui uitt in Fig. Fig. 4 does does Maintained Start-Stop — The exactly what its name implies — starts and stops a motor by depressing the maintained start and stop pushbuttons START OL
STOP M
L1
L2
Fig. 4. Maintained Start-Stop Circuit. Motor is started by pushing button. Starter drops out whenever voltage is below hold-in value.
This circuit has undervoltage release. Should the voltage on the coil drop below the hold-in value, the starter will drop out. When the voltage is restored, the starter will immediately pick up since the pushbutton has remained closed. Maintained start-stop circuits should only be used in the applic applicati ation on of heatin heating, g, lighti lighting ng and other other such such nonnonmechanical applications. This circuit would not be used with rotating or moving equipment due to the potential hazard of an unwanted restart when power is restored. UNDERVOLTAGE UNDERVOLTAGE PROTECTION may be provided in a circuit and assures that a magnetic controller will not rest restar artt afte afterr a powe powerr inte interr rrup upti tion on unti untill the the oper operat ator or initiates the action with a pushbutton or other device. Momentary Start-Stop . In Fig. 5 we have a momentary start-stop control circuit. Here, the safety feature of undervoltage protection is provided. The operator must push the start button to reenergize the starter after it has opened due to undervoltage release as compared to the starter in Fig. 4 which will energize as soon as voltage is restored. This is accomplished by a holding interlock on the starter and momentary actuated pushbuttons — as differentiated from the maintained type used in Fig. 4.
Starter coil M is energized when the start pushbutton is depres depressed sed.. This This closes closes contac contactt M which which is connec connected ted around the start pushbutton, thus electrically “sealing” the circuit. The start pushbutton, being of the momentary type, spring-re spring-returns turns to the open position position when released. released. The starter, however, remains energized due to completion of the circuit through the now-closed M contact. This contact is referred to as the “seal-in” or “holding” interlock, and would be the left contact on the starter in Fig. 2. Should the starter coil circuit be interrupted for any reason such as power failure, insufficient coil voltage, overload trip, or operation of the stop button, the starter will drop out or be de-energized. The seal-in interlock opens and prevents an unwanted restart until the start button is again operated. This is where the protection feature comes into play, since operation of the motor is completely under the operator’s control. Multiple Start-Stop Stations. Extra start-stop pushbutton stat statio ions ns can can be adde added d as show shown n in Fig. Fig. 6. The The stop stop pushbuttons should be connected in series and the start butt buttons ons in parall parallel. el. Note that that only only a single single seal-in seal-in M contact is required around the multiple start pushbuttons to maintain the circuit to the motor starter coil. STOP
STOP
START
M
M
L1
L2
L2
START
START
M
Fig. 6. Multiple stations used with momentary start-stop circuit. Stop buttons are wired in series and start buttons in parallel.
TWO & THREE WIRE CIRCUITS The terms two-wire and three-wire control are frequently used — but not always understood. Using the basic circuits shown in Figs. 7 and 8, let us clarify the origin of these two expressions. 1
2
MOTOR STARTER COIL M
FLOAT SWITCH
OL
OL
L1
L1
STOP
START
STOP
L2 OL CONTACTS
Fig. 7. Two-wire control circuit. Two wires are connected to the float switch energizing the magnetic starter.
M
Fig. 5. Momentary Start-Stop Circuit showing holding interlock and momentary actuated pushbuttons .
Page 3 of 8
Schematics Made Easy Two-wire control is so named because only two wires (as shown in Fig. 7) are connected to the pilot device that energizes the magnetic controller. In the diagram, the pilot device shown is a normally open float switch used with a pump motor starter. It energizes the motor starter only as long as It remains closed. This type of circuit provides undervoltage release but not undervoltage protection, since a holding interlock is not used. The motor starter would drop out on loss of voltage and then immediately pick up again (without operator control) upon restoration of power — provided the float switch had remained in the closed position. 1
3 START
STOP
closed contact of the reverse (R) contactor is used in the forward (F) contactor coil circuit. Figure 10 shows an interlock which mounts on the starter and is actuated whenever the starter is operated. These normally closed electrical interlocks are shown mounted betwe between en the forwar forward d and revers reversee starte starterr arc boxes, boxes, in Figure 9.
MOTOR STARTER COIL M
L1
L2 OL CONTACTS
2 M
Fig. 8. Three-wire control circuit. Three wires must be connected to the pilot devices to energize the magnetic starter.
Three-wire control, (Fig. 8), gets its name from the three wires that must be connected to the pilot device used to operate the motor starter. Notice here we have the basic moment momentary ary startstart-sto stop p circui circuitt shown shown in Fig. Fig. 5 which which provides the undervoltage protection feature.
REVERSIN REVERSING G CIRCUITS CIRCUITS
— Thre Threee phas phasee squirrel-cage motors are particularly suited to reversal of rota rotati tion on by simp simply ly inte interc rcha hang ngin ing g two two of the the line line conductors supplying the motor. This is commonly done by using two separate contactor assemblies — one for forward rotation and the other to reconnect for reverse rotation.
Fig. I Normally closed (NC) auxiliary electrical interlock as mounted on reversing starter in Fig. 9 between both arc boxes. May also be normally open (NO) for other applications .
There There are two types types of revers reversing ing circui circuits: ts: Forwa ForwardrdReverse Compelling and Forward-Reverse Forward-Reverse Optional. Forward-Reverse Compelling — Compelling circuits are used with motors which are not instantly reversible. These motors are brought to a stop before changing direction of rotation. In Fig. 11 depressing the forward push button will energize the forward contactor coil (F), causing the motor to rotate forward. At the same time, it opens the normally closed (F) contact in the reverse contactor coil (R) circuit and closes closes the normal normally ly open open (F) (F) contac contactt around around the forward pushbutton to seal-in the circuit. As long as the forward forward contactor is picked picked up, depressing depressing the reverse pushbutton will have no effect. This is because the (F) contact is open in the reverse coil circuit.
The circui circuitt derive derivess its its name name becaus becausee the operat operator or is compelled to depress the stop pushbutton before he can change direction of rotation. Once the forward contactor has dropped out and reclosed its normally normally closed contact (F) in the reverse coil circuit, the rotation of the motor can be started in the reverse direction. Limit switches (LS) are shown in this circuit since it is sometime sometimess used for equipment equipment such as overhead overhead doors, doors, which are stopped with a limit switch at the end of the door travel. STOP
FOR. F
L1 LS
Fig. 9. Horizontal reversing magnetic starter with mechanical and electrical interlocks.
R
F REV.
LS
L2 OL
F R
A revers reversing ing starte starterr is electr electrica ically lly and mechan mechanica ically lly interlocked so that both contactors cannot close at the same time time and and caus causee a dead dead shor shortt circ circui uit. t. Mech Mechan anic ical al inte interl rloc ocki king ng is done done by mean meanss of an inte interf rfer eren ence ce mech mechan anis ism m whic which h bloc blocks ks the the oper operat atio ion n of the the open open cont contac acto torr when when the the othe otherr one one is clos closed ed.. Elec Electr tric ical al interlocking — known as “cross electrical interlocking” -is done done by auxili auxiliary ary interl interlock ockss on each each contac contactor tor.. A normally closed contact of the forward (F) contactor is used in the reverse (R) contactor coil circuit. A normally Page 4 of 8
R
Fig. 11. Forward-Reverse Compelling Circuit. Operator is compelled to depress stop button before changing motor rotation.
Schematics Made Easy Forward-Reverse Optional — The optional circuit in Fig. 13 is similar to the compelling circuit with one exception It util utiliz izes es a push pushbu butt tton on with with both both norm normal ally ly open open and and normally closed contacts such as shown in Fig. 12. Each pushbutton with two sets of contacts is indicated by the dotted lines connecting the two parts of the single buttons. The normally closed contact on the forward push button is connec connected ted in the revers reversee contac contactor tor coil coil circui circuitt and the normally closed contact of the reverse pushbutton in the forward coil circuit.
LOW
L1 STOP
HIGH
L2
L H
HIGH
L-OL H-OL H
CR
L
CR
L
H
CR
Fig. 14. Two Speed Compelling Circuit. Motor must be started in low speed. Stop button must be pushed b efore going from high to low.
A control control relay relay (Fig. (Fig. 15)— designat designated ed as CR in the diagram and referred to as a compelling relay — ensures that the motor is started in the low speed. The relay has two normally open contacts. One is to seal it in after being energized through a contact on the low-speed starter (L). The other is located in the high-speed starter coil circuit to prevent initial start on high speed. Upon changing from low to high high speed speed note note that that the low speed speed start starter er coil coil circuit is opened by the normally closed contact of the high- speed pushbutton.
Fig. 12. Pushbutton with Double Contacts.
With ith the the motor motor runnin running g forwar forward, d, pushin pushing g the revers reversee pushbutt pushbutton on will open the circuit circuit to the forward forward contactor contactor and cause it to drop out and close its normally closed (F) contact in the reverse contactor coil circuit. The motor will immediately be connected for reverse rotation. It will sealin and operate continuously in this direction until either stoppe stopped d with with the stop pushbu pushbutt tton, on, or change changed d to the the forward direction again. Note that it is not necessary to push the stop button before changing directions as in the compelling circuit shown in Fig. Fig. 11. Thus Thus the the term termin inol ology ogy,, “For “Forwa ward rd-R -Rev ever erse se Optional.” For this application, motors must be de signed to go directly from full speed in one direction to full speed in the other direction. STOP
STOP
FOR. F
L1 LS
R
Two-Speed Non-Compelling — These circuits are used in applications where the motor may be started in either high or low speeds (Fig. 16). Speed can also be changed during operation between low and high by the operator, without having to first bring the motor to a stop. This circuit is similar to the Forward-Reverse Optional circuit shown in Fig. Fig. 13 and utili utilizes zes both both normal normally ly open open and normal normally ly clos closed ed cont contac acts ts on each each of the the low low and and high high spee speed d pushbuttons.
OL
F REV.
L2
Fig. 15. Control Relay (CR) with 2-pole contact block and magnetic operator. operator. Poles may be ei ther Normally Open or Normally Closed .
STOP LS
HIGH H H
R FOR.
STOP
L1
F
L
L2 L-OL
H-OL
R
Fig. 13. Optional Forward-Reverse Circuit. Stop button need not be pushed when changing direction.
MULTI-SPEED CIRCUITS. Control of two-
HIGH
LOW
H L
L
Fig. 16. Two-Speed Non-Compelling Circuit. For applications where motor may be started in high or low speeds.
speed peed motor otors, s, bot both sing ingle and and twotwo-w windin nding, g, is accomplished by the following two circuits — depending on application requirements. The common types of twospeed speed circui circuits ts are known known as “Comp “Compell elling ing”” and “Non“NonCompelling.” Two-Speed Compelling — The circuit in Fig. 14 is used in applications requiring: ( that the motor be started in low speed before going to high speed, and (2) that the motor not be switched from high speed to low speed with out first depressing the stop pushbutton. This is known as TwoSpeed Compelling because the operator is compelled to start in the lower speed. Page 5 of 8
Schematics Made Easy
SEQUENCE SEQUENCE ST STAR ARTING TING.
Ofte Often, n, motor otorss cont contro roll lled ed with with sepa separa rate te star starte ters rs must must be star starte ted d in sequence from a single start-stop pushbutton station. This can be done in two ways — as shown in Figs. 17 and 19— depending on application requirements. Sequence Start by Auxiliary Sequence Auxiliary Contacts Contacts — In Fig. 17, auxiliary contacts on the motor starters are used to provide automatic sequence start from a single pushbutton station. This This ensure ensuress that that motor motor No. 1 must must be runnin running g before before motor No. 2 is started and that motors 1 and 2 must be in operation before motor No. 3 can start. All the overload relay relay contac contacts ts are wired in series series so that that an overlo overload ad condition on any one of the motors will shut down the complete system. START
STOP
M1
L1 M1 M1
M2
L2 OL OL OL
M2
M3
Fig. 17. Sequence start with auxiliary contacts on the motor starters. Sequence start activated from a single pushbutton station.
Timed Sequence Start — The timed sequence start circuit in Fig. Fig. 19 employ employss time time delay delay relay relayss with with their their coils coils connected in parallel with the motor starter coils. Their time time delay delay contac contacts ts provid providee the autom automati aticc sequen sequence ce starting of the motors.
STOP
START M1
L1 M1
M1
L2 OL OL OL
M2 TR1
M2 M3
TR2
Fig. 19. Timed Sequence Start Circuit uses adjustable time delay relays. One motor comes up to speed before second is started.
This type of circuit is used to permit one motor to come up to speed before the second motor is thrown on the line. Its action prevents heavy line surges which result when more than one motor is started at once on lines that do not have sufficient capacity. Proper adjustment of the time delay relays permits the power regulating equipment to recover betwe between en automa automatic tic start starting ing of multip multiple le motors motors — and prevents serious dips in line voltages. Individual pneumatic timing heads, operated directly from the movement of the magnetic starter armatures, can also be used on some types of starters (Fig. 20). They eliminate the need for time delay relay coils.
Fig. 20. Starter Starter with timer head accessory operated operated from starter starter armature.
Fig. 18. Solenoid operated adjustable time delay relay.
In summary . . . it can be readily seen that although wiring diagrams may seem to be complicated and unwieldy they need not be. Much of the confusion and mystery can be removed by simply breaking the over-all control diagrams into their basic circuits. The power circuit furnishes power for the motor and load. The individual control circuits usually operates only one motor controller function.
Page 6 of 8
Schematics Made Easy
SYMBOLS CHART CIRCUIT DISCONNECT INTERRUPTER
LIMIT SWITCH
CIRCUIT BREAKER
SPRING RETURN
MAINTAINED
Normally Closed
Normally Open
Neutral Position
Thermal
NP
Held Closed
LIQUID LEVEL Normally Open
TEMPERATURE ACTIVATED
VACUUM & PRESSURE
Normally Closed
Normally Closed
Normally Open
Held Open
Normally Open
FLOW (AIR, WATER, ETC._ Normally Open
Normally Closed
PUSH BUTTONS Normally Open
Normally Closed
Double Circuit
SELECTOR SWITCH J
K
J K L A1 x A2 x B1 x B2 x
B1 B2
R
GENERAL CONTACTS
Normally Open
Normally Open
Normally Closed
Normally Closed
TC
TO
TO
TC
OR
DENOTE COLOR BY LETTER
x INDICATES CONTACTS CLOSED
CONDUCTORS
Normally Closed
Normally Closed
TIME DELAY CONTACT
PUSH TO TEST
A1 A2
Normally Open
FOOT SWITCH Normally Open
Maintained
Mushroom Head
LAMPS
J–K-L
L
Normally Closed
OR
MAGNET COIL
OR
OR
CONTROL TRANSFORMER
METER
Connected
Not Connected
H1
H3
X2
H2
H4
VM
X1 AM
FULL WAVE RECTIFIER
GROUND
HORN, SIREN
BELL, BUZZER
FUSE
3 Phase
AC
DC
OVERLOAD RELAY
MOTOR
DC MOTOR AC
AUTO TRANSFORMER
RESISTOR Adjustable
LOCATION OF RELAY CONTACTS 1CR
Fixed 1
1CR
2 RES
1 RES
1CR 3 1CR 4
(2 – 3 – 4) NUMBERS IN PARENTHESIS DESIGNATE THE LOCATION OF RELAY CONTACTS. A LINE UNDERNEATH A LOCATION NUMBER SIGNIFIES A NORMALLY CLOSED CONTACT.
Page 7 of 8
Schematics Made Easy
EXAMPLES: L1
START STOP
1
2
L2
OL
3 M
M
Fig. 1. Three Wire Control Giving Low Voltage Protection Single Two Button Pushbutton Station L1 STOP
1
L2
START
STOP
STOP
1
STOP
FOR LS
FOR
REV
2
L2 OL
6 R F
F
OL
3
2
L1
M
REV LS
REV
START
FOR 4
5
7 F
R
R START
Fig. 8. Three Wire Control for Instant Instant Reversing Reversing Applicati Applications ons Using Using Single Three Button Pushbutton Station
M
Fig. 2. Three Wire Control Giving Low Voltage Protection Multiple Two Button Pushbutton Station
L1 1
L1 1
STOP
2
1
L2 OL
3
F 7
5
4
M
2 M
L1 1
SLOW
STOP
4
FAST
2
F CR
L2
JOG STOP
2
R
Fig. 9. Three Wire Control for Reversing Using Single Three Button Pushbutton Station
Fig. 4. Three Wire Control for Jog or Run Using Start- Stop Pushbuttons and Jag-Run Selector Switch
1
9
R
RUN
L1
REV LS
REV
START JOG
F
M
M
STOP
L2 OL
8
F
Fig. 3. Three Wire Control Giving Low Voltage Protection with Safe-Run Selector Switch L1
R 6
3
OL
3
RUN
2
L2
START
SAFE
FOR LS
FOR
STOP
OL
4 M
6
FOR LS
L2 OL
8
S
5 CR S F
S
CR
Fig. 10. Control for Two Speed with a Compelli Compelling ng Relay to Insure Insure Starting on Slow Speed
CR L1 START 1 STOP
3
REV
2
CR CR
FOR
REV 4
AUTO
HIGH
M
3
REV
5
R
F
6
7
FOR LS
REV LS
L2 8
OL F
5
F 7
10
9
REV LS 9 R
11
LO HI HI LO
Fig. 11. Control for Two Speed Reversing Starter Forward, Reverse, Stop, with High, Law Selector Switch RUN-JOG
L1 1
STOP
R
OL
3
L2
M 2
Fig. 7. Two Two Wire Control Control for Reversing Reversing Jogging. Jogging. Using Single Single Two Button Pushbutton Station
Page 8 of 8
F
LOW
Fig. 6. Two Wire Control Giving Low Voltage Release Only Using HandOff-Auto Selector Switch FOR
OL
OL
3
AUTOMATIC SWITCH
1
L2 8
R
L2
OFF
L1
FOR LS
M
OFF HAND
3 R 6
F
Fig. 5. Control for Jogging-Start-Stop Jogging-Start-Stop All with Push buttons L1
FOR
M
Fig. 12. Selector Push Contacts as shown for “Run” (three wire operation rotate switch sleeve and selector contact opens between “2’ and “Stop” button (two wire operation).