Schematics Made Easy

Schematics Made Easy

SCHEMATICS MADE EASY CONTENTS POWER & CONTROL CIRCUITS.............................................................................................................................................2 CONTROL CIRCUITS................................................................................................................................................................3 TWO & THREE WIRE CIRCUITS.............................................................................................................................................3 SYMBOLS CHART.....................................................................................................................................................................7 EXAMPLES:................................................................................................................................................................................8

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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 circuits. The overall diagram, and basic machine functions, are then easier to understand. Control systems are usually designed so that an individual circuit controls only one function of a machine. For example, this could be the starting and stopping of an electric motor by means of pushbuttons, or controlling 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 Diagram — Figure 1 shows the wiring or connection diagram of a magnetic starter with a start-stop pushbutton station. The location of each wire and terminal identifications are shown. L2

L1 2

L3

1 A

3 M

C

V

W

The control circuit is shown in light lines and consists of the stop-start pushbuttons, holding interlock, magnetic starter coil M, and overload relay contact X2.

Fig. 2. Pushbutton station and 3-phase magnetic starter with arc box cover removed to show contacts. Heater elements shown installed.

Schematic Diagram — A schematic or elementary 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 that the same terminal identification letters and numbers are used in both the wiring and schematic 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.

X2

HEATER ELEMENTS L1

T1

MOTOR

T2

M T3

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.

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T1

L2

T2 T1

M

T3

T2

MOTOR

T3

L3 M START L1

1

2

3 V

STOP M

M

MOTOR STARTER COIL

L2 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 portion and its variations for different machine functions.

CONTROL CIRCUITS Maintained Start-Stop — The circuit in Fig. 4 does 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 application of heating, lighting and other such nonmechanical 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 PROTECTION may be provided in a circuit and assures that a magnetic controller will not restart after a power interruption until the operator 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. STOP

START

STOP

STOP

L2

START

STOP

OL M

L1

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

L1 FLOAT SWITCH

OL M

L1

Starter coil M is energized when the start pushbutton is depressed. This closes contact M which is connected around the start pushbutton, thus electrically “sealing” the circuit. The start pushbutton, being of the momentary type, spring-returns to the open position when 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 stations can be added as shown in Fig. 6. The stop pushbuttons should be connected in series and the start buttons in parallel. Note that only a single seal-in M contact is required around the multiple start pushbuttons to maintain the circuit to the motor starter coil.

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.

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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 between the forward and reverse starter arc 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 momentary start-stop circuit shown in Fig. 5 which provides the undervoltage protection feature.

REVERSING CIRCUITS

— Three phase squirrel-cage motors are particularly suited to reversal of rotation by simply interchanging two of the 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 are two types of reversing circuits: ForwardReverse Compelling and 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 the normally open (F) contact around the forward pushbutton to seal-in the circuit. As long as the forward contactor is picked up, depressing the reverse pushbutton will have no effect. This is because the (F) contact is open in the reverse coil circuit. The circuit derives its name because the operator 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 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 sometimes used for equipment such as overhead doors, which are stopped with a limit switch at the end of the door travel. STOP L1

FOR.

F LS

Fig. 9. Horizontal reversing magnetic starter with mechanical and electrical interlocks.

F REV.

LS

R

L2 OL

F R

A reversing starter is electrically and mechanically interlocked so that both contactors cannot close at the same time and cause a dead short circuit. Mechanical interlocking is done by means of an interference mechanism which blocks the operation of the open contactor when the other one is closed. Electrical interlocking — known as “cross electrical interlocking” -is done by auxiliary interlocks on each contactor. 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 utilizes a pushbutton with both normally open 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 connected in the reverse contactor coil circuit 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 before going from high to low.

A control relay (Fig. 15)— designated 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 speed note that the low speed starter coil circuit is opened by the normally closed contact of the high- speed pushbutton.

Fig. 12. Pushbutton with Double Contacts.

With the motor running forward, pushing the reverse pushbutton will open the circuit to the forward 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 stopped with the stop pushbutton, or changed to 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. 11. Thus the terminology, “Forward-Reverse 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. L1

STOP

STOP

FOR.

F LS

F REV.

LS

R

L2 OL

Fig. 15. Control Relay (CR) with 2-pole contact block and magnetic operator. Poles may be either Normally Open or Normally Closed.

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. 13 and utilizes both normally open and normally closed contacts on each of the low and high speed pushbuttons. L1

F

STOP

HIGH H

R FOR.

STOP

H L

R

Fig. 13. Optional Forward-Reverse Circuit. Stop button need not be pushed when changing direction.

MULTI-SPEED CIRCUITS. Control of two-

HIGH

LOW

L2 L-OL

H-OL

H L

L

Fig. 16. Two-Speed Non-Compelling Circuit. For applications where motor may be started in high or low speeds.

speed motors, both single and two-winding, is accomplished by the following two circuits — depending on application requirements. The common types of twospeed circuits are known as “Compelling” and “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 STARTING.

Often, motors controlled with separate starters must be started 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 Contacts — In Fig. 17, auxiliary contacts on the motor starters are used to provide automatic sequence start from a single pushbutton station. This ensures that motor No. 1 must be running 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 contacts are wired in series so that an overload condition on any one of the motors will shut down the complete system. L1

START

STOP

M1 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. 19 employs time delay relays with their coils connected in parallel with the motor starter coils. Their time delay contacts provide the automatic sequence starting of the motors.

STOP L1

START M1 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 between automatic starting of multiple 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 with timer head accessory operated from starter armature.

Fig. 18. Solenoid operated adjustable time delay relay.

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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.

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

J

K

LAMPS

J K L A1 x A2 x B1 x B2 x

B1 B2 x INDICATES CONTACTS CLOSED

GENERAL CONTACTS Normally Open

R

Normally Open

Normally Open

Normally Closed

Normally Closed

TC

TO

TO

TC

OR

DENOTE COLOR BY LETTER

CONDUCTORS

Normally Closed

OR

MAGNET COIL

CONTROL TRANSFORMER

HORN, SIREN

BELL, BUZZER

H3

H2

H4

X1

OVERLOAD RELAY

MOTOR

VM

AM

FUSE

3 Phase

AC

DC

METER

Connected

Not Connected

X2

FULL WAVE RECTIFIER

OR

OR

H1

GROUND

Normally Closed

TIME DELAY CONTACT

PUSH TO TEST

J–K-L A1 A2

L

FOOT SWITCH Normally Open

Maintained

Mushroom Head

SELECTOR SWITCH

Normally Closed

DC MOTOR AC

AUTO TRANSFORMER

RESISTOR Adjustable

LOCATION OF RELAY CONTACTS 1CR

Fixed 1

RES

1 RES

2

1CR 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.

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Schematics Made Easy

EXAMPLES: L1

START

STOP

1

2

3 M

M

L2

OL

Fig. 1. Three Wire Control Giving Low Voltage Protection Single Two Button Pushbutton Station L1 STOP

1

STOP 2

STOP

START

L2

L1 1

STOP

2

M START

REV LS

REV

FOR

4

5

L2

OL

6 R

F

F

OL

3

FOR LS

FOR

REV

7 F

R

R START

Fig. 8. Three Wire Control for Instant Reversing Applications Using Single Three Button Pushbutton Station

M

Fig. 2. Three Wire Control Giving Low Voltage Protection Multiple Two Button Pushbutton Station L1 1

STOP

2

1

M

L2 OL

3 JOG

R 6

5

F 7

4

L1 1

L2

JOG 2

REV LS

9

R

M

2 M

STOP

4

SLOW 2

CR

STOP

F

Fig. 9. Three Wire Control for Reversing Using Single Three Button Pushbutton Station

FAST 6

Fig. 4. Three Wire Control for Jog or Run Using Start- Stop Pushbuttons and Jag-Run Selector Switch

1

L2 OL

8

R

RUN

L1

FOR LS

F REV

START

STOP

3

M

Fig. 3. Three Wire Control Giving Low Voltage Protection with Safe-Run Selector Switch L1

FOR

STOP 2

OL

3

RUN

1

L2

START

SAFE

L1

OL

4

M

F

S

FOR LS

L2 OL

8

S

5 CR S

F CR

Fig. 10. Control for Two Speed with a Compelling Relay to Insure Starting on Slow Speed

CR L1

START

1 STOP

3

REV

2

CR CR

AUTO

HAND

FOR

4

OL

3

HIGH

M

OFF AUTOMATIC SWITCH

FOR 1

3

REV

5

R

F

6

7

FOR LS REV LS

L2 8

OL F

OL

F

REV

5 F 7

REV LS 9 R

10

LO

11

HI

HI LO

Fig. 11. Control for Two Speed Reversing Starter Forward, Reverse, Stop, with High, Law Selector Switch RUN-JOG

L1 1

STOP

3

M

OL

L2

2 M 9

R

Fig. 7. Two Wire Control for Reversing Jogging. Using Single Two Button Pushbutton Station

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L2 8

LOW

Fig. 6. Two Wire Control Giving Low Voltage Release Only Using HandOff-Auto Selector Switch L1

FOR LS

R

L2

OFF

3 R 6

F

M

Fig. 5. Control for Jogging-Start-Stop All with Push buttons L1

FOR

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).