Notes Electric Workshop

8/12/2019 Notes Electric Workshop

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1.1 Components in Control Circuit

1.1.1 Biased Switch / Push Button

- Push button is used as a control element in Control Circuit, to manually cut

the power supply.

- Symbol

Stop Start

- Two distinct colors for identification. Red indicates Push Button Stop whereas

Green indicates Push Button Start.

- To operate the switch, an external force must be exerted to the button. It

returns to normal position when there is no more force exerted.

1.1.2 Latching Switch- Latching Switch is used as a control element in Control Circuit, to manually

cut the power supply. The contact is mechanically latched closed.

- Symbol

LatchingSwitch

- Two distinct colors for identification. When the red colored switch is pressed,

it will cut the power supply. When the green colored switch is pressed, it will

close its contact and allowing current through it.


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1.1.3 Relay

- A relay is used to isolate one electrical circuit from another. It allows a low

current control circuit to make or break an electrically isolated high current

circuit path. The basic relay consists of a coil and a set of contacts. The most

common relay coil is a length of magnet wire wrapped around a metal core.

When voltage is applied to the coil, current passes through the wire and

creates a magnetic field. This magnetic field pulls the contacts together and

holds them there until the current flow in the coil has stopped. The diagram

below shows the parts of a simple relay.

1 2 3/4

R5/7

orNormally Closed conta

Relay Coil3/4 6/8

Normally Opened conta

1.1.4 Contactor

- Behaves as a magnetic switch. It will either allow or cut the power supply to

electrical instruments. The operation is not much different from relay but

bigger in size. It has higher current ratings which allow various power rating

applications and control.

- Symbol:

CContactor Coil

Contactor Contact (Normally Opened / N.O)

Contactor Contact (Normally Closed / N.C)


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- Contains a Coil and Contacts.

- There are two types of contact:

Main Contacts Normally opened contact type with high current

rating (30A ~ 40A). Behave as an electrically latched switch in a

main circuit.

Auxiliary Contacts Have a lower current rating (5A). Behave as

an electrically latched switch in a control circuit. Divided into two

types; Normally Opened and Normally Closed contacts and could

be added according to the application needs.

1.1.5 Timer Relay

- Timer relay consists of a coil, contacts and time adjustable knob.

- When current flows through the coil, it becomes electromagnet. When it

reaches a preset time interval, the induced magnetic field will attract the

contacts.

- It has two types of contacts; normally opened and normally closed contacts.

- Symbol:

T Timer Coil

Timer Contact (Normally Opened / N.O)

Timer Contact (Normally Closed / N.C)


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1.1.6 Thermal Over load Relay

- To protect electrical motor and main circuit from over load current / over

current.

- An over current imposed high current through overload relay. This current

exceed rating limit and hence, operate contacts.

- The tripping mechanism is made of a bi-metal. The bi-metal consists of two

metal strips of different temperature coefficients of expansion which are rolled

one on the other. It deflects when heated by the current flowing through it

which depends on the current magnitude and duration.

- Symbol:

Normally Open Contact (N.O)

Normally Close Contact (N.C)

1.1.7 Circuit Breaker

- Miniature Circuit breaker (MCB) will cut off the circuit whenever the currentexceeds its rated value. Normally, it will take one to two hours to trip if the

current passing through it is 130% to 145% of the rated current (BS EN

60898:1991).

- Tripping mechanism is similar as Thermal Over load Relay but with an

additional mechanism called electromagnetic trip for handling tremendous

inrush current.

- Both 3-phase MCB and 1-phase MCB will be used for a control circuit and a

main circuit respectively

- Abbreviation: MCB SPfor single phase MCB

MCB TPfor three phase MCB

- Symbol:

Circuit Breaker


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1.1.8 Indicator Lamp

- As an indicator to indicate the state of operation of the circuit.

- Green or yellow indicates that the circuit is running in a normal mode.

- Red indicates that there is an overload or tripping in the circuit. In this

situation, the main circuit has been cut off from the main power supply.

- Symbol:

Indicator Lamp


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1.2 Electrical instruments for trouble shooting

1.2.1 Digital or analog Multimeter.

- The functions of Multimeter are as follows:

i. To measure DC Volts, AC Volts

ii. To measure DC Current, AC Current

iii. To measure Resistance

iv. To measure Continuity

v. To test Diode

vi. To measure Capacitance and Inductance

vii. To measure Frequency

- Below is a specification of a Multimeter Sanwa CD800a

DC VOLTS 400mV- 600V

AC VOLTS 4V-600V

DC CURRENT Max 400mA

AC CURRENT Max 400mA

RESISTANCE 400, 4k,40k, 400k, 40M

CONTINUITY Audible indication: 10-120 normal

CAPACITANCE 50nF, 500nF, 5F, 50F,100F

INDUCTANCE

FREQUENCY 5Hz, 50Hz, 500Hz, 5kHz, 50kHz,100kHz

TRANSISTOR


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- Start-up/Inspection procedure

- Voltage measurement

i. Set the function/range switch to desired voltage range

ii. Select AC/DC fortype of voltage range

iii. Connect the red and black test probes to the measured circuit


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iv. If unknown voltage, set switch to the highest range until the

satisfactory reading is obtained

v. For DC, ensure the correct polarity

vi. After measurement, release the probes.

- Frequency and duty cycle measurement

i. Set the function switch at Hz/% function

ii. Apply the probes to the measured circuit and read the display

- Resistance and continuity measurement

i. Set the function/range to the desired resistance or continuity position


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ii. Remove power supply from the circuit under test

iii. Connect probes to the test point

iv. Resistance: Value will be displayed

v. Continuity: Beeper sounds continuously

- Capacitance measurement

i. Set the function/range to the desired capacitance position

ii. Remove power supply from the circuit under test

iii. Connect probes to the test point

iv. Capacitance value will be displayed


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- Current measurement

i. NEVER apply voltage to the input terminalsii. Be sure to make SERIES connection to load

iii. Set the function at mA and select either AC or DC

iv. Apply the probes to the measured circuit in series with load

v. If unknown current, set switch to the highest range until the

satisfactory reading is obtained

vi. For DC, ensure the correct polarity

vii. After measurement, release the probes.

- Safety precautions

i. Turn off power supply for certain measurements


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ii. Use caution when working above 60vdc or 30vacrms

iii. Dont measure voltage exceeding the limits

iv. Keep your finger behind the finger guard on the probe

v. Do not use the meter if the meter or test probes is damaged

- Diode measurement

Clamp Tester

- The functions of clamp tester are as follows:

i. To measure AC Volts

ii. To measure AC Current

iii. To measure Resistance

- Below is a specification of a clamp tester model Kyuritsu

- Test Current - Frequency - Accuracy

- AC 200A- 50-60Hz - 1.5% + 4dgt

- 40-1kHz - 2.0% + 5dgt

- AC 600A

- 50-60Hz - 1.5% + 3dgt

- 40-1kHz - 2.0% + 5dgt

- Current measurement

i. Set the range switch to desired position

ii. Press trigger to open transformer jaws


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iii. Clamp onto a conductor

iv. Keep the transformer jaws fully closed

v. Read and record reading

- Safety precaution

i. Never use on a circuit above 600VAC

ii. Do not apply at a current above 600A. It will damage the instrument

due to overheat

iii. Do not open battery compartment cover when make measurement

iv. Remove test leads when make current measurement

1.2.2 Insulation tester

- The function of this tester is to measure insulation resistance level between:

i. Live conductor to other live conductor

ii. Live conductor to earth

iii. Live conductor to neutral

iv. Equipment insulation

- Below is a specification of an insulation tester model Kyuritsu model 3165

-

Test Voltage 500VDC 1000VDC

Measuring Range 0-200M 0-200M

Terminal Voltage 500V +20%, -0% from 0.5M to 1000V +20%, -0% from

1M to

Output Short Circuit

Current1.4mA 1.6mA

Usage

General purpose

Natural installation

Electronics components

High voltage installation

Acceptable Insulation

Resistance>1M >1M


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- Insulation measurement

i. Always off the circuit under test

ii. Connect the crocodile clip:

iii. BlackEarth

iv. RedPhase, Neutral

v. Press test button

vi. Power On lamp will light

vii. Read and record the reading

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