Components & SymbolsUsed in Electrical Circuits

Schematics & Notations

• Understanding drawings, especially electrical drawings requires a keen understanding of schematics, symbols & other details used to represent components.

• Continual practice & a habit of reading the starter Panel drawing before attempting to troubleshoot it is the key to comprehending Electrical Starter Panels & machinery in general

Schematics & Notations

• The bibliography of starter Panels on board generally contains vital information as regard how the maker has represented various components of the starter or panel internals.

• Due reference to this is a must though generally parallels may be drawn form experience.

ContactsNormally Open Contact

Normally Closed Contact

Normally Open Contact with terminals

Normally Closed Contact with terminals

Start Push Button ( NO Contact )

Stop Push Button ( NC Contact )

Relays & Coils

Relay coil / Contactor Coil

Timer Relay Coil ( on Delay )

Timer Relay Coil ( off Delay )

Other Components

Operated by pushing

Operated by turning ( Latch )

Emergency Switch ( Mushroom Head Safety Feature )

Operated by pushing ( & then latches on )

Operated by turning ( spring return )

Other Components

Potential Transformer / Control Transformer

Secondary winding has less turns and Primary has more turns. Used for connecting volt meters

Current Transformer

Secondary has more turns and Primary has less turns. Used for connecting Amp meters.

Indicating Lamp ( Transformer Type )

Other Components

Heater

Device fitted outside the panel ( indicated by dashed lines )

SV Solenoid Valve

THE RELAYS

A very important component in an Electrical Circuit.

A relay is a simple electromechanical switch made up of an electromagnet and a set of contacts. Relays are found hidden in all sorts of devices. In fact, some of the first computers ever built used relays to implement.

Relay ConstructionRelays are amazingly simple devices. There are four parts in every relay:1.The Electromagnet2.The Armature that is attracted by the electromagnet3.A Spring4.A Set of electrical contacts

Relays

Factors when looking at a relay

•Coil terminals

•Number of switching contact sets

•Rating of the coil

•Rating of the contact

•Structure of the pin layout

Relay representation

Coil

N/O Contacts

N/C Contacts

N/O are called ‘a’ contacts

N/C are called ‘b’ contacts

This relay represented has a coil which operates on ac voltage & has 2 N/O contacts & two NC contacts.

Many such possibilities may be presented depending upon the number of contacts

220 V ac

Relay Format

The coil is found between terminals 2 & 10. Contacts 1 & 4 and 8 & 11 and 5 & 6 are normally closed.

When the coil is energized the contact switch over to 1 & 3 ( closed ) & 11 & 9 and 6 & 7 (closed).

Relay Format

1

2

34 5

6

78 The coil is found between

terminals 2 & 7. Contacts 1 & 4 and 8 & 5 are normally closed.

When the coil is energized the contact switch over to 1 & 3 ; & 8 & 6

8 pin format

Analyzing the requirement Timer Relays

A delay timer that waits for a predetermined amount of time before closing a set of contacts once the power is applied.

Terminals 2 & 7

Terminals 1 & 3

Terminals 1 & 4

Coil

Set of contacts 1

Set of contacts 2

T

Analyzing the requirement Timer Relays

Relay that closes/switches on a set of contacts immediately when a coil is energized, then waits for a pre determined time to switch them off/open after the power is removed.

Terminals 2 & 7

Terminals 1 & 3

Terminals 1 & 4

Coil

Set of contacts 1

Set of contacts 2

T

Contactor - Example

Contactor

Main Contacts or

power contacts

Auxiliary contacts

Starter Routines

Motor starter panel routines includes inspection and maintenance of motor’s starter panel.

Starter panel routines mean inspection and cleaning of contactor’s (the switch inside the starter panel box controlling the on and off of the motors) contacts.

Checking of connections in the starter panel

Cleaning of the complete starter panel

Checks in the terminal box on the motor for looseConnections

Visual inspection of overall starter panel

Safety Measures to be Followed While Starting Motor Starter Panel Routines

• Switch off the main power supply from the circuit breaker• Take out the main fuses in the starter panel, and if required control fuses

as well• Put the lock out tag• Inform the engineer who is in charge of that particular machinery

Fixed Contacts

THE FIXED AND MOVING CONTACTS

EFFECTS OF EXCESSIVE USE OF CONTACTS

Thermal Overload Relay

Heater coils of the thermal overload Relay

Contact operated by the overload relay with resetting option.

NC NO

Reading Diagrams

• Diagrams are always drawn in a state of rest i.e. without power – also known as the de-energized state.

• They are read from left to right.

• Generally devices with the same power level are drawn at the same horizontal level.

• The components are identified by two ways, Cartesian system ( co-planar ) or by a line diagram system.

• If the diagram runs into a few pages then reference to pages is made near the component.

MAGNETIC CONTACTOR OPERATION

Star Delta Starter

QUERIES ?

There are two other differences between an AC and DC solenoid: 

1) The iron core of the coil, the electromagnet, is solid for DC and layered for AC to avoid loss in iron due to eddy current. 

2)The AC solenoid has a "split pole" - a part of the layered pole (around 30% of cross sectional area) has a copper ring as short circuit so that in this part of the magnet the maximum and minimum of magnetic intensity is phase shifted by 90° in respect to the main part of the pole, and therefore avoiding chattering of the solenoid by having a total magnetic force that is always well above zero when actuated. 

Comparison of AC vs. DC Solenoids

As a general rule, DC solenoids are typically preferred over AC solenoids for several reasons. For example, when space constraints are tight, a DC solenoid will usually give better performance in a smaller package than will a comparable AC solenoid. In addition, DC solenoids have the capability of being modified in such a fashion as to prevent the pole faces from making contact at the end of the stroke. In AC solenoid applications, this type of noise dampening would very likely cause premature overheating and failure. AC solenoids require great care to ensure precise alignment of the plunger's pole face to the stator pole face. This facilitates the pole faces making contact with as much surface area as possible in the energized state which will reduce the amount of hum or chattering in the AC unit. However, in a similar DC application, a slight gap between the pole pieces at the end of the stroke can have a drastic effect on improving the overall life of the solenoid.

When looking inside the AC solenoid stator cavity at the stator pole piece, there is a small ring inserted into the face of the stator pole. This ring is known as a shading coil and is designed to obtain minimal pulsing in force. What this means is that the coil splits the pole into two separate parts which causes the flux of these parts to be out of phase. If not for this shading coil, the chattering sound that is commonly associated with AC solenoids would be more noticeable.

Bottoming out of the pole pieces during each stroke is a requirement on AC solenoids. Naturally, with this metal-to-metal contact, some deformation of the pole pieces is possible. As the pole pieces deform and less surface area makes contact between the pieces, the AC solenoid hums louder and louder. In contrast, a DC solenoid does not have to have the pole pieces come in contact during each cycle. Rather, if the pole pieces do not make contact, life of the solenoid is extended. Life of an AC solenoid is typically lower than that of a similar DC counterpart.