hampson, j 2013, electrotechnology practice: section 5

PowerPoint

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Section 5

Control circuits

Copyright ©2011 Pearson Australia (a division of Pearson Australia Group Pty Ltd) – 9781442523258/Hampson/Electrotechnology Practice/2nd edition

Ladder diagrams

Ladder diagrams are specialised circuit diagrams commonly used to illustrate industrial control systems.

They are called ‘ladder’ diagrams because they resemble a ladder, with two vertical parallel lines (rails) to represent the main power bus.

Ladder diagrams are only used on control circuits and not for the power circuits of the loads.


Ladder diagrams


Ladder diagrams

Drawing conventions

The unified rule for ladder diagrams is that the power flow is from left to right and the progression of events is from top to bottom.

All ladder diagrams are drawn in the de-energised state with the contacts drawn to operate in a clockwise direction.

Always draw coils, indicating lamps, and contacts to line up either vertically or horizontally.


Ladder diagrams

Line diagrams

Line diagrams are made up of two circuits, the control circuit drawn in ladder diagram form and the power circuit.

Electrical conductors in a line diagram are represented by dark lines.

Control-circuit wiring is represented by a lighter-weight line and power-circuit wiring is represented by a heavier-weight line.


Ladder diagrams


Ladder diagrams

Symbols

The symbols used for the circuit elements in a ladder diagram are taken from HB3:1996 Electrical and electronic drawing practice for students and AS/NZS 1102.102:1997 Graphical symbols for electrotechnical documentation series.


Ladder diagrams

There are basically two types of control devices.

One is a normally open (NO) contact and requires an action to close and complete the circuit.

The other is a normally closed (NC) contact and requires an action to open the circuit.


Ladder diagrams

A ladder diagram is a logical way to show how all of the elements of a control system operate.

The difficult part in reading a ladder diagram is that the different control contacts with respect to a control device may be spread out on different rungs of the ladder diagram.


Ladder diagrams

Manufacturers’ terminal numbers

Manufacturers of electrical contactors, relays, timers, counters, and so on include numbers on the terminal connection points.

These terminal numbers are used to identify and separate the different component parts (coil, NC, NO contacts, etc.) included on the individual pieces of equipment.


Ladder diagrams


Ladder diagrams

Manufacturers’ terminal numbers

The letters ‘K1/4’ indicate that the coil ‘K1’ controls four contacts: K1.1, K1.2, K1.3 and K1.4 (these are not on the contactor but would be found on a circuit diagram).

The terminals for the coil are marked ‘A1’ and ‘A2’.The mini contactor is a small, compact unit typically with 3 NO main contacts (power contacts) and, if attached, a built-in normally open (NO) or normally closed (NC) auxiliary contact.


Ladder diagrams


Control circuits

The term ‘control’, as applied to control circuits, is a wide-ranging term that means anything from a simple rocker switch to a complex system of devices (which may include contactor relays, contactors, timers, various types of switches and indicating lamps).

Every electrical circuit for light or power has control devices.


Control circuits


Control circuits

Contactors and contactor relays

An air break contactor is an electromagnetic-operated device, for repeatedly establishing and interrupting an electrical power circuit.

A contactor contains a set of main (heavy-duty) contacts for switching high-load currents at load rated voltages and may have a set of auxiliary contacts (light duty) for the control circuit.


Control circuits


Control circuits Note that an ac coil will draw more current when in

the unlatched state than when in the latched state. The coils designed for alternating current have relatively low resistance (R). This causes a high closing current when the coil is energised.

As the electromagnetic effect of the coil starts closing the armature, inductive reactance (XL) starts to build up (the air gap is getting smaller). The coil now has impedance (Z) which is the phasor sum of resistance and inductive reactance.

Maximum impedance (MZ) occurs when the armature has closed (smaller air gap in its magnetic path). MZ limits the holding current of the coil.


Control circuits

Contactors and contactor relays

The purpose of contactors and contactor relays is to latch and unlatch associated contacts and to provide no-volt and under-volt protection.

A letter or letter/number is used to designate the coil (e.g. ‘K1’); the associated contacts have the same identifying letter (e.g. K1.1, K1.2, etc.).


Control circuits

Coils require a holding-in voltage to ensure that the contacts remain latched.

If the supply voltage falls below the required holding-in voltage the contacts will unlatch.

This is what is meant by the term ‘no-volt protection’.


Control circuits


Control circuits

When the coil is energised the magnetic field developed within the armature reverses in unison with the ac current that is applied. As the electromagnetic field reverses it induces an e.m.f in the shading ring, causing a current to flow.

This current develops a secondary reversing electromagnetic field. This field is out of phase with that of the applied coil power and holds the armature faces continuously closed between coil current reversals and minimises electromagnetic noise.


Control circuits

A contactor relay is an electromagnetic-controlled device that opens and closes electrical contacts to effect the operation of other devices in the same or another circuit.

The contactor relay is suitable for switching small control or auxiliary circuit currents at low voltages.


Control circuits


Control circuits

Direct current coils

Instead of using an ac-type contactor or contactor relay it is sometimes preferable to use a dc-type electromagnet. The power in a dc coil is independent of the position of the armature. Direct current coils compensate for this condition; they contain three to four times as much copper as an ac coil and more powerful solenoids (an ac magnet is laminated steel, a dc magnet is made of solid steel) are used.


Control circuits

Utilisation categories

Utilisation categories are important because they help the systems electrician identify the correct contactor for a particular application.


Control circuits

The designation of the utilisation category is made up of three parts:

The prefix ac or dc, which indicates the nature of the current.

A one-digit number or a two-digit number, which indicates the type of application the contactor is designed for.

The suffix ‘a’ or ‘a’, which indicates whether the contactor is suitable for frequent (a) or infrequent operation (b).


Control circuits

Utilisation categories

The utilisation categories allow for initial selection of a contactor that can meet the demands of the circuit purpose.

As can be seen from the utilisation categories there are a number of ratings that can be applied to any given contactor.

The thermal or heating current is the maximum continuous current that the contactor can carry. The rating of the contacts within the contactor is a thermal rating.


Control circuits

Contactors designed for controlling motors are rated AC3 and are suitable for regular overloads of 600% of full-load current every time the motor is started. The contacts will heat more due to the increase in current during the overload (I2r) and so the continuous current is reduced to ensure that overheating does not occur during the maximum temperature of the contact material. The frequency of overload and the duration of the overload is a very important part of the AC3 rating. If there is a very frequent overload, the AC3 rating should be further reduced.


Control circuits


Control circuitsThe various combination of materials for the contact tips

results in different contact properties.

flame resistance

high resistance to arc erosion

high resistance to contact welding

cool contact surface with the arc extinguished automatically.

low contact resistance

self-lubricating behaviour

good electrical thermal conductivity


Control circuits

Thermal overload

Thermal overload relays prevent an electric motor from drawing too much current and overheating.

Thermal overload relays are three-pole bi-metallic devices


Control circuits


Control circuits

When selecting a thermal overload you need to know:

the full-load current rating of the motor

the overload setting range in amperes (fixed or adjustable)

whether there is or is not single-phase protection

suitability for two- or three-phase thermal sensing

whether resetting is automatic or manual

whether there is direct on contactor (piggy back) or independent mounting

direct sensing or CT type


Control circuits

Stop/start circuits

A starting device is a control device used primarily to start machinery and hold it in operation.

A stopping device is a control device used to shut down machinery, while an isolator is a device which holds and locks out the machinery from operation.


Control circuits


Control circuits

When choosing a pushbutton control device factors that should be considered include:

mounting requirements

number and type of contacts

current and voltage rating

colour of the button

illumination requirement

shape and physical size

IP rating

security requirements


Control circuits


Control circuits

Remote stop/start circuit

A set of start and stop pushbuttons may be located in a remote position from the controlled machinery in order to energise or de-energise it as long as all the start and stop buttons remain in series with the circuit control current flowing through them.


Control circuits


Control circuits

Emergency stop

Emergency stop circuits are one of the most common safety systems found in commercial and industrial installations.

Emergency stop buttons should be located within reach of any point where an operator of machinery is exposed to a hazard (e.g. cutting hazard).


Control circuits


Control circuits

Make-before-break

In some applications it is important to ensure one switch closes before the other switch opens.

A make-before-break switch has a movable contact that makes contact (shorting) with the next circuit before breaking the first circuit.


Control circuits


Control circuits

Break-before-make

A break-before-make switch is valuable in applications where it is undesirable to have two independent sources short-circuited while switching.


Control circuits


Control circuits

Limit switches

The function of a limit switch is to switch an electrical signal when physical contact occurs between the device to be detected and the operating head of the switch.


Control circuits


Control circuits

Temperature switch

The bi-metal temperature switch finds applications in temperature monitoring/control and signalling in cooling and heating systems, compressors and motors.


Control circuits


Control circuits

Pressure switch

Pressure switches are suitable for many liquid and gaseous mediums and are used in vacuum technology (pump control), refrigeration technology (compressor control), gas technology (stock monitoring, leak detection), filter monitoring (dirt detection), level measurement (overfill protection, dry-running protection), and is also used for a whole variety of measurement tasks in hydraulics and pneumatics, in machine construction and in building technology.


Control circuits


Control circuits

Dead man controls

The name derives from the concept that if the operator were to die, fall asleep, suffer a heart attack or become incapacitated the device the operator was controlling would stop.

The dead man system is an electrical control unit (handle, foot pedal and switch) designed to provide a means of switching off electrical equipment.


Control circuits

Jogging circuits

Jogging describes the repeated starting and stopping of machinery at frequent intervals for any short periods of time necessary for obtaining limited movement.

An electric motor would be jogged when a piece of driven machinery has to be positioned accurately.


Control circuits


Control circuits

Two-wire control

The term ‘two wire control’ obtains this description because only two wires are required to wire to the control device.

Two-wire control wiring is used for remote or inaccessible installations such as pumping stations, refrigeration plants and compressors.


Control circuits


Control circuits

Electrical interlocking

Electrical interlocking refers to the use of the normally closed auxiliary contacts of one relay in series with another relay coil to make certain that this second relay cannot be energised at the same time and vice versa.


Control circuits


Control circuits

Mechanical interlock

A mechanical interlock is used for horizontal interlocking of two contactors.

The mechanical interlock is either a front-mountable, top-mountable or side-mountable contactor accessory device that interlocks two contactors of the same frame size.


Control circuits


Control circuits

Timer-controlled circuit

A pneumatic timer (mechanical operation), timing relays (electrical operation) or electronic relays are designed to operate at a pre-set time interval after the coil is energised or de-energised.


Control circuits

A delay on energisation is also referred to as ‘on delay’.

On-delay timers actuate the contacts at a pre-set time after the coil is energised and reset when the coil current is removed.

A time delay on de-energisation is called an ‘off delay’.

Off-delay timers actuate the contacts instantly when the coil is energised and reset at a preset time after the coil is de-energised.


Control circuits


Control circuits

Other types of safety control circuits include through-beam light curtains (which act as an emergency stop) and safety door interlock switches.

These devices ensure that the machine operator will not be allowed to run the system if the control circuit experiences a detectable fault.

Pilot devices used in control circuits include float, temperature and pressure and limit switches.


Motor starter circuits

Motor starter circuit diagrams show the connections of both the power and the control circuits as clearly as possible with all conductors drawn neatly as straight lines.

The actual layout of the components is usually quite different from the circuit diagram.



There are two drawing conventions used for setting out motor starter circuit diagrams. They are:

horizontal layout

vertical layout



Two-hand control

For working environments where it is prudent for an operator to account for the position of both hands some control devices provide for two start switches.

This means that the controlled process is hazardous and its operation requires concurrent pressure from both of the operator’s hands during a substantial part of the machine process.


Motor starting circuits


Fault-finding techniques

Risk assessment

Prior to engaging in fault finding techniques conduct a risk assessment. This assessment should include:

Using PPE such as: fire rated safety clothing, safety footwear, safety glasses.

Check/secure area adjacent to work.

Check for working at heights

Check whether extra operative needed.



Risk assessment (cont.)

Consider risks and hazards in work area. Are relevant permits in place?

Be familiar with the component to be changed before carrying out any work.

Check availability of plans/drawings of systems.


Fault-finding techniques Hazards

falls from heights

eye injury

electrocution

electric shock

explosion.


Fault-finding techniques Harm

Burns or fatalities may arise from contact with a live conductor.

Back injuries, fractures to fatality from falls.

Eye injury to fatality may arise from explosion.


Fault-finding techniques Control measures

All electrical fault finding to be carried out by qualified tradesmen.

Be familiar with all safety and operating controls such as pressure switches, thermostats, time-delay relays, motor starters, motor overloads, etc.

Check faults in a planned and concise manner.

Isolate the supply as soon as is practically possible.

Start testing from the source of supply and work by a process of elimination.

Use insulated tools and ‘in service’ test equipment.



On completion of a job

Tidy up, remove and dispose of redundant materials.

Sign off any relevant permits.

Sign off job on completion.



Faults

The range of faults that could exist in a power and control circuit include:

lack of control circuit power

lack of supply power to the load

timer settings

overload settings

short-circuit faults

open-circuit faults



Faults (cont.)

low insulation resistance

high earth resistance

high-resistance joints (terminal connections)

contact deterioration (can be caused by machine vibration)

Check the voltage drop across the coil while it is energised to see that it is actually the specified voltage for that coil as delivered through all the control circuits and connections.



When fault-finding always use visual, sound, smell, and heat sensing where appropriate, observe timing of occurrences, operational sequence of contactor switching and any departure from normal operation. Symptoms are verified by direct observation.

All faults are located and diagnosed using logical, systematic analysis, supported by observation, measurements and testing. Finally the fault repair is confirmed by operational testing.


END

hampson, j 2013, electrotechnology practice: section 5

Education

ladder diagramscopyright

ladder diagram form

control circuits

specialised circuit

controlcircuit wiring

different control contacts

term control

electrical circuit