ELECTRICAL DRIVES

BEF35803

WEEK 1

BASICS OF INDUSTRIAL MOTOR CONTROL

LECTURER INFO

1) Mohd Aifaa bin Mohd Ariff

2) B. Eng. (UTM), M. Eng. (UTM), PhD (ICL), DIC

3) Room: D1-213 (By appointment only)

4) Contact Number: 017-7326074

5) Email: aifaa@uthm.edu.my

6) Notes: https://www.edmodo.com/ (Code: w3xsfg)

COURSE INFO

Assignment 1 : 10%

Assignment 2 : 10%

Group Project : 10%

Test 1 : 10%

Test 2 : 10%

Final : 50%

Total : 100%

1.0 INTRODUCTION

• The methods used to control the performance of an electrical system.

• Involves the starting, acceleration, reversal, deceleration, and stopping of a motor.

• This section covers the conventional electrical control of 3-phase AC motor

1.1 CONTROL DEVICES

• A control circuit is composed of a number of basic components connected together to achieve the desired performance.

• The basic components are the following:

1) Disconnecting switches

2) Manual circuit breakers

3) Cam switches

4) Pushbuttons

5) Relays

1.1 CONTROL DEVICES

6) Magnetic contactors

7) Thermal relay and fuses

8) Pilot lights

9) Limit switches

10) Other special switches

11) Resistor, reactors, transformer, and capacitors

1.1 CONTROL DEVICES

1) Disconnecting switches

To isolate the motor from the power source.

2) Manual circuit breakers

Opens and closes a circuit.

Figure 1.1 – Three phase fused disconnecting switch

Figure 1.2 – Three phase circuit breaker

1.1 CONTROL DEVICES

3) Cam switches

To control the motion and position of machine tools.

4) Pushbuttons

A switch activated by finger pressure.

Figure 1.3 – Three phase, surface mounted cam switch

Figure 1.4 – Mechanical-interlocked pushbuttons

1.1 CONTROL DEVICES

5) Control relays

An electromagnetic switch that operates when the relay coil is energized.

6) Thermal relay

A temperature sensitive device that operates when the motor currents exceed a pre-set limit.

Figure 1.5 – Single phase relay Figure 1.4 – Three phase thermal relay with variable current setting

1.1 CONTROL DEVICES

7) Magnetic contactors c

A large control relay designed to operate a power circuit.

8) Pilot lights

Indicates the state of a remote components.

Figure 1.7 – Three phase magnetic contactors

Figure 1.8 – Pilot light

1.1 CONTROL DEVICES

9) Limit switches

A low-power snap-action device that opens and closes a contact..

10) Other limit switches

Sensitive to pressure, temperature, liquid level, or direction of rotation.

Figure 1.9 – Limit switch with one NC contact

Figure 1.10 – Liquid level switch

1.1 CONTROL DEVICES

11) Proximity detectors

A sealed devices that can detect objects without coming in contact with them

Figure 1.11 – Proximity detector to monitor the loading of a conveyor belt

1.2 NORMALLY OPEN AND CLOSED CONTACTS

• Control circuit diagrams always show components in a state of rest.

• Contacts that are open when not energized are called normally open contact (NO).

• Contacts that are closed when not energized are called normally closed contact (NC).

Figure 1.12 – NO and NC symbols

1.3 RELAY COIL EXCITING CURRENT

• The relay coil is excited by a fixed AC voltage.

• The coil requires much higher magnetizing current in the open than in the closed contactor position.

• In case of magnetic contactor:

The magnetic circuit has longer air gap in the open position compared in the closed position.

• In case of AC contactor:

The inductive reactance is lower when the contactor is open than when it is closed.

1.3 RELAY COIL EXCITING CURRENT

Example 1.1

A three phase NEMA size 5 magnetic contactor rated at 270 A, 460 V possesses a 120 V, 60 Hz relay coil. The coil absorbs an apparent power of 2970 VA and 212 VA, respectively, in the open and closed contactor position. Calculate the following:

a) The inrush exciting current (Answer: 24.75 A).

b) The normal sealed exciting current (Answer: 1.77 A).

c) The control power needed to actuate the relay coil compared to the power handled by the contactor (Answer: 215120 VA).

1.4 CONTROL DIAGRAMS

A control system can be represented by four types of circuit diagrams:

1) Block diagram

2) One-line/Single-line diagram

3) Wiring diagram

4) Schematic diagram

1.4 CONTROL DIAGRAMS

Block diagram

• Composed a set of rectangles, each representing a control devices with a description of its function.

• Connected by arrows that indicate the signal flow.

Figure 1.13 - Block diagram of a combination starter

1.4 CONTROL DIAGRAMS

One-line/Single-line diagram

• A block diagram, except that the components are shown by their symbol rather than by rectangles.

• Yield more information.

Figure 1.14 - One-line diagram of a combination starter

1.4 CONTROL DIAGRAMS

Wiring diagram

• Shows the connection between the components.

• Taking into account the physical location of the terminals and the colour of wire.

Figure 1.15 - Wiring diagram of a combination starter

1.4 CONTROL DIAGRAMS

Schematic diagram

• Shows all electrical connections without their physical location or terminal arrangement.

• Indispensable when troubleshooting a circuit.

Figure 1.16 - Schematic diagram of a combination starter

1.5 STARTING METHODS

• A three-phase, squirrel-cage motors can be started by connecting them directly across the line.

• Across-the-line starting is simple and inexpensive.

• It has several disadvantages:

1) High starting current

2) Mechanical shock

• The fuses must be designed to carry the starting current during the acceleration period.

1.5 STARTING METHODS

• A motor control circuit contains two basic components:

1) A disconnecting switch

2) A starter

• Sometimes mounted together to make a combination starter.

• The fuses in the disconnecting switch are rated at about 3.5 times full-load current

• They do not protect the motor against overload.

1.6 MANUAL ACROSS-THE-LINE STARTERS

Manual 3-phase starters

• Composed of a circuit breaker and three thermal relays.

• All mounted in an appropriate enclosure.

• The thermal relays trip the breaker whenever the current in one of the phases exceeds the rated value for a pre-set time.

1.6 MANUAL ACROSS-THE-LINE STARTERS

Manual single-phase starter

• Built along the same principles with the 3-phase starters.

• Only contain one thermal relay.

Figure 1.17 - Manual starters for single-phase motors rated

1.7 MAGNETIC ACROSS-THE-LINE STARTERS

• Employed when a motor has to be controlled from a remote location.

• Also used when the power rating exceeds 10kW.

Figure 1.18 - Three-phase, across-the-line magnetic starter

1.7 MAGNETIC ACROSS-THE-LINE STARTERS

• The disconnecting switch is external to the starter.

• The starter has three main components: a magnetic contactor, a thermal relay, and a control station.

Figure 1.19 - Schematic diagram of a 3-phase, across-the-line magnetic starter

1.7 MAGNETIC ACROSS-THE-LINE STARTERS

1) The magnetic contactor

• Has three heavy contacts A and one auxiliary contact AX.

• Contacts A carry the starting current and the nominal full-load current.

• Contact AX carries the current from relay coil A.

1.7 MAGNETIC ACROSS-THE-LINE STARTERS

2) The thermal relay

• Protects the motor against sustained overload.

• Consists three individual heating elements, connected in series with the three phases.

• A small NC contact opens when the thermal relay gets too hot and stays open until the relay is manually reset.

1.7 MAGNETIC ACROSS-THE-LINE STARTERS

3) The control station

• Composed of start-stop pushbuttons

• To start the motor:

1) Close the disconnecting switch.

2) Depress the start button to energize Coil A, causing contacts A and AX to close.

3) The full line voltage appear across the motor.

• To stop the motor:

• Push the stop button, which open the circuit and the coil