MOTOR CONTROLS Contactor, Relays and Overloads
UNIT OBJECTIVES After studying this unit, the reader should be able to
• Describe the differences between relays, contactors and starters •List the component parts of contactors and starters • Name two types of external motor overload protection • Explain the differences between external overload protection devices • Explain the conditions under which motor safeties can be reset
What is a Switch? • A switch is a device that allows you to stop the flow of current entirely. These are usually mechanical devices that separates two bits of metal (contacts). When the metal doesn't touch, current doesn't flow. When the metal touches, is called a closed circuit. When the metal doesn't touch, is called an open circuit. (closed = ON, open = OFF)
Basic Switch Schematic Symbol S1
Single Pole, Single Throw = SPST A simple on-off switch. This type can be used to switch the power supply to a circuit.
Circuit Symbol
Example
SPST toggle switch
Single Pole, Double Throw = SPDT • This switch can be on in both positions, switching on a separate device in each case. It is often called a changeover switch. For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in the other position. Circuit Symbol
Example
SPDT rocker switch
Double Pole, Single Throw = DPST •
A pair of on-off switches which operate together (shown by the dotted line in the circuit symbol).
Circuit Symbol
Example
DPST rocker switch
Double Pole, Double Throw = DPDT •
A pair of on-on switches which operate together (shown by the dotted line in the circuit symbol).
Circuit Symbol
Example
DPDT slide switch
Push Button Switches (NO/NC) • A Normally Open (NO) switch returns to its normally open (off) position when you release the button. Circuit Symbol
Example
Push-to-make switch
• A Normally Closed (NC) switch returns to its normally closed (on) position when you release the button. Circuit Symbol
Example
Push-to-break switch
INTRODUCTION TO MOTOR CONTROL DEVICES •
Relays, contactors and starters pass power to the motor by closing sets of contacts
•
Explain why the locked rotor amperage (LRA) affects the choice of a motor starter Contacts controlled by coils in the control circuit Starting relays are only in the active circuit for a short period of time The type of motor control used is determined by the size and application of the motor used
• • •
Contactors Contactors are relays that switch high current loads a.k.a magnetic starters
Manual Motor Starter
L1
L2
RELAY OR CONTACTOR
CONTROL CIRCUIT
MOTOR RUN START
START RELAY
L1
L2
RELAY OR CONTACTOR
CONTROL CIRCUIT
MOTOR RUN START
START RELAY
MOTOR AMPERAGES •
•
•
Running load amperage (RLA) – Similar to full load amperage (FLA) – Amperage drawn by the motor while operating Locked rotor amperage (LRA) – Amperage drawn by motor on startup – Five to seven times greater than RLA or FLA Both LRA and RLA must be considered when choosing a control device
Contactor, Relays and Overloads Upon completion of this chapter the student will be able to: • • • • • • • • • •
Explain the parts and operation of contactors and relays. Explain the application of contactors and relays in control systems Correctly install a contactor or relay in a control system Draw a simple schematic wiring diagram using contactors and/or relays to control load in a control system. Understand the types and application of overloads Troubleshoot contactor and relays. Identify the common types of overload used to protect loads. Explain the operation of the common overloads. Determine the best type of overload for a specific application. Draw schematic wiring diagrams using the proper overload to protect loads.
Key Terms • • • • • • • • • • • • • • • •
Coil Contactor Contacts Current Overload Fuse Inductive Load Internal Compressor Overload Line Break Overload Magnetic Overload Magnetic Starter Mechanical Linkage Overload Pilot Duty Overload Push-Button Station Relay Resistance Load
Relays •
Relays are used to open and close a circuit to allow the automatic control of a device or circuit.
THE RELAY • Uses a magnetic coil to open or close one or more sets of electric contacts • The most common control voltage for both relays and contactors is 24 volts. • Relays are not repaired. Replace on failure. • Used for light duty applications • Can be used as a pilot-duty relay • The relay contacts must be able to handle the amperage draw of the load being controlled • Pilot relays are designed to switch on and off larger contactors or starters. They are very light duty and are not designed to start motors directly.
NORMALLY OPEN CONTACTS
COIL
NORMALLY CLOSED CONTACTS
Relay Hysteresis •
Relays and contactors naturally tend to provide a differential gap for On-Off control because of the hysteresis effect inherent in their operation. To cause a magnetic relay to energize: – The coil current must rise beyond the pull-in current or pick-up current Once a relay has been pulled in: − the coil current must drop below a certain value called the hold-in current or drop-out current.
Why is there a need for a strong magnetic field to establish to pull the armature?
1. There is an air gap in the magnetic loop; this cause the magnetic field to be weaker. 2. The attractive force between the core and armature (opposite magnetic poles) is weak because of the distance between the poles.
Relay Logic
Basic Relays • A relay is an electronic control switch used to open or close a mechanical contact whenever a voltage is applied to its coil.
Basic Relays • A basic relay usually contains two sets of contacts, normally open and normally closed. The schematic will show the normal contact position for a de-energized coil. Normally Open Contact
Normally Closed Contact
1
2
1
2
3
4
3
4
Relay Labeling • •
Always look at the relay case to identify the coil, coil voltage, max current, NO and NC contacts. Labeling is always shown in a de-energized state.
NC Common
Coil
NO
Contacts
V2 12V J1
1
2
Key = Space 3
4
R1 1.0kohm
V1 48V
1
2
3
4
R2 1.0kohm
1
2
3
4
1
2
3
4
LED1 LED_red
LED2 LED_red
Simple Relay Logic Ckt.
Which LEDs will illuminate in this circuit with switch 1 open?
R3 1.0kohm
LED3 LED_red
R4 1.0kohm
LED4 LED_red
With switch 1 closed?
NAND – Burglar Alarm
Relay Logic? (AND, OR, NAND, NOR)
Relay Logic? (AND, OR, NAND, NOR)
Complete the ckt (AND gate)
Complete the ckt (OR gate)
Complete the Truth Table
Complete the truth table
Complete the truth table
Relay Timers •
ON Delay
•
OFF Delay
Wired ON Delay OFF
X1
X2
NO ON
NC
1
1.
Energy applied to power rails
Wired ON Delay - NCTO ON
X1
X2
1. 2.
NC ON
NC
2
Energy applied to power rails Start PB is pressed - Coil is energized - Holding contact close - Timer contact stays closed, lamp stays on. - Count begins (5 sec)
Wired ON Delay - NCTO ON
X1
X2
3. Timer count ends - Coil is still energized - Timer contact open - lamp goes off.
NC NO
The
OFF 3
4. Timer contacts remain open until the coil is deenergized
Normally Closed contact will take 5 seconds To Open when the coil is energized.
X1
OFF
ON Delay - NOTC
NO
X2
1.
Power is applied to rails
OFF
1 The
Normally Open contact will take 5 seconds To Close when the coil is energized.
X1
X2
ON
ON Delay - NOTC
2.
Start PB is pressed – –
NC OFF
– – –
Coil energizes Holding contacts close Timer contacts stay open Lamp stays off Counter starts to count (5 sec)
2 The
Normally Open contact will take 5 seconds To Close when the coil is energized.
X1
X2
ON
ON Delay - NOTC
3. Counter finishes count – – –
NC NC
Coil stays energized Timer contacts close Lamp goes on
ON
4. Timer contacts will open when relay coil is de-energized.
3 The
Normally Open contact will take 5 seconds To Close when the coil is energized.
OFF Delay - NCTC 1. Power is applied to rails 2. Coil is off, contacts are closed, lamp is on
The timer contacts will close 5 seconds after the coil is de-energized
OFF Delay - NCTC 3. Start PB is pressed 4. Timer contacts open 5. Counter will start to count only when coil is de-energized.
OFF Delay - NOTO 1. Power is applied to rails 2. Coil is off, contacts are closed, lamp is on
OFF Delay - NOTO 3. Start is pressed. 4. Contacts close, lamp on 5. Counter only starts when coil is de-energize
Relay Applications •
Relays can be used to control indoor fan motors, condenser fan motors, damper motors, starting capacitors, and control lockouts.
THE CONTACTOR • Larger version of the relay • Has movable and stationary contacts • Often times only one set of contacts opens which opens only one side of the power to condensing units. This provides power for off-cycle heat to the compressor. • Holding coils are rated at different voltages • Can have one or more sets of contacts • Some are equipped with auxiliary contacts, which are usually rated at a lower amperage than the primary contacts. • Contacts and coils can be replaced, the material most used on the contacts is silver. • Use the exact replacement whenever possible
STATIONARY CONTACTS
MOVABLE CONTACTS AND ARMATURE
COIL CONNECTIONS
HOLDING COIL
STATIONARY CONTACTS
The most common coil voltage is 24 volts.
WHEN THE COIL IS ENERGIZED, THE CONTACTS ARE PULLED CLOSED
Contactors • •
A contactor is used to control an electric load in a control systems. Contactors make or break a set of contacts that control the voltage applied to some load in cooling systems.
Contactors
Coils • • •
Coil Characteristics depends on the type of wire and the manner in which it is wound. Coils are typically designed to operate on 24 volts, 120 volts, 208/240 volts and occasionally 480 volts. The coil is identified by the voltage marked on it.
Contacts • • • • •
The contacts of a contractor make a complete circuit when the contactor is energized, allowing voltage to flow to the controlled load. Contractors are rated by the ampere draw they can carry. There are two types of loads that a contractor can control: an inductive load, and a resistive load. Contacts are made of silver and cadmium which resists sticking. The chemical composition of contacts is such that they operate at cool temperatures of up to 125% of their current-carrying capacity.
Contacts •
The contactor contacts must be in good condition to ensure that proper voltage reaches the load.
Overloads • • • •
An overload is an electrical device that protects a load from a high ampere draw by breaking a set of contacts. The simplest form of overload protection is the fuse. Fuses can be used to protect wires and non-inductive loads, but they provide inadequate protection for inductive loads. A load that is purely resistive in nature with no coils to cause induction is called resistive or non-inductive load.
Over Load Protection
Fuses •
Fuses consists of two ends or conductors with a piece of wire that will melt and break the circuit if the current passing through it exceeds the amperage rating of the fuse.
Line Break and Pilot Duty Overloads • • •
Overloads can be divided into two basic groups: Line break and Pilot duty. The line break overload breaks the power to a motor. A pilot duty overload breaks an auxiliary set of contacts connected in the control circuit.
Line Break Overload • •
One of the most common types of line voltage overloads is the metal disc mounted between two contacts. This is called a bimetal line break overload.
Internal Compressor Overload • • •
The most popular line break overload for use in small central residential system is an internal compressor overload. The internal compressor overload is a small device inserted into the motor windings. This overload can sense the current draw of the motor, as well as the winding temperature, more effectively than external overloads.
Internal Compressor Overload
Pilot Duty Overload • • •
The pilot duty overload breaks the control circuit when an overload occurs, which would cause a contractor to be de-energized. This type of overload is common on larger systems and still exists on smaller systems currently in the field. Two basic pilot duty overloads are being used in the industry today: – The current overload – Magnetic overload
Current Overload •
Works similarly to the line break overload excepts that a pilot duty set of contacts is opened rather than the line.
Magnetic Overload • • • • •
Consists of a movable metal core in a tube filled with silicone or oil. Surrounding the metal tube is a coil of wire. When the current increases, so does the magnetic field of the coil. The overload operates by the magnetic field created by the coil. The device is designed to create a magnetic field that is strong enough to pull the coil up, opening the pilot contacts on overload.
Magnetic Overload
