Test 3 Flashcards
Motor Starter Size
Motor starters are typically larger than contactors and control relays
Motor Starter Auxillary Contacts
Starters often feature auxiliary contacts for additional control functions
Motor Starter Overload Protection
- Uses overload heaters placed in series with each phase of line voltage.
- Senses electrical overload and breaks continuity to shut down the motor
Manual Motor Starter Switch is…
Operated by hand
Manual Motor Starter Advantage
Remains on after power failure; motor restarts automatically
Manual Motor Starter Disadvantage
Auto restart can be dangerous as equipment (lathes, drills, presses) may harm people
Manual Motor Starter Low Voltage Protection
- Prevents automatic restart after power loss.
- Uses a continuous-duty solenoid:
- Energized when line voltage is present and start button is pressed.
- If power is lost, solenoid de-energizes, opens contacts.
- Contacts won’t reclose when power is restored; must be manually reset.
Overloads/Heaters Purpose/Function
- Motor cannot detect overloads, so heaters prevent it from burning out.
- Overloads or heaters are placed on each phase to monitor current draw and trip the motor before damage.
Overloads/Heaters Provide…
No short-circuit protection
Main Overload Types
- Melting Alloy Thermal Unit
- Bimetallic Overload Relay
- Magnetic Overload Relay
Melting Alloy Overload Consists of…
- Heater element
- Solder pot
Melting Alloy PO
- Excess current melts the solder, releasing the ratcheted wheel.
- The wheel trips normally closed contacts, stopping the motor.
Melting Alloy in 3 Phase
- All 3 contacts are in series.
- Motor stops if any phase detects excessive current.
Heaters are interchangeable based on…
Horsepower/current rating
Melting Alloy Overload Resetting
Overload can be reset after solder cools, but motor must be restarted
General Purpose Enclosures (NEMA-1)
Construction: Made of sheet steel.
Purpose: Prevent accidental contact with live parts.
Features:
- Latches with padlock provisions (Figure 3-37).
- Designed for indoor use in normal conditions.
- Provides protection from light splash, dust, and falling debris (e.g., dirt).
Bimettalic Overload Relay Components
- Bimetallic Strip
- Built-in heating element
Bimettalic Overload Relay Functions
- As current rises, temperature increases.
- Bimetallic strip bends, breaking a normally closed contact.
Bimettalic Overload Relay Resetting
Automatic reset after cooling; contact closes
Bimettalic Overload Relay Notes
- Not recommended for two-wire control in automatic restart.
- Adjustable to different settings.
Magnetic Overload Relay Components
- Coil of Wire
- Current Transformer
Magnetic Overload Relay Function
- Coil creates a magnetic field based on current flow.
- Current transformer detects changes in load current (rise or fall) and raises or drops them
Magnetic Overload Relay Use
Primarily used to protect very large motors
Magnetic Overload Relay Notes
- Protects by sensing current intensity, not heat.
- Requires a timing device to prevent instant tripping.
