EC (Motor Starters) Ch 14 Flashcards
(14 cards)
Why are overload relays used in motor starters?
Overload relays protect motors from excessive current over time, preventing overheating and damage by disconnecting the motor during prolonged overcurrent.
How does the normally closed overload relay contact deenergize the motor starter coil?
The NC overload contact is wired in series with the starter coil. If an overload occurs, the contact opens, cutting power to the coil and deenergizing the motor.
What effect does high ambient temperature have on overload relays?
High temperatures can cause thermal overload relays to trip prematurely, leading to nuisance tripping even if the current is within limits.
Why should the overload relay be trip-free?
A trip-free design ensures the relay will trip regardless of the condition of the reset mechanism, enhancing safety.
Draw the basic power and control circuit for a magnetic full-voltage reversing motor starter.
Power Circuit: L1 L2 L3 → F1 F2 F3 → M1 M2 M3 → T1 T2 T3 → Motor; Control Circuit: L → [Stop] → [Forward PB] → [NC Rev Aux] → (Fwd Coil); [Reverse PB] → [NC Fwd Aux] → (Rev Coil).
How is reversing achieved in an AC induction motor?
Reversing is achieved by swapping any two phases, typically T1 and T3, causing the motor’s magnetic field to rotate in the opposite direction.
What torque will be developed at 65% starting voltage?
Torque is proportional to the square of voltage: T = (0.65)² × 100 = 42.25% of full-load torque.
How does a resistor-type reduced-voltage starter work?
Series resistors are added to drop voltage at motor start. After starting, the resistors are shorted out, restoring full voltage.
What is the difference between no-voltage release and no-voltage protection?
No-voltage release requires manual restart after power loss; no-voltage protection detects power drop and disconnects motor to prevent damage.
What is jogging in motor control?
Jogging means quickly energizing the motor starter to produce short bursts of motion for positioning or alignment.
What protection does a solid-state reduced-voltage starter use for overcurrent heating?
Thermal sensors monitor internal temperature and shut down before damage occurs.
How is voltage controlled in solid-state reduced-voltage starters?
Voltage is controlled through the conduction of SCRs, which phase-control the voltage waveform to gradually ramp up voltage at startup.
What is the difference between open and closed transition in reduced-voltage starters?
- Open Transition: Disconnects motor briefly during switching, causing voltage dips or torque surges. * Closed Transition: Keeps motor connected during transition, avoiding voltage drop for smoother operation.
