Flashcards

DC Machines, AC Motors and Motor Starters & Control Revision Flash Cards

Oliver Balay English 17 cards

Cards in this deck (17)

1DC Machine Basic Principle: What key law explains how a DC machine works (both motor & generator)?Electromagnetic Induction (Faraday's Law) — a conductor cutting magnetic flux induces an e.m.f., and current in a magnetic field produces force/torque.
2DC Motor Operation: How does a DC motor produce rotation?Current through the armature in a magnetic field produces force (F = BIL), generating torque; the commutator keeps torque in one rotational direction.
3DC Generator Operation: How does a DC generator create electricity?Mechanical rotation moves armature conductors through a magnetic field, inducing an e.m.f. which the commutator converts to DC.
4DC Motor Types: What are the 3 main DC motor types and their characteristics?Series – very high starting torque, variable speed.
Shunt – good, almost constant speed.
Compound – mix of both, used for lifts/elevators.
5Back EMF: What is back e.m.f. in a DC motor?A voltage generated in the armature that opposes the supply — it regulates motor speed and current.
6DC Machine Common UsesBattery‑powered tools
Portable appliances
Small lifting equipment (DC motors)
Emergency lighting charging systems (generators)
HVAC actuators and control motors
7AC Motor Basic Principle: What is the fundamental principle behind all AC motors?A rotating magnetic field (RMF) is produced in the stator and induces current/torque in the rotor.
8Three‑Phase Induction Motor: Why are 3‑phase induction motors self‑starting?A naturally produced three‑phase rotating magnetic field induces rotor currents, creating torque automatically.
9Squirrel Cage Rotor: What is special about squirrel‑cage rotors?They are robust, simple, low‑maintenance, and operate at nearly constant speed.
10Wound Rotor Machine: What advantage does a wound‑rotor induction motor have?External resistances can be added for high starting torque and speed control.
11Single‑Phase Motor Issue: Why is a single‑phase induction motor not self‑starting?A single‑phase supply only produces an alternating field, not a rotating one. Phase‑shift methods (capacitors, shading coils) create starting torque.
12Capacitor‑Start Motor: What does the capacitor do in a capacitor‑start motor?Creates a phase shift to generate a rotating magnetic field for high starting torque.
13Shaded‑Pole Motor: What is the operating principle of a shaded‑pole motor?A copper shading ring delays flux in part of the pole, producing weak rotation — simple, cheap, low torque.
14AC Motor Common Uses: Where are AC motors most commonly used in electrical installation?- Fans & extractors
- Pumps (heating, water boosting)
- HVAC units
- Conveyor systems
- Workshop machinery
- Boiler fans & circulation motors
15Motor Starters & Control: What is a DOL starter used for?Direct Online Starter (DOL): Starting small motors (<5 kW). Simple, cheap, full‑voltage start.
16Star‑Delta Starter: Why use a star‑delta starter?Reduces starting current for motors 5–20 kW by initially starting in star, then running in delta.
17Soft‑Start / Variable Frequency Drive (VFD): What does a VFD allow?Precision speed control, reduced starting current, torque smoothing, and energy efficiency.