Electrical and Mechanical Theory, Components, and Controls
Domain focus
Electrical and mechanical theory, components, and controls tests the physical basis of elevator operation: AC/DC circuits, relays and contactors, motors and drives, transformers and rectifiers, encoders, safety chains, and mechanical advantage. Expect calculations and component identification, not just definitions.
Core rules and concepts
- Ohm's law and power: V = IR, P = VI = I²R. In series, current is common and voltages add; in parallel, voltage is common and currents add.
- Safety chain: normally closed switches in series; any open device drops the chain and removes run power. The chain is not a substitute for lockout.
- Contactors and relays: coil voltage, contact rating, normally open/normally closed, and arc suppression matter. A contactor picked up does not prove power is present downstream.
- Traction drives: VVVF control varies voltage and frequency; encoder feedback closes the speed loop. Loss of encoder feedback can cause rough ride or fault.
- Mechanical: counterweight balance is typically car weight plus 40-50% of rated load. Sheave diameter and rope ratio affect rope speed and tension.
Worked example
A car weighs 2,500 lb and rated capacity is 3,000 lb. At 45% counterbalance, add 0.45 × 3,000 = 1,350 lb. Counterweight = 2,500 + 1,350 = 3,850 lb. This leaves the machine working against 55% of load when fully loaded and against 45% when empty, reducing motor torque and improving ride.
Traps
Treating a safety chain as a power source; assuming a 24 VDC coil will operate on 24 VAC; ignoring contact ratings; using car weight alone for counterweight; and diagnosing a drive fault by replacing the encoder before checking wiring and shield grounding.