Quick questions on DC and AC motors for lifting: HSC Engineering Studies Lifting Devices

DC motor torque is proportional to armature current; speed is proportional to applied voltage. They give excellent low-speed torque and are easy to control with simple electronics. The disadvantage is brush wear (typical replacement every 2000 to 5000 hours).

The workhorse of industrial lifting. Construction:

Used at very large power (above about 200 kW). The rotor has DC-excited field windings (or permanent magnets) that lock to the rotating stator field, so the rotor runs at synchronous speed exactly. No slip. Used in heavy industrial winders and some large ship-to-shore container cranes.

$$P = T \omega = T \times \frac{2 \pi N}{60}$$

A VSD (also called variable-frequency drive, VFD, or inverter) electronically synthesises a three-phase AC waveform at a controllable frequency and voltage. By varying the supply frequency, the VSD changes the synchronous speed of the motor and so the operating speed. The volts-per-hertz ratio is held roughly constant to keep the magnetic flux constant.

Sydney CBD high-rise lifts (Salesforce Tower, International Towers Barangaroo) use gearless permanent-magnet synchronous motors driving the sheave directly, with VSD control. Mid-rise commercial lifts use geared induction motors. Industrial cranes at Port Botany use induction motors with VSD control for both lifting and trolley travel. Mining hoists for underground shafts (Cadia, Olympic Dam) use very large synchronous motors driven by cycloconverter drives.

$N_s$ depends on pole count. A 2-pole motor on 50 Hz runs at 3000 rpm; a 4-pole at 1500; a 6-pole at 1000; an 8-pole at 750.

Starting torque is much higher than rated. Sizing a hoist on starting current alone is wrong.

VSDs are 95 to 98 percent efficient, but harmonic distortion in the AC line and the resulting motor heating must be considered for large drives. :::