Describe the construction and operating principle of DC, AC induction and three-phase synchronous motors, calculate motor torque and power, and identify the role of variable-speed drives in modern lifting

What this dot point is asking

NESA wants you to describe the construction and operating principle of the three motor families used in lifting (DC, AC induction, AC synchronous), apply the synchronous speed and slip relationships, calculate motor torque and power, and identify why variable-speed drives are used.

The answer

DC motors

A DC motor has:

• Stator. Permanent magnets (small motors) or wound field coils (industrial DC).
• Rotor (armature). Wound conductors that carry current.
• Commutator and brushes. Mechanical switch that reverses current direction as the rotor turns, keeping torque always in the same direction.

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).

Historic lift drives (Sydney Town Hall, the QVB) used DC motors with Ward-Leonard control until the late 20th century.

AC induction motors

The workhorse of industrial lifting. Construction:

• Stator. Three-phase windings on a laminated iron core. Powered from the mains.
• Rotor. A squirrel cage of aluminium or copper bars short-circuited at both ends, embedded in a laminated iron rotor.
• No brushes or commutator.

Operating principle: the three-phase stator currents create a rotating magnetic field that turns at the synchronous speed:

where $f$ is the supply frequency (50 Hz in Australia) and $p$ is the number of poles. A 4-pole motor on 50 Hz has $N_s = 1500$ rpm.

The rotor turns slower than the field. The difference is called slip:

Typical full-load slip is 2 to 5 percent. The relative motion between rotor and field induces currents in the rotor bars; these currents interact with the field to produce torque. Without slip, no torque.

Three-phase synchronous motors

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.

Motor torque and power

Mechanical power output:

For a motor at rated power and rated speed, the rated torque is $T = P / \omega$.

The torque-speed curve of an induction motor has the following key points:

• Starting torque at zero speed (typically 1.5 to 2.5 times rated)
• Peak torque (breakdown torque) at 70 to 90 percent of synchronous speed (typically 2 to 3 times rated)
• Rated point at the slip corresponding to rated power
• Synchronous point at zero load

Variable-speed drives (VSDs)

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.

VSDs are now standard on industrial lifting because they give:

• Soft start. Reduces inrush current from 6 times rated to about 1.5 times rated, reducing motor and grid stress.
• Speed control. Smooth speed regulation from creep to full speed, useful when positioning loads.
• Regenerative braking. Most modern VSDs feed deceleration energy back to the grid or burn it in a brake resistor.
• Reduced energy use when lifting at less than full speed.

Australian application

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.