Motors, Motor Controls, and Industrial Equipment
Motor circuit design
Motors, Motor Controls, and Industrial Equipment tests the difference between overload protection and short-circuit protection, the selection of contactors and starters, and the safety functions around industrial machinery. Motor starting current is several times full-load current, so the short-circuit device must ride through starting while the overload device protects the winding.
Core rules
- Use the motor full-load current from the nameplate or a recognised table, not the kW rating alone. The overload setting follows the full-load current and the service factor; a 1.15 service factor may allow a setting up to 115 percent of nameplate current, subject to the manufacturer.
- Contactors are selected by utilisation category. AC-3 suits squirrel-cage motor switching; AC-1 is for resistive loads and is not a motor rating.
- Star-delta starting reduces starting current and torque. In star-delta, the main and delta contactors carry about 58 percent of line full-load current, while the star contactor carries about 33 percent. An overload placed in the delta branch is set to about 58 percent of line full-load current; an overload in the line is set to line full-load current.
- VFDs require motor cable suited to PWM waveforms, correct earthing and screened cable where specified. Do not use the drive as a motor isolator unless the manufacturer permits it.
- Emergency stops must be hard-wired and latching where required, not implemented only through software. Isolation and lock-off points must be identifiable.
Worked example
A 22 kW, 400 V motor has a nameplate full-load current of 40 A and is started star-delta. Main and delta contactors: 0.58 x 40 = 23.2 A, so a 25 A AC-3 contactor is selected. Star contactor: 0.33 x 40 = 13.2 A, so a 16 A AC-3 contactor is selected. If the overload relay is in the delta branch, set it to 23.2 A; if it is in the line, set it to 40 A. The reasoning is that the phase current in delta is lower than the line current, and the overload must see the current that actually flows through its element.
Traps that cost marks
Sizing the overload to the star contactor current; using an AC-1 contactor for motor duty; assuming a VFD removes the need for motor earthing; and overlooking the difference between a motor isolator and an emergency stop.