Motor Control and Protection Equipment Selection Guide
How to size and coordinate motor protection switches, thermal overload relays, direct-on-line starters, and soft starters under IEC 60947-4-1.
Three-phase induction motors draw 6 to 8 times their rated current during direct-on-line starting and require dedicated protection against overload, phase loss, and short circuits. Motor Control Equipment is sized according to IEC 60947-4-1 based on motor full-load current ($I_n$), utilization category (AC-3), and Type 1 / Type 2 coordination tables.
Core components in an industrial motor feeder
- Motor protection switches and overload relays: Provide phase-failure sensitive thermal tripping (Class 10 / Class 20) and magnetic short-circuit interruption matched to the motor's nameplate current.
- Direct-on-line (DOL), star-delta, and soft starters: Enclosed DOL starters handle small-to-medium motors, while star-delta assemblies and thyristor soft starters reduce starting current and mechanical torque surges on pumps, fans, and compressors.
Motor control equipment comparison table
| Equipment Family | Standard | Primary Function | Key Sizing Parameter | Category Link |
|---|---|---|---|---|
| Motor control & protection units | IEC 60947-4-1 | Thermal overload, phase loss, and feeder coordination | Motor full-load current ($I_n$) adjustment range | Motor Control Equipment |
| Direct-on-line & enclosed motor starters | IEC 60947-4-1 | Pre-assembled start/stop switching and overload protection | Motor kW rating at 400 V and coil voltage | Motor Starters |
| Soft starters | IEC 60947-4-2 | Stepless voltage ramp for smooth starting and stopping | Motor current, starts per hour, and internal bypass | Soft Starters |
Frequently asked questions
How should the thermal overload setting be adjusted for a motor?
For direct-on-line (DOL) feeders, set the dial to the exact full-load current ($I_n$) on the motor nameplate; for a thermal relay placed inside the delta loop of a star-delta starter, set it to $0.58 \times I_n$.
What is the difference between Type 1 and Type 2 coordination?
Under IEC 60947-4-1, Type 1 ensures safety under a short circuit but allows contactor or overload relay damage requiring repair, whereas Type 2 guarantees that the starter remains operational after clearing the fault.