Motor Starters & Control Circuits
Guide to DOL, star-delta, soft starters, and VFDs with wiring diagrams and selection criteria
Motor Full Load Current
Calculating the full load current from the motor nameplate data is the starting point for cable sizing, protection, and starter selection.
IL = P / (sqrt(3) x VL x eta x cos(phi))- IL
- = Full load line current (A)
- P
- = Motor rated power (W)
- VL
- = Line voltage, typically 400V
- eta
- = Motor efficiency (e.g. 0.88)
- cos(phi)
- = Power factor (e.g. 0.85)
Motor nameplate data
Example: A 7.5kW motor, efficiency 0.88, power factor 0.85 at 400V draws 7500 / (1.732 x 400 x 0.88 x 0.85) = 14.5A full load current.
Starting Methods Comparison
Motor starting method characteristics
General motor starting guidance| Starter Type | Starting Current | Starting Torque | Typical Use |
|---|---|---|---|
| DOL | 6-8x FLC | 100% rated | Up to 7.5kW (depends on supply) |
| Star-Delta | 2-3x FLC | 33% rated | 7.5kW - 75kW, light start loads |
| Soft Starter | 2-4x FLC | Adjustable | Pumps, conveyors, compressors |
| VFD | 1-1.5x FLC | Adjustable 0-150% | Speed control, energy saving |
FLC = Full Load Current. Starting current is the peak inrush during acceleration.
Supply Capacity
MCB and Protection Selection
MCB type selection for motor circuits
BS 7671 and BS EN 60898| MCB Type | Trip Range | Motor Application |
|---|---|---|
| Type B | 3-5x In | Not suitable for motor circuits |
| Type C | 5-10x In | DOL starters, soft starters, VFDs |
| Type D | 10-20x In | Star-delta transition, high inertia loads |
Motor circuits require both short-circuit protection (MCB/MCCB) and overload protection (thermal or electronic overload relay). The overload is set to the motor FLC and trips on sustained overcurrent. The MCB handles fault current only.
VFD Cable Considerations
EMC and Cable Selection
Cable Derating with VFDs