Sizing The DOL Motor Starter Parts (Contactor, Fuse, Circuit Breaker and Thermal Overload Relay)
Calculate size of each part of DOL motor starter for the system voltage 415V, 5HP three phase house hold application induction motor, code A, motor efficiency 80%, motor RPM 750, power factor 0.8 and overload relay of starter is put before motor.
Basic Calculation of Motor Torque and Current
- Motor Rated Torque (Full Load Torque) = 5252xHPxRPM
- Motor Rated Torque (Full Load Torque) = 5252x5x750 = 35 lb-ft.
- Motor Rated Torque (Full Load Torque) = 9500xKWxRPM
- Motor Rated Torque (Full Load Torque) = 9500x(5×0.746)x750 = 47 Nm
- If Motor Capacity is less than 30 KW than Motor Starting Torque is 3xMotor Full Load Current or 2X Motor Full Load Current.
- Motor Starting Torque = 3xMotor Full Load Current.
- Motor Starting Torque = 3×47 = 142Nm.
- Motor Lock Rotor Current = 1000xHPx figure from below Chart/1.732×415
Locked Rotor Current
| Code | Min. | Max. |
| A | 1 | 3.14 |
| B | 3.15 | 3.54 |
| C | 3.55 | 3.99 |
| D | 4 | 4.49 |
| E | 4.5 | 4.99 |
| F | 5 | 2.59 |
| G | 2.6 | 6.29 |
| H | 6.3 | 7.09 |
| I | 7.1 | 7.99 |
| K | 8 | 8.99 |
| L | 9 | 9.99 |
| M | 10 | 11.19 |
| N | 11.2 | 12.49 |
| P | 12.5 | 13.99 |
| R | 14 | 15.99 |
| S | 16 | 17.99 |
| T | 18 | 19.99 |
| U | 20 | 22.39 |
| V | 22.4 |
- As per above chart Minimum Locked Rotor Current = 1000x5x1/1.732×415 = 7 Amp
- Maximum Locked Rotor Current = 1000x5x3.14/1.732×415 = 22 Amp.
- Motor Full Load Current (Line) = KWx1000/1.732×415
- Motor Full Load Current (Line) = (5×0.746)x1000/1.732×415 = 6 Amp.
- Motor Full Load Current (Phase) = Motor Full Load Current (Line)/1.732
- Motor Full Load Current (Phase) = 6/1.732 =4Amp
- Motor Starting Current = 6 to 7xFull Load Current.
- Motor Starting Current (Line) = 7×6 = 45 Amp
1. Size of Fuse
Fuse as per NEC 430-52
| Type of Motor | Time Delay Fuse | Non-Time Delay Fuse |
| Single Phase | 300% | 175% |
| 3 Phase | 300% | 175% |
| Synchronous | 300% | 175% |
| Wound Rotor | 150% | 150% |
| Direct Current | 150% | 150% |
- Maximum Size of Time Delay Fuse = 300% x Full Load Line Current.
- Maximum Size of Time Delay Fuse = 300%x6 = 19 Amp.
- Maximum Size of Non Time Delay Fuse = 1.75% x Full Load Line Current.
- Maximum Size of Non Time Delay Fuse = 1.75%6 = 11 Amp.
2. Size of Circuit Breaker
Circuit Breaker as per NEC 430-52
| Type of Motor | Instantaneous Trip | Inverse Time |
| Single Phase | 800% | 250% |
| 3 Phase | 800% | 250% |
| Synchronous | 800% | 250% |
| Wound Rotor | 800% | 150% |
| Direct Current | 200% | 150% |
- Maximum Size of Instantaneous Trip Circuit Breaker = 800% x Full Load Line Current.
- Maximum Size of Instantaneous Trip Circuit Breaker = 800%x6 = 52 Amp.
- Maximum Size of Inverse Trip Circuit Breaker = 250% x Full Load Line Current.
- Maximum Size of Inverse Trip Circuit Breaker = 250%x6 = 16 Amp.
3. Thermal Overload Relay
Thermal Overload Relay (Phase):
- Min. Thermal Overload Relay setting = 70%x Full Load Current(Phase)
- Min. Thermal Overload Relay setting = 70%x4 = 3 Amp
- Max. Thermal Overload Relay setting = 120%x Full Load Current(Phase)
- Max. Thermal Overload Relay setting = 120%x4 = 4 Amp
Thermal Overload Relay (Phase):
- Thermal Overload Relay setting = 100% x Full Load Current (Line).
- Thermal Overload Relay setting = 100%x6 = 6 Amp
4. Size and Type of Contactor
| Application | Contactor | Making Cap |
| Non-Inductive or Slightly Inductive ,Resistive Load | AC1 | 1.5 |
| Slip Ring Motor | AC2 | 4 |
| Squirrel Cage Motor | AC3 | 10 |
| Rapid Start / Stop | AC4 | 12 |
| Switching of Electrical Discharge Lamp | AC5a | 3 |
| Switching of Electrical Incandescent Lamp | AC5b | 1.5 |
| Switching of Transformer | AC6a | 12 |
| Switching of Capacitor Bank | AC6b | 12 |
| Slightly Inductive Load in Household or same type load | AC7a | 1.5 |
| Motor Load in Household Application | AC7b | 8 |
| Hermetic refrigerant Compressor Motor with Manual O/L Reset | AC8a | 6 |
| Hermetic refrigerant Compressor Motor with Auto O/L Reset | AC8b | 6 |
| Control of Restive & Solid State Load with opto coupler Isolation | AC12 | 6 |
| Control of Restive Load and Solid State with T/C Isolation | AC13 | 10 |
| Control of Small Electro Magnetic Load ( <72VA) | AC14 | 6 |
| Control of Small Electro Magnetic Load ( >72VA) | AC15 | 10 |
As per above chart:
- Type of Contactor = AC7b
- Size of Main Contactor = 100%X Full Load Current (Line).
- Size of Main Contactor = 100%x6 = 6 Amp.
- Making/Breaking Capacity of Contactor = Value above Chart x Full Load Current (Line).
- Making/Breaking Capacity of Contactor = 8×6 = 52 Amp.