Calculate Size of Contactor, Fuse, C.B, Over Load Relay of DOL Starter

Calculate Size of Contactor, Fuse, C.B, O/L  of DOL Starter

• Calculate Size of each Part of DOL 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 , Overload Relay of Starter is Put before Motor.

Basic Calculation of Motor Torque & Current:

• Motor Rated Torque (Full Load Torque) =5252xHP/RPM
• Motor Rated Torque (Full Load Torque) =5252×5/750=35 lb-ft.
• Motor Rated Torque (Full Load Torque) =9500xKW/RPM
• Motor Rated Torque (Full Load Torque) =9500x(5×0.746)/750 =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

• 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:

• 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:

• 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 over Load Relay:

• Thermal over Load Relay (Phase):
• Min Thermal Over Load Relay setting =70%xFull Load Current(Phase)
• Min Thermal Over Load Relay setting =70%x4= 3 Amp
• Max Thermal Over Load Relay setting =120%xFull Load Current(Phase)
• Max Thermal Over Load Relay setting =120%x4= 4 Amp
• Thermal over Load Relay (Phase):
• Thermal over Load Relay setting =100%xFull Load Current (Line).
• Thermal over Load Relay setting =100%x6= 6 Amp

(4) Size and Type of Contactor:

• 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.