Calculate Size of Contactor / Fuse / CB / OL Relay of Star-Delta Starter

• Calculate Size of each Part of Star-Delta starter for 10HP, 415 Volt Three Phase Induction Motor having Non Inductive Type Load, Code A, Motor efficiency 80%, Motor RPM 600, Power Factor 0.8. Also Calculate Size of Overload Relay if O/L Relay Put in the wingdings (overload is placed after the Winding Split into main and delta Contactor) or in the line (Putting the overload before the motor same as in DOL).

Basic Calculation of Motor Torque & Current:

• Motor Rated Torque (Full Load Torque) =5252xHPxRPM
• Motor Rated Torque (Full Load Torque)=5252x10x600=88 lb-ft.
• Motor Rated Torque (Full Load Torque) =9500xKWxRPM
• Motor Rated Torque (Full Load Torque)=9500x(10×0.746)x600 =119 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=3x Motor Rated Torque (Full Load Torque).
• Motor Starting Torque==3×119=356 Nm.
• Motor Lock Rotor Current =1000xHPx figure from below Chart/1.732×415

• As per above chart Minimum Locked Rotor Current =1000x10x1/1.732×415=14 Amp
• Maximum Locked Rotor Current =1000x10x3.14/1.732×415=44 Amp.
• Motor Full Load Current (Line) =KWx1000/1.732×415
• Motor Full Load Current (Line) = (10×0.746)x1000/1.732×415=13 Amp.
• Motor Full Load Current (Phase) =Motor Full Load Current (Line)/1.732.
• Motor Full Load Current (Phase) ==13/1.732=7 Amp.
• Motor Starting Current (Star-Delta Starter) =3xFull Load Current.
• Motor Starting Current (Line)=3×13=39 Amp

(1) Size of Fuse:

• Maximum Size of Time Delay Fuse =300% x Full Load Line Current.
• Maximum Size of Time Delay Fuse =300%x13= 39 Amp.
• Maximum Size of Non Time Delay Fuse =1.75% x Full Load Line Current.
• Maximum Size of Non Time Delay Fuse=1.75%13=23 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%x13= 104 Amp.
• Maximum Size of Inverse Trip Circuit Breaker =250% x Full Load Line Current.
• Maximum Size of Inverse Trip Circuit Breaker =250%x13= 32 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%x7= 5 Amp
• Max Thermal Over Load Relay setting =120%xFull Load Current(Phase)
• Max Thermal Over Load Relay setting =120%x7= 9 Amp

Thermal over Load Relay (Line):

• For a star-delta starter we have the possibility to place the overload protection in two positions, in the line or in the windings.
• If O/L Relay Placed in Line: (Putting the O/L before the motor same as in DOL).Supply>Over Load Relay>Main Contactor
• If Over Load Relay supply the entire motor circuit and are located ahead of where the power splits to the Delta and Star contactors, so O/L Relay size must be based upon the entire motor Full Load Current.
• Thermal over Load Relay setting =100%xFull Load Current (Line).
• Thermal over Load Relay setting =100%x13= 13 Amp
• Disadvantage: O/L Relay will not give Protection while Motor runs in Delta (Relay Setting is too High for Delta Winding)
• If O/L Relay Placed In the windings: (overload is placed after the Winding Split into main and delta Contactor).Supply>Main Contactor-Delta Contactor>O/L Relay
• If overload is placed after the Point where the wiring Split into main and delta Contactor, Size of over load relay at 58% (1/1.732) of the motor Full Load Current because we use 6 leads going to the motor, and only 58% of the current goes through the main set of conductors (connected to the main contactor).
• The overload then always measures the current inside the windings, and is thus always correct. The setting must be x0.58 FLC (line current).
• Thermal over Load Relay setting =58%xFull Load Current (Line).
• Thermal over Load Relay setting =58%x13= 8 Amp.
• Disadvantage: We must use separate short-circuit and overload protections

(4) Size and Type of Contactor:

• Main and Delta Contactor:

• The Main and Delta contactors are smaller compared to single contactor used in a Direct on Line starter because they Main and Delta contactors in star delta starter are controlling winding currents only. The currents through the winding are 1/√3 (58%) of the current in the line. These two contactors (Main contactor and Delta Contactor) are close during run. These rated at 58% of the current rating of the motor.

• Star Contactor:

• The third contactor is the star contactor and that only carries star current while the motor is connected in star in starting. The current in star winding is 1/√3= (58%) of the current in delta, so this contactor can be rated at 1/3 (33%) of the motor rating. Star contactor can be selected smaller than the others, providing the star contactor pulls first before the main contactor. Then no current flows when third contactor pulls.
• In star connection at start, the motor draws and delivers 1/3 of its full rated power.
• When the starter switches over to Delta, the motor draws full power, but since the contactors and the overload relay are usually wired within the Delta, you need to use contcators and relay which are only rated 1/√3 =58% of the full rated power of the motor.

• As per above Chart
• Type of Contactor= AC1
• Making/Breaking Capacity of Contactor= Value above Chart x Full Load Current (Line).
• Making/Breaking Capacity of Contactor=1.5×13= 19 Amp.
• Size of Star Contactor (Starting Condition) = 33%X Full Load Current (Line).
• Size of Star Contactor =33%x13 = 4 Amp.
• Size of Main Contactor (Starting-Transition-Running) = 58%X Full Load Current (Line).
• Size of Main Contactor =58%x13 = 8 Amp.
• Size of Delta Contactor (Running Condition) = 58%X Full Load Current (Line).
• Size of Delta Contactor =58%x13 = 8 Amp.

Summary:

•  Type of Contactor= AC1
• Making/Breaking Capacity of Contactor=19 Amp.
• Size of Star Contactor =4 Amp.
• Size of Main Contactor = 8 Amp.
• Size of Delta Contactor =8 Amp.