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

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

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 =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:

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%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:

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%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 …