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

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

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 MotorTime Delay FuseNon-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 MotorInstantaneous TripInverse 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 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:

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.

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