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EXAMPLE- To 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 ( Ihp=746 watts)
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
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
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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
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.
HOW TO CALCULATE THE SIZE OF CIRCUIT BREAKER
CIRCUIT BREAKER AS PER NEC 430-52
Type of Motor
Inverse Time
Single Phase
250%
3 Phase
250%
Synchronous
250%
Wound Rotor
150%
Direct Current
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.
HOW TO SIZE 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).
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Thermal Overload Relay setting = 100%x6 = 6 AmpHOW TO SIZE 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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EXAMPLE- To 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 ( Ihp=746 watts) • 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 Code A B C D E F G H I K L M N P R S T U V Min. 1 3.15 3.55 4 4.5 5 2.6 6.3 7.1 8 9 10 11.2 12.5 14 16 18 20 22.4 Max. 3.14 3.54 3.99 4.49 4.99 2.59 6.29 7.09 7.99 8.99 9.99 11.19 12.49 13.99 15.99 17.99 19.99 22.39 • 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 FUSE AS PER NEC 430-52 Type of Motor Time Delay Fuse Single Phase 300% 3 Phase 300% Synchronous 300% Wound Rotor 150% Direct Current 150% Non-Time Delay Fuse 175% 175% 175% 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. HOW TO CALCULATE THE SIZE OF CIRCUIT BREAKER CIRCUIT BREAKER AS PER NEC 430-52 Type of Motor Instantaneous Trip Single Phase 800% 3 Phase 800% Synchronous 800% Wound Rotor 800% Direct Current 200% Inverse Time 250% 250% 250% 150% 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. HOW TO SIZE 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 AmpHOW TO SIZE Size and Type of Contactor Application Non-Inductive or Slightly Inductive ,Resistive Load Slip Ring Motor Squirrel Cage Motor Rapid Start / Stop Switching of Electrical Discharge Lamp Switching of Electrical Incandescent Lamp Switching of Transformer Switching of Capacitor Bank Slightly Inductive Load in Household or same type load Motor Load in Household Application Hermetic refrigerant Compressor Motor with Manual O/L Reset Hermetic refrigerant Compressor Motor with Auto O/L Reset Control of Restive & Solid State Load with opto coupler Isolation Control of Restive Load and Solid State with T/C Isolation Control of Small Electro Magnetic Load ( <72VA) Control of Small Electro Magnetic Load ( >72VA) Contactor AC1 AC2 AC3 AC4 AC5a AC5b AC6a AC6b AC7a AC7b AC8a AC8b AC12 AC13 AC14 AC15 Making Cap 1.5 4 10 12 3 1.5 12 12 1.5 8 6 6 6 10 6 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. Name: Description: ...
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