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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.
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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
Min. Thermal Overload Relay setting = 70%x4 = 3 Amp
Max. Thermal Overload Relay setting = 120%x4 = 4 Amp
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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
AC7a
1.5
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.