Sizing The DOL Motor Starter Parts (Contactor, Fuse, Circuit Breaker and Thermal Overload Relay)

Sizing The DOL Motor Starter Parts (Contactor, Fuse, Circuit Breaker and Thermal Overload Relay)


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
  • 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

CodeMin.Max.
A13.14
B3.153.54
C3.553.99
D44.49
E4.54.99
F52.59
G2.66.29
H6.37.09
I7.17.99
K88.99
L99.99
M1011.19
N11.212.49
P12.513.99
R1415.99
S1617.99
T1819.99
U2022.39
V22.4
  • 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 Phase300%175%
3 Phase300%175%
Synchronous300%175%
Wound Rotor150%150%
Direct Current150%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 Phase800%250%
3 Phase800%250%
Synchronous800%250%
Wound Rotor800%150%
Direct Current200%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 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 Amp

4. Size and Type of Contactor

ApplicationContactorMaking Cap
Non-Inductive or Slightly Inductive ,Resistive LoadAC11.5
Slip Ring MotorAC24
Squirrel Cage MotorAC310
Rapid Start / StopAC412
Switching of Electrical Discharge LampAC5a3
Switching of Electrical Incandescent LampAC5b1.5
Switching of TransformerAC6a12
Switching of Capacitor BankAC6b12
Slightly Inductive Load in Household or same type loadAC7a1.5
Motor Load in Household ApplicationAC7b8
Hermetic refrigerant Compressor Motor with Manual O/L ResetAC8a6
Hermetic refrigerant Compressor Motor with Auto O/L ResetAC8b6
Control of Restive & Solid State Load with opto coupler IsolationAC126
Control of Restive Load and Solid State with T/C IsolationAC1310
Control of Small Electro Magnetic Load ( <72VA)AC146
Control of Small Electro Magnetic Load ( >72VA)AC1510

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.

About Author //

author-pic

Jignesh Parmar

jiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Assistant Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.



16 Comments


  1. Sanjul Seth
    Jan 31, 2015

    Dear Sir/Mam ,

     I have completed bachelor’s degree in Electrical & Electronics engineering branch in 2014 . I have 6months experience  in designing electrical panels ( APFC, MCC & PCC panels)& switchgears and software skills in electrical autocad.
    Can you please suggest doing PLC, SCADA automation course will hike up my career or not
    to get a higher package. If yes please do suggest Institutes for the same
    Regards
    Sanjul


  2. Mohamed
    Jan 08, 2015

    The starting current is 45A , maximum size of none time delay fuse is 11A ,
    Why it will not blow?


  3. Mohamed
    Jan 08, 2015

    Why Code F minimum locked rotor current =5A , maximum locked rotor current =2.59A

Leave a Comment

Tell us what you're thinking... we care about your opinion!
and oh, not to forget - if you want a picture to show with your comment, go get a free Gravatar!


− 1 = five

CONNECT //
Electrical Engineering Daily Dose
Daily dose of knowledge and news from
Electrical Engineering World

Support our work //

Feel free to donate and support our work and contribution to the #EE society.
Get
PDF