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 only. The currents through the winding are 1/√3 (58%) of the current in the line. These two contactors (Main contactor and Delta Contactor) are close during run. These rated at 58% of the current rating of the motor.
  • Star Contactor:

  • The third contactor is the star contactor and that only carries star current while the motor is connected in star in starting. The current in star winding is 1/√3= (58%) of the current in delta, so this contactor can be rated at 1/3 (33%) of the motor rating. Star contactor can be selected smaller than the others, providing the star contactor pulls first before the main contactor. Then no current flows when third contactor pulls.
  • In star connection at start, the motor draws and delivers 1/3 of its full rated power.
  • When the starter switches over to Delta, the motor draws full power, but since the contactors and the overload relay are usually wired within the Delta, you need to use contcators and relay which are only rated 1/√3 =58% of the full rated power of the motor.
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= AC1
  • Making/Breaking Capacity of Contactor= Value above Chart x Full Load Current (Line).
  • Making/Breaking Capacity of Contactor=1.5×13= 19 Amp.
  • Size of Star Contactor (Starting Condition) = 33%X Full Load Current (Line).
  • Size of Star Contactor =33%x13 = 4 Amp.
  • Size of Main Contactor (Starting-Transition-Running) = 58%X Full Load Current (Line).
  • Size of Main Contactor =58%x13 = 8 Amp.
  • Size of Delta Contactor (Running Condition) = 58%X Full Load Current (Line).
  • Size of Delta Contactor =58%x13 = 8 Amp.

Summary:

  •  Type of Contactor= AC1
  • Making/Breaking Capacity of Contactor=19 Amp.
  • Size of Star Contactor =4 Amp.
  • Size of Main Contactor = 8 Amp.
  • Size of Delta Contactor =8 Amp.

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 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 Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “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.

7 Responses to Calculate Size of Contactor / Fuse / CB / OL Relay of Star-Delta Starter

  1. KOTADIYA HARDIK says:

    Please post about cost ratio of overhead line and underground cable. And relation between them what is the ratio of between them??

    Thank You

    On Sat, 1 Sep 2018, 00:14 Electrical Notes & Articles, wrote:

    > Jignesh.Parmar posted: ” 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” >

  2. S B koli says:

    Thank you very much sir, Electrical notes shared are very useful. please share short circuit calculation ( base KVA method) in excel format.

  3. Wafaa Saber says:

    thanks aloootwith my best

  4. ENGR.MD.JAMIRUL ISLAM JEWEL says:

    Thank you sir for your valuable mail regards

    On Sat, Sep 1, 2018 at 12:44 AM Electrical Notes & Articles wrote:

    > Jignesh.Parmar posted: ” 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” >

  5. How to calculate the size of power contactor /fuse/over load relay/power cable size for DOL Starter

    On Sat 1 Sep, 2018, 00:16 Electrical Notes & Articles, wrote:

    > Jignesh.Parmar posted: ” 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” >

  6. mushtaq ahmad says:

    thank u

    Best regards

    Mushtaq Ahmad OM Engineering Arena Pakistan

  7. shahbaz hussain says:

    Hi Mr.Jignesh,

    I would like to know about the selection of cable for motors. As the starting current is very high and if we select the cable on full load amps so can it bear the high inrush current? Kindly let me know.

    Thanks & Regards,
    Syed Shahbaz Hussain

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