Calculate Size of Contactor, Fuse, C.B, Over Load Relay of DOL Starter
June 2, 2013 49 Comments
Calculate Size of Contactor, Fuse, C.B, O/LÂ 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 |
Â |
- As per above chart Minimum Locked Rotor CurrentÂ =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:
FuseÂ 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.
(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%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.
thanks……sir
Thanks & regards
Asam Ali
Al Mariah United Group Abu Dhai – U.A.E Cell No.: +971 567077962
________________________________
very nice usefull message
Thank s sir
well………..
why did you eliminate the efficiency of the motor & the p.f value from your calculations ?
I think the rated current =(5*746)/(1.732*415*0.8*0.8)=8 Amp.
Thanks
send me diploma cet question papers
good materil
very useful notes u provided, keep doing this
Dear Sir,
What is mean by Instantaneous Trip and Inverse Trip for Size of Circuit Breaker?
Regards
Arun S
What does 3.31,3.7,4.6,0.37,2.95 values indicate sir in DOL Starter sizing calculations
Interesting&knowledgeable hope to see more quality& informative articles
Motor name plate do not indicate locked rotor current code, how to know that?
Why normally relay setting is 1.3 times of the motor HP.Is there any exact formula to calculate the same ?
How to calculate relay setting for motors with different RPM.Ex.10 HP/1000 RPM,10HP/1500 RPM, 10 HP/3000 RPM.
Sir may you tell me to setting of relay 3 phase 10 hp motor 380 volts cos.8 And how to calculate please tell formula’s
superbbbbbbbbbbbbbbb
please tell me about overload, mcb, mccb,fuses selection about dol ,star delta starter in
Refer the related Post of this Blog..
which code u considered sir ?
Need to know how to come up with an motor protection. ie mcb, fuses for various sizes of motors
Dear sir
Thanks for the valuable information.For 5 HP motor the letter code is A or F. This is my doubt. please make it clear if you are free sir.
warm regards
Hi, first of all, thanks for sharing this information. Second, I have a question: Doesn’t the contactor should be sized in comparison with the conductor size?, which must be 125% of the FLA of the load (If PF is higher or equal than 0.8). I make this cuestion because if the conductors are sized 125% of the FLA, and the contactor sized on 100%, there might be a voltage drop, or a hot spot in the contactor. What do you think, sir?
Thank You Sir.
Tuhin
From
Bangladesh
Mobile:+8801712427773
3+N
Thank you Sir…Greetings from West Texas
What is calculation of 5hp ,415v induction motor protection panel ?
Hi there I have an under floor heating matt with a full load of 10000kw/ 42 amps.
Wanted to no what size contactor I would need
Thanks Henry
how it can be possible that a 5 hp motor is rated for 47 A. as u have taken 47A. for calculating motor starting torque (i.e. at the Basic Calculation of Motor Torque & Current topic). Plz tell me sir..
Hello Sir, I am Mr.Sovann,I am living in Cambodia. I would like to ask u some question. My country use 380v for three phase, and 220v for single phase, 50Hz, and Pf 0,85.
Question: i have a motor thee phase, its power is 3kw. but i don’t know how to choose a Motor Circuit Breaker, a Contactor, an overload relay,and its cross-section area of wire to be suit with it. Could u show me how to choose all of that devices? or could i have some of formulas about them?
Thanks in advance
Good luck!
Well.I hope,I will learn more….
good
wat is d difference between slip ring induction motor and squirrel cage.??
what is the different will occure if we use method of starting like star delta or soft starter
thanks
sir pls tell me the overload relay size for 45 kw 3 ph motor
Actually, it is very useful.
Hi Sir,
Good to know that you are tremedous on Electrical calculations tools in excel itself.
Would like to have help on Motor FLA Calculation Sheet,Contactor sizes ( NEMA ).
Sharing your knowledge in such a platform is appreciable matter.
Thanks,
Jignesh vaghela
Jignesh22
1085@gmail.com
how to calculate power contactor coil current.
it is very good for improving knowledge
thanks
Super sir
Thaink you
plz give 11 KW slip ring induction motor installation,commissioning and installed equipment list?
sir, the fuse rating formula come ac and dc load or same?
please give me the rating of MCCB contractor and types of starter for the squirrel cage induction motor of 112Kw ??? that need to operate blending blower in cement industry.
how to find fuse rating of 3 phase induction motor
Your information about fuse size for motors was helpful as I have been searching for a simple table to give me some sense of the accuracy of my calculations. Thank you. But I must inform you that I looked further and I now know that there is an error in your NEC 430.52 table. You have the multiplier factors under the wrong headings. In other words, they are reversed. For a Single Phase Motor (for example), your table shows a Non-Time Delay Fuse value of 175% of FLC and 300% for a Dual Element (Time Delay Fuse). In fact the NEC 430.52 table shows that For a Single Phase Motor a Non-Time Delay Fuse should be 300% of FLC and 175% for a Dual Element (Time Delay Fuse). Your math example below your table is also incorrect because it follows the reversed data from the table. I only point this out because I have much respect for you and have read good advice from you many times. I checked a few other sources and your table is the only one with the reversal. I would not want your readers to be misled as I know you would not either. Thank you for sharing your wisdom online. Be well,
I agree, the table is wrong
How to size conatctors for wound rotor motor 400V 132KW?
thank u soo much it, s was too good!
Thank you Sir.
I have feed it in to Excel sheet with formulae for future reference.
Similarly I want to know for resistive (heater) load. Kindly help me.
Sir,
How do i calculate the Contactor Ratting by only with the known values oh KW,V,& A