# Calculate TC Size & Voltage Drop due to starting of Large Motor

## Calculate TC Size & Voltage Drop due to starting of Large Motor

• Calculate Voltage drop in Transformer ,1000KVA,11/0.480KV,impedance 5.75%, due to starting of  300KW,460V,0.8 Power Factor, Motor code D(kva/hp).Motor Start 2 times per Hour and The allowable Voltage drop at Transformer Secondary terminal is 10%.

## Motor current / Torque:

• Motor Full Load Current= (Kwx1000)/(1.732x Volt (L-L)x P.F)
• Motor Full Load Current=300×1000/1.732x460x0.8= 471 Amp.
• Motor Locked Rotor Current =Multiplier x Motor Full Load Current
 Locked Rotor Current (Kva/Hp) Motor Code Min Max A 3.15 B 3.16 3.55 C 3.56 4 D 4.1 4.5 E 4.6 5 F 5.1 5.6 G 5.7 6.3 H 6.4 7.1 J 7.2 8 K 8.1 9 L 9.1 10 M 10.1 11.2 N 11.3 12.5 P 12.6 14 R 14.1 16 S 16.1 18 T 18.1 20 U 20.1 22.4 V 22.5
• Min Motor Locked Rotor Current (L1)=4.10×471=1930 Amp
• Max Motor Locked Rotor Current(L2) =4.50×471=2118 Amp
• Motor inrush Kva at Starting (Irsm)=Volt x locked Rotor Multiplier x Full Load Currentx1.732 / 1000
• Motor inrush Kva at Starting (Irsm)=460 x 4.5x471x1.732 / 1000=1688 Kva

## Transformer:

• Transformer Full Load Current= Kva/(1.732xVolt)
• Transformer Full Load Current=1000/(1.732×480)=1203 Amp.
• Short Circuit Current at TC Secondary (Isc) =Transformer Full Load Current / Impedance.
• Short Circuit Current at TC Secondary= 1203/(5.75/100)= 20919 Amp
• Maximum Kva of TC at rated Short Circuit Current (Q1) = (Volt x Iscx1.732)/1000.
• Maximum Kva of TC at rated Short Circuit Current (Q1)=480x20919x1.732/1000= 17391 Kva.
• Voltage Drop at Transformer secondary due to Motor Inrush (Vd)= (Irsm) / Q1
• Voltage Drop at Transformer secondary due to Motor Inrush (Vd) =1688/17391 =10%
• Voltage Drop at Transformer Secondary is 10% which is within permissible Limit.
• 471 Amp <=65%x1203 amp = 471 Amp<= 781 Amp
• Here Voltage Drop is within Limit and Motor Full Load Current<=TC Full Load Current.
•  Size of Transformer is Adequate.

Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 14 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. 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.

### 9 Responses to Calculate TC Size & Voltage Drop due to starting of Large Motor

1. senthil says:

Sir,

I am from india, I want to download the unlocked versions.I have paypal account ,but when provide the details of pay pal,its blocked and transactions not succeed.
Pls provide relevant info for buy the unlocked version of earth mat design .

thanks

Nathan.

2. Er. S K Pal says:

Electronic Energy meters are based on current pulses and calibrated for standard PF (0.8 PF) and standard voltage so by improving PF we may reduce Electricity bills.

Hi Jignesh bhai,

Thanks for your wondeful notes on electrical systems.

can you please check and clarify the below point.

■Short Circuit Current at TC Secondary= 1203/5.75= 20919 Amp

which you have posted under

Calculate TC Size & Voltage Drop due to starting of Large Motor

4. Umesh MM says:

Dear Sir,

As per excel sheet, we have selected 3.5Cx120 Sq.mm Al. Cable for 125Kw motor (DOL)(I,e 415VAC, 50HZ) and its starting current 600%.

And now our cable vendor telling it will not meet , bcz starting current is more.

Regards,
Umesh MM
9986006728

• It is totally depend upon
(1) S.C capacity of your system as well as Cable.
(2) allowable starting Voltage Drop
(3) No of run of cable (Cable current capacity and distance)

All above this will decide size of cable

5. Ganesh says:

Hi Jignesh,

My query is, generally we check the voltage dip of the largest starting motor only in the motor terminals to ensure that motor starts and its torque overcomes the load torque with a min. possible voltage i.e 85% of rated motor voltage.

Which is more essential to check, whether voltage drop at transformer terminal or voltage dip at the largest motor terminal ?

Rgds,
Ganesh

6. Shiv Kumar says:

Hi Jignesh ,

Motor locked current and starting current of the motor both are same . If same than it’s also depends upon the selection of startor of the motor .