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# Calculate size of transformer and voltage due to starting of large size motor

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## Easy way to calculate transformer and motor details

### Transformer details:

1. Transformer size
2. Transformer secondary voltage (v2)
3. % Impedance (z)
4. Allowable voltage drop in system

### Motor details for one and more motors:

1. Motor full load current (ILM)
2. Locked rotor current multiplier (Min)
3. Locked rotor current multiplier (Max)
4. Locked rotor current (L1) (Min)
5. Locked rotor current (L2) (Max)
6. Motor inrush KVA at starting (IrsM)
7. How many times motor starts per hour

### and Calculations:

1. Full load current of transformer (ILX)
2. Short circuit current at transformer secondary (Isc)
3. Maximum KVA of transformer at Isc (Q1)
4. Motor inrush KVA At starting (IrsM)
5. Voltage drop at transformer secondary due to motor inrush (Vd)
6. Transformer secondary voltage

### Results you get:

• Voltage Drop at Transformer Secondary Due to Motor inRush
• Transformer Size:(20% Added due to motor starts more than once/Hour)

#### Calculation formulas used:

• ILX = KVA / (1.732 x v2)
• ILM = (Kw x 1000) / (1.732 x Vm x P.F)
• Isc = ILx / z
• Q1 = (v2 x ISC x ILM x 1.732) / 1000
• IrsM = (Vm x L2 x 1.732) / 1000
• Vd = IrsM / Q1
 Software: Calculate size of transformer and voltage due to starting of large size motor Version: 10.6.13 Developer: Jignesh Parmar Size: 40 KB Price: Free Download: Right here | Video Courses | Membership | Download Updates

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1. Baryalai
Oct 05, 2020

I need the size of the Generator and Transformer for 400 kw motor

2. Shrikant
May 24, 2020

IEEE Standards provide correct formula. Please refer and accordingly revise Excel sheet tabulated formulas. Regards.

3. C.S.NAMBISAN
May 23, 2020

Motor KW is 300KW
Motor KVA at 0.8 PF is 300/0.8 = 375 KVA ,not 240 KVA given in the sheet
Also Motor KW is out put KW ,not input KW which needs to consider efficiency ,say 90% for 300KW motor
Input KW = 375/0.9 = 417 KVA
motor full load current = 417000/ 1.732×460 = 522 Amp ,not 470 Agiven ,so starting current and volt drop also will increase proportionately also.
The sheet needs such major corrections , CS NAMBISAN

4. Khaled Ellithy
Jan 25, 2018

Excellent example. Could you please modify it to include 75 HP motor with pf of 0.8 lag and starting pf of 0.3. and constant load 20 KVA, pf = 0.8 lag

Thanks

Feb 01, 2017

What is the rating of transformer which can feed motor 1500 HP , 4160 V and 60 HZ ?

6. Dr Khaled Ellithy
Apr 02, 2016

Ganesh, I agree with you, It should include the KVA of the loads connected to the transformer before starting the motor. Including the loads KVA to the starting KVA of the motor may end up with the transformer size is incorrect (VD% is above the limit. Comment of Jignesh Parmar is highly appreciated.

7. mohsin raza
Apr 14, 2015

Please note that in thi volatge drop calculation existing load on transformer is not considered. only motor starting KVA is considered for voltage drop calculation. Kindly tell me how to consider other conneted load in this calcullation.

8. Bhagaban Barik
Sep 22, 2014

Kw=KVA*p.f
KVA=Kw/p.f
To calculate sizing of Transformer 30% future expansion should be considered.

• Faisal Nawaz
Dec 01, 2018

Kw=KVA*p.f
KVA=Kw/p.f
In the excel sheet he multiply the KW by Power Factor but actual KVA= KW/Pf

9. Ruon
Nov 21, 2013

motor kVA from kW and cos phi will be rather E11/E13 than E11/E13
isn’t it ?

• Jay
Mar 06, 2019

Yes, it should be

10. R.Ganesh
Jul 13, 2013

This calculation has two mistakes.
1. You have not considered the existing load in the transformer during motor starting, as if we are starting the motor alone without any other load. Normally transformers will be loaded 50% of its rating. So atleast you have to consider 30 to 40 % load in the transfomer and add that to the motor starting KVA. Here it is around 300 to 400 KVA. IF you add that the transformer selection will be wrong.
2. Motor starting KVA can be calculated directly from starting KVA/HP, converting KW to HP, which will be 400HP*4.5 = 1800KVA, which is correct way of doing. In that case your figure is wrong. In both cases the transformer will not be suitable.

Usually in LV MCC, motor KW is restricted to 160KW the maximum due to the above reasons.

• Faisal Nawaz
Dec 01, 2018

otor starting KVA can be calculated directly from starting KVA/HP, converting KW to HP, which will be 400HP*4.5 = 1800KVA, which is correct way of doing. In that case your figure is wrong. In both cases the transformer will not be suitable.

where from u get this.??

11. vins231
Jul 11, 2013

Yup they are right it is not downlodable. i am trying since two months and for your information when we registered ourself then only we are able to give comments here. pls do the needful

12. Eduardo Vega
Jul 10, 2013

Muchas gracias.

13. Jatin Joshi
Jul 08, 2013

Calculation is good but i understand that it does not include starting method of motor??? or does it include this effect in form of motor code. This is one gray area I am not clear about. Can you please confirm the same please….

Thank you
J

14. nirbhik.modi
Jul 08, 2013

15. celebrationcometh
Jul 08, 2013

• Edvard
Jul 08, 2013