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.
• Motor Full Load Current<=65% of Transformer Full Load Current
• 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.

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Calculate Size of Contactor, Fuse, C.B, Over Load Relay of DOL Starter

Calculate Size of Contactor, Fuse, C.B, O/L  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

• 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 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.

Calcualte Number of Lighting Fixtures & Lux Level

• Calculate required No of Fixtures.
• Calculate required No of Lamps.
• Calculate Watts / Sq.foot.
• Calculate Energy Cost / Year.
• Calculate Actual Lux Level.

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