BASIC ELECTRICAL CALCULATIONS FOR FRESHERS INTERVIEW QUESTIONS AND ANSWERS
In this post we can learn about Basic Electrical calculations for fresh
engineering students. this Pdf very helpful to students who are preparing
for their first dream electrical job.
Computing Watts When Volts and Amps are Known
POWER (WATTS) = Volts x Amps
• For example, a small computer has a nameplate that shows 2.5 amps. Given a normal
120 Volt, 60 Hz power source and the ampere reading from equipment, make the
following calculation: POWER (WATTS) = 2.5Amps x 120Volts = 300 WATTS
Generally: P=IE
P= Power(WATTS)
I = Current(Amps)
E = Voltage(Volts)
So: I = P/E and E = P/I
Therefore: 1 Watt = 1 Ampere x 1 Volt
Kilovolt-Amps Calculation (kVA)
kVA is an abbreviation for "Thousand Volt-Amps."
A 2-Pole Single Phase 208-240 power source necessitates the use of two
hot wires from two different circuits.
(also known as poles) from a power distribution panel
SINGLE STAGE
Volts x Amps / 1000 = KILOVOLT-Amps (kVA).
Continuing with the previous example: 300 VA = 120 x 2.5 300 volts / 1000
volts equals.3 kVA
SINGLE-PHASE 208-240 (2-POLE SINGLE-PHASE)
• As an example, consider a 4.7 amp enterprise computer bvserver that
requires a 208-volt power supply.
240 volt power supply For our calculations, we'll use 220 volts.
Kilovolt-Amps (kVA) = Volts x Amps divided by 1000
220 times 4.7 equals 1034 1034 / 1000 equals 1.034 kVA
THREE-PHASE
• As an example, consider a large disk storage system loaded with disks.
According to the equipment documentation, a 50-amp 208-240 VAC power
source is required. Make no calculations for the plug or receptacle. For
the calculation, use 220 volts.
Kilovolt-Amps (kVA) = Volts multiplied by Amps multiplied by 1.73 / 1000
220 multiplied by 50 multiplied by 1.73 equals 19,030 19 kVA = 19,030 /
1000 This is rounded to 19.
Converting The difference between kW and kVA
• The power factor is the only distinction between kW and kVA. Unless
taken from the manufacturer's specifications, the power factor is an
approximation. We'll use a power factor of.95 in this example. The kVA
value will always be greater than the kW value.
kW to kVA kW / .95 = kVA
kVA TO kW kVA x .95 = kW
How to Convert kW-Hours to kVA
• There is NO kWH-to-kVA conversion. These are two distinct
measurements. kWH stands for energy, and kVA stands for power (not
necessarily dissipated). There is a relationship between kW (power) and
kVA (power). The power factor of the load is the result of this
relationship.
• Known Standard: 1 kW = 3413 BTUs (or 3.413 kBTUs) • Dividing the
electrical nameplate BTU value by 3413 may not yield the published kW
value. If the manufacturer provides BTU information, use it; otherwise,
use the above formula.
Horsepower Calculations for Beginners
Horsepower is defined as work done per unit of time. One horsepower
equals 33,000 ft-lb of torque per minute.
The work done by a torque source (T) to produce (M) rotations about an
axis is: radius x 2 x rpm x lb. or 2 TM
When the rotation rate is N rpm, the HP delivered is:
33,000 = TN / 5,250 HP = radius x 2 x rpm x lb.
For hoisting or vertical motion:
W x S / 33,000 x E = HP
W = total weight in pounds to be lifted by motor S = hoisting speed in
feet per minute
E denotes the overall mechanical efficiency of the hoist and gearing.
For the purpose of calculating E =.65 for the eff. of the hoist and
"connected gear."
Mechanical General Approximations - THUMB RULES
For quick approximations in the field, use these:
At rated HP output, a motor develops 1.5 lb-ft of torque per HP at
3600 rpm.
At rated HP output, a motor produces 3 lb-ft of torque per HP at 1800
rpm.
At rated HP output, a motor produces 4.5 lb-ft of torque per HP at
1200 rpm.
At rated HP output, a motor develops 6 lb-ft of torque per HP at 900
rpm.
A 3-phase motor draws 1 amp per horsepower at 575 volts.
A 3-phase motor draws 1.25 amps per horsepower at 460 volts.
A three-phase motor draws 2.5 amps per horsepower at 230 volts.