## Thursday, October 28, 2010

### Ohm's Law

The formulas of Ohm's Law

Ohm's Law can be rewritten in three ways for calculating current, resistance, and voltage.

If a current I should flow through a resistor R, the voltage V can be calculated.

V = R × I

If there is a voltage V across a resistor R, a current I flows through it. I can be calculated.

I = V / R

If a current I flows through a resistor, and there is a voltage V across the resistor R can be calculated.

R = V / I

Name Formula sign Unit Symbol
voltage V or E volt V

current I ampere (amp) A

resistance R ohm Ω

power P watt W

Eddy Current Inspection Formula - Ohm's Law
I = Current (amp)

V = Voltage (volt)

Z = Impedance (ohm)

Ohm's Law is a mathematical equation that shows the relationship between electric voltage (V), current (I) and impedance (Z). Ohm's Law is often seen with impedance replaced by resistance, which is fine for electricians wiring a house with only resistance in the circuits. However, in nondestructive testing the circuits also have inductive reactance and capacitance which also impede the flow of electricity. Therefore, impedance is used in this version of Ohm's Law. Ohm's Law can be expressed in the following three ways.

V = I x Z

Z = V / I

I = V / Z

Ohm's law is used in eddy current testing to determine the change in impedance caused by some change in the material or loading of the test coil. Eddy current instruments typically apply a constant AC current and monitor the voltage or visa versa. A change in the voltage (or current) in an eddy current test system can be directly related to a change in impedance.

### Standard Electrical Formulas Used for Power Consumption Calculations

Standard Electrical Formulas Used for Power Consumption Calculations

TO DETERMINE SINGLE-PHASE THREE-PHASE DIRECT CURRENT Amperes (when HP is known)

HP x 746 E x %EFF x PF HP x 746 1.73 x E x %EFF x PF HP x 746 E x %EFF

Amperes (when kW is known)
KW x 1000 E x PF

KW x 1000 1.73 x E x PF

KW x 1000 E Amperes (when KVA is known) KVA x 1000 E KVA x 1000 1.73 x E Not Applicable

Horsepower
I x E x %EFF x PF 746

I x E x 1.732 x %EFF x PF 746

I x E x %EFF 746 Kilovolt/ Amperes I x E 1000 I x E x 1.73 1000 Not Applicable

Kilowatts
I x E x PF 1000

I x E x 1.73 x PF 1000

I x E 1000

I = amperes E = volts KW = Kilowatts KVA = kilovolt/amperes HP = horsepower % Eff. = percent efficiency PF = power factor

### Symbols and terms used in the measurement of compressed air volumes

Common symbols and terms used in the measurement of compressed air volumes

M3/Min = Cubic metres per minute
M3/Hr = Cubic metres per hour
L/S = Litres per second
L/Min = Litres per minute

To convert from Metric Measurement to C.F.M use the following factors:

M3/Min x 35.32 = C.F.M.
M3/Hr x 0.589 = C.F.M.
L/S x 2.12 = C.F.M.
L/Min x 0.0353 = C.F.M.
L/S x M/Min x 0.06
L/S x M/Hr x 3.6

To calculate volume of air from kilowatt / Horsepower

KW x 5.2 = C.F.M.
HP x 4.2 = C.F.M.

To calculate number of AMPS drawn by electrical motors (Indication Only)

A) 380 volt supply KW x 2 = AMPS
B) 525 volt supply KW x 1.8 = AMPS

To calculate starting AMPS
A) Star Delta AMPS x 3

B) Direct on load AMPS x 6