Current Units

Electrical current is the flow of electrons. Together with voltage, it determines the power consumed by compressors.

What is Current?

Current ($I$) is the amount of electrical charge flowing per second. Think of it as:

• Water flow rate in a pipe → Current in a wire
• Higher flow = more water per second
• Higher current = more electrons per second

Current Units

Unit	Symbol	Equivalence
Ampere (Amp)	A	Base unit
Milliampere	mA	0.001 A
Kiloampere	kA	1,000 A

Current in Compressors

Rated Current vs Starting Current

Type	Description	Typical Value
FLA (Full Load Amps)	Current at full load	Nameplate value
LRA (Locked Rotor Amps)	Starting current	5-8× FLA
RLA (Running Load Amps)	Actual operating current	80-100% FLA

Caution

Starting current can be 6-8 times higher than rated current:

$$I_{starting} = 6 \times I_{rated}$$

This affects cable and protection sizing.

Typical Currents by Power (480V Three Phase)

Power	Approximate Current
5 HP	7 A
10 HP	14 A
25 HP	34 A
50 HP	65 A
100 HP	124 A
200 HP	240 A

Power Factor

Not all current does useful work:

$$P_{real} = V \times I \times \cos\phi \times \sqrt{3}$$ $$P_{apparent} = V \times I \times \sqrt{3}$$ $$PF = \frac{P_{real}}{P_{apparent}} = \cos\phi$$

Power Factor	Meaning
1.0	Ideal - all current is useful
0.85-0.95	Typical for loaded motors
0.5-0.7	Underloaded motor or problems

Improve PF

A low power factor means you pay for current that doesn't do work. Capacitor banks can correct it.

Ohm's Law

The fundamental relationship between voltage, current, and resistance:

$$V = I \times R$$

Where:

• $V$ = Voltage (Volts)
• $I$ = Current (Amps)
• $R$ = Resistance (Ohms)

Current Measurement

Clamp Meter

Essential tool for compressor diagnostics:

Measurement	Indicates
High current	Overload, damaged bearings, low voltage
Low current	Underload, faulty valves
Unbalanced current	Phase or motor problem
Fluctuating current	Mechanical problem

Current Imbalance

In three-phase systems, all three phases should have similar current:

$$\text{Imbalance (\%)} = \frac{I_{max} - I_{average}}{I_{average}} \times 100$$

Imbalance	Status
< 10%	Acceptable
10-20%	Problematic
> 20%	Critical

Overcurrent Protection

Device	Function
Fuses	Short circuit protection
Breakers	Overload protection
Thermal relay	Motor protection
Motor protector	Integral protection