Starting Current and Running Current
In electrical machines, especially single-phase and three-phase motors used in refrigeration and air conditioning systems, understanding starting current and running current is very important. These two parameters help in analyzing motor performance, safety, and efficiency.
Starting Current
Definition
Starting current is the current drawn by a motor at the moment it is switched ON. At this stage, the rotor is stationary and the motor behaves like a short circuit, drawing a large amount of current from the supply.
Characteristics of Starting Current
- Very high in magnitude (typically 5 to 7 times the running current)
- Flows for a short duration during starting
- Depends on motor type and load conditions
Effects of High Starting Current
- Voltage drop in the supply system
- Heating of motor windings
- Mechanical stress on motor components
Methods to Reduce Starting Current
- Using capacitors (in single-phase motors)
- Star-delta starters (in three-phase motors)
- Soft starters or variable frequency drives (VFD)
Running Current
Definition
Running current is the current drawn by the motor during its normal operation after it reaches its rated speed.
Characteristics of Running Current
- Lower than starting current
- Remains nearly constant during steady operation
- Varies depending on load on the motor
Factors Affecting Running Current
- Load on the motor
- Supply voltage
- Motor efficiency
- Condition of windings and components
Difference Between Starting Current and Running Current
| Feature | Starting Current | Running Current |
|---|---|---|
| Definition | Current at the time of starting | Current during normal operation |
| Magnitude | Very high | Low |
| Duration | Short time | Continuous |
| Effect | Heating and voltage drop | Normal operation |
Importance in RAC Systems
In refrigeration and air conditioning systems, compressors and fan motors draw high starting current. Proper control and protection devices such as overload protectors, relays, and capacitors are used to manage starting current and ensure smooth operation.
Conclusion
Starting current and running current are key parameters in motor operation. Starting current is high and short-lived, while running current is lower and continuous. Understanding these concepts helps in proper motor selection, protection, and maintenance in RAC systems.