Generator Sizing and Selection
Generator sizing is a crucial part of power system design, ensuring the generator can handle both continuous load demands and peak start-up conditions. Proper sizing and selection require evaluating power requirements, efficiency, load types (resistive, inductive, or capacitive), and potential future expansion.Key factors to consider in generator sizing:
1.Load Demands: This includes all electrical
devices, equipment, and systems that the
generator will power.
o Continuous Load: The normal operating
load that the generator must supply.
o Peak Load: Temporary surge loads, such as
the starting current of motors.
2.Start-up Conditions: Particularly important for inductive loads like motors, which require a
much higher starting current (typically 3-7
times the running current).
3.Efficiency and Power Factor: Generators
should be sized considering their efficiency
(typically around 80-90%) and the power factor
of the loads (usually 0.8 for industrial loads).
Generator Sizing Formula
Generator Size (kVA) = Total Load (kW) / Power
Factor (PF)
Given Values
Lighting Load: 15 kW
Motor Load: 20 kW (with a starting current
factor of 5)
Resistive Load: 10 kW
Power Factor (PF): 0.8
Calculations
Step 1: Calculate Total Load (kW)
Total Load (kW) = Sum of Individual Loads
(kW)
= Lighting Load + Motor Load + Resistive Load
= 15 kW + 20 kW + 10 kW
= 45 kW
Step 2: Calculate Peak Load due to Motor Start-Up
Starting Load (kW) = Running Load (kW) x
Starting Current Factor
= 20 kW x 5
= 100 kW
Total Peak Load (kW) = Total Load (kW) +
Starting Load (kW)
= 45 kW + 100 kW = 125 kW
Step 3: Apply the Power Factor (PF)
Generator Size (kVA) = Total Peak Load (kW) /
Power Factor (PF)
= 125 kW / 0.8
= 156.25 kVA
Step 4: Round Up for Safety Margin
Select a 160 kVA generator for safety and future expansion.