Result
Preliminary calculation: This tool is for quick sizing and information. Electrical installation designs and cable and protection selection must be done according to the relevant standards and by an licensed electrical engineer. Results are approximate.
Formula
P_input = P_shaft / η, I_n = P_input / (√3 × V × cosφ)I_start ≈ k × I_n × f_methodFor compressor and pump motors, the starting method affects the sizing of the grid connection and generator. Star-delta starting reduces the starting current but also reduces starting torque; it may be inadequate for high-inertia loads. Soft starters and inverters smooth the start; an inverter also provides speed control.
- The voltage drop during starting may affect other equipment
- When powered by a generator, the largest motor start is the limiting factor
- For compressors, the starting method recommended by the manufacturer takes precedence
Frequently asked questions
How many times the rated current is the motor starting current?
With direct-on-line starting it is generally several times the rated current; the exact value is taken from the motor nameplate or the catalog.
Why does star-delta reduce the current?
At start-up the windings are connected in star, the phase voltage drops and the current falls to about one third; torque also decreases.
How does an inverter affect starting current?
By raising voltage and frequency gradually, it keeps the starting current close to the rated current.