Penbro Kelnick

Engineering guide

Motor starting resistor calculator and guide

Review stator starting resistors and wound-rotor resistor duty for LV and MV industrial motors, including starting current, voltage drop, start duration, pulse power and start energy.

Motor starting resistor context

Stator starting

Stator starting resistors sit in series with the motor supply. They reduce motor terminal voltage and inrush current during direct-on-line alternatives or reduced-voltage starting schemes, but available starting torque also reduces, so the load acceleration check is critical.

Rotor starting

Rotor starting resistors apply to wound-rotor or slip-ring motors. External resistance is inserted in the rotor circuit to shape motor starting current and torque, then progressively shorted out as the motor accelerates.

Steps and duty

Single-stage banks suit simple current limiting. Multi-step banks soften the transition during acceleration. The step values shown here are indicative and still need motor data, load inertia and starts-per-hour review.

Stator starting

Series stator resistor estimate

Full-load current

54.2 A

Start current

162.5 A

Total resistance / phase

8.21 ohm

Approx. R in circuit / phase

Step 18.21 ohm

Pulse duty

650.2 kW

Start energy

6502 kJ

Rotor starting

Wound-rotor resistor estimate

Total external R / phase

2.31 ohm

Approx. R in circuit / phase

Step 12.31 ohm
Step 21.54 ohm
Step 30.77 ohm

Pulse duty

623.5 kW

Start energy

7482 kJ

Rotor resistor values must be confirmed from the motor datasheet and required torque-speed curve. Multi-step starters, slip-ring ratings and start frequency can change the final resistor bank design.

Request motor starting resistor review

Adjust the stator and/or rotor section before clicking. The contact message will copy the relevant calculator section only; if all values are still at their defaults, no calculator values are prefilled.

These calculations estimate resistor duty from the selected current, voltage and start duration only. Equal step values are preliminary. Final design must be checked against motor data, acceleration time, load torque, starts per hour, cooling time, switching sequence, enclosure ventilation, insulation level, site conditions, applicable standards and manufacturer data before procurement or commissioning.