Motor starting presents unique voltage drop challenges due to inrush currents 6-8 times higher than running current. Understanding and managing this temporary but significant voltage sag is essential for reliable motor operation and avoiding disturbances to other equipment.

The Motor Starting Challenge

When an induction motor starts across-the-line (DOL starting), it draws locked-rotor current—typically 6-8 times its full load running current—until it accelerates to near operating speed. This high current creates a temporary but severe voltage drop that can:

Reduce starting torque (torque varies with voltage squared)
Cause visible light flicker in nearby lighting
Trip sensitive electronic equipment
Affect other motors on the same system

Calculation Example

50 HP Motor Starting Analysis

480V 3Φ, 200ft run, 4 AWG copper (R = 0.308 Ω/kft)

Running (65A FLC)

Vd = (1.732 × 65 × 200 × 0.308) / 1000

Vd = 6.93V (1.44%)

Starting (455A @ 7× FLC)

Vd = (1.732 × 455 × 200 × 0.308) / 1000

Vd = 48.5V (10.1%)

While 1.44% running voltage drop is excellent, the 10.1% starting drop would cause significant disturbance to other equipment and might prevent the motor from starting under load.

Mitigation Strategies

Soft Starters

Limit starting current to 2-4× FLC by ramping up voltage gradually. Reduce starting drop by 50-70%.

VFDs

Limit starting current to 1-1.5× FLC. Virtually eliminate starting voltage drop concerns.

Larger Conductors

Upsize feeders to reduce both running and starting drops. Higher upfront cost but simpler system.

Analyze Your Motor Circuit

Use our voltage drop calculator to evaluate both running and starting conditions for your motor circuits.

Motor Circuit Calculator

Managing Motor Starting Voltage Drop