How do different starting methods reduce motor inrush current?

Star-Delta starting reduces line voltage across windings, lowering inrush to one-third (2.0-2.5x FLC). Soft starters ramp voltage via thyristors to 2.5-4.0x FLC. VFDs regulate frequency and voltage to deliver rated torque at only 1.0-1.5x FLC.

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Motor Full Load & Starting Current

Full load current from the nameplate rating, then the starting current that the cable, protective device and any generator upstream actually have to cope with. Three-phase uses I = P / (√3 × V × pf × η).

Results

Full load current -

Electrical input power -

Apparent power -

Starting current -

Starting multiple applied -

Starting kVA -

Why the efficiency figure matters. A motor nameplate says what comes out of the shaft, not what goes in at the terminals. A 7.5 kW motor at 90 % efficiency draws about 8.3 kW of electrical power, and it is that larger figure the cable has to carry. Leaving efficiency out undersizes the circuit by roughly a tenth.

Worked example

7.5 kW three-phase motor, 400 V, pf 0.85, efficiency 90 %, DOL start.

Input power = 7.5 / 0.90 = 8.33 kW

FLC = 8330 / (1.732 × 400 × 0.85) = 14.1 A

Starting current = 14.1 × 6.5 = 91.9 A

On star-delta the same motor starts at roughly a third of that, about 30.6 A - which is the whole reason star-delta exists.

Starting multiples are typical values, not standard figures. Where the starting current is load-bearing - generator sizing, discrimination, or a marginal supply - get the actual figure from the motor data sheet. Verify protective device selection against BS 7671.

Regulation reference: BS 7671 Section 552 covers rotating machines, including the requirement that circuits supplying motors are suitable for both starting and running conditions.

Frequently Asked Questions

Why must motor shaft output power (kW) be converted to electrical input power?

Under BS EN 60034-1, the kilowatt rating stated on an electric motor nameplate represents mechanical shaft output power, not electrical input power. Because motors incur internal electrical and mechanical losses (copper, iron, friction, and windage), electrical power drawn from the supply is P_elec = P_shaft / η. Sizing cables or overcurrent protection using shaft power alone underestimates circuit loading by 8% to 15%.

What is the difference between locked-rotor starting current and full-load running current (FLC)?

Full-load current (FLC) is continuous steady-state current drawn when the motor operates at rated shaft power and voltage. At stand-still (zero rotor speed), the motor produces zero back-EMF and behaves like a short-circuited transformer. On Direct-on-Line (DOL) starting, locked-rotor inrush current reaches 6.0 to 8.0 times FLC before decaying to rated FLC as rotor speed accelerates towards synchronous speed.

How do different starting methods (Star-Delta, Soft Starter, VFD) reduce inrush current?

Star-Delta starting reconnects motor windings in star during acceleration, reducing phase voltage by 1/√3 and inrush current to one-third (approx 2.0–2.5× FLC). Electronic Soft Starters use thyristors to ramp voltage smoothly, limiting inrush to 2.5–4.0× FLC. Variable Frequency Drives (VFDs) control frequency and voltage together (V/f), generating 100% full breakaway torque while drawing only 1.0–1.5× FLC from the mains.