Induction motor starting methods: DOL, star-delta, soft starter, VFD

Why the starting current is so high

While the rotor is at rest, slip is 100%. The rotating field cuts the rotor conductors at full speed, so the induced rotor current is at its maximum. This is reflected into the stator, and the only limit is the leakage impedance of the windings. Typical results:

• Locked-rotor current: 5 to 8 times rated current for standard cage motors
• Starting torque: 1.5 to 2.5 times rated torque
• Duration: until the motor approaches full speed, typically 1 to 10 seconds

A large inrush causes a voltage dip on the supply, trips protection and stresses the windings. A reduced-voltage start protects the network but costs torque, because torque varies with the square of the voltage:

T ∝ V²

The choice of method is a trade-off between inrush current, starting torque and cost.

Direct on line (DOL)

The motor is connected to full voltage with a single contactor. It gives the full starting torque and the highest inrush current. It is the cheapest and simplest method and is used for small motors, often up to a few kW, or whenever the supply is strong enough to absorb the inrush. Utilities often limit DOL starting to a given motor size.

Star-delta starting

A motor that runs in delta on the supply voltage, for example 400 V on a 400/690 V plate, is first connected in star, then switched to delta after a few seconds. In star each winding receives 1/√3 of the line voltage, so

• phase current is 1/√3 of its delta value,
• line current is 1/3 of the direct-on-line value,
• torque is 1/3 of the direct-on-line value.

It needs three contactors and a timer and a motor with six terminals brought out (see the star-delta guide). The switch-over briefly interrupts the supply and can cause a current and torque spike. Star-delta suits light starts such as fans, pumps and unloaded machines, but not loads that need more than a third of the full starting torque.

Autotransformer and soft starter

An autotransformer starter feeds the motor from a tap of a three-phase autotransformer, such as 50%, 65% or 80% of line voltage, then bypasses it. At a tap k, the motor current and torque scale as k and k², and since the transformer ratio also reduces the line current, the line current is k² of the DOL value. A 65% tap gives about 42% of the DOL line current and torque. It is robust and suited to large motors but is bulky and expensive.

A soft starter uses back-to-back thyristors in each line. The voltage ramps up from an initial value to full over a set time, typically 2 to 30 seconds, then a bypass contactor closes. It gives smooth acceleration, an adjustable current limit and soft stops, but the line current equals the motor current, so at a voltage ratio k the torque is k² and the current k.

VFD and rotor resistance starting

A variable frequency drive starts the motor at low frequency and raises frequency and voltage together. The motor operates near the steep, stable part of its torque–speed curve throughout, so it can produce high torque at start (roughly rated torque or more) while drawing about rated current or somewhat more. A VFD is the best option for torque and smoothness, and also gives speed control (see the V/f control guide), but it costs the most.

A wound-rotor (slip-ring) motor takes a different route. A resistor in the rotor circuit, connected through the slip rings, raises the starting torque while reducing the current. The torque at breakdown stays the same, but its slip increases in proportion to the total rotor resistance. When the rotor resistance (referred to the stator) is approximately equal to the standstill leakage reactance, the breakdown torque occurs at standstill and the starting torque is at its maximum. The resistance is cut out in steps as the motor speeds up. This was once the standard way to start heavy loads such as cranes, mills and compressors, and it is now largely replaced by VFDs.

Worked example and comparison

Take a 4 kW, 400 V motor with a rated current of 8.2 A and a rated torque of 26.5 N·m, with typical starting values of 6 times rated current and 2 times rated torque on DOL.

• DOL: 6 × 8.2 = 49 A, 53 N·m
• Star-delta: 49 / 3 ≈ 16 A, 53 / 3 ≈ 18 N·m
• Autotransformer 65%: 0.65² × 49 ≈ 21 A, 0.65² × 53 ≈ 22 N·m
• Soft starter at 50% initial voltage: 0.5 × 49 ≈ 25 A, 0.5² × 53 ≈ 13 N·m
• VFD: roughly rated current (about 8 to 12 A) at around rated torque or more, at low frequency

In summary: use DOL when the grid allows it, star-delta for cheap inrush reduction on light starts, autotransformer where a higher torque is needed, a soft starter for smooth, adjustable starts, a VFD where speed control or full torque at start is needed, and rotor resistance for wound-rotor drives with heavy starts.

Try it in the induction motor simulators

In the three-phase induction motor simulator, the Starting method control offers Direct on line (DOL), Star-delta, Autotransformer, Soft starter and Variable frequency drive (VFD). The related settings are Switch-over time to full voltage (default 3 s), Autotransformer tap (default 65%), Soft start initial voltage (default 40%) and Ramp time (default 5 s). Read the Peak starting current, Peak starting current (× In) and Start time readouts, and press Power on in the Speed and current curves experiment to see the time curves.

• Run with DOL and note the peak current ratio.
• Select Star-delta. The simulator applies 1/√3 of the voltage during the first stage, with a Y→Δ marker on the chart where it changes to full voltage after the Switch-over time.
• Select Autotransformer and Soft starter. Compare the peak currents with the k and k² rules above (the peak can occur at the switch-over to full voltage), and note the longer Start time.
• Select Variable frequency drive (VFD). Frequency and voltage ramp together from a low value (about 1 Hz) to the set values over the Ramp time.
• Raise Mechanical load or try a different Load type, and see which methods accelerate slowly or fail to start.

The wound-rotor induction motor simulator has the same controls plus External rotor resistance, Rotor resistance mode (Fixed or Stepped starter), Starter resistance at switch-on, Number of steps and Time per step. Compare the Starting torque and Starting torque (rings shorted) readouts as you increase the resistance, and watch Slip at breakdown torque move.

Try it in the simulator