The soft starter

A soft starter is a voltage controller: it lowers the voltage at the motor terminals during starting, then raises it to full. It does this without transformers, using power semiconductors that switch on part way through each half-cycle.

Phase-angle control

In each phase, two thyristors are connected in anti-parallel, one for each half-wave. A thyristor conducts from the moment it receives a gate pulse until its current falls to zero. Delay the pulse by the firing angle α after each zero crossing, and the motor only receives the rest of each half-wave.

On a resistive load the RMS output voltage is

$$V_\text{out} = V\sqrt{1 - \frac{\alpha}{\pi} + \frac{\sin 2\alpha}{2\pi}}$$

At α = 0 the full wave passes; at α = 180° nothing does. During a start, the controller begins at a large α and reduces it over a few seconds.

Firing angle α

60°

• Supply voltage
• Voltage on the motor

Output RMS voltage

206 V (90 %)

Fundamental (50 Hz part)

193 V

Harmonic distortion (THD)

38 %

3rd harmonic

28 %

Starting torque (Lab I motor)

59 N·m (70 %)

Starting current

88 A (84 %)

Resistive load shown for clarity. On a motor, current lags the voltage, so each thyristor keeps conducting a little past the zero crossing, but the principle is the same: a later firing angle means less voltage. Torque scales with the square of the fundamental voltage, current with the voltage.

FoundationStart here if this is new to you

Imagine a door that opens for every wave of people passing, but only after a delay. Open it straight away and everyone goes through. Open it half-way through each wave and only half the people get in. The soft starter is a very fast door for current, opening a little later or earlier in every half-cycle, a hundred times a second.

What it changes, and what it does not

The motor's torque depends on the square of the fundamental voltage, its current on the voltage itself:

$$T \propto V^2, \qquad I \propto V$$

At α = 90° on the widget, the 50 Hz part of the output is 136 V, 59 % of the supply: the Lab I motor's starting current falls to 62 A (59 %) but its starting torque falls to 30 N·m (35 %).

Predict first

A soft starter holds the voltage at 60 % during a start. What starting torque does the motor give, compared with direct on line?

60 %36 %77 %

A soft starter does not change the frequency, so it cannot control speed: the synchronous speed stays at $120 f / p$. Once the motor is up to speed, lowering the voltage only increases the slip and the losses.

Features that matter in practice

• Current limit. Instead of a voltage ramp, the controller adjusts α to hold the current at a set value, for example 3 × rated.
• Kick start. A short pulse of high voltage to break the load away from standstill, then the normal ramp.
• Soft stop. Ramping the voltage down when stopping a pump, so the water column decelerates gently. Stopping a pump abruptly can cause water hammer: a pressure surge that bangs valves and can burst pipes.
• Gentle handling. A typical ramp takes the voltage from about 30 % to 100 % in a few seconds. The course's example is a bottling-plant conveyor: direct on line or star-delta would jerk the belt and topple the bottles, while a 5 s ramp holds the torque just above friction.
• Bypass contactor. Each thyristor drops about 1 to 1.5 V while conducting, which on a large motor means hundreds of watts of heat. Once the motor is at speed, a contactor bypasses the thyristors.
• Harmonics. The chopped waveform is rich in harmonics while α is large (at 90°, THD 65 % on a resistive load). They only last during the ramp, so they rarely matter to the supply.

ExplorerGo deeper: derivations and open questions

Derive the RMS formula. Integrate $v^2 = 2V^2\sin^2\theta$ from α to π and divide by π. Check your result against the widget at α = 60° (206 V on 230 V).

Inductive load. On a motor, the current lags the voltage by the angle φ. Show that for α < φ the thyristors conduct continuously and the output is the full sine: a soft starter has no control below α = φ. What sets φ during a start?

Three-phase controllers. Some soft starters control only two of the three phases, the third being wired straight through. What does that save, and what does it cost in terms of current balance and motor heating?