Starting methods · DriveLab

At standstill an induction motor draws 5 to 8 times its rated current (Chapter 1, lesson 2). That inrush dips the supply voltage for everyone on the line, heats the cables and protection, and the matching torque jolt stresses couplings, belts and pipes. Every starting method trades some of that current for something else: starting torque, time, cost or complexity.

Direct on line

The contactor closes and the motor sees full voltage at once (the seal-in circuit of lesson A6). It is the simplest, cheapest and gives the highest starting torque. It is the right choice whenever the supply is strong enough and the machine can take the jolt. Many utilities limit the size of motor allowed to start direct on line on a given supply. It is also called full-voltage non-reversing (FVNR) starting. In practice it is kept to smaller motors, typically below 5 to 10 kW depending on the network: a small dewatering pump on a building site, for example.

Star-delta

A motor whose windings are rated for the line voltage in delta (for example 400 V Δ on a 400 V network) is started in star, then switched to delta. In star each winding sees $1/\sqrt{3}$ of its voltage, so: The timer changes over to delta once the motor has reached about 75 to 85 % of its rated speed.

I_\text{line, star} = \tfrac{1}{3}\,I_\text{line, DOL}, \qquad T_\text{star} = \tfrac{1}{3}\,T_\text{DOL}

The current falls by 3, not by $\sqrt{3}$ , because two effects combine: each winding carries $1/\sqrt{3}$ of its delta current, and in star the line current equals the winding current instead of being $\sqrt{3}$ times it. Torque goes as voltage squared: one third.

Two weaknesses follow. The starting torque is only a third, so a heavily loaded machine may never accelerate in star. And the changeover briefly disconnects the motor (open transition), then reconnects it at full voltage: a second current spike.

Autotransformer

An autotransformer feeds the motor a fraction $x$ of the line voltage (taps of 50, 65 and 80 % are common). The motor current falls to $x$ , and because the transformer steps the current down again on the line side, the line current falls to $x^2$ . The torque also falls to $x^2$ . For the same line current, it gives more torque than star-delta and lets you choose the tap, at a higher cost. At the 65 % tap, current and torque both fall to $0.65^2 \approx 0.42$ of direct on line. The course's example is a 75 kW centrifugal compressor: too large for direct on line, with too much inertia for star-delta's fixed third, started on the 65 % or 80 % tap.

Soft starter and VFD

A soft starter ramps the voltage up with thyristors (next lesson): current scales with the voltage, torque with its square. A VFD ramps the frequency instead, so the motor runs on the stable part of its curve from the first instant. It gives full torque at about rated current, and its line current is even lower at low speed, because the drive only draws the power the motor actually uses.

Method	Line current at start (× DOL)	Torque at start (× DOL)	Remarks
Direct on line	1	1	simplest; full jolt
Star-delta	1/3	1/3	cheap; spike at changeover; delta-rated motor
Autotransformer, tap $x$	$x^2$	$x^2$	adjustable; bulky, costly
Soft starter, voltage $k$	$k$	$k^2$	smooth; torque falls fastest
VFD	≈ rated current	up to rated or more	best control; highest cost; also controls speed

Reading the starter schematics (Tutorial 2.2)

DOL starter. Power circuit: L1-L2-L3 through the breaker QF1, the main contacts of contactor KM1 and the thermal elements of overload relay F2, to motor M1. Control circuit, in series: QF1 auxiliary contact, STOP S1 (NC), START S2 (NO) with the seal-in contact KM1 (13-14) in parallel, overload contact F2 (95-96), coil KM1.

KM1 (13-14) is the seal-in: once KM1 closes it bypasses START, so the coil stays energised when START is released.
KM1 energises only if QF1 is on, STOP is not pressed, F2 has not tripped, and START is pressed (or the seal-in is already closed).
Once running, STOP, an F2 trip or a QF1 trip stops the motor.

Automatic open-transition star-delta starter. START energises the main contactor KM1, which seals in; the timer KT1 starts and the star contactor KM2 closes: the motor starts in star. After the delay, KT1's NC contact drops KM2 and its NO contact closes the delta contactor KM3. The NC contact of KM3 in the star rung and of KM2 in the delta rung are interlocks: they make it impossible for KM2 and KM3 to be closed together, which would be a phase-to-phase short circuit.

Try both in the simulator: press START and STOP, trip F2, and switch to star-delta.

Try it: ladder logic starter

Overload relay tripped (F2)

Motor stopped

Hold START briefly, release it, and watch the KM1 seal-in contact keep the coil energised.

The race

Same Lab I motor, same load, five methods. The readouts give each method's peak line current as a multiple of full-load current, and its time to reach speed.

Try it: the starting-methods race

Load torque at full speed60 N·m

Inertia J0.40 kg·m²

Autotransformer tap65 %

Soft-starter initial voltage40 %

Direct on line
Star-delta
Autotransformer
Soft starter
VFD

Direct on line: 5.4× · 0.48 s
Star-delta: 3.7× · 1.66 s
Autotransformer: 3.7× · 1.22 s
Soft starter: 3.0× · 1.92 s
VFD: 0.9× · 4.63 s

Each value: peak line current as a multiple of full-load current (I_n = 19.3 A), then time to reach 90 % speed. Star-delta and autotransformer change over at 80 % speed, or after 6 s at the latest. Soft starter ramps from its initial voltage to 100 % in 4 s; VFD ramps 0 to 50 Hz in 5 s.

With the default fan load, direct on line peaks at 5.4 times full-load current and is up to speed in half a second. Star-delta starts at 1.8 times but spikes to 3.7 times at the changeover. The soft starter stays at about 3 times, and the VFD never exceeds full-load current.

Predict first

Switch to a constant-torque load of 50 N·m (a loaded conveyor). What happens with star-delta?

It starts, just more slowlyIt stays stuck in star, then takes the full DOL current when it switches to deltaIt trips at once

ExplorerGo deeper: derivations and open questions

Closed-transition star-delta. A fourth contactor and a set of resistors can keep the motor connected during the changeover. Why does the open transition cause such a spike? (Hint: the rotor field is still spinning when the supply is reconnected, and it may be out of phase with it.)

Acceleration time. For each method the time to speed follows from $J\,d\omega/dt = T_m(\omega) - T_L(\omega)$ (lesson A3). Why does the soft starter on a fan take longer than star-delta, even though its current is similar? Look at the torque it gives in the first second.

Voltage dip. A 5 × 20 A starting current through a supply impedance of 0.2 Ω per phase drops 20 V. What does that do to the torque of the starting motor, and to other motors on the same bus?