Troisième année Lesson 7 of 13 · about 13 min
Motor starting methods
What you’ll learn
- Why a motor’s starting current is a problem worth solving, and the trade every reduced-voltage method makes.
- How across-the-line, primary resistor, autotransformer, wye-delta, soft starter and VFD starts work.
- How to work the line current and torque each one gives, as a fraction of a full-voltage start.
- Where each one fits, and where the motor-control labs let you run one.
Before this: Induction motors · Motor control course
At standstill an induction motor draws several times its running current. Every starting method other than across-the-line exists to reduce that surge — and every one that does it by lowering the voltage pays for it in torque, because a motor’s torque falls with the square of its voltage. The drive is the exception, and that is why drives took over.
Across-the-line (direct-on-line)
A contactor connects the motor straight to full voltage. Full starting current, full starting torque, the fastest acceleration, the simplest and cheapest gear. This is the start/stop and seal-in circuit you have already built. It is the right answer for most small motors. It becomes the wrong one when the voltage dip upsets the rest of the building, when the utility limits starting current, or when full torque would snap a belt or hammer a pipe.
The reduced-voltage family
| Method | How it lowers the voltage | Line current | Starting torque |
|---|---|---|---|
| Primary resistor (or reactor) | Resistors in series with each line, shorted out after a time | = the fraction of voltage (e.g. 65%) | = fraction² (e.g. 42%) |
| Autotransformer | A tapped autotransformer feeds the motor, then is switched out | = tap² (e.g. 42% on 65%) | = tap² (e.g. 42%) |
| Wye-delta | Starts the delta-run windings in wye, then reconnects | 1/3 | 1/3 |
| Part-winding | Energises half of a split winding first, then the rest | reduced — depends on the motor | reduced — depends on the motor |
| Soft starter | SCRs chop the voltage and ramp it up | adjustable | falls with voltage² |
The autotransformer beats the resistor at the same motor voltage because it is a transformer: stepping the voltage down on the motor side steps the current down on the line side by the same ratio. The resistor just burns the difference.
Worked: one motor, four starts
- A motor draws 600 A locked-rotor across the line (a practice figure) and makes 100% of its locked-rotor torque.
- Primary resistor to 65% voltage: 0.65 × 600 = 390 A line current; torque 0.65² = 42%.
- Autotransformer, 65% tap: the motor draws 390 A at 65% voltage; the line draws 0.65 × 390 = 253.5 A (42%); torque 42%.
- Wye-delta: 600 ÷ 3 = 200 A; torque 33%.
- Question the numbers ask: will 33% or 42% of locked-rotor torque still accelerate the load? A fan with its damper closed, yes. A loaded conveyor, maybe not — and a motor that cannot accelerate sits at high current until the overload trips.
Wye-delta in a little more detail
The motor must be built to run in delta on your supply, with both ends of each winding brought out (six leads, or twelve on a dual-voltage motor). The starter connects the windings in wye so each sees 1/√3 of its run voltage; a timer lets the motor come up to speed, then the wye contactor drops and the delta contactor pulls in. Mechanical and electrical interlocks stop both being in at once — that would short the supply. The wye-delta lab has the whole control circuit to run and break.
In an open-transition starter the motor is disconnected for a moment during the switch and drifts out of step with the supply, so reconnecting it causes a spike of current and torque. Closed-transition starters bridge the gap through resistors to smooth it.
Soft starters and drives
A soft starter uses back-to-back SCRs to chop each phase and ramp the voltage up over a set time, often with a current limit. It is smooth and adjustable, and most bypass themselves with a contactor once up to speed. It still lowers torque with voltage — it is a better-behaved resistor starter. A VFD lowers the frequency and voltage together, so the motor starts on the steep part of its speed-torque curve at low speed: near full torque at around rated current. It also controls speed and stopping. The fourth-year VFD lesson picks it up from there.
Sur le chantier
Tips
- Before changing a starting method, ask what the load needs to break away and accelerate. Less current always comes with less torque (except on a drive).
- On a wye-delta, confirm the motor is delta-run for your voltage from the nameplate, and match the lead numbers to the maker’s diagram.
- Set the wye-delta timer so the motor is close to full speed before transition; switching early gives a near full-voltage start anyway.
- Starter protection and overload settings are code-book and nameplate work, not rules of thumb.
Common mistakes
- Wiring a wye-delta starter to a motor built to run in wye.
- Mixing up the lead numbers so one winding is reversed at transition — the motor growls and draws heavy current in delta.
- Assuming a soft starter gives full torque at low current. That is a drive.
- Forgetting the interlocks between the wye and delta contactors.
- Blaming the overload when a reduced-voltage start simply cannot accelerate the load.
Check yourself
-
1.A motor is started wye and then switched to delta. During the wye start, each winding sees what fraction of its run voltage?
In wye each winding is line to neutral point, so it gets line ÷ √3 = 58% of the voltage it sees in delta.
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2.Compared with an across-the-line start, a wye-delta start reduces the line current and the starting torque to about…
Each winding gets 1/√3 of the voltage, so winding current falls to 1/√3 — and in wye the line current IS the winding current, where in delta it would have been √3 times bigger. Combined: 1/3. Torque goes with voltage squared: (1/√3)² = 1/3.
-
3.An autotransformer starter on its 65% tap. How does the LINE current compare with an across-the-line start?
The motor sees 65% voltage and draws 65% of its locked-rotor current. The autotransformer then steps that current DOWN on the line side by the same 0.65 ratio: 0.65 × 0.65 = 0.42. That is the autotransformer’s advantage over a resistor.
-
4.A primary resistor starter drops the motor voltage to 65%. Line current and starting torque are about…
A resistor is in series with the motor, so line current = motor current = 65%. Torque goes with voltage squared: 0.65² = 42%. Same torque as a 65% autotransformer, but more line current for it.
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5.What does a wye-delta starter need from the motor?
The starter has to reconnect the windings, so it needs both ends of each. And the motor must be rated to run in delta on that supply; start a wye-run motor this way and it would run in delta at √3 times its rated winding voltage.
-
6.Why do reduced-voltage starting methods reduce starting torque?
Torque goes with voltage squared. That is the trade: less current means less torque, so a heavily loaded machine may not accelerate on a reduced-voltage start. A VFD avoids the trade by lowering voltage AND frequency together.
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7.Which starting method can give full rated torque from standstill while drawing only around rated current from the line?
A drive starts the motor at a low frequency with a matching low voltage, so the motor runs on the steep part of its speed-torque curve from the first moment — full torque with small slip and modest current. A soft starter only lowers the voltage, which lowers torque.
-
8.An open-transition wye-delta starter is known for…
In open transition the wye contactor opens before the delta contactor closes. The spinning motor drifts out of step with the supply in that gap, so reconnection causes a spike. Closed-transition starters bridge the gap with resistors to avoid it.
Questions fréquentes
Why not start every motor across the line?
Most small motors are started across the line, and it is the simplest and toughest method. The trouble comes with bigger motors: the starting surge dips the voltage for everything else on the supply, can trip upstream protection, may breach a utility limit, and slams the driven machine (belts, gearboxes, pipes full of water) with full torque. Reduced-voltage starting trades some torque for a gentler start.
Soft starter or VFD?
A soft starter only ramps the voltage up at start (and often down at stop), then usually bypasses itself — cheaper and simpler, but it cannot run the motor at reduced speed and it gives less torque while starting. A VFD controls speed the whole time and gives near-full torque at low current, at a higher price and with harmonics and heat to manage.
Does reduced-voltage starting reduce the energy used?
Not meaningfully. The start lasts seconds. What it reduces is the peak current and the mechanical shock. Energy savings come from running at reduced speed (a VFD on a fan or pump), not from a gentler start.
Voir aussi
- Wye-delta starting lab — the timer, three contactors and interlocks — run it
- Start/stop and the seal-in — the control circuit every starter builds on
- Forward / reverse lab
- How an AC induction motor works — the lesson before
- How a VFD works
- Reading a motor nameplate — code letter and lead markings
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