VSD or soft starter: choosing the right motor starting method

A three-phase induction motor started direct-on-line draws a very large current for a short time and delivers a torque step to the driven machine. Which of those is the problem determines whether the answer is a soft starter, a variable speed drive, or neither — and choosing on habit produces installations where a soft starter is fitted to a load it fundamentally cannot start.

What direct-on-line actually does

At the instant of energisation a squirrel-cage motor is, electrically, a short-circuited transformer: the rotor is stationary, slip is 1, and the current drawn is the locked-rotor current. For general-purpose industrial motors this is typically in the region of six to eight times full-load current, but the actual figure is a characteristic of the specific motor, given on its nameplate — read it there rather than assuming a multiple.

That current has two consequences. The electrical one is a voltage dip whose severity depends on the source impedance; on a weak supply or a generator it can disturb other equipment. The mechanical one is a torque transient: the motor develops its starting torque abruptly and the driven machine, its coupling, belts and gearbox absorb the step. On a pump the downstream effect is a pressure surge. For a small motor on a stiff supply driving a robust load none of this matters and DOL is the right answer.

Soft starters: what the physics allows

A soft starter uses back-to-back thyristors in each phase, controlling the conduction angle to reduce the r.m.s. voltage applied to the motor and ramping it up over a set time. The relationship that governs everything else is that motor torque varies with the square of applied voltage while current varies roughly in proportion to it. Halve the voltage and you get roughly half the current — and a quarter of the torque. That single fact is the whole selection criterion:

• A soft starter helps only if the load’s breakaway torque is comfortably below the torque the motor develops at the reduced voltage. Centrifugal pumps and fans, whose load torque rises with speed, are ideal. A loaded conveyor, a crusher, a positive-displacement pump or a mixer starting into settled product may demand near-full torque from standstill, and a soft starter will fail to break it away or stall on the ramp.
• The current reduction is bounded by the torque the load needs. You cannot specify a start current independently of the load; you can only reduce it as far as the remaining torque still accelerates the machine.
• Once the motor is at speed the job is done, and most units close a bypass contactor so the thyristors stop dissipating heat. A soft starter saves no energy during running. Most also offer a controlled stop ramp — the standard mitigation for check-valve slam and water hammer on pumps.

Sizing is not done on motor kW. Because the thyristors pass a raised current for the whole ramp, the rating is a starting duty — the AC-53 code in IEC 60947-4-2, stating a start current as a multiple of rated current, a start time, an on-load duty factor and starts per hour. A unit adequate for a ten-second start twice an hour may be inadequate for six starts an hour, and ambient temperature and altitude derate it further.

Variable speed drives: what you gain and what you take on

A VSD rectifies the incoming supply to a DC link and inverts it back to a three-phase output at a frequency and voltage the drive chooses. The starting problem disappears as a side effect: frequency ramps from zero, so current never approaches locked-rotor current. Controlling frequency controls the motor’s synchronous speed; controlling voltage alongside it holds the flux roughly constant and so produces rated torque across the speed range. With vector control it produces full torque at zero speed.

The larger prize is on variable-torque loads. For a centrifugal fan or pump obeying the affinity laws, flow varies with speed, torque with the square of speed and shaft power with the cube — so 80 % speed takes shaft power to roughly half. Where flow is currently throttled with a damper or control valve, replacing the throttle with speed control is usually the largest energy saving available in a plant. The saving depends on the duty cycle of the specific installation and must be calculated, not assumed.

What a drive brings with it:

• Harmonics. A six-pulse diode front end draws non-sinusoidal current, loading the supply and heating transformers and neutrals. Mitigation — a line reactor, DC-link choke, passive filter, active front end or multi-pulse arrangement — is a site question answered against the applicable limits, with IEC 61000-3-12 and IEEE 519 the usual references.
• EMC. The inverter switches fast and the motor cable becomes an antenna. Screened cable, terminated at both ends with a 360° gland, is not optional. IEC 61800-3 defines categories C1 to C4 by intended environment, and the drive’s declared category must match the installation.
• Motor stress. Fast-rising output pulses stress winding insulation, more severely as cable length increases, and can drive currents through the bearings. Long runs, older motors and larger frames may need output filters, dV/dt reactors or an insulated bearing.
• Motor cooling and panel heat. A totally-enclosed fan-cooled motor is cooled by a fan on its own shaft, so sustained low-speed operation at high torque needs a derating, forced ventilation or an inverter-duty motor. The drive itself dissipates a percentage of the power it passes, and the enclosure has to shed it.

Sizing a VSD is done on current, not kW: the motor’s full-load current at the actual supply voltage is the number that matters, and the kW badge assumes a typical motor at a typical voltage. Check also whether the rating is normal duty or heavy duty — the same hardware usually carries both, differing in sustained overload, and a constant-torque load needs the heavy-duty figure.

The third option: star-delta

Star-delta reduces both starting current and starting torque to approximately one third of their DOL values, and for the right load is inexpensive. Its weakness is the changeover: the motor is briefly disconnected, and on reconnection there is a transient that can approach the DOL values the arrangement was fitted to avoid.

Matching the method to the load

How to check this on the product page

Soft starters and drives on this site carry a specification table under the product image. The fields that decide the selection are:

• Rated operational current — size against this, not the kW figure.
• Rated power, with the supply voltage it assumes, and the supply voltage and frequency range.
• Overload / duty rating — the AC-53 code on a soft starter; normal-duty and heavy-duty figures on a drive.
• Control method on a drive (V/f, sensorless vector, closed-loop), which decides whether it holds torque at low speed.
• EMC category and integrated filter, IP rating, and ambient temperature.

Every value is a link that filters the catalogue to everything sharing it. Where a field is absent we do not hold a verified figure — take it from the manufacturer’s datasheet and ask us if you would like it confirmed. Parameter lists, derating curves and harmonic data live in the manufacturer’s documentation, which we can send you for a specific unit. Add your shortlist to a quote request and the sales desk will confirm stock, lead time and price.

A closing note on responsibility

The right starting method depends on the driven machine’s torque-speed and inertia characteristics, the motor’s own data, the strength of the supply, the duty cycle and the environment — none of which a catalogue can know. Harmonic and EMC assessment, upstream protection co-ordination and motor thermal verification are part of the design, not the purchase. Final selection is the responsibility of the designing engineer, working from actual application data and the manufacturer’s published data for the specific product. RMS can assist with selection, supply datasheets and application documentation, and sanity-check a proposed arrangement — but that assistance supports competent design, it does not replace it.