Soft Starter vs VFD: Which Should Your Pump Use?

Use a soft starter when the pump runs at one fixed duty and you only need a gentler start and stop. Use a variable frequency drive (VFD) when the flow or pressure the pump must deliver changes through the day, because only a drive can slow the pump down and cut running energy. A soft starter limits starting current to roughly 3 to 4 times motor current and then steps out of the circuit; a VFD keeps starting current within its own rating and controls speed continuously, at the cost of a higher price, around 3–4% losses at full speed and current harmonics that must be managed.

Below: how the two differ, a decision matrix by pump duty, worked energy numbers, how fire pumps fit in, and what we check on site, using manufacturer data and our own Singapore projects.

In this article

• What is the difference between a soft starter and a VFD?
• Should I use a soft starter or variable frequency drive for my pump?
• When to use a soft starter vs a VFD: decision matrix
• Variable speed drive vs soft starter: the energy maths
• Fire pumps: what our LTA projects used
• Harmonics, cables and panel space
• Common mistakes and what we check on site
• Soft starters and drives we supply for pumps
• Frequently asked questions about soft starters vs VFDs
• Sources

What is the difference between a soft starter and a VFD?

A soft starter is a solid-state motor starter covered by IEC 60947-4-2. It uses thyristors to raise the voltage to the motor gradually, limiting the current and torque during acceleration, then closes a bypass contactor so the motor runs directly from the supply. The Himel HASXS we supply is a typical example. Its 132 kW datasheet lists voltage-ramp starting, an integrated bypass, and start and stop times adjustable from 2–100 s and 1–50 s.

A VFD rectifies the incoming AC supply to DC and then rebuilds an AC output at whatever frequency and voltage the motor needs. Because motor speed follows frequency, the pump can run at any speed for as long as it runs. Pump drives such as the Himel HAVSP Smart Pump Series add pressure control and protection functions on top.

The starting problem both devices solve is the direct-on-line (DOL) start. Schneider Electric’s guide to DOL starters puts the inrush of a motor switched straight onto the supply at typically 6 to 8 times its rated running current. Carlo Gavazzi’s RSGD brochure adds that newer high-efficiency motors can draw more: about 10 × for IE3 and up to 15 × for IE4 motors.

Should I use a soft starter or variable frequency drive for my pump?

Decide from the pump’s duty, not the price list. The Hydraulic Institute and Europump guide to variable speed pumping recommends starting with a duration diagram: how many hours a year the system needs each flow rate. Then work through these steps:

1. Map the duty. Log flow, pressure, running hours and starts for a typical week, or take them from the BMS.
2. Find the static head, the height and fixed pressure the pump must overcome even at zero flow. The Hydraulic Institute warns that in systems with high static head, “a small turn down in speed greatly reduces flow rate and pump efficiency”, and that applying the affinity laws there “can also lead to major errors”.
3. Check the supply. Both devices remove the 6–8 × DOL inrush, so a weak supply alone is no reason to buy a drive.
4. Decide how the pump must stop. The US DOE sourcebook Improving Pumping System Performance notes that rapid acceleration of large volumes of fluid can create damaging water hammer. Both devices can ramp down under control.
5. List the control functions. Constant-pressure PID, cascading, sleep and dry-run protection are drive functions; a float or pressure switch start suits a soft starter.
6. Check harmonics, heat and environment. The Hydraulic Institute notes that drive electronics cope less well with corrosive and damp locations than conventional starters.
7. Check the regulations for fire pumps (see below).

A fixed duty or high static head points to a soft starter. Varying demand on a friction-dominated system, such as a chilled-water loop, points to a drive, whose capital cost the Hydraulic Institute notes can often be offset by removing control valves, bypass lines and conventional starters.

When to use a soft starter vs a VFD: decision matrix

Applied to common pump duties in Singapore buildings and plants (a starting point, not a substitute for the consultant’s calculations):

The Carbon Trust’s VSD technology guide agrees: where demand already matches the motor’s full-load capacity, a VSD “would only add to the overall system losses”.

Variable speed drive vs soft starter: the energy maths

Take a 37 kW centrifugal pump motor on 400 V, drawing 30 kW at its duty point, priced at the SP Group regulated tariff of 28.59 cents/kWh before GST (October–December 2026). Your contracted rate may differ.

Example 1: fixed-duty transfer pump, 3,000 hours a year

On bypass, the soft starter adds almost nothing. A VFD at 50 Hz adds its own losses: US DOE Motor Systems Tip Sheet #11 gives 97% typical efficiency at full load for a 50 hp drive, so 30 kW ÷ 0.97 = 30.9 kW. The extra 0.9 kW over 3,000 hours is about 2,800 kWh, roughly S$800 a year more, before the higher purchase price.

Example 2: variable-demand circulating pump, 6,000 hours a year

Now the same pump serves a friction-dominated loop that could run at 80% speed. Power varies with the cube of speed (0.8³ = 0.51). Allowing 95% drive efficiency at part load (DOE table): 30 kW × 0.51 ÷ 0.95 ≈ 16.2 kW, about 13.8 kW less than full speed, or roughly 83,000 kWh and S$23,700 a year.

Treat that as a ceiling: a throttled pump does not draw full power at reduced flow, and static head cuts the saving. Our article on how the Schneider ATV630 cuts pump and fan energy covers the measurement.

Starting current on the same motor

Full-load current is about 67 A (assuming 0.86 power factor and 93% efficiency; check the nameplate):

Fire pumps: what our LTA projects used

A fire pump’s duty is simple, but a failed start is serious. SCDF’s Fire Code 2023, clause 5.2.3 sets rules for these motor circuits. Cable connections must be protected by HRC fuses or MCCBs with magnetic release, able to carry the motor’s stalled current for at least 75% of the time that would damage the windings. Any no-volt release must reset automatically, and thermal overload trips are not permitted. Clause 6.8 requires the fire pumping system to keep performing when a pump or its control system is out of service.

Our fire pump references show both approaches:

• Singapore Rail Test Centre, Tuas (LTA, 2023): a 132 kW Himel HASXS Expert Series soft starter, configured for the fire pump application, tested and commissioned.
• Kim Chuan Depot Extension (LTA, 2024): a 160 kW HASXS soft starter for the fire pumps of an underground rail and bus facility.
• HDB estates across four Town Councils (2022): more than 100 Himel HAVSP drives replacing ageing drives on fire pump systems. The HAVSP datasheet lists a fire override mode.

Neither device is right for every fire pump; the settings must follow the Fire Code and the design. On chilled-water plant, our Gateway East project used more than 10 HASXS units (37–55 kW) as standby starters for motors in service for over 10 years.

Harmonics, cables and panel space

A drive’s rectifier draws current in pulses, adding harmonic distortion. IEEE 519 is commonly used to set limits, and specifications may add their own: on our ST Aerospace project, each drive had to stay below 35% THDi (total harmonic current distortion), and we measured every one.

US DOE Motor Systems Tip Sheet #15 adds that cable runs of 50 feet (about 15 m) or more from a PWM drive almost always need mitigation, such as a load reactor or dV/dt filter, and the Hydraulic Institute recommends insulated non-drive-end bearings above 100 kW.

Both need a proper enclosure: the HASXS is IP00 and the HAVSP IP20, each rated for non-condensing humidity only.

Common mistakes and what we check on site

• A drive on a pump that never slows down: 3–4% losses for no saving.
• Affinity-law savings quoted for high static head pumps.
• Soft starter current limit set too low, so the pump never reaches speed. Schneider’s typical centrifugal pump figures are 300% and 5–15 s.
• Ignoring starts per hour. Carlo Gavazzi notes soft starters are typically designed for 10–20 starts per hour; more may need oversizing.
• Thermal overload trips on fire pump circuits, which clause 5.2.3 does not permit.
• Removing a throttling valve without replacing its shut-off role. The Carbon Trust notes a VSD pump cannot act as a shut-off or back-flow device at zero flow.
• Long motor cables with no output filter on retrofits.

Soft starters and drives we supply for pumps

We supply, test and commission both:

• Soft starters: the Himel HASXS Expert Series (11–600 kW, integrated bypass), the compact Himel HASES, the Schneider Electric Altistart ATS22 and Altistart ATS480, and the Carlo Gavazzi RSGD for compact panels. Browse all soft starters.
• Pump drives: the Himel HAVSP Smart Pump Series (multi-pump, dry-run and pipe-burst protection, PID with sleep), the Himel X Series and the Schneider Altivar Process ATV630. Browse all variable speed drives.

For product-level detail, see our guides to the Schneider ATS480 soft starter, the Schneider ATS22, Carlo Gavazzi soft starters, the Himel Smart Pump Series and how VSDs improve motor efficiency. For a recommendation, send the pump duty and motor nameplate through our contact page.

Frequently asked questions about soft starters vs VFDs

What is the difference between a soft starter and a VFD?

A soft starter ramps the voltage up to start the motor smoothly, then bypasses itself so the motor runs at full speed. A VFD controls the motor’s frequency and voltage for the whole run, so it can change the speed. Only a VFD can save running energy.

Should I use a soft starter or variable frequency drive for my pump?

Use a soft starter if the pump runs at one fixed duty, such as a transfer or fire pump, and you only need to reduce starting current and water hammer. Use a VFD if the flow or pressure demand varies, such as on a booster set or a variable-flow chilled-water loop.

When should I use a soft starter instead of a VFD?

Choose a soft starter when the motor must run at full speed whenever it runs, when the budget or panel space is tight, or when you want to avoid adding drive harmonics. On a fixed-duty pump, a VFD adds roughly 3–4% losses and saves nothing.

Does a variable speed drive save more energy than a soft starter?

Only when the pump can run slower. On centrifugal pumps, power falls with the cube of speed, so running at 80% speed needs about half the power. At full speed, a soft starter on bypass is slightly more efficient than a drive.

Can a VFD replace an existing soft starter on a pump?

Yes, if the duty varies enough to justify it. Check the motor insulation and cable length, add a pressure or flow sensor for control, plan for harmonics and heat in the panel, and keep a check valve or other shut-off function at the pump discharge.

Are soft starters used on fire pumps in Singapore?

Yes. We have supplied 132 kW and 160 kW Himel HASXS soft starters for LTA fire pump systems. Drives with fire override settings are also used on fire pump systems. Either way, the settings must follow SCDF Fire Code 2023 and the consultant’s design.

Sources

• Schneider Electric, DOL starter: types and working principle.
• Schneider Electric, Altivar Soft Starter ATS480 catalogue (DIA2ED2210602EN), normal and heavy duty application table.
• Himel, HASXS4T1320G product data sheet (132 kW soft starter).
• Himel, HASES series soft starter catalogue.
• Carlo Gavazzi, RSGD/RSGT self-learning soft starters brochure.
• Himel, HAVSP4T0370P product data sheet (37 kW SMART Pump VSD).
• US Department of Energy, Motor Systems Tip Sheet #11: Adjustable Speed Drive Part-Load Efficiency.
• US Department of Energy, Motor Systems Tip Sheet #15: Minimize Adverse Motor and Adjustable Speed Drive Interactions.
• Hydraulic Institute, Europump and US DOE, Variable Speed Pumping: A Guide to Successful Applications (Executive Summary).
• US DOE and Hydraulic Institute, Improving Pumping System Performance: A Sourcebook for Industry.
• Carbon Trust, Variable speed drives technology guide (CTG006).
• Singapore Civil Defence Force, Fire Code 2023, clause 5.2 and clause 6.8.
• IEC, IEC 60947-4-2: semiconductor motor controllers, starters and soft-starters.
• IEEE, IEEE 519-2022 Standard for Harmonic Control in Electric Power Systems.
• SP Group, Tariff information.
• Trans-Digi Global case studies: Singapore Rail Test Centre, Kim Chuan Depot Expansion, Gateway East, HDB estates across four Town Councils and ST Aerospace Aviation Centre.