What Are Soft Starts?

Soft starts, also called soft starters, are motor control devices that reduce the voltage applied to an AC motor during starting and, in some models, during stopping. Instead of applying full line voltage instantly, the unit ramps voltage over a set time. That lowers inrush current and reduces the sudden torque hit that comes with direct-on-line starting. Once the motor reaches speed, some soft starters continue conducting through their power electronics while others transfer to an internal or external bypass path to reduce heat. Soft starts are generally chosen for fixed-speed motors where the goal is controlled starting and stopping rather than continuous speed variation.

Where Are Soft Starts Used?

Soft starts are widely used anywhere a motor starts hard on the electrical system or the driven equipment. Common applications include centrifugal pumps, booster pumps, irrigation systems, HVAC fans, exhaust fans, blowers, screw conveyors, belt conveyors, crushers, mixers, compressors, and some woodworking or material handling equipment. They are especially useful where nuisance voltage dip, belt slip, pipe shock, or product spillage is a concern. In commercial buildings, they are often used on larger HVAC motors and pumping systems. In industrial plants, they are often selected where a motor runs at one main speed and the process does not justify a VFD.

How To Choose Soft Starts

Start with the motor nameplate and the application. Confirm supply voltage, phase, motor full load current, horsepower as a cross-check, service factor if relevant, and the type of load. A pump or fan with relatively light starting duty may allow a more straightforward selection than a loaded conveyor or compressor. Check whether the soft starter is rated for normal duty or heavy duty and whether the manufacturer sizes by current, horsepower, or both. Then review required features: current limit, voltage ramp, kick start, soft stop, built-in bypass, motor overload protection, phase loss or phase imbalance monitoring, communications, and keypad or remote control options. Also consider enclosure location, ambient temperature, ventilation, and whether the unit will be mounted in a panel with other heat-producing devices. If the application needs frequent starts, reversing, plugging, or speed control during operation, a soft start may not be the right tool.

Trade Rules Of Thumb

As a typical rule of thumb, soft starts are a good fit when the process runs at full speed after startup and only needs gentler acceleration or deceleration. Pumps often benefit from a controlled ramp-up and soft stop to reduce pressure shock. Fans and blowers often benefit from lower belt and coupling stress during startup. Conveyors may need a longer ramp to reduce product shift or mechanical shock, but loaded starts can require more torque than a lightly loaded fan. Another practical rule is that a soft starter is not a cure for an undersized feeder, weak utility service, or a mechanically overloaded machine. If the motor struggles to accelerate under normal conditions, review the load and system design before assuming a larger soft starter will solve it. These are practical guidelines only and not a substitute for manufacturer data or applicable code requirements.

Sizing Guidelines

Soft starters are commonly sized from motor full load current first, with horsepower used as a secondary check. For many standard applications, matching the unit to the motor FLA and voltage class is the starting point. For heavier starts, high inertia loads, or frequent cycling, it is common practice to move up to a heavier duty rating or a larger frame size if the manufacturer allows or recommends it. Confirm the allowed starts per hour, ramp time, overload class, and whether the published rating assumes an internal bypass, external bypass, or open mounting with specific ventilation. If the motor is on a pump, fan, or unloaded conveyor, the current limit and ramp settings may be moderate. If the motor starts under load, the unit may need more current capacity or different settings to avoid stall conditions. Always coordinate the soft starter with the motor, branch protection, contactors if used, and the panel thermal design. Final sizing and protection should be verified against the manufacturer documentation and the applicable Canadian Electrical Code requirements.

Common Installation Practices

In practice, installers usually mount soft starters in a clean, ventilated enclosure with attention to heat rise, line and load conductor routing, and service access to terminals and keypad settings. It is common to verify line-side and load-side wiring carefully because a mislanded motor lead can create commissioning problems that look like a programming fault. Many installations include upstream short-circuit protection, an isolation means, and separate control wiring for start, stop, fault reset, and status indication. Where the design uses an external bypass contactor, the control sequence has to be coordinated correctly so the starter handles acceleration and the bypass closes only after the motor is up to speed. Grounding, conductor sizing, and SCCR coordination should be reviewed as part of the panel design. Follow the manufacturer instructions for clearances, torque values, ambient limits, and any required line reactors, bypass components, or suppression accessories.

Common Mistakes

One common mistake is choosing by horsepower alone and ignoring motor full load current, duty cycle, and load type. Another is using a soft starter where the process really needs a VFD for speed control or torque management across a wide operating range. Installers also run into trouble when they set the ramp too short, causing a hard start, or too long, causing excess heating and possible stall conditions. On pump jobs, soft stop settings that are too aggressive can create poor process results instead of reducing hydraulic shock. Another frequent issue is overlooking enclosure heat, especially in compact panels with contactors, overloads, power supplies, and drives nearby. Finally, replacing an existing unit without checking control voltage, bypass arrangement, terminal layout, and fault logic can turn a simple swap into a longer shutdown than expected.

Brand Comparisons

Noark is often cross-shopped against established motor control brands such as ABB, Schneider Electric or Telemecanique legacy installations, Eaton, and Lovato. In many standard applications, Noark can be a sensible value-oriented choice when the electrical ratings, control features, and support expectations fit the project. ABB and Schneider are often strong where a site is already standardized on those ecosystems, where staff want familiar programming and accessories, or where matching existing installed gear reduces commissioning time. Eaton is commonly preferred in facilities already built around Eaton distribution and control hardware. Lovato is often considered on compact control panel and OEM work. If a facility already has a large installed base of one brand, staying with that platform may simplify spare parts, documentation, and technician familiarity. If the job is a new panel or a cost-sensitive replacement and Noark meets the required ratings and functions, it may be a practical alternative for many routine applications. Brand choice should be based on fit, support, and compatibility rather than name alone.

Related Products

Soft starts are commonly purchased with contactors, overload relays where required by the design, motor circuit protection devices, disconnects, control transformers, pilot devices, enclosure cooling components, terminal blocks, and control wire. Depending on the application, buyers may also need current transformers, line reactors, bypass contactors, surge protection, and panel accessories for status indication or remote reset. For pump and fan systems, pressure switches, float controls, level controls, and building automation interface components may also be part of the package. If the application may later require speed control, it is worth comparing the soft starter approach with a VFD before finalizing the bill of materials.

Frequently Asked Questions

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

A soft start mainly controls how the motor starts and sometimes stops. A VFD controls motor speed during operation as well as startup. If the motor will run at one main speed and you only need reduced inrush and smoother acceleration, a soft starter is often the simpler option.

Can a soft starter be used on pumps?

Yes. Pumps are one of the most common soft starter applications. A controlled start can reduce mechanical shock, and a soft stop function may help reduce water hammer in some systems when set correctly.

Do I size a soft starter by horsepower or amperage?

In practice, amperage is usually the main sizing reference, with horsepower used as a cross-check. Always verify the motor full load current, supply voltage, duty rating, and the manufacturer selection tables before ordering.

Does a soft starter replace overload protection?

Not always. Some soft starters include motor overload functions, but the overall protection scheme still has to match the equipment design and code requirements. Confirm what protection is built in and what external devices are still required.

When should I choose a larger soft starter?

A larger or heavier duty unit may be needed for high inertia loads, loaded starts, long acceleration times, frequent starts, or hotter enclosure conditions. Follow the manufacturer duty tables rather than assuming one size up is always correct.

Can I retrofit a soft starter into an existing motor control panel?

Often yes, but check available panel space, heat dissipation, line and load wiring layout, control voltage, bypass method, short-circuit coordination, and the existing protection scheme. Retrofit work is usually easier when the replacement matches the site control philosophy and available spare parts strategy.