Soft Starters - Two Phase Control vs Three Phase Control

Soft starters in modern motor control

Soft starters remain one of the simplest and most cost effective ways to protect motors, reduce mechanical shock, and limit inrush currents during start up. Instead of applying full line voltage instantly, the soft starter ramps the voltage and current so that the motor accelerates gently and the driven load sees less impact and wear.

Internally, this is achieved with SCRs (thyristors) that regulate the voltage supplied to the motor during start and often during stop. There are two main control topologies used in three phase soft starters:

• Two phase control - only two motor phases are controlled by SCRs.
• Three phase control - all three motor phases are controlled by SCRs.

At first glance they appear to do the same job, but their internal behaviour is very different, especially in terms of smoothness, torque delivery, heat generation and overall efficiency.

What is two phase control?

In a two phase controlled soft starter, only two of the three motor phases are controlled by SCRs (thyristors). The third phase is directly connected to the supply throughout the start.

Because that direct phase is not being reduced in the same way as the controlled phases, it runs at a higher current for a longer duration during the start. This can lead to higher heating, slightly lower efficiency, and greater motor losses, so the motor uses marginally more energy for the same start profile.

Benefits of two phase control

1. Lower purchase cost
   Two phase soft starters use fewer power semiconductors and supporting components, which reduces product cost.
2. Simple and compact design
   The reduced component count often allows for smaller enclosures that are better suited to tight control panels.
3. Good for smaller basic loads
   Centrifugal pumps, fans and other easy to start loads will generally start reliably with two phase control.

Drawbacks of two phase control

The uncontrolled phase leads to current imbalance during the start, especially at low firing angles. This affects both the motor and the soft starter.

• Causes current imbalance and more heat
  Because one phase is uncontrolled, the current waveform becomes distorted at low speeds and the motor runs less efficiently, therefore consuming more energy. This increases:
  • Harmonics.
  • Motor heating.
  • SCR heating.
  • Copper losses.
• Higher torque ripple
  Unbalanced current causes pulsing acceleration, which can lead to vibration, belt flutter or nuisance mechanical noise in some systems.
• Limited load handling capability
  Two phase control is not well suited to higher torque, frequent start or longer start applications such as:
  • High inertia loads.
  • Positive displacement pumps.
  • Compressors.
  • Loaded conveyors.
  • Applications with difficult breakaway torque conditions.

What is three phase control?

In a three phase controlled soft starter, all three motor phases are controlled by SCRs (thyristors). This allows true symmetrical control of the voltage waveform during ramp up and ramp down, higher motor efficiency and therefore lower energy usage.

Benefits of three phase control

1. Smoothest torque and acceleration
   Balanced SCR firing across all phases eliminates most torque pulsation. This helps protect:
   • Couplings.
   • Gearboxes.
   • Belts and chains.
   • Pumps.
   • Drive trains and connected pipework.
2. Lower heat generation
   Full control across all three phases results in:
   • Lower harmonic distortion.
   • Balanced current.
   • Lower peak current.
   • Reduced motor copper and iron losses.
   • Lower overall SCR heating.
   As a result, three phase control is inherently cooler running and more efficient.
3. Supports hard to start and frequent start loads
   Three phase control is ideal for:
   • Heavy conveyors.
   • High inertia machines.
   • Crushers and mixers.
   • Positive displacement pumps.
   • Hammer mills.
   • Compressors and other demanding machinery.
4. Better voltage and current protection
   Because the current is more consistent and better measured, protection functions can be more accurate and performance under voltage dips or unbalanced supplies is improved.

Drawbacks of three phase control

• Typically higher price due to more SCRs and associated componentry. In many cases, the efficiency and protection benefits offset this over the life of the machine.
• Slightly larger physical size in some power ratings, as additional thermal management may be required.

Comparison summary

Which should you choose?

Choose two phase control if:

• You need a cost effective solution.
• The load is light, infrequent start and predictable.
• You are driving centrifugal pumps or fans.
• Panel space is extremely limited.

Choose three phase control if:

• The load requires frequent starts or high breakaway torque.
• Smoothness and vibration control matter.
• You want the lowest heating and highest efficiency.
• Long ramp times or precise acceleration are needed.
• You are replacing existing star delta starters.
• The motor is part of a high value or high duty system.

Final summary

Both two phase and three phase soft starters have their place, but they behave differently in real engineering conditions.

Two phase control systems are a good fit for simple, predictable loads where budget and space are tight. However, they introduce waveform imbalance, which leads to greater heat generation, higher harmonic distortion and increased torque pulsation. Over time this can result in higher energy usage and reduced motor life.

Three phase control systems, by contrast, ensure balanced current, smoother starts, less vibration and significantly lower heating. They are the preferred solution for heavy duty, high inertia or vibration sensitive applications and are generally regarded as the more advanced and electrically correct approach for modern industrial motor control.

How Charter Controls can help

As a manufacturers agent specialising in low voltage control and automation products, Charter Controls can help you select suitable soft starters for both new projects and retrofit work. If you have a particular application or load profile you would like to review, you can contact the team here: Contact Charter Controls.

Frequently asked questions about two phase and three phase soft starters

Does three phase control really generate less heat than two phase control?

Yes. Three phase controlled soft starters regulate all three motor phases and produce a more balanced current waveform with lower harmonic content. For the same starting duty this reduces losses in the motor and SCRs, so the overall system runs cooler and more efficiently than an equivalent two phase design.

Is a two phase soft starter bad for the motor?

A correctly sized and applied two phase soft starter is not inherently bad, but it does run the motor slightly less efficiently, especially on heavier or longer starts. Over time this additional heating can contribute to reduced insulation life if the application is demanding and not properly engineered.

When is two phase control a sensible choice?

Two phase control is a sensible choice when the load is easy to start, such as standard centrifugal pumps and fans, start frequency is low, and budget or panel space is a major constraint.

When should I insist on three phase control?

Three phase control is strongly recommended for high inertia or heavily loaded machinery, frequent starts, long ramps, or any system where vibration, energy usage and equipment life are important commercial considerations.

Can I replace an existing star delta starter with a soft starter?

Yes. In many retrofit projects, a three phase soft starter is an excellent replacement for a star delta starter, offering smoother starts, reduced mechanical stress and better current control. Panel layout, protection settings and cable sizes should be checked as part of the upgrade design.