Comparing Soft Starters and a.c. Inverters.
Which One is Right for You?

Correct application of soft start and variable speed technology is often poorly understood. Both technologies fulfill their own important yet distinct roles in the field of motor control, and correct selection and application of each is vital in order to realise the full benefit of each and avoid unnecessary expense.

Fixed Speed: Four to One

Sources suggest that nearly 80% of worldwide AC induction motor applications are in fact fixed speed applications. If an application truly requires speed control, then an a.c. Inverter is the only choice, however, in fixed speed applications, soft starters are typically the ideal selection. Here's three reasons to consider:

1. The Efficiency Equation

In fixed speed applications using soft start control, the soft starter is bypassed and is >99.9% efficient while the motor is running. In contrast, a.c. Inverters are always in circuit with the motor during start, stop and run in order to achieve speed control. This causes energy losses of anywhere up to 5 percent, a significant monetary cost over time.

2. Harmonics

In simple terms, harmonics are integer multiples of some fundamental frequency. As impedance of a power system is a function of frequency, harmonics cause additional current flow which leads to heating effects and reduction in torque produced by motors.

Both soft starters and a.c. Inverters generate harmonics while operating, but as a soft starter is bypassed while the motor is running, these harmonics are never an issue. a.c. Inverters generate harmonics continually during start, stop and run. These harmonics can induce further energy losses in motors, transformers, and power factor correction capacitors, and affect the electrical network. Harmonic filters can significantly increase the cost, and physical size, of the motor control system.

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3. Less is more

In addition to harmonic mitigation and energy costs, the capital cost of a variable speed control solution is considerably higher than a fixed speed one. Matching the technology to the need is critical to delivering a motor control solution that is not only functional, but also efficient and economical.

Generally speaking, a soft starter is the optimum motor control solution for fixed speed applications. When variable speed or an especially low starting current is a necessity, select an a.c. Inverter.

Looking for More Information? We Can Help With That...

With AuCom Soft Starters, Mitsubishi Electric a.c. Inverters and TEC Electric Motors, LC Automation now has a Complete Motor Control Solution. If you are looking for expert, impartial advice about which motor control solution is the best fit for your application, call our Technical Support Engineers on 01254 685900. They will be happy to answer your questions and advise you on the right solution for you.

Should I Use a Soft Starter or a Variable Speed Drive?

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