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 all AC induction motor applications worldwide are in fact fixed speed applications. If an application truly requires speed control, then a variable speed drive is the only choice, however for 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, variable speed drives 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.
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 variable speed drives generate harmonics while operating, but as a soft starter is bypassed while the motor is running, these harmonics are never an issue. Variable speed drives 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.
3. Less is more
In addition to harmonic mitigation and energy costs, the capital cost of a variable speed control solution is notably 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 efficient and economical.
Generally speaking, soft start is the optimum motor control solution for fixed speed applications. When variable speed or an especially low starting current is a necessity, select a variable speed drive.
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