Choosing between a VSD and a Soft Starter

VSD vs Soft Starter

Soft Starters utilise silicon-controlled rectifiers (SCRs) to perform phase angle control, while Variable Speed Drives (VSDs) use insulated-gate bipolar transistors (IGBTs) to perform pulse-width-modulation (PWM) control. VSD technology continues to evolve, while Soft Starters are considered a mature product by most. Each new generation of VSDs provide higher performance, smaller size, and more advanced control.

Soft Starters are used to limit inrush current associated with motor start-up. It lowers the initial voltage by adding solid-state series impedance and ramp up until full speed is achieved. Doing this extends the life of the motor and mechanical components that are connected to it. Large inrush currents also place a high demand on the electrical supply system, which often results in extra costs.

VSDs deliver much higher frequency source power during start-up to reduce current due to the increased inductive impedance. This can be advantageous if control is desired after full speed is reached and allows for continuing control as the load on the motor is changing. VSDs are more efficient and produce less heat, while being able to start motors under considerable loads.

For most applications, the main deciding factor between using a Soft Starter or a VSD boils down to the need for continuous speed control. If your application is going to be a set speed and across-the-line starting is not acceptable, or an application requires adjustable start and stop times for electrical or mechanical reasons, then a Soft Starter can be an excellent solution.

A Soft Starter helps to protect the motor and connected equipment from damage by controlling the voltage supplied to the motor. This limits the initial inrush of current, extending the life of the motor, and reduces mechanical shock sustained during starting by providing a more gradual ramp-up to full speed (for applications such as conveyors, belt-driven systems, gears and couplings; and where pressure surges or water hammer may occur in piping systems). Soft Starters are also capable of providing a gradual ramp to stop, where sudden stopping may create problems in the connected equipment. Soft Starters are also valuable where electrical systems have limited current capacity (Soft Starters limit inrush current more than conventional across-the-line motor starter). Further to this, limiting peak current input may provide considerable energy cost savings, especially by avoiding utility company charges for peak demand levels.

Soft Starters are generally easier to setup than VSDs and much lower in cost – and are thus often the most economical choice when considering only the starting condition. If you are however going to need or want to be able to match the speed of other machines, or have the flexibility to continually ensure optimal machine operation, then a VSD is the way to go. VSDs offer more options on how you can fine-tune your process and flow to optimise your process. You also have considerably more feedback available.

VSDs provide all the Soft Starter benefits and more. Not only do VSDs provide an extremely smooth and controlled start, but they also efficiently control motors to nearly any desired level of speed, torque, or position, including over-speed and full torque at zero speed. Special applications and safety features are also included, providing even greater benefit. In most cases, variable speed control greatly improves processes and yields impressive energy savings.

VSD benefits, when compared to using a Soft Starter, include power-factor correction and low power consumption, along with full-speed control and not just start/stop control. Other benefits include dynamic braking, PID control, built-in I/O and logic, circulatory control mode, multi-motor control, communication interfaces etc., but the additional cost of the VSD must be justified. Another capability that some VSDs have is the ability to run 3-Phase motors when only Single-Phase power is available (act as a power converter). This is since VSDs have an internal filter circuit that takes Single- or 3-Phase power and converts it to DC before converting it back to AC power for the motor.

Some disadvantages of VSDs, compared to Soft Starters, include the fact that VSDs introduce a small amount of extra heat in the motor (imperfect Sine Wave) and VSDs draw nonlinear current, known as harmonic distortion, from the power source. However, in most cases, the benefits of VSDs greatly offset these things and special VSD countermeasures exist to greatly reduce them when necessary.  The main advantage of a Soft Starter over a VSD in this instance lies in its ability to bypass the power electronics in the Soft Starter once the motor is up to full speed and operating – benefits include lower Watt loss, no harmonics (except during acceleration/deceleration) and less maintenance.

A big disadvantage of a Soft Starter is that full torque is not available at zero speed or very low speeds. Load types for which a Soft Starter would thus not be recommended for include extruders, positive displacement pumps, inclined conveyors, lifts and non-hydraulic elevators. VSDs on the other hand can produce rated torque from zero to rated speed and can typically handle all load types.

The biggest advantage of a Soft Starter over a VSD is however the lower initial cost of the product. If matching of the speed of the process is not required, then the initial cost of the product becomes a bigger factor, which might make a Soft Starter a more optimal choice. If the process can be optimised or additional costs can be saved on electricity by matching the process speed, or by reducing maintenance costs of the overall system (extending system lifetime), then a VSD will most likely be the optimal choice.

It therefore all comes down to user intent. The goal of each system is to reduce overall system and usage costs, which includes initial product costs as well as year-over-year savings.

For more information regarding Soft Starters, please refer to the following related blog posts:

For more information regarding VSDs, please refer to the following related blog posts:

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