Inverters/ Variable Speed Drives

Variable speed or frequency drives (VSD or VFD), also called inverters, are typically composed of a power supply that converts fixed frequencies and voltage input to variable output. Using solid-state components on a PCB, this power supply drives a three-phase AC induction motor, whose speed and torque can change according to the needs of the application it's running.

This means that the speed of the motor can be accurately controlled across a broad demand spectrum, thus offering considerable energy savings. Applications such as fans or pumps typically demand a wide range of operating speeds – and much less energy is used with a VSD than with a drive operating at a constant speed or frequency. VSDs are commonly found in electro-mechanical drive systems such as HVAC and refrigeration applications, as well as manufacturing processes such as packaging and pick-and-place.

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VSD benefits

In addition to energy savings, the benefits of using VSDs to run your applications include a controlled starting current, which puts less stress on the mechanical parts of your motor. Instead of requiring a large power input to start up, which can strain the motor windings, a VSD has an initial zero voltage and frequency at start-up.

This builds up gradually and magnetises the motor windings without putting excessive stress on them, thus extending motor life. It also reduces the power surges on motor start-up. These can adversely affect the rest of your power distribution system and cause voltage-sensitive equipment such as contactors, proximity switches and sensors to drop out.

Using a VSD eliminates this problem, plus any extra cost for possible power surges in peak usage periods. The motor's acceleration and stopping are also controlled smoothly, greatly reducing mechanical stress, and its speed and torque can be adjusted remotely.

Another substantial advantage of using a VSD is that it eliminates the need for more expensive mechanical drive components, especially gearboxes. Its infinitely variable speed capacity can deliver whatever speeds are required by the load, without mechanical gearing or other intermediary speed controls. It’s also capable of reverse operation. This capability also substantially reduces floor space requirements and maintenance costs.

Inverters: Size, Cost & Performance

Advances in power electronics technology have enabled the size and costs of today's VSDs to be substantially reduced. Their performance has also greatly improved with similar advances in drive topologies, control hardware/software and inverter switching devices. They are produced in a range of different low and medium voltage options, for AC-AC conversion as well as DC-AC.

At Rowse Automation we have seen the development of many new VSDs, and we stock a wide range of units in different configurations. These are generally logic-controlled, modular units designed for mounting on a DIN rail or control panel, and come with or without filter. The Siemens V20 range offers a large selection of frame sizes, motor horsepower and current, as well as two voltage ranges. A range of mounting accessories is available for the unit, together with single or three-phase line reactors, shield connection kits and expansion modules.

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