Industrial Motors

The basics of variable-frequency drive installation

Here's what you need to know and do to realize the potential benefits of a VFD

By Dave Perkon for Control Design

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The VFD, often called an ac drive or inverter, takes a single- or three-phase signal and varies the speed of a three-phase ac induction motor. This is its main benefit. Running a motor more slowly can save significant energy, and speed changes may be useful to the application. Another big benefit is adjustable acceleration and deceleration. Less acceleration can soften the mechanical forces at motor start and reduce inrush current. The VFD also has built-in overload protection and motor start/stop control functions.

There are both physical and electrical installation basics to be aware of when using a VFD. When mounting the VFD on a back panel, be sure to check the specifications. It is common for multiple devices to be installed in one location, but all VFDs need proper air flow, so check the installation instructions carefully when laying out a control panel. Mount the drives vertically. Some drives can be mounted with no clearance, but it's common to have a minimum side-to-side spacing of 50 mm or more and to have vertical clearance above and below the drive of 100 mm to 150 mm.

It's not uncommon to hear about noise problems in VFD applications. However, proper shielding and grounding and the use of filters or line reactors can help. If multiple VFDs are installed in a single location, don't daisy-chain the ground wire; it creates ground loops. Connect each ground to a single ground point, connected in parallel. The line reactor can help to protect from transient voltages and reduce harmonics to or from the drive. 

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