How to control the manual input of a VFD

To increase the flexibility of motor control systems, variable frequency drives (VFDs) are a common device used to solve many motor control problems. In this article, learn how to control the manual inputs of a VFD.

Variable frequency drives solve many of the challenges associated with driving three-phase motors, including adjustable speed, networking multiple motors to achieve the entire process, and ultimately the ability to adjust operations without changing the wiring. However, for simple operations, these drives can still use the same manual push button control as in the motor starting circuit, but with additional features.

Driving a three-phase motor is a very simple process with the right equipment. Manual control circuits, including contactors and starters, can be very cheap, but they are limited in many ways. First, these circuits will not drive the motor at less than full speed. Also, if you want to change the mode of operation, you have to rewire the entire control circuit.

To increase the flexibility of motor control systems, variable frequency drives (VFDs) are commonly used to solve many motor control problems. In particular, when the operating speed needs to be changed, VFDs are one of the only options.

Fortunately, using a VFD does not require a PLC or large network control system. But if a larger control system does not exist, the VFD provides an easy option for integrating the motor into a future control system. Future-proofing a system is very useful, especially if it is not very expensive.

It is not difficult to connect to a VFD using simple manual control elements such as a start/stop button, directional switch, and speed control. Although there are many makes and models, the overall strategy is the same for all VFDs. For proper operation, two different elements must be considered: wiring and parameters.

Inverter I/O Wiring

Each VFD will include a set of terminals that allow the user to connect the I/O wires. Typically these will be on the same 24 vDC system as the rest of the control system equipment. The wires can be connected directly to screw-in terminal blocks or to a set of one or more pre-wired connectors on a circuit board. Be sure to use at least the minimum wire gauge size for these control wires, but they can be quite small since very little control power is used.

With a NO button, an NC button, selector switches and a potentiometer, almost any kind of control scenario can be connected.

First, determine the wiring diagram, which you may find online or in the accompanying documentation. The diagram should show several terminal functions:

The power terminals should be marked with 24 volts

Common Ground, or COM

Typically provides 10 volt output for analog speed control devices

The next group of terminals are the digital inputs, which may be labeled "DI" or "In." There are usually four to six of these terminals.

A single analog input, which may be labeled "AI" or "V" for voltage

Many VFDs also include an electromechanical relay to provide output information that switches based on certain operating points.

Digital Input

If the VFD is set up for either a "source" or "sink" wiring type, care must be taken to connect the wires correctly. Some VFDs have a toggle switch to select which type, while others are set up simply by connecting the power source in the most appropriate scheme.

If the VFD is set up for "sink" wiring, each control device should be connected to a positive voltage. Each device is then also connected to its corresponding digital input terminal. Usually, but not always, the first terminal is always connected to the STOP normally closed button. The actual function of each digital input will be configured by parameters later.

If the VFD is set up for "source" wiring, the above is the same, except that COM is connected to each device instead of +V.

One important note: Some VFDs require a wire or NC switch to be connected to the stop terminal for safety reasons. In these models, a jumper must be connected from the +V terminal to this input, even if it is used with network control.

Analog Input

Analog inputs, such as rotating a potentiometer knob, are a classic, simple way to select motor speed. For a three-terminal potentiometer, terminals 1 and 3 will be connected to the 10 volt supply and COM. The order of these will determine whether clockwise or counterclockwise rotation will speed up the motor. The second terminal (2) should be connected to the "AI" or "V" terminal.

Sometimes this analog voltage signal can be sent from a PLC or a simple 0-10v controller signal, allowing the VFD to work in a variety of situations.

parameter

The customization of each VFD is not based on I/O wiring, but rather on a menu of parameters that control how each terminal affects operation.

A set of buttons on the front of the unit is almost certainly the way to access the parameters, but the manual should be consulted to determine exactly how the structure works. Every VFD is completely different in terms of how many parameters there are, how to access them, what the defaults are, and how each operating mode is represented.

Figure 3. Excerpt from the PowerFlex 40 VFD manual showing the digital input terminals, and the selector switch that toggles between sink (SNK) and source (SRC) wiring at the bottom. The function of the digital input is the function of the parameters (specifically A051 through A054 in this unit).

Regardless of make and model, there are many similarities.

First, the control parameters must be set to use the input terminals—NOT local control (keypad) and NOT network control (eg Modbus RS-485 or Ethernet).

Next, the digital input parameters can determine several operating modes:

One DI terminal is forward, the other DI terminal is reverse

Stop and start, one-way

Stop, with toggle switch for forward or reverse movement

Stop, with start and manual direction control switching

Stop and start using several toggle switches for selecting between different preset speeds

Use Start or Jog to stop, selectable by toggle switch

These digital inputs may even have some additional benefits:

Fault reset

Increase or decrease speed

Summary

If flexibility is a requirement for motor control, then a VFD may be the best choice. It can also be integrated into almost any existing control system, or a new control system can be built around an existing VFD.