If you want to control a motor, you can easily select the drive by looking at the motor's power and voltage. Remember to check the actual supply you are using as dual voltage motors (allowing star or delta connections) are suitable for two supplies e.g. B. 230 or 400 V. The frequency converter must be chosen for one or the other voltage.

It is always worth looking at the rated current of the motor and that of the drive. With high efficiency motors it is sometimes possible to choose a smaller drive than the nameplate rating would suggest. That means a 15kW AC drive can supply enough power for an 18.5kW motor. it is the electricity that counts!

If you have not yet selected your motor, you must do so before selecting a frequency converter.

Once you have decided on the power and voltage, you need to consider the protection level of the frequency converter. If the drive is mounted in a cabin or in a machine, you can use an IP20 drive. However, if the frequency converter is in any way exposed to dust, dirt or liquids, protection to IP55 or higher (e.g. IP66) is required. If you choose this option, consider whether you need a drive with integrated controls and isolators. Adding these externally increases installation costs.

European law requires all frequency converters to meet the requirements of various electromagnetic compatibility regulations. In practice, this means using a frequency converter with a built-in filter; Choosing a drive without a filter is wrong economics - even for export outside the EU - unless you really know what you're doing.

Almost all manufacturers of frequency converters offer products for different application classes. A low power VFD can be categorized as general purpose so the choice is easy. However, larger and more specialized drives are offered for "industrial" applications or "pump and fan applications". Sometimes these are referred to as constant torque and variable torque applications respectively. The industrial frequency converters (constant torque) are designed in such a way that the drive has a high overload capacity and a high torque in the entire speed range. This is suitable for applications such as many machines, conveyors, mixers, hoists, etc. It can also include vector and torque control functions - more of that in later articles.

The "pump and fan" (variable torque) drive is optimized for (you guessed it!) pumps and fans. Most of these applications do not require high torque at low speed and by adapting the drive to this requirement, the energy savings are even better. Other features such as cascade control, torque monitoring, etc. are often added to this drive class. Be careful though; There are many pumps (and some fans or compressors) that may require low speed torque and therefore require an "industrial" frequency converter to power them. Invertek offers the E3 drive as a general purpose drive (up to 22kW), the P2 for heavy duty "industrial" constant torque applications and the Eco drive to control pumps and fans.

That narrows down the choices pretty well. If you work at high temperatures or altitudes, or require specific features or functionality, you will need to speak to the supplier to help with drive selection.

Installing a variable speed drive is fairly easy, but remember that it needs to be kept cool. This means that there should be good airflow around the frequency converter and the manufacturer's recommendations (read the manual!) for clearance around the unit should be followed (read the manual!). Do not block the air flow with cable ducts! Make sure the air supply is clean and free of dust, dirt, liquids, etc. Figure 1 shows the location of the main parts on an IP20 E3 drive. IP66 units naturally have closed terminals and larger power drives have the terminals on the bottom.

Fig. 1: Important installation features E3 IP20

When the drive is closed, consider the worst case temperature rise. The drive dissipates heat, which can cause other losses through devices in the same enclosure and heat everything up. A ten or twenty degree rise in a cabinet may be fine in Britain but can be excessive in an Indian summer. Application Note AN-ODE-2-070, available on the Invertek website (invertekdrives.com), contains a lot of information on temperature rise calculation and other installation matters.

Once the frequency converter is installed, we should think a little about EMC and plan the cabling accordingly. All electrical devices generate electromagnetic interference (EMI) and most devices are designed to be compatible with other devices. This is what EMC (Electromagnetic Compatibility) is all about. Installation and wiring can mean the difference between reliable operation and expensive site visits, although this is generally not as big of a problem as it has been in the past.

However, make sure you follow basic guidelines when it comes to wiring. There is a lot of information in the application note, but modern AC drives are all about using shielded or armored cables for the motor connection (grounded at both ends) and ensuring a good ground all round.

Figure 2 from the Application Note shows the basic principles of good EMC practice. Note that the control cables are separate from and at right angles to the mains cables, which are shielded or armored.

Fig. 2. Basic EMC guidelines (from AN-ODE-2-070)

With the mains voltage and motor connected, simply double check that:

• The supply matches the drive. As already mentioned, some frequency converters are designed for a 230V supply and will certainly fail when connected to 400V.
• The motor voltage also matches the supply and in the case of a dual voltage motor it is connected in star for low voltage and delta for high voltage operation.
• You have not connected the supply to the motor terminals and vice versa. Happens!
• The frequency converter and the motor are properly grounded.

Now you can connect (or check) the control wiring. If the drive has built-in controls, this may not be necessary. For non-controller drives, this usually consists of at least one switch or connection between terminals 1 and 2 to enable (start) the drive. This is required in almost all control modes, even if the frequency converter is to be started from a serial link or front panel.

Drives such as the P2 and Eco drives (but not E3) require additional connections (e.g. links) from the Safe Torque Off terminals to 0V and 24V terminals. If you haven't done this, the display will show INHIBIT.

The output frequency (and therefore motor speed) is often controlled by a 0 – 10V signal applied to the analogue input. A potentiometer can be connected directly to the control terminals or a 0 – 10V signal supplied from an external source such as a PLC. Figure 3 from the manuals shows the basic connections to get you started.

Fig. 3: Basic Control Connections

In any case, refer to the manual or help card (built into the drive) for basic wiring information.

Now you are ready to commission the frequency converter.