Choosing the right type of Variable Frequency Drive

22nd July 2021  |  Drive systems

There are a variety of variable frequency drives (VFDs) available, each suited to a unique application. In this article, we review the key factors that should guide your buying decision.

There are a number of factors you need to consider when selecting a variable frequency drive for any sort of drive system. These include factors related to the end-application, the motor you are using, and even the promised longevity of the VFD. Here are our four biggest factors to consider when selecting a VFD:

1. The incoming power supply

The power supply to a drive system s the biggest definer of capacity in your system. Variable frequency drives should be selected based on compatibility with its power supply. In terms of voltage and frequency, current capacity and available kVA.

Your VFD rated voltage needs to match the available voltage, phase number and frequency at the site. These could be, for example, 200V/3-phase/60Hz, 400V/3-phase/50Hz, or even higher voltages (575V or 690V).

2. The motor type

Generally many induction motors can benefit from having a VFD these include higher efficiency. Permanent magnet synchronous motors (PMSMs) are not self starting and need a VFD in order to operate. Some VFDs come with permanent magnet motor control options as a standard feature, allowing operators to control the permanent magnet motor to drive the machine efficiently.

3. The motor rated current, voltage and frequency

The motor rated current, voltage and frequency. A VFD should never be selected based upon the power rating (kW or HP) of the motor alone. It is more important to ensure that the VFD can deliver the motor’s rated nameplate current continuously. Allowing for any required overload.

4. The type of application

The type of application – HD or ND for example. Many VFDs are only designed to be used on normal duty or light duty loads like centrifugal fans or pumps and will only tolerate lighter overloads for a short period of time. Some VFD’s are ‘Dual Rated’ and can be used on either HD or ND/LD loads in which case it is usually possible to drop down one VFD power rating when used on an ND/LD application.

5. The installation environment

  • The VFD will need to be derated if installed above 1000m.
  • Most VFD’s can operate up to 50⁰C but some de-rating will be necessary. It is important that the VFD’s maximum operating temperature is not exceeded or premature failure could occur. If a VFD is to be mounted in an enclosure, the heat created by switching losses in the VFD must be removed from the enclosure to prevent the VFD maximum operating temperature being exceeded. This can by achieved by installing extract fan(s) and air intake(s) or a bespoke chiller pack.
  • Choose the correct IP rating for the VFD determined by if it will be installed in an enclosure or directly onto a wall, pump skid or machine leg.
  • Be aware of condensation which can occur if the VFD is exposed to extreme temperature changes like daytime sun to night time frost. Consider using a thermostatically controlled panel heater, for example.

6. Harmonics

When connecting VFD’s or other non-linear loads the effect on the power supply types must be taken into consideration. Non-linear loads with diode rectifier input designs tend to ‘bite’ current from the mains in chunks and at specific multiples of the fundamental supply frequency rather than sinusoidally as with linear loads which in turn causes distortion to the supply voltage waveform.

Harmonic currents can have an adverse effect on transformers, cables and sensitive electronic equipment, so network operation companies (NOC’s) will insist that certain harmonic limits are adhered to before connecting VFD loads onto a new or existing network.

In the UK for example Engineering Recommendation G5/5 came into force on 17th June 2020 and is effectively an installation standard to limit harmonics in the public electricity supply so as to not to cause problems for other users.

Many islanded consumers only have an option to use diesel generators to supply their power and because of their inherently higher source impedance careful consideration must be given to ensure the generator capacity is of significantly higher rating than the total KVA of the connected non-linear loads. This is equally true when diesel gensets are used as a backup in case of mains power failure. As the harmonic content of the VFD’s supply current can cause ‘flat-topping’ of the generator’s output voltage and other types of voltage distortion that can cause the VFD to trip off line or otherwise malfunction.

Looking for an inverter drive? Speak to our specialists

Dalroad supports end users and electrical contractors in a wide variety of industries to dimension their drive system and choose the best variable frequency drive for their project. Whether you are working on a fan or pump, or a heavy-duty application, we’re able to offer our customers best-in-class support from dimensioning to installation.

Get in touch with an inverter drives expert at Dalroad today for no-obligation advice and product recommendations.

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