VFD selection experiences

When we are assisting with selection of our Variable Frequency Drives we look at your application, is it being used for Constant or Variable Torque (Standard Pumps are considered a Variable Torque), the Full load current of the motor, Supply voltages, ambient temperatures, ETC. The VFDs are rated and sized by Amperage and it will come down to making sure it is not undersized, Talking with your suppliers engineers are the safest way to make sure you get the best value for your money without getting problems from being undersized.

Please bear in mind that all voltage source inverters have a problem at the corner - ie if your primary energy input is 480VAC RMS - the maximum output voltage of the inverter is 5% below that input voltage #.... Some VFDs, [eg ABB drives], go into 2 phase operation, which allows more effective output voltage at the higher output fundamental frequencies. You need to ask the question about maximum output voltage to any drive supplier of choice. Also in any VFD application proper coordination of the voltage vs frequency curve is essential in order to allow the motor to develop the torque at the proper current level. {ie you can under excite or over excite the motor which will not produce the desire torque per ampere.}

It is not always appropriate to select variable speed control system and there are many applications that can be run with a fixed speed control method (Motor Contactors or Soft Starters for example). A fixed speed control should be selected where:

• The load demands maximum motor speed
• Mechanical constraints do not permit speed variation
• Financial considerations (e.g. Energy costs) do not justify payback of a variable frequency drive

In these cases, the simplest and most effective method to reduce energy consumption is to switch off the motor when it is not required.