When selecting a frequency converter, the load conditions of the application should always be taken into account first. A fundamental distinction is made between quadratic and constant load characteristics, which are the most common in practice. When a frequency converter controls a motor, torque limits can be set for that motor. Selecting a frequency converter with an apparent power rating that matches the rated current or power of the motor ensures that the required load can be driven reliably. However, an additional reserve is necessary in order to enable smooth acceleration of the load and also cater for occasional peak loads.
Below are examples of a constant load torque characteristic. If a load is placed on a conveyor belt, the torque that must be applied to transport the load is constant over the entire speed range.
With a constant load, an over-load reserve of approximately 50 to 60% for 60 seconds is typically used. If the maximum overload limit is reached, the response depends on the frequency converter used. Some types switch off their output and lose control of the load. Others are able to control the motor at the maximum over-load limit until they trip for thermal reasons.
A quadratic load characteristic usually occurs in applications where increasing speed leads to an increasing quadratic load torque. Fans and centrifugal pumps are amongst the types of equipment that display behavior of this kind. Furthermore, most applications with a quadratic torque characteristic, such as centrifugal pumps or fans, do not require rapid acceleration phases. For this reason, excess load reserves of 10 % are usually chosen for quadratic torques.
Here are some examples of a quadratic load torque characteristic.
Even with quadratic load and an over-load capacity of 10% modern frequency converters can be set up to have a higher break-away torque at the start to ensure the proper start of the application.
Remember to consider whether the application will always require a quadratic torque. For example, a mixer has a quadratic torque requirement when it is used to mix a very fluid medium, but if the medium becomes highly viscous during processing, the torque requirement changes to constant.
It is important to also remember that the most energy-efficient solution is where the machine, the motor and the frequency converter are selected for the best system efficiency. For example, fan speed will typically differ from nominal speed, and so the motor, but many motors have their highest efficiency at a speed between 75 and 100% of nominal speed. Some brands of frequency converters have a built-in software function that secures the best motor shaft power related to the frequency converter’s input power.