Factors to be considered in sizing variable frequency drives

Thid article gives an outline of the factors to be considered in the sizing of a VFD for more straightforward application. Other parameters such as environmental conditions, harmonic considerations, multiple pump applications, load inertia, need for braking, control capabilities and communication may also affect the selection of the Variable Frequency Drive. It is recommended in all cases to consult the VFD manufacturer for an engineedred solution for the specific applciation.

Motor nameplate full load amps (FLA)

380V The motor full load current is the most important factor in sizing a VFD and for electronic motor protection. The motor power rating (kW or hp) value may be used only as an approximation as the power factor and efficiency of the motor can vary substantially, especially for slow speed and electro submersible pump motors. The VFD size will normally be selected with a maximum continuous output current rating that is greater than the motor full load current. It is not required to grossly oversize a VFD for a pumping application unless motor uprating is foressen in the future.

If the duty is well specified, and the actual maximum running current is known then there are possible cost saving opportunities when retrofitting a constat speed application with a VFD. This is achieved by selecting a drive with lower rated current than the motor nameplate. Case Study 1 provides an example, but this selection must be undertaken with considerable care.

Motor load torque characteristic

VFD manufacturers will often allocate several ratings to a particular converter. For example, a rating with small overload capability, typically 10% or 20% for a short duration, (often one minute in ten), and also a lower rating, with rather higher short-duration overload capability of 150% or 200%. The purpose of these overload is to allow short period transients required for acceleration and overcoming starting stiction.

The very different torque-speed curves for rotodynmaic and positive displacement pumps are shown in Figure 7.3. These two load charateristic are sometimes refferred to as variable toruqe and constant toruqe loads respectively. In general, a rotordynamic pump can be powered by a VFD with small overload (overcurrent) capability, whereas a PD pump may weill require a higher overload capability to achieved staring requirements.

In general, if a VFD is rated for variable torque loads it will have a higher maximum continuous current output than with the same VFD rated for a constant torque application. Conversely, if a VFD is rated for variable torque loads it will have a lower percentage of over current capacity than the same VFD rated for a constant torque application.

Motor nameplate 3 phase voltage

It is always important to match the volage that can be fed from teh VFD to the motor being used. A VFD will be quoted as having an input supply tolerance of +/-10%, but in general it cannot produce more volts than it is supplied with. In face, generally the maximum output voltage from a VFD will be lower than the supply volts, due to volt drops across the inductors, used to reduce harmonics, and in output line (dU/dt) filters.

In some cases step up transformers may be appropriate, but the advice of the motor and VFD manufacture should be obtained, as these transformers may require a sine filter in the inverter output, and a special core construction.

If the voltages are mismatched, the VFD may have to pass a higher current to the motor to achieve a required torque, with consequent additional heating of the motor.

Single phasing

In rural areas, threee phase power is not always available. Generally single phase supply VFDs are limited to powers up to around 2.2kW (3hp).

Some VFD manufactures will allow their standard three phase VFDs to be supplied at single phase, providing they are suitably de-rated, (typically by a factor of two), and with the phase loss protection inhibited.

Some manufactures offer VFDs for higher power single-phase applications. Proprietary series harmonic filters are available which will provide a three phase supply, with reduced net harmonics, allowing a conventional VFD to operate off a single phase supply.

While single phase supplies are used to feed the VFD, the output to the motor is always three phase, allowing lower cost three phase motors to be used. Single phase capacitor started motors must never be used with a VFD.

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