Selection of a variable frequency drive

In this post we walk through some aspects regarding selection of a variable frequency drive.

Variable frequency drives, often abbreviated as VFD, VSD or ASD, are means to achieve energy savings [1] and to enable position, speed or torque control of electric motors. Meanwhile, VFDs can be found in practically any industry. But how to select the most suitable solution? What factors to consider? This article may serve as a very basic guide to start with.

VFD offers many benefits, such as significant energy savings, smooth motor start or precise and dynamic process control (just to mention some of them). In order to maximize the benefits that VFD can bring to the user it is crucial to select the right fit. Therefore, we start with some basic considerations everyone shall do prior to purchasing.

For selection of a variable frequency drive some of the key technological aspects are:

♦ Technological purpose

♦ Environment and location/installation

♦ Rated current, maximum overload and duty cycle

♦ Rated output frequency and frequency range

♦ Requirement on reversible operation

♦ Requirements on dynamics

♦ Max. allowable current and voltage distortion

♦ Torque pulsations (ripple)

♦ Grid connection, reactive power and harmonic distortion

Technological purpose

The purpose of the VFD application is naturally a major point to consider. it may significantly impact the selection of VFD – hardware dimensioning, control requirements, communication with upper controller, specific features etc. Technological purpose usually determines whether e.g. active braking is required or not. For specific technological processes a multidrive solution may be of a great benefit. Safety aspects must not be underestimated either.

Environment and installation

Environmental conditions may be decisive when selecting e.g. cooling type or enclosure ingress protection (see also [2] regarding impact of environment on dimensioning). Based on environment the manufacturer can decide whether e.g. a space heater is needed or not. When the equipment is located outdoors, typically in an E-house, many other things become relevant, such as wind load, solar irradiation, snow etc.

Installation also plays an important role. Some places may have risk of increased seismic activity. In others the FD will be exposed to certain level or roll and pitch (e.g. marine drives). A limited space may call for tailor made VFD layout, e.g. L-shape or U-shape.

Rated current, voltage and power

Obviously, the power/current/voltage rating influences the selection of VFD [3]. There are different VFD topologies and solutions when we talk about low power, medium power or high power range. The electric power is a product of current and voltage. Sometimes there is a preferred motor voltage. Other times you are free to select the best option.

Some applications require light or even heavy overloads. For optimal design it is always good to know the duty cycle. Moreover, it is good not know the corresponding output frequency or machine speed. Overload at very low speed may have other implication on design than overload at nominal speed or maximum speed.

Output frequency (range)

Thanks to the versatility of VFD applications we can find practically all examples for very low speed up to truly high speed. Of course, motor speed does not directly tell anything about output frequency of the VFD as it depends on number of pole pairs and presence of gearbox.

Although the VFD nominal output frequency can be set almost arbitrary, many standard applications (called general purpose) have nominal frequency at or around 50 Hz or 60 Hz. However, there are also special purpose drives that often use different output frequency. For instance, large mills in mining industry have low speed (in rpm). Also rolling mills in metal processing plants use motors with lower rated frequency. On the other side of the spectrum is the turbomachinery where either a gearbox solution or a direct drive is used.

Reversible operation

Is reversible operation required? Many applications do not need such option, but for some this is a must. It requires reversing the torque, active braking and possibly accelerating in opposite direction of rotation. Current source inverters can achieve it by reversing the polarity of voltage. Voltage source inverter needs to change the direction of current [4]. Therefore, an active rectifier is needed. For occasional active braking a chopper might be an alternative.

With respect to reversible operation we often talk about “2-quadrant” and “4-quadrant” drives referring to a torque-speed diagram divided into 4 sectors.

Dynamic requirements

Dynamics is something that often differentiates a “general-purpose” and “special-purpose” drive. Majority of VFD applications do not require much dynamics. In fact, it is often desired to have a smooth speed control and no abrupt changes of torque. However, there are applications where dynamics is crucial. In rolling mill drives the dynamics directly affects the productivity and also quality of final product.

Dynamics is linked with power hardware (e.g. short time overoadability), but also the control. Simple scalar control is not a suitable option as it works for (quasi) steady state operation, but not during transients. Instead, a more sophisticated field oriented control or direct torque control (DTC) is needed. DTC is well-known to have unrivaled dynamic performance that had been proven over last three decades in the most demanding applications.

Current and voltage distortion

The power quality applies to both side – motor side and grid side. We focus more on the motor side in this paragraph.

The requirements on quality of power delivered to the motor depends on several factors. It is partly a question of approach [5]. Obviously, you can either use less motor-friendly VFD in combination with a specially designed motor (inverter duty). The opposite approach is to use a very motor-friendly VFD in combination with practically standard motor.

Both approaches are fully legitimate and the choice depends on the system. Motor-friendly VFD is a must for retrofit projects with other machines not designed for VFD duty. In contrast, for high performance applications with superior dynamics the natural choice is the other approach with specifically designed motors.

Torque pulsations

Torque of a VFD driven motor is never perfectly smooth. The fluctuations around the mean value are referred to as torque pulsations or torque ripple. The torque ripple affects the motor itself and also the driven load. Many people just set certain limits on peak to peak torque in normal operation. However, more important is typically to look at the frequency spectrum of the torque. When a component in the torque matches a mechanical resonance then an issue can appear (depending on magnitude of excitation component, system damping and dwell time). See also [6] as good starting point for torsional considerations.

Torque pulsations are partly linked to a VFD topology, but partly also depend on modulation strategy. Finally, active damping algorithms can significantly improve the situation.

Grid connection

The characteristics of the grid at the point of common coupling (PCC) or point of interconnection impact the VFD selection as well. Refer to our series on network harmonics and some considerations on rectifier type (AFE vs DFE) [7]. A general trend is that the higher the short circuit power at PCC the stronger grid and the less potential issues. However, more accurate analysis uses frequency scans or frequency sweeps to identify grid resonances.

Harmonics [8] are one part of the equation, reactive power consumption [9] may be another one. DFE can offer you input power factor of 0.95 to 0.97 at nominal load (inductive, lagging) while AFE typically controls the power factor to unity and optionally can be used for VAR compensation.

Obviously, these are not the only criteria for selection of a variable frequency drive. The user shall definitely consider other essential points, such as availability and reliability [10], engineering support [11], after sales support, local/regional service capabilities and so on. What you want is a high quality support throughout the entire lifecycle of your VFD or drive system, contributing to profitability and sustainability [12].

Summary

Selection of a variable frequency drive is an important step. The lifetime of the equipment is typical 20 years or more. Therefore, it is worth to spend some time before making the choice. In this blog we have listed multiple topics and areas to consider when specifying or purchasing a VFD. Specific topics are elaborated in more detail in the references below.

We wish you all to make the right decision that you will never regret.