Variable Speed Drives

The Accelerated Capital Allowances (AC) list of eligible products includes Variable Speed Drives (VSDs).

CONTENTS LIST

Introduction

Variable Speed Drives (VSDs) – also commonly known as Variable Frequency Drives or Inverters – are used to control the speed of AC induction motors.

This guide

•  provides an overview of the operation of VSDs, their benefits, suitable applications and potential savings
• outlines the key issues to be addressed when you are considering the design and installation of a VSD

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What is a Variable Speed Drive (VSD)?

A VSD is a device that is used to control the speed of an induction motor. An AC induction motor is a constant-speed motor. In many motor applications, energy use can be reduced considerably if the speed of the motor varies in response to the changing process conditions. A VSD makes this possible.

 
Figure 1: Variable Speed Drive (courtesy of Danfoss)

In applications where motors are used to drive pumps, fans and compressors, the mechanical power needed is proportional to the cube of the fluid flow. Thus, for instance, reducing the flow from 100% to 80% of the nominal value would halve the mechanical power required.

Where fluid flow is controlled by dampers or valves, much of this potential for energy saving is lost. The application of a VSD, on the other hand, enables the motor to respond to the changing flow requirement, and therefore directly translates this potential into electricity savings.

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How does a VSD work?

A VSD converts the 50Hz fixed-frequency and fixed-voltage AC power supply into a DC supply, using an integrated rectifier. Integrated power electronics then convert the DC supply into a sinusoidal output with continuously variable frequency and voltage, which is used to drive the motor. In other words, a fixed sinewave in is converted into a variable sinewave out.

This variable output enables the VSD to quickly change the speed and the torque of the motor in response to the changing load. The required variation in output is controlled by onboard microprocessors. Modern VSDs have no moving parts and are therefore highly reliable and efficient.

A VSD typically has an operator interface that enables manual operation of the drive and the configuration of parameters to match the VSD to the motor and to the application.

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What types of systems are best suited to a VSD?

Systems that require control of flow or pressure are most suited to the use of a VSD. This is because the consumed power is roughly proportional to the cube of the flow, or speed of the motor . Such applications may include:

• Fans and pumps: In applications where the flow of fluid is variable, considerable energy savings can be achieved by replacing existing throttling valves and dampers with VSDs.
• Conveyors: For conveyors with varying speed or with varying material flow, a VSD can adjust to the changing load requirements.
• Compressors and chillers: In the same manner as fans and pumps, compressors can take advantage of the energy saving that is achieved by varying the flow with a VSD.

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Are some systems unsuited to a VSD?

Systems with a high static head are unsuited to the use of a VSD. Static head is a measure of the pressure needed to make a fluid flow. This is a constant pressure and is independent of motor speed. When a high motor speed is needed to overcome the static head, the motor speed is no longer proportional to the flow rate. A small reduction in motor speed could result in a large drop in flow and a big reduction in machine efficiency.

In applications where the motor load remains more or less constant, a VSD is unlikely to achieve a significant saving, as the opportunity to reduce power is diminished.

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Will you be able to retrofit a VSD to an existing motor?

An existing AC induction motor will be compatible with the application of a VSD. The viability of retrofitting the VSD will depend on:

•  the cost
• the space available
• the suitability of the application to variable speed control

You should also consider the additional energy savings that could be achieved by replacing the existing motor with an equivalent energy-efficient motor.

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How do you match a VSD with an existing motor?

It’s best to start with the motor and ensure it is suited to its task, i.e. that it’s not overloaded or oversized.

The first step is to define the operating profile of the load. This involves measuring the required torque and current consumption under all operating conditions.

Then, this profile needs to be examined to ensure, first, that the motor is correctly sized; secondly, that the installation is suited to the application of a VSD. The VSD must be sized to the motor load, based on the maximum current requirements under peak torque demands. A VSD cannot be sized using the kilowatt rating of the motor alone.

Suppliers of VSDs included in the ACA specified products list should help you to choose the correct product.

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How do you perform simple savings and payback calculations?

To perform a useful payback calculation, application-specific data must be gathered from the installation. The load profile under all conditions must be measured or estimated.  As each motor application is unique, only a costing exercise will reveal the potential savings of installing a VSD in a system.

For a simplified example of a payback calculation, see Example of a VSD payback calculation. Note that participation in the ACA scheme further reduces the payback period.

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Should you be concerned about harmonics?

All VSDs produce harmonics from pulsed current being drawn from the supply. Harmonics are multiples of the 50Hz supply frequency which become superimposed on the supply system. Harmonics can cause problems such as equipment and capacitor overheating, voltage distortion and equipment malfunction. However, the use of filters, sometimes available as part of a VSD, helps to minimise the effects of harmonics.

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How can you further improve the energy efficiency of your drive installation?

If practical, motors and drives should be switched off when not in use.

For all new installations, energy-efficient motors should be used. The combination of a VSD and an energy-efficient motor ensures an economical and future-proof installation.

For retrofit applications, you should consider replacing standard-efficiency motors with equivalent energy-efficient ones. Energy-efficient motors are also included in the ACA, and those that have qualified included on the specified list.

The mechanical efficiency of the driven equipment and transmission system (i.e. pumps, fans, belts, etc) directly influences the efficiency of the overall drive system. It’s important that this equipment be regularly maintained and lubricated and that wear-and-tear of mechanical parts be monitored.

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Example of a VSD payback calculation

Consider an 11kW 2-pole EFF1 motor driving a product transfer fan for a milk powder processing plant. The fan motor operates 6,000 hours per annum. Air flow is controlled via a manual damper set to 80% open. The motor efficiency is 90.5%.

From the curve representing the system in Figure 2 below, we see how the damper setting reduces the input power requirement by a factor of about 0.9. A cost of electricity of €0.14 per kWh is assumed.

Figure 2: Damper vs. VSD control efficiencies (courtesy of The Carbon Trust)

Without VSD

The annual cost of running the motor without VSD is as follows:

With VSD

If the damper is replaced with a VSD, the curve in Figure 2 shows that the input power is now reduced to 58% of maximum when running at 80% of full load. If we assume that the combined efficiency of the motor and the VSD is now 86%, then the annual running cost of the motor combined with VSD can be calculated as follows: 

Thus the annual cost savings achieved by replacing the damper with the VSD are as follows:

Cost savings with VSD = €9,188.95 - €6,231.63 = €2,957.32 p.a.

If we assume a cost of €6,000 to supply and install the VSD, taking support from the ACA scheme into account, this gives us the following payback period:

Payback period = €6,000 / €2,957.32 = 2.03 years

In this simplified example, a payback of two years has been calculated. The load profile has been simplified to a constant 80% of full load. In practice, a more detailed examination of a varying load profile would be needed to calculate the true annual running costs.