Energy Efficiency Tip 1 — Energy Saving From AC Variable Speed Drives

Last edition, there was an article giving an overview of how Members can increase the energy efficiency in their business. Over the coming editions we will look at how these ideas can be applied to construction material processing businesses.

The advantage gained in both productivity improvements and reduced energy consumption by using AC variable speed drives (VSD) on pumps, fans, compressors and other equipment has been widely documented in the past few years.

Variable speed drives can reduce energy costs and prolong the life of equipment by adjusting motor speed to meet load requirements.

For example, by lowering fan or pump speed by 15% to 20%, shaft power can be reduced by as much as 30%.

There are many advantages to variable speed drives over other forms of control.

The main advantages are:

• Energy is saved by replacing mechanical fluid flow controllers with integrated motor speed control. Generally, these energy savings translate into cost savings and a reduction in greenhouse gas emissions for a given level of production. The soft start characteristic of VSDs eliminates voltage dips and reduces starting shock on motor, couplings, gears and driven equipment, in turn reducing maintenance.
• Running at reduced speed reduces wear on all drive train components and reduces the need for high maintenance items such as dampers, throttling valves, etc.
• The range of flow control is generally higher with VSDs compared to mechanical controllers and the likelihood of surges and vibration is reduced.
• Operating speeds in excess of 3000rpm for 2-pole AC motors are possible without the use of gearing where possible.

There are a wide range of motors ranging from 0.75kW to 500kW due to their cost and ability to perform under extreme conditions.

This, combined with continuing advances in power electronics and microprocessor technology, allows AC variable speed drive designers to incorporate greater control, greater power handling capability and reduce switching losses.

As a result the following changes have occurred:

• Increased drive efficiency (typically 97-98%)
• Reduced volume and weight
• Lower price (Eg. 55kW VSD unit with filters costs ~ $18,000 installed)
• Reduced audible noise level
• Improved power factor
• Reduced harmonic distortion to supply
• Improved reliability
• Larger voltage and current rating
• High switching speeds and lower losses

Applications

There are three major areas where drives and motors are applied.

• Constant torque applications: This is where the same amount of torque is required at low speed as at high speed. Power is directly proportional to speed. Typical applications include conveyors, mixers, screw feeders, extruders and positive displacement pumps.
• Constant power applications: This is where a high torque is required at low speed and a low torque at high speed. Typical applications include machine tools and traction.
• Variable torque applications: This is where a low torque is required at low speed and higher torque at high speed. These applications are generally centrifugal loads such as fans, pumps and blowers. (These are good targets for energy savings both during planning/construction or for retrofitting electric VSDs at a later stage to existing installations.)

Pumps, Fans and Blowers

Pumps (except positive displacement pumps) have the following physical principles of operation:

• Flow rate (change in volume/change in time) varies in proportion to drive speed. The relationship is expressed (Flow 1/Flow 2) = (Speed 1/Speed 2)
• Pressure and torque vary in proportion with the square of the flow rate and hence speed. The relationship is expressed as (Pressure 1/Pressure 2) = (Speed 1/Speed 2) ²
• Motor shaft output power (neglecting motor and drive losses) varies in proportion with the cube of the flow rate and hence speed. The relationship is expressed as (Power 1/Power 2) = (Speed 1/Speed 2)³

Pumps

In centrifugal pumping applications, a control valve is usually employed for flow control. By contrast, for positive displacement pumps, the traditional way to control flow was to return part of the fluid back into the pump by means of a bypass valve.

VSDs have been successfully applied to large boiler feed water pumps in power plants, hot water circulation pumps in commercial buildings and for waste water treatment plants.

The figure above depicts a typical application of a variable speed drive unit within a pumping installation where the pumping rate is adjusted by variation of the pump motor speed rather than the more traditional throttle and bypass valve systems.

Comments from the Experts:

The only negative with such equipment that is that this technology is very expensive. CEC Electrical just did a budget price for a crusher starter for twin 300kW VSD drives which totaled around $200,000. A standard starter would cost about $80,000. Although the VSD drives allow you to do a lot more, faster crusher speeds produce more dust. VSD Drives can be used to alter your product with ease as well as reduce running costs when running at lower speeds. Wear and tear on equipment is greatly reduced.

Craig Cahill, CEC Electrical, Ph 0417 379 062

A few issues are that such equipment generates a lot of heat and as such requires a significant amount of room or a controlled temperature. There is also the issue that it is not just a matter of installing the equipment, the installation may also require a specific type of cable to be rerun to overcome emissions, the length of this cable can effect performance.

Neville Palmer, Gordyn & Palmer, Ph 03 9703 2477

There are a lot of drives in a quarry where they may add unwanted costs. The cost of a variable speed drive is not cheap especially on larger drives. But they can save energy and protect motor and gearboxes from overload, as well as provide the flexibility of being able to change the speed

Alan Reints, Select Transmission & Motors Australia, Ph 03 9761 7355

You must be logged in to post a comment Login