Variable Speed Drives

What are variable speed drives?

Variable speed drives (VSDs) allow loads driven by AC induction motors (such as fans and pumps) to operate in a wide range of speeds compared to the motor fixed speed. VSDs are also called variable-frequency drives, adjustable-speed drives, variable-frequency inverters, or frequency converters.

VSD installations can increase energy efficiency (in some cases energy savings can exceed 50%), improve power factor and process precision, and provide other performance benefits such as soft starting and over-speed capability. They also can eliminate the need for expensive and energy-wasting throttling mechanisms such as control valves and outlet dampers.

Typical applications of variable speed drives

VSDs can provide significant savings in applications such as:

• Variable air volume air conditioning systems
• Chilled water pumping
• Exhaust air systems, such as dust extraction, paint shop exhaust, and fume cupboards
• Refrigeration systems
• Some modern compressors (including air and refrigeration compressors) 

Variable speed drives case study

Hotel saves energy and improves comfort 
A major Perth hotel installed variable speed drives on its air conditioning fans, improving conditions, reducing noise and cutting fan-related energy costs by about 40%.

Project description

The hotel has a large number of units supplying air conditioning to the guest rooms. As more rooms are occupied, more air is needed to maintain a comfortable temperature in each room. As a result, the fans supplying the air need to vary their output, based on the number of occupied rooms at any given time.

The initial installation had throttling dampers installed in the supply air ductwork. As the requirement for airflow decreased, these dampers closed to reduce airflow to the rooms.
Not only was this method of flow control inefficient in terms of energy use, it was noisy and required regular maintenance.

The hotel determined that installing variable speed drives (VSDs) on the fans would save energy, improve system performance and lower maintenance costs.

After a trial run, the hotel installed VSDs on each of the guests' rooms supply air fans. The work included disconnecting the existing supply air dampers, upgrading the air conditioning control systems and installing the new VSDs.

Savings achieved were around 40% of fan energy costs. This equated to approximately $23,000 per annum. A typical result (for one fan) is shown in the figure below.

Figure 1: Before and after installation 

Based on the monitored results the initial payback period will be 36 months. This does not include the maintenance savings.

This case demonstrates that it is possible to improve energy performance and service delivery at the same time. The upgrade was very well received by both hotel management and their guests.

Summary

Efficiency measure(s)	Installation of VSDs on fans
Potential users	Offices, hotels, hospitals, schools
Site	Hotel in Perth.
Cost of implementation	$67,000
Expected payback period	36 months

Note: Whilst care has been taken in the preparation of this material, it is intended to provide a general idea about what can be achieved in a particular situation. Quantitative indicators may vary with time and changing circumstances. Each situation is different, requiring its own evaluation; and therefore the results given above should not be taken as being directly transferable to other circumstances. Names and addresses have been withheld for privacy reasons.

Variable speed drive tips

Full load operation

VSDs provide dramatic energy savings by optimising the system, not by improving the actual efficiency of the motor in isolation (as an energy efficient motor retrofit would). In fact, a VSD system is about 4% to 6% less efficient at full load than an induction motor alone. This is mainly due to the losses in the VSD itself. However, it doesn’t take much operation at reduced load to save more energy than is lost at full load. Average loading as high as 90% can justify a VSD retrofit for high-duty applications.

Low speed operation

Most induction motors can operate with modern VSDs through moderate speed ranges (around 30% to 100% speed). Sustained operation at low speeds and, in particular, high load at low speeds may require a special or larger drive and special measures to cool the motor.

AC induction motors operate hotter with a VSD because of harmonics, impurities in the electric power they provide to the motor and also the slower rotating speed of the motor's integral cooling fans. This is usually not a problem if speeds are continuously above 40% or where there are brief periods of slow-speed operation. However, prolonged operation at or below about 30% speed, especially when driving significant loads, can cause rapid and potentially damaging heat in some motors.

Starting Torque

In VSD/motor systems, starting torque is typically determined by the drive (not the motor). For conventional VSD applications, the VSD/motor system will have a peak starting torque of about 130% of rated full-load torque, which is significantly less than what the motor could develop by itself. This level of starting torque is acceptable for most variable speed loads, but some loads (especially constant-torque loads such as conveyers, escalators, augers, or reciprocating compressors) may require greater starting torque.

Harmonics and power factor

Although they can improve displacement power factor (DPF), modern VSDs also create harmonics, which reduce real power factor. (Real power factor includes harmonics and DPF.) For instance, while a VSD can improve DPF to close to 1.0, the harmonics generated by the VSD can cause the real power factor to decline to between 0.75 and 0.80. These harmonic currents (most often the fifth and seventh harmonics) tend to exacerbate resistance losses and can even negate the benefits of improved DPF.

To minimise this problem, more and more VSD manufacturers are packaging harmonics-mitigating equipment (such as line reactors or isolation transformers) with drives. This lets users enjoy the full benefits of power factor improvement. What’s more, this added equipment can significantly reduce the impact of VSD-generated harmonics on other electronic equipment.

VSDs located too far from motor

Pulse-width modulated drives can cause significant damage to motors if the length of cable between the VSD and the motor exceeds 15 to 30 metres. (The number seems to differ by manufacturer.) Using pulse-width modulated VSDs on older motors with long cable runs may shorten the life of the motor.

Mechanical resonance

It is important to determine any mechanical resonance frequencies and to program the VSD to avoid steady operation at those speeds. These resonance frequencies, common in large fans, gears, and belt-driven systems, can cause significant damage through vibration.

Motor compatibility

To ensure that your VSD and motor are compatible, either buy them from the same company, or ask the manufacturer to test the VSD to make sure it’s compatible with another company's line of motors.

Further information

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