AC drives and motors: Selection matters

Variable speed drives (VSDs) have been around for a long time. Is the market now saturated?

VSDs have been around for over 40 years. Yet despite electric motors accounting for over 65% of industrial energy consumed, only 5% of the installed base currently use a VSD. Of course, not all motors are suitable for VSD use, but the majority of direct-on-line motors controlling pumps and fans can certainly be retrofitted with a drive. And the energy savings alone can be staggering.

However, the awareness of just how effective VSDs and high efficiency motors are at saving energy is still not getting through. When asked, in a survey, what is the most effective way to reduce energy costs, both finance managers and engineers responded with: “Change energy supplier.” Yet this only moves the problem from one supplier to another. A VSD is proven to substantially reduce energy bills. 

What applications are best suited to variable speed drives (VSDs)?

Applications like pumps and fans are referred to as variable torque loads. Reducing the speed of such a load with a VSD will save between 20 and 60% of energy. 

Other applications like conveyors, mixers and air compressors are referred to as constant torque loads. With such applications, the torque requirement throughout the speed range is the same. While there are still energy savings to be made, they are not as dramatic as those seen in variable torque applications. However, applications such as air compressors and hydraulic power packs can achieve significant savings depending upon the on-load and off-load cycle times.

The main benefits of VSDs in constant torque applications are precise process speed control and starting/stopping with controlled acceleration/deceleration. This is why VSDs were originally invented: to control process speeds and improve productivity. The energy saving aspects are, in effect, a hugely beneficial side line.

Variable torque loads	Constant torque loads
– Air handling units – Industrial process cooling pumps – Chilled water pumps – Hot water circulation pumps – Cooling tower fans – Return air fans – Circulation and supply pumps – Combustion blower fans	– Air compressors – Hydraulic power packs – Conveyors – Mixers – Positive displacement pumps – Rotary kilns – Agitators – Crushers – Surface winders

What needs to be done to raise the awareness of VSDs?

What tends to hold back the market is a lack of appreciation as to what a VSD does. As new engineers graduate, they are not always made aware of VSD technology and how it applies to motor-driven applications. Embracing VSD technology could be the one decision that totally changes the performance of a business.

Few realise that, for example, a moderately sized supermarket of 4645m2 will use about 143,000kWh per year for cooling alone. Applying VSDs to the cooling system might, conservatively, reduce energy consumption for cooling by 20%. This equates to 28,600kWh in savings. In other words, it would take 24 'average' homes to convert entirely to compact fluorescent lighting to produce the same energy savings as one drive system in a relatively small application.

What are the benefits or replacing old VSDs or constant-speed kit with new technology?

Replacing old drives and/or motors brings many benefits:

• Better motor control – today’s drives enable higher quality output waveforms, reducing motor losses by 10-15%
• Lower running costs - it is highly likely that older motors and drives from any manufacturer will start to suffer breakdowns so it could be beneficial to cut your maintenance bill and replace the motors and drives 
• Greater reliability - lower component count means new drives last longer and help reduce downtime. Likewise, high efficiency motors tend to run cooler, thereby extending the life time and intervals between maintenance
• Smaller size - a new drive delivers much more power, size for size. A modern ABB drive can, in some cases, be just one third of the size of a drive only a few years older 
• Latest technology - users with new drives and motors can benefit from recently introduced features, for instance flux (energy) optimisation, which can reduce the energy consumption in pump and fan applications by 10%
• Production improvement - with a VSD, speed increases of 5 to 20% can be easily attained without changing the mechanics. The production increase can be achieved without any extra investment. The accurate speed control obtained with VSDs results in process optimisation.

When it comes to motors, what are the basic choices for pump and fan applications? 

Electronically commutated motors (ECM) are combined with a brushless controller onto the fan assembly. However, if one component fails, the entire unit – including fan, motor and controller - must be replaced. ECM fans have a poor harmonic signature, typically do not come with building management system (BMS) fieldbus protocols and cannot catch spinning loads. 

Synchronous reluctance motors (SynRM) are controlled by variable speed drives. They offer the same efficiencies as ECMs but brings advantages such as improved harmonic mitigation, reduced noise, seamless power dip ride-through, fieldbus connectivity, built-in control features to enhance the application, accurate speed control and ease of retrofit.

Permanent magnet (PM) motors - the losses and absence of heat due to current in the rotor means the PM motor is a more efficient alternative to induction motors in some applications. However, PM motors can be difficult to handle (they magnetise themselves to any ferrous surface), have non-standard motor shapes and fixing dimensions and generate dangerous back EMF voltages when freewheeling. 

Induction motors (IM) technology is well understood, the motors are easy to control and to repair and they have become more efficient over time.