Motors and variable speed drives


Electric motors have a range of industrial applications, including the operation of pumps, compressors and fans. Motors use substantial energy and can incur significant running costs. While they are generally inefficient, there is often significant potential for saving energy.

The largest segment of motor electricity consumption in Australia is mid-size motors with output power between 0.75kW to 375kW. Motors in this range have been subject to minimum energy performance standards since 2002, and have become more efficient over time.

Variable speed drives (VSDs)

Motor systems always require a means of control, from a simple switch to a modern VSD. VSDs control speed to precisely match the load, thereby improving system performance. Combining high-efficiency motors with VSDs can save energy and increase productivity.

Along with system optimisation and motor efficiency, VSD motor control is one of the three main areas where energy savings can be achieved. Savings will depend on the nature and variability of the load and total operating hours. Where process output requirements vary by 30% or more, matching the load with a VSD can reduce energy use significantly.

Aside from substantial energy savings, motor systems fitted with VSDs can bring other benefits to improve productivity and profit. Benefits may include:

  • reduction in maximum power demand and associated charges
  • reduced stress on system components
  • accurate system control of parameters such as pressure, flow and temperature
  • improved workplace safety and amenity, through reduced heat and noise levels
  • ability to interface VSDs to wider process control systems such as building management systems (BMS)

Assessing motor loads and savings potential

Pumps and fans typically have variable torque loads subject to the ‘cube law’ — meaning that reducing the motor speed by 20% can reduce the power required by 50%. Constant torque loads occur where torque is independent of speed, as is commonly the case in cranes, hoists, conveyors, extruders, mixers, reciprocating air compressors, and rotary screw air compressors. In such cases, the speed/power relationship is proportional, meaning that a 50% reduction in speed delivers a 50% reduction in power.

To estimate the energy savings when a VSD is applied to a variable or constant torque load, you will need to:

  • establish the percentage of time the equipment operates under various load conditions 
  • determine the efficiency of the potential VSD and motor combination when operated under similar conditions

Efficiency values for motor drive systems when connected to various kinds of loads can usually be obtained from equipment manufacturers.

System configuration and optimisation

On average, motor systems lose over half their input energy before delivering end-use service. So, while energy efficiency of individual motors may be high, the efficiency of the system as a whole can be low. Improving the service delivery system can bring the greatest savings.

Once the system serviced by the motor is optimised, motors and drives can be selected to most efficiently meet final requirements.

General areas for system optimisation include:

  • rationalisation or separation of existing production lines or processes
  • arrangement of machinery to minimise system distribution losses and facilitate energy recovery
  • ensuring pipes and ducts are of proper diameter to minimise friction
  • minimising pressure drop, including reducing flow obstructions, sharp bends, expansions and contractions
  • selecting low-loss valves and fittings
  • ensuring driven machinery is operated at optimal efficiency points, and specified for task
  • ensure all system components and filters are clean

Motor management and maintenance

A properly maintained motor can perform up to 15% more efficiently. It’s worth implementing a proactive motor management and maintenance program.

A large industrial facility can contain thousands of motors, with most energy consumed by a few essential systems. Improving motors in a targeted, strategic manner is likely to deliver the most benefits with the fastest payback.

Motor energy use can be monitored by installing dedicated electricity metering equipment. Modern sensor systems assist in implementing a cost-effective motor management system by automatically identifying potential problems before they arise (see the Industry 4.0 page for more information).

In the absence of detailed data, inspections of motor and drive components should be based on such factors as run time, environmental conditions and consequences of failure. Inspection can also be carried out using an infra-red camera to identify motors that are running hot.

There are a number of maintenance tasks to help ensure motors are functioning optimally:

  • Clean motors of dirt and grease, particularly fans on fan-cooled motors.
  • Check for excessive vibration which may be a sign of motor misalignment.
  • Check for connections or wires that might be loose or damaged.
  • Keep motors cool, by providing adequate ventilation, and keeping cooling fins and fan vents clean. 
  • Lubricate motors, bearings, gearboxes and chain drives according to the manufacturer’s recommended intervals and lubricant specifications.
  • Ensure belt drives are correctly tensioned, and free of dirt and abrasives. Ensure multiple belts are evenly matched. 
  • Re-assign motors within a factory according to actual measured loads.
  • Replace old motors before they break down.

Right-sizing motors

Motors should be correctly sized for the maximum intended duty.

Motor sizing must consider:

  • the running load and the power required to start the machine
  • speed and torque requirements of the driven equipment
  • the ability of the motor to respond to load changes

Electric motors are generally inefficient when operated at loads below 40% of their rated output. They’re often most efficient when operating between 70% and 80% of rated output.

The consequences of oversizing motors include lower efficiency and power factor, as well as higher equipment, installation and operating costs. Undersizing a motor for the intended application means the motor will have to spin faster and work harder, leading to higher temperatures, reduced efficiency and shortened life.

When right-sizing motors, assessing the actual required load will help avoid replacing like-with-like. This process can facilitate replacement or reassignment of motors based on actual loads rather than rated motor capacity.

High-efficiency motors

Most motors operating in Australia are older, standard efficiency motors, with particularly poor efficiency at part load.  In most cases, these can be replaced with high-efficiency motors.

High-efficiency motors can cost up to 40% more, but the payback period can be less than 2 years with the associated energy savings.

The Australian Government’s Energy Rating website has an exhaustive database of mid-sized electric motors sold in Australia since 2002, including currently available models. The database includes the tested efficiency of each motor at low, medium and high loads.

While no energy star rating label is required on motors, all motors subject to regulation must comply with labelling requirements that indicate whether the motor is standard or high efficiency. In 2018, MEPS raised the threshold for what can be declared as a high-efficiency motor.

Motor speed control options

Motor speed control options can vary from simple voltage-controlled DC motors to fully functional electronic VSD systems driving AC motors. Motor speed control allows motors to be oversized to meet extreme requirements without energy wasted during low demand. In general, the best option for variable motor speed is to combine a high-efficiency AC motor with an electronic VSD.

Depending on the application, alternatives to complex VSDs are available at a lower cost. AC motors can be designed with controls that switch between discrete speed settings. Several smaller motors can be controlled with a switch to power just enough motors to meet the demand. Mechanical and hydraulic devices can also be used to convert a fixed-speed motor output into a variable speed output. However, these suffer from inherent losses and are therefore not as energy-efficient as electronic controls.

Variable frequency drives

Modern electronic VSDs are also known as variable frequency drives (VFDs) as they work by varying the AC electrical input frequency to control drive speed.

VFD technology has widespread uptake and application with AC induction motors. VFDs are increasingly favoured by industry due to their versatility and control where speeds can be accurately varied from zero rpm to over 100% of the rated speed. VFDs also enable motor control in either direction.

VFDs may be of little benefit where precise motor speed control does not assist the production process or where hours of reduced demand are few. VFDs are also not recommended for applications in which slowing down the machine causes operating problems, such as insufficient torque or poor cooling.

Retrofitting VSD/VFD

VSDs can often be retrofitted to existing motors. Evaluation of motors and load requirements should be conducted on a case-by-case basis to see where this is a feasible. Combining an in-service AC motor with an electronic VSD provides effective speed-control technology that does not require the use of a different type of motor.

The performance of modern AC motors with VSDs now matches the performance of DC systems and will have significantly lower maintenance costs, making DC motor replacement potentially cost-effective. In some cases, existing single-speed motors can be retained to cover the variable baseload. These can be complemented with a VSD on an appropriately sized motor dedicated to providing the additional variable load.

Voltage and power factor

AC induction motors can cause a facility to have low-power factor, resulting in additional electrical current needed to perform the required work.

Power factor can be improved by:

  • minimising oversized and inefficient motors
  • avoiding idling or lightly loaded motors
  • adding power factor correction devices


Permanent magnet motors

Permanent magnet motors use magnets instead of traditional metallic motor components, resulting in improved torque and efficiency. Permanent magnet motors will generally only operate when connected to a dedicated VSD which has been optimised for their control. Hybrid permanent magnet motors are available as direct replacements for conventional AC induction motors in most applications.

Synchronous reluctance motors

Synchronous reluctance motors feature sophisticated geometry that keeps internal resistance consistently low in all motor shaft positions, thereby increasing the available flux and motor output. Synchronous motors are inherently efficient as the current does not need to flow to the rotor, further minimising energy losses. Compared with an AC induction motor, synchronous reluctance motors produce more power at a smaller size and can deliver superior low-speed torque and efficiency.

Magnetic bearings

An increasingly common innovation in motor systems is the use of magnetic bearings to support spinning shafts in motors and driven equipment. The technology is based on a system of active electromagnets that can adjust for any wobble and keep the shaft perfectly supported without physical contact. Magnetic bearings allow faster motor speed and improved reliability. They also reduce friction, heat and vibration, and do not require lubrication.

Alternative materials

While manufacturers are offering permanent-magnet and other high-efficiency motor drive combinations as standard components, there are concerns about the ongoing availability and cost of required rare-earth metals such as neodymium. Research is identifying rare-earth replacements in combination with alternative motor configurations that retain the efficiency of permanent magnet motors.

Software controls

VSD control software can be included with new systems, or purchased separately to complement existing systems. Modern VSD software includes settings designed to deliver the maximum possible amount of energy savings. Modern VSD software also provides self-learning and motor modelling capabilities that can minimise the use of traditional sensors as inputs into the control system.

Internet-enabled systems

The next generation of VSDs are designed for internet connectivity to gather and analyse large amounts of data. With a trend towards standard internet communications protocols, VSDs will offer a single point of connection for a multitude of sensors and data points. Internet connectivity allows for rapid data analysis of system performance and predictive scheduling of equipment maintenance and repairs. It can allow users to be notified by smartphone before a failure occurs.

Regenerative capability

Regenerative drives recover motor braking energy rather than it being completely lost to heat, using inverters to convert the resulting DC power into AC power. The extra cost of a regeneration is worthwhile in VSDs only where the system requires frequent braking and starting.

Integrated packages

VSDs are usually purchased as standalone devices, but are available as part of integrated packages. Integrated motor and drive packages have a number of advantages including:

  • eliminating separate enclosures and reducing required floor space
  • decreasing costs
  • eliminating long, costly cable runs between motor and drives
  • opening many more motor systems to the potential of variable-speed control

More information

Electric motors efficiency regulations Council of Australian Governments

Energy Efficiency: Pumping Systems Government of Victoria

Improving Motor and Drive System Performance US Department of Energy

Guide to Adjustable Speed Drive Part-Load Efficiency US Department of Energy