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SERIES 21 / Module 08 Drives & Motors


only, while a variable speed drive (VSD) refers to either AC or DC drives. VSDs can also implement soft


motor starting and control based on external parameters such as temperature, flow, humidity, and proximity sensors which detect when a system or motor is required to operate. A VSD saves energy based on the


Cube Law. By the Cube Law, power is proportional to the speed cubed; this means that a small increase in speed requires a lot more power, and a modest speed reduction can give significant energy savings. The Carbon Trust state that a 20% reduction in fan speed can result in a 50% reduction in fan energy use. The main components of a VSD are: ● Rectifier – this changes the


incoming alternating current (AC) supply to direct current (DC). ● Intermediate circuit – the rectified


DC supply is then conditioned in the intermediate circuit, normally by a combination of inductors and capacitors. ● Inverter – the inverter converts


the rectified and conditioned DC back into an AC supply of variable frequency and voltage. This is normally achieved by generating a high frequency pulse width modulated signal of variable frequency and effective voltage. ● Control unit – this monitors and


controls the rectifier, the intermediate circuit and the inverter to deliver the correct output in response to an external control signal, i.e., temperature sensor. Energy consumption of motors


depends on the system load, configuration, and duty-cycle; therefore knowledge of the specific duty and utilisation is key to defining the savings opportunity. It should be noted applications like crushers, elevators, and hydraulic pumps enjoy lower benefits from adopting VSDs. VSD Control – there are two main


ways to control the speed of a VSD; networked or hardwired. Networked involves transmitting the intended speed over a communication protocol such as Modbus, Modbus/TCP, EtherNet/IP, or via a keypad, while hardwired involves a pure electrical means of communication. Premature failures of motor


insulation systems have been noted with fast-switching VSDs. To avoid motor failure when considering a VSD upgrade: 1. Locate the VSD controller as


A VSD saves energy based on the Cube Law


equipped with multiple fans, the fans can be staged for part-load operation, delivering energy savings of up to 30%. Refrigeration compressor motor


–compressors currently used in self- contained commercial refrigeration are single-speed, hermetically-sealed reciprocating compressors – there is an opportunity for improved control at scale.


Air compressors – a variable speed-


close as possible to the motor, ideally less than 15 feet. 2. Contact motor manufacturer for


guidance on motor/drive compatibility. 3. Consider replacing the old motor


with a premium efficiency ‘inverter- duty’ motor. 4. Prioritise the use of a single VSD controller for each motor.


Opportunities for energy saving in commercial and Industrial sectors It is always useful to first consider low- cost options for energy saving prior to installing costly frequency drives. Reduce motor size – motor sizes can


be reduced if the drive train is efficient, the load is well understood, and installed motor is deemed as oversized. Although motors sized too large stay relatively cool in operation, they waste energy through inefficient operation. Motor replacement – it is sometimes


more cost-effective to replace a failed motor with a Higher Efficiency Motor (HEM), rather than repair it. This is because rewind cost, especially for smaller motors can be high. Also a failed motor that has been rewound can be 0.5-2% less efficient than it was


Schematic of a VSD


previously, therefore reduction in energy efficiency results in increased running costs. The replacement decision should be based on a comparison of the annual cost to own and operate the motors. For small motors and HVAC applications consideration should be given to EC motors.


Upgrade belt driven fans with


direct plug fans – belt driven centrifugal fans commonly found in air handling units can be upgraded with direct plug drives, where the impeller is directly connected to the motor shaft. The benefit is that there are no power transmission losses, the energy the electric motor develops is transmitted directly to the impeller, which passes into the airflow and pressure development. Furthermore, cost savings in maintenance are achieved by removing the need to regularly replace belts and pulleys. Direct plug fans can also be VSD controlled. Cooling water circulation and cooling towers – VSDs are commonly used for cooling tower fans but variable flow cooling water loops with cooling towers are a recent innovation. Fans for air-cooled chillers –


because air-cooled chillers are typically


controlled or frequency-controlled air compressor automatically adjusts its motor speed to the air demand, whereas fixed speed or load/unload compressors are either on full throttle or off. VSDs can deliver energy savings of 35% on average in this application9. Grid peaks reduction – with VSD,


you avoid peak currents at start up and therefore reduce the peak load on the industrial or commercial facility, avoiding excess capacity charges and lower DUOS costs.


Alternatives to VSDs Sequential controllers – where multiple fans, pumps or compressors are installed, sequential controllers can be used to sequentially start and stop equipment according to the process demands. Whilst not as precise as variable speed control, significant energy savings can still be achieved. PLCs (Programmable Logic


Controllers) can be used to build ‘intelligence’ within a system. They can be programmed to predetermined schedules, or react to sensor inputs, and to ensure the motors only operate when required. ▄


References


1. US Department of Energy. Energy saving potential and opportunities for high efficiency motors in residential and commercial equipment 2. https://commission.europa.eu/energy-climate- change-environment/standards-tools-and-labels/ products-labelling-rules-and-requirements/energy- label-and-ecodesign/energy-efficient-products/ electric-motors-and-variable-speed-drives_en


3. Widmar, Martin (2015). “Electric vehicle traction motors without rare earth magnets”. Sustainable Materials and Technologies. 3: 7–13


4. “Wells to wheels: Electric car efficiency”. 22 February 2013


5. https://www.zap-map.com/ev-stats/ ev-market#:~:text=With%20increasing%20 consumer%20demand%20and,further%20 600%2C000%20plug%2Din%20hybrids.


6. Carbon Trust Motors Overview 7. https://www.manualslib.com/manual/1321460/ Allen-Bradley-Powerflex-523. html?page=11#manual


8. https://www.energy.gov/sites/prod/files/2014/04/ f15/motor_tip_sheet14.pdf


9. https://www.thecompressedairblog.com/ compressed-air-systems-the-vsd-advantages


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EIBI | MARCH 2024


21


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