August 2016 TTM-0: The relationship of system efficiency and speed control using VSDs vs. EC motors Kunal Pental, Sales Manager NSW & QLD, ebm-papst A&NZ 1 Introduction In an effort to reduce energy consumption, variable airflow is utilised in many HVAC&R applications. This can be achieved in different ways. This article investigates the use of induction motors with Variable Frequency Variable Voltage (VVVF) drives, also called Variable Speed Drives (VSDs) and EC direct-drive motors. Significant energy savings can be achieved in a variable torque application such as fans and pumps utilising affinity laws (also called power laws). 2 Affinity Laws The theory of affinity laws for fans and pumps states that the power consumed by the impeller is proportional to the cube power of the change in speed; e.g. reducing the impeller speed by 50 per cent reduces the power input to 12.5 per cent, as shown in Figure 1. If the fan speed can be modulated to meet a demand, significant energy savings can be achieved. As with any theoretical formula, the affinity laws do not consider losses, and therefore caution should be used when predicting efficiency. Figure 1: Power consumption following affinity laws In particular, three main factors are often overlooked that can significantly affect an application s overall cost of ownership - hidden costs, the effect on complete system efficiency and costs associated with ongoing maintenance, as discussed in the following paragraphs. The technical specification IEC6003-31 Selection of energy-efficient motors including variable speed applications advises that the addition of a VSD increases motor losses by two to five percent; while in adverse circumstances it can increase motor losses up to 15 to 20 percent. 1 Figure 2 gives an overview of the losses associated with fan operation. The losses in motors are radiated as heat losses. 1 IEC TS 6003-31:2010: Rotating electrical machines - Part 31: Selection of energy-efficient motors including variable speed applications - Application guide, International Electrotechnical Commission 2010. ebm-papst A&NZ Pty Ltd 10 Oxford Road Laverton North VIC 3026 Phone +61 (03) 9360 600 Fax +61 (03) 9360 66 1
Figure 2: Fan input power vs. output power as defined in AS/NZ ISO 12759: 2013 - Fan efficiency classification for fans 3 Complete System Efficiency Calculation of the efficiency of a driven fan system is defined within AS/NZS ISO 12759, Section B, and is the product of the efficiencies of the component parts: η " = η $ η & η ' η ( C & C ( Where: ηe is overall efficiency [%] ηr is optimal fan impeller efficiency according to Pu(s) / Pa, as given in ISO 5801[%] ηm is motor efficiency [%] ηt is drive mechanism (transmission) efficiency [%] ηc is the variable speed drive efficiency [%] Cm is the compensation factor to account for matching components: Cm = 0.9 Cc is the part load compensation factor (defined in Section 6.3 in AS/NZS ISO 12759). ebm-papst A&NZ Pty Ltd 10 Oxford Road Laverton North VIC 3026 Phone +61 (03) 9360 600 Fax +61 (03) 9360 66 2
Alternatively, ISO 12759 gives the option to simply measure total efficiency in a system, which may be preferable to get a more accurate result. 3.1 Hidden Installation Costs of VSDs There are many hidden costs associated with VSD-based speed control compared to speed control with EC fans, of which the following list gives an overview. EC fans have motor electronics integrated into the fan, whereas e.g. an IP 20 VSD often needs complete electrical cabinet enclosure with other ancillary devices such as line filters and output chokes. Electrical panel designers also need to consider the cooling requirements of variable speed drives. Alternatively, IP5/55 VSDs with in-built line filters and possibly output chokes are typically costly. These VSDs are still installed in partial enclosures to protect against environmental conditions. Whereas EC fans do not require shielded cables, the majority of VSD manufacturers asks for shielded cables for induction motors in order to reduce harmonics. 2 This can result in a restriction of the maximum cable length to 100m to 150m. In a typical commercial building this can be challenging. In an EC fan, the cooling requirements of motor and electronics are optimised by design. In contrast, AC squirrel cage motors, commonly called induction motors, are often TEFC Totally Enclosed Fan-Cooled, where an axial impeller is fitted to its non-drive end. The cooling of the motor needs to be considered, especially when the motor is operated below 20-25 Hz. 3 On the other hand, if the motor is operated above the base frequency in the so-called field weakening range, it is important to keep in mind that torque decreases in inverse proportion to frequency, as shown in Figure 3. Figure 3: In the field weakening range torque decreases in inverse proportion to frequency 2 IEC TS 6003-25:201: Rotating electrical machines - Part 25: AC electrical machines used in power drive systems - Application guide, International Electrotechnical Commission 201. 3 AS 1359.30-1997: Rotating electrical machines - General requirements - Preferred outputs and frame sizes, Standards Australia 2007. ebm-papst A&NZ Pty Ltd 10 Oxford Road Laverton North VIC 3026 Phone +61 (03) 9360 600 Fax +61 (03) 9360 66 3
EC motors have integrated thermal protection. In a VSD setup, additional wiring between the motor and VSD panel is required to monitor the motor s thermal conditions. When it comes to ease of supply, often the VSD, motor and impeller need to be obtained from three different manufacturers. This can result in requiring additional time to source, set up and program even a simple 0-10V control system. Programming costs and on-site support may lead to increased project costs. Additional components such as belts and pulleys further complicate this process. This stands in contrast with EC fan products, which are supplied as one complete and optimised unit. In an established installation using an older motor, where the insulation system was not designed to accommodate the voltage and thermal requirements of operation with a VSD, it is possible that operation with a VSD is not suitable. It is then recommended to replace the motor with a motor that is MEPS-compliant (Minimum Efficiency Performance Standards). Further, with existing external rotor motor installations fan manufacturers like ebm-papst recommend the use of all-pole sine filter at the output of VSD. The addition of a VSD to an existing fan system reduces the total efficiency. Savings are achieved only if the fan speed is reduced. In contrast, EC technology offers higher system efficiencies form the start, and additional savings when speed controlled. 3.2 Maintenance The previous points have outlined common pitfalls in the selection and operation of fans. the potential issues associated with fan and motor maintenance are considered below. 3.2.1 Direct Drive System on a VSD Set Up The maintenance of a direct drive system using a VSD setup involves visual inspections as well as replacement of consumables. Preventive Maintenance often asks for regular inspections of electrical cabinets where VSDs are installed. In addition, filters for electrical cabinets require frequent cleaning or replacement. Figure : Direct-drive centrifugal fan with standard AC motor and VSD AS IEC 61800.5.1-2013: Adjustable speed electrical power drive systems - Safety requirements - Electrical, thermal and energy, Standards Australia 2013. ebm-papst A&NZ Pty Ltd ABN 33 115 927 556 10 Oxford Road Laverton North VIC 3026 Phone +61 (03) 9360 600 Fax +61 (03) 9360 66 NSW Phone +61 (02) 9827 600 NSW Fax +61 (02) 9827 66 NZ Phone +6 (09) 525 025 NZ Fax +6 (09) 525 026
3.2.2 Belt-Drive Systems on VSD Set Up The maintenance of belt-drive systems using VSDs includes all the above factors, as well as additional ones, such as frequent belt changes, regular lubrication of bearing systems and belt dust removal. This has flow-on effects, such as more frequent air filter changes, reduced airflows, and increase in power consumption. 3.2.3 No hidden surprises with EC technology Figure 5: Belt-drive centrifugal fan with standard AC motor and VSD Electronically commutated motorised impellers are supplied as one complete system. This increases system efficiency and reduces power consumption which increases with speed control. It also leads to higher performance and improved reliability. The motors and impellers are made for each other, which means that there is no programming time required for a simple 0-10V control set up. Even a closed-loop sensor control or two-parameter set up can be done in only a few minutes. Most EC fans offer a serial interface and programmable analogue and digital in- and outputs. This makes them more accessible and increases their controllability. Figure 6: Direct-drive centrifugal fan with GreenTech EC motor Conclusion While the use of AC fans with VSDs has traditionally been an accepted choice to achieve desired airflows, practical alternatives have been emerging recently. One of these are fans with integrated EC technology which are simpler to source, program and speed control (cf. Fig. 7). Figure 7: The use of EC motors eliminates the need for many additional accessories used in a VSD setup This article illustrates the effects of speed control on efficiency in HVAC systems using VSDs versus direct-drive EC motors. It was shown that the use of VSDs, while being able to speed control fans to the required airflow, does not increase the efficiency of the fan itself and can bring with it a range of problems from optimisation and commissioning to maintenance. EC products on the other hand are usually one complete unit, which avoids hidden costs and ensures guaranteed performance. ebm-papst A&NZ Pty Ltd ABN 33 115 927 556 10 Oxford Road Laverton North VIC 3026 Phone +61 (03) 9360 600 Fax +61 (03) 9360 66 NSW Phone +61 (02) 9827 600 NSW Fax +61 (02) 9827 66 NZ Phone +6 (09) 525 025 NZ Fax +6 (09) 525 026 5