Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
D R I V E S & S W I T C H G E A R A ny initiative that can contribute to a significant national energy savings would be very advantageous in South Africa currently. Energy savings with AC VSDs and motors on pumps This paper presents one aspect of energy savings that can be shown to realistically save an amount of power roughly equal to the output of a single coal fired power station. Electric motors account for 50 – 65% of the total national electrical consumption of South Africa. In turn, a significant percentage of these motors are used on pumps. In most industries this percentage varies from 20 – 80% of all motors. Many of these may be VSD driven giving significant energy savings and reduced life-cycle costs. This paper deals with the following factors that are part of correct application of AC VSDs for effective life cycle cost reduction: l Pump energy savings calculation l Practical considerations of VSD applications: - VSD compatible motor selection - Voltage peaks, dv/dt, harmonics and other VSD effects on motors - Motor speed range - Harmonics l High efficiency motors: - The real facts on motor efficiency requirements - VSDs and high efficiency motors This paper will assist with giving a clearer understanding of the above factors, many of which have been a cause of concern within industry. Pump energy savings calculation It is relatively common knowledge that there is a large potential for energy savings when a pump motor is VSD driven. Though the general concept is by Johan van Niekerk, Zest Electric Motors & Drives Fig.1: Torque load. Fig. 2: Comparison. Zest pump VSD energy saving calculation Input data - yellow blocks only Customer: Test Application absorbed power (kW): 180 Application: Test Installed motor power (kW): 200 Flow % Time % Days of operation per year: 360 100% 10% Hours operation per day: kWhr cost in Rands: Estimated cost of VSD installation: Duty 24 90% R0,15 80% 20% R150 000,00 70% 35% 60% 20% 543745 40% 0% R81 561,69 30% 0% 20% 0% 10% 0% Throttling KkWhr VSD kWhr cost Throtting kWh cost Calculation results 15% 50% Annual savings in kW hour Annual savings in Rands Payback period 22 0% Calculation Total hours per year 8640 Total duty percentage 100% Flow % Running hours VSD absorbed power (kW) Throttled abbsorbed power (kW) Difference between absorbed power values VSD kWhr Annual saving kWhr Annual savings Rands 100 864 180 180 0 155520 155520 R23 328,00 R23 328,00 0,00 R0,00 90 1296 131 172,8 42 17061 223949 R25 509,17 R33 592,32 53 887,68 R8 083,15 97 165,6 69 167215 R42 923,52 118 841,70 R17 841,25 70 3024 81 158,4 77 244944 479002 R36 741,60 R71,850,24 234 057,60 R35 108,64 60 80 1728 1728 72 151,2 79 124416 261274 286157 R18 662,40 R25 082,27 R39 191.04 136,857,60 R20 528,64 50 0 63 144 81 0 0 R0,00 R0,00 0,00 R0,00 Totals: 862156 1405901 R129 323,43 R210 885,12 543745 R81 561,69 All flow below %0% absorbs power equivalent to 50% flow Estimated cost of VSD installation R150 000,00 Payback period - months 22 Important notes: In a centrifugal pump, the outflow varies along with the speed, and the pressure varies with the square of the speed. As the pressure varies along with the square of speed, at 70% of speed the pressure has a theoretical value of 50% of the rated pump pressure. Fig. 3: VSD energy saving. accepted the actual facts when quantified in terms of kW/hours and Rands are often unclear. Also, unrealistically September 2008 - Vector - Page 37 optimistic scenarios are often given. We present here a straightforward method to calculated energy savings. The benefit is that the end user is empowered to make an informed decision regarding the cost effectiveness of a VSD installation. Centrifugal pumps use power that is proportional to the speed cubed.[P∝N3]. This means that at reduced speed, the power consumption is greatly reduced. When 100% flow is not required, speed can be reduced in order to reduce flow, resulting in power savings as well. Another method would be to throttle the pump, thereby reducing flow. Throttling however also uses much more power than speed control. Where possible, the best method of controlling flow and power is by variable speed drive. The power savings can be simply calculated using an Excel based software from Zest. The possible power savings are illustrated. (Fig. 1 to 3) It is common for several pumps to be used together in order to maintain a required flow rate or pressure. In such an application it is possible to do PID control on one such pump by VSD. The VSD may be used to start and stop the other pumps as and when required. Using this method the VSD energy savings benefit is also realised. Practical considerations of VSD applications Not all standard AC motors are truly suitable for VSD applications. There are two main issues to consider: l Motor heat rise – a motor on a VSD will run at higher temperature than a DOL motor. It is important for this additional heat rise to be minimal and to be precisely known. l Motor insulation – VSD output pulses have peak values (Vpeak) and sharp rise times (dv/dt) far above the normal grid 50 Hz sine wave. These values as produced by the VSD and the corresponding motor insulation must be known and must be compatible. It is well known that motors in VSD applications will have a higher heat rise than normal and must therefore be de-rated for VSD applications. The reasons and details of this are however not well known. There are three reasons for increased heat rise: l Current distortion on the VSD output. l Reduced cooling at reduced speeds. (N < Nnominal) < LI>) l Reduced torque at increased speeds. (N > Nnominal) The basic operating principle of all standard VSDs is to convert the 50 Hz AC supply voltage to DC and then convert the DC back to AC. For this the voltage cannot increase but the frequency does. This results in reduced flux and consequently reduced torque. Fig. 5 illustrates this. Harmonics Fig. 4. reason VSDs are sometimes referred to as frequency converters. The conversion from DC back to AC uses a system of calculated and controlled pulses referred to as pulse width modulation (PWM). This system allows the VSD output frequency and voltage to be controlled, thereby controlling motor speed. However the PWM results in current distortion. This can be seen in Fig. 4. The current distortion causes additional heat in the motor. A motor depends on a shaft mounted cooling fan on its non drive end for cooling air flow. When the motor runs at speeds slower than the normal 50 Hz speed, this fan turns slower and the motor has reduced cooling. Due to this the motor thermal capacity and hence its allowable output torque is reduced. Motors torque is dependant on magnetic flux. Flux is a part of motor design and is directly proportional to the ratio of voltage to frequency. For a 400 V 50 Hz motor this ratio is 400/50 = 8. For a 525 V 50 Hz motor this ratio is 525/50 = 10,5. At full speed, e.g. 1460 rpm on a 4 pole motor, the VSD output is full voltage and nominal frequency. This gives full flux and torque. Above this speed, To an ever increasing extent, users are becoming aware of the reality and negative impact of VSD induced harmonics. These harmonics are inherently a result of the VSD method of operation. The initial rectification of the AC supply voltage to DC causes an irregular or distorted current to be drawn. This causes harmonics. All standard VSDs produce very similar levels of harmonics. Harmonic current are currents at multiples of the fundamental frequency. For example the 5th harmonic is at 5 x 50 Hz = 250 Hz. These harmonics cause additional heating in the supply transformer and at sufficiently high levels may cause interference with other equipment. Most large end users find a harmonic level that causes 5% distortion on the voltage acceptable, i.e. 5% voltage THD. Fortunately this level of distortion is seldom reached. It is important to note that vurrent THD and voltage THD are at very different values. Also it is possible to accurately predict the harmonic distortion prior to proceeding with an installation. Fig. 6 shows an example of such a calculation. Once this calculation has been made the user can determine if the harmonic distortion level will be acceptable or not. The simplest method to reduce the harmonic distortion is by adding line reactors. High efficiency motors In a relatively short period the use of high efficiency motors has become a matter of much increased consideration within industry. There is generally not a Fig. 5. September 2008 - Vector - Page 38 Fig. 6: Calculation of harmonics. clear understanding as to what constitutes a “high efficiency” motor. Also, what are the international standards specifying efficiencies? What is the effect of using a high efficiency motor when it is VSD driven? We will also deal with ground breaking technology regarding the use of VSDs and high efficiency motors. There are several international standards dealing with high efficiency motors. There is however no international standard that is complete and in line with local standards. l Europe – CEMEP standard covers from 1,1 kW to 90 kW only (EFF1,2&3) l Australia & New Zealand – MEPS covers 0,75 to 185 kW only USA – NEMA Premium – frame sizes and other aspects that are not in accordance with SABS l IEC & SABS 60034-30 – workgroup drafts in progress – the drafts will only be tabled in 1 to 3 years. These standards are not mandatory in South Africa, nor are they complete. This means that many motors are being marketed as high efficiency, when in fact they are only higher than similar motors from the same manufacturer. Though the above standards are not complete, the CEMEP and MEPS do correspond on the motors covered by both standards. This means that the MEPS standard can reasonably be applied as a definition of high efficiency until such time as the l September2008 - Vector - Page 39 IEC and SABS standards are complete. There is only one motor supplier locally that complies with both the current SABS motor standards and the MEPS high efficiency standard. When VSD driven, both normal and high efficiency motors suffer a reduced efficiency. However, the high efficiency motor retains its efficiency advantage. This means that the use of high efficiency motors in VSD applications is still advantageous. Furthermore, when Optimal Flux VSD technology is used the high efficiency motor and VSD combination offers thermal characteristics far better than currently possible with a standard motor. Contact Johan van Niekerk, Zest Electric Motors & Drives, Tel 011 723-6157, [email protected] D