Dynamic Performance Assessment for Water Pump System using different

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*Corresponding Author: Reynato Andal Gamboa,Email id :[email protected] Article History: Received: Aug 15, 2018, Revised: Sep 10, 2018, Accepted: Oct 04, 2018

Dynamic Performance Assessment for Water Pump System using different

controllers

Reynato Andal Gamboa^*,1, Aravind CV¹, Gowtham Raj¹, Rajmal Joshi²

1Electrical & Electronic Engineering, School of Engineering Taylor’s University

2Panimalar Engineering College, Chennai India

Abstract. The performance of VSD system was analysed through simulation as well as experiments and compared to the DOL system. The simulation was conducted to identify the behaviour of motor at the starting of the system because the overshooting and oscillations could not be determined from the experiments.

Experiments were conducted to measure the voltage, current, power consumed, power factor, total harmonic distortion (THD) and total demand distortion (TDD). Results showed that the motor had high overshoot at the starting of the system and that overshoot could be reduced by changing the voltage frequency with the help of power electronics. The frequency could be gradually increased at the starting of the system so the motor speed would slowly increase without overshooting too much. The experiment results proved that the usage of VSD helped reduced power consumption by allowing the motor to run at its desired speed according to the application needed. Using VSD also increased the power factor of the system. However, using VSD increased the harmonics in the system which might affect the equipment using the same line.

1 Introduction

Pump systems are commonly used for various industrial applications such as pumping water from wells, fuel injection in cars, pumping oil and natural gas. Generally, pump systems are operated by induction motors which convert electrical energy into mechanical energy to drive the loads. The motors can be connected to the power supplies in two ways, direct on line (DOL) starter or through a variable speed drive (VSD)[1]. DOL method connects a motor directly to the power supply and therefore its configuration is easier compared to using VSD.

DOL starter has high staring current as the motor is directly connected to the power supply. Full line voltage is applied to the motor windings when the contactor is closed. The drawback of DOL starter is its high staring current which requires more expensive molded case circuit breaker (MCCB) with higher current ratings. Other than that, the system may have a huge voltage drop at the beginning due to the high starting current draw by the motor. The high staring current will also produce extra heating in the motor and thus reducing the lifespan of the motor. In order to overcome all these flaws of DOL starter, VSD is introduced.

VSD increases efficiency of pump systems by providing the required the motor speed and torque at the right timing under different load conditions and thus reducing the energy losses in conventional constant speed motors which can only operate either in stop or maximum speed condition. A typical VSD is composed of three components, which are the electric motor, control system and power converter [2]. The power converter and control system control the AC supply from the supply transformer to the motor, while the motor is connected to the load. VSD provides control of motor speed and torque by varying the frequency of the AC supplies to the motor according to the load condition at that time, which is different to DOL method where the motor is constantly operating at full speed. Thus, by implementing VSD in a pump system, the electrical energy from the supply side can be utilized and thus reducing the heat losses in the motor windings, this extends the lifetime of the equipment and as a result the overall cost of the pump system is reduced. Table below showed the potential savings on electrical energy by using VSD.

Table 1. Potential energy savings by using VSD [3].

Average speed reduction (%) Potential energy savings (%)

10 22

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20 44

30 61

40 73

50 83

60 89

In 2016, fertilizer manufacturer Terra Nitrogen faced an issue of having different operating frequencies between its 40 Hz pumps and the 50 Hz mains supply [4]. VSD was implemented in the system to allow the conversion of 50 Hz to 40 Hz frequency by replacing the less reliable conversion equipment that requires high maintenance cost. By implementing VSD, the 40 Hz pumps as well as other 50 Hz equipment could be used at the same time and thus saving the cost of buying new equipment.

In 2009, ABB’s Jukka Tolvanen claimed that VSD can save 50% or more of the energy in applications that use pumps and fans on a 20% speed reduction [5]. In [5], it was stated that the need for power increases with the cube of the pump speed, meaning a slightly increase in fan speed would require more power. From another perspective, speed reduction in the motor would result in significant energy savings, for example a motor that runs at half of its maximum speed will only require 1/8 of the power used during full speed operation [5]. Other than that, it also claimed that VSD could produce less mechanical wear and thus increasing the lifespan of the equipment in the system. A case study also showed that the food manufacturer Northern Foods in United Kingdom was saving over £30,000 per year in electricity costs by installing ABB’s variable speed drives at its Riverside Bakery in Nottingham [5]. A review done by R.Saidur et al. in 2012 also claimed that VSDs required little maintenance and were able to reduce pump failure caused by pump cavitation, thus providing a reliable and cost effective solution for motor speed control [2].

In 2013, VSDs were installed on South Africa mine cooling pump systems to study the potential of energy efficiency improvement of VSD. In the published paper, it stated that an electrical energy saving of 144,721 MWh per year, cost saving of US$ 6,938,148 per year and a reduction in carbon dioxide emission by 131,812,158 kg per year were estimated if VSDs were implemented on all the pumps and fans [6]. The conducted pilot study used to support the findings above also shown an electrical energy saving of 250 MWh, cost saving of US$ 11,996 and a reduction in carbon dioxide emission by 227,893 kg, thus proving the benefits of VSD in terms of energy efficiency improvements and cost savings [6].

In 2015, Eaton developed VSD pump solutions and claimed that they could offer energy savings of up to 70% depending on the duty cycle of the machines [7]. In Brazil, a study was conducted by L.Roberto Valer et al. in 2016 regarding the implementation of VSDs in photovoltaic pumping system (PVPS) for irrigation [8].

The study showed that the issues such as high initial costs and low availability of PVPS were solved by installing VSDs on the pump system.

In 2017, VSDs were installed in the hot water supply system of Daresbury Park Hotel & Spa in Warrington, UK to solve the issue of excessive stress in the pipes of the pump systems due to the high water supply demand of the hotel [9]. After the installation of VSDs in the existing pump system, the energy usage for the hot water supply system was reduced significantly by 53% [9]. In 2018, VSD was implemented in hydraulic clamping system in a Computer Numerically Control (CNC) machine to reduce the energy consumption of the system [10]. The electric power consumption of the hydraulic clamping system with VSD implemented was compared to conventional hydraulic clamping system and the results showed that the system with VSD installed had lower energy consumption.

Other than that, Jie Cai and James E. Braun have conducted assessments of variable-speed equipment for rooftop units (RTUs) in the United States [11]. Three variable-speed retrofits for the RTUs were studied in assessments, the first was replacement of fixed speed fans with variable-speed fans for single-compressor units, the second was retrofits of fixed speed RTUs with new RTUs which had two-speed compressors with variable- speed fans, while the third one was replacement of RTUs with new RTUs which had both variable-speed compressors and fans [11]. The results showed that all the three variable-speed retrofits had significantly reduced the fan energy consumption while less obvious savings in compressor energy consumption [11].

The objective of this paper was to analyse the performance of a VSD water pumping system through software simulations as well as experiments. The obtained results were analysed and then compared to a DOL system.

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2 Methodology

The experiment was conducted by first simulating the DOL system and VSD system to observe the waveform produced. That was important because the values obtained in the experiment would be from a power meter which would not help determining the behaviour of the motor at the starting of the system. Therefore, simulations were done on MATLAB Simulink to for DOL system (Fig. 1) and VSD system (Fig. 2) to identify the starting behaviour of the system.

Fig. 1. DOL System.

Fig. 2. VSD System.

For VSD system, the supply voltage would first be rectified and the fed into a 3-phase IGBT system which converts the DC voltage to AC voltage with a desired frequency, that was controlled by the PWM generator.

The simulations were done at frequency of 10 Hz, 40 Hz and 50 Hz to determine the behaviour of the motor at different frequencies. The voltage with a different frequency would then be fed into the motor.

Then, the experiment was conducted to physically measure the power consumption of motor under DOL system and VSD system. The measurements would be taken from a power meter connected to the supply. The average voltage, current, power consumed, power factor, total harmonic distortion (THD) and total demand distortion were recorded. Besides, the speed at which the motor was running was measured using a stroboscope.

The results from simulation and experiment would be analysed to identify the performance of DOL system and VSD system.

3 Results and Discussion

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3.1 Simulation Results

The simulated result for DOL was as shown in Fig. 3.

Fig. 3. DOL Simulation.

Results showed that the speed of the motor oscillated before reaching a steady state. It was noticed that the power in DOL systems would overshoot before reaching its steady state.

Then, simulations were done on the VSD system to investigate the effect of changing the voltage frequency on the power consumption. Fig. 4, Fig. 5 and Fig. 6 showed the simulation results at frequency of 10 Hz, 40 Hz and 50 Hz respectively.

Fig. 4. VSD Simulation - 10 Hz.

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The simulation results showed that the increasing frequency increased the speed of the motor. However, it was noticed that when at low frequency, i.e. low speed, the power overshoot reached about 15 kW while the power overshoot at overshoot at 50 Hz reached about 30 kW, which was similar to that in DOL system.

However, the power electronics added to control the frequency introduced harmonics to the system. These could be observed from the ripples in the waveform in VSD systems and was not observable in DOL system.

3.2 Experimental Results

The experimental results were tabulated as shown in Table 2.

Table 2. Experimental Results.

Parameter DOL VSD

40 Hz 50 Hz

Vavg, V 237.94 236.24 236.31

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Iavg, I 2.493 1.297 2.144

Ptot, kW 1.34367 0.73785 1.32227

Qtot, kVAR 1.16373 0.55868 0.77058

Stot, kVA 1.77728 0.9255 1.53042

PF, Lag 0.75575 0.79729 0.86842

THD I Ia, % 2.3772 66.874 49.955

THD I Ib, % 1.8877 75.024 54.982

THD I Ic, % 2.2643 75.934 54.872

THD I In, % 0 0 48.081

TDD, % 2.6033 34.563 45.573

Speed, rpm 940.2 762.3 935.3

From the experimental results, it was noticed that controlling the frequency not only controlled the speed of the motor. It also helped reduced the power consumed by the system. When the frequency was at 40 Hz, the motor was operating at 762.3 rpm while consuming power of 0.9255 kVA. At 50 Hz frequency, the motor was operating at 935.3 rpm while consuming power of 1.53042 kVA, slightly lower than that of DOL system which was 1.77728 kVA.

The VSD functioning at 50 Hz exhibit similar trait with DOL system in terms of the power consumed and motor speed. However, using VSD introduced harmonics to the system. It could be seen from the THD and TDD. It was noticed that using VSD system increased the THD from about 2% to about 50% and the THD was worse when the system was functioning at 40 Hz instead of 50 Hz. That would be due to the power electronics to convert the input supply to signal with desired frequency.

From the simulation and experimental results, it was justified that VSD system would be a better system to reduce overshooting of current at the starting of the system as the frequency of the signal could be adjusted to start from 0 Hz to 50 Hz gradually with the help of power electronics. Besides, with VSD, the motor could be set to function at a specific speed by altering the frequency of the supply. That was especially important when the system was not at full load condition. Changing the motor speed would help save the energy as well by reducing power consumption. However, the THD would increase as harmonics would be introduced to the system. The effect of harmonics to the surrounding equipment should be taken into consideration when designing a VSD system so that the other equipment would not be affected by the harmonics created.

4 Conclusion

In conclusion, the results revealed that the VSD system was a better option. It could be used as a drive starter so that the motor would operate gradually without overshooting at the starting of the system by feeding the motor with low frequency and gradually increased until it reached the desired value. Using a lower frequency would reduce the overshoot of the motor at the starting. Therefore using power electronics to increase the frequency gradually until the desired value would prevent the motor from overshooting too much. Besides, the usage of VSD allowed the change of motor speed to the desired value according to applications. This way, the power consumption would be reduced and not wasted unnecessarily. However, VSD systems had relatively higher THD than DOL systems due to the usage of power electronics for the frequency conversion introduced harmonics to the system. These harmonics might affect the performance of other equipment in the same line.

Acknowledgement

The authors acknowledge the support for the project from the Taylor’s University flagship research grant TUFR/2017/001/01, 2017- 2020.

Authors also acknowledge students Chong Meng Joe, Sam Yee Thong Wah, for the preparation of this original report and also perforing the experiments in lab.

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