In today’s power scenario we are facing a major power crunch. Day by day gap between demand and supply of electric energy is widening. Bridging this gap from supply side is very difficult and expensive proportion. The only viable way in handling these crises, in addition to capacity addition, is the efficient use of available energy sources. Because of limited resources, huge capital investment limits.
Fossil fuel based generation is harming the environment. Hydel power is capital intensive and poses threat to the ecological balance. The generating cost is increasing day by day. Unless the above crises are properly managed power scenario will be bleak. Conservation of the energy is the call of the day.
Fossil fuel based generation is harming the environment. Hydel power is capital intensive and poses threat to the ecological balance. The generating cost is increasing day by day. Unless the above crises are properly managed power scenario will be bleak. Conservation of the energy is the call of the day.
On the same line an investigation of the electrical energy saving obtained by reducing the terminal voltage of Induction Motors when they are not loaded or only partially loaded is presented. The purpose is to obtain information to help decide whether to install an energy saving device in plants using Induction Motors. At no load operation, the saving can be as high as 50%. The saving decreases with the increase of load. Switching-off one phase of the three phase supply during no load operation does not contribute to energy saving. Intermittent disconnection of the supply voltage can lead to considerable energy saving if the interruption interval is sufficiently long.
2. INTRODUCTION
French novelist Emile Zola once said, “The day must come when electricity will be for everyone, as the waters of the reveres and the winds of heaven. It should not merely be supplied, but lavished, that men may use it at their will, as the air they breathe”. Thomas Alva Edison said after his electric bulb invention, “Only rich people can use candles in the future” As electrical engineers we hope that the said day will come soon in India
Electric motors are industry’s basic need. Industries consume about 50% of the power generated in the country and electric motors consume around 76% of the total electricity used in the industrial sector. It is a trend in the industry include material, labor and energy costs. Material and labor cost reduction has its own limitations and a manufacturer does not have a direct control many times. But the manufacturer himself can influence the energy costs through energy conservation measures and effective energy management.
3. ENERGY CONSERVATION TECHNIQUE
ü The first principle of energy conservation is,
“When you don’t need it, just turn it off”
3.1 The modified principle is:
We should minimize losses or use energy efficiently.
The Indian power sector more than tripled capacity from 30 GW in 1981 to 110 GW in 2003-04; however the country is still plagued by severe peaking and energy shortages. These shortages are exacerbated by inefficiencies in power generation, distribution and end use system (mostly in induction motors). The inefficiencies in the end use system are due to irrational tariffs, technical obsolescence of industrial equipment and lack of awareness, a nascent energy service industry and in adequate policy drivers.
When we consider the present energy scenario in India
Ø Installed capacity nearly 1,10,000MW
Ø 36%of installed capacity wasted now.
Ø 70% of the population lives in villages.
Ø 70% of power consumptions are in cities.
Ø 70,000 villages have no electric poles.
This article presents an analysis of partial loaded induction motor with special reference to power electronics.
Power electronics is one of the broadest growth areas of electrical technology. In recent years field of power electronics has experienced a large growth due to confluence of general factors. Revolutionary advances in semiconductor fabrication technology have made it possible significantly to improve the voltage and current handling capability and the switching speed of power semiconductor devices, industrial and power system applications. At one time, the growth was pushed by energy conservation goals. In many applications constant speed operation, induction motor [operate under no load or light load for prolonged time periods, such as in pressing machine conveyers, rock crushers, centrifuges, drill presses, wood saw and, some machine tools. In such applications saving in energy can be achieved by operating the motors at low voltages while running at no load and light loads.
When a motor operates at full voltages at no load, core loss has a large value. Reduction in voltage increases the copper loss but reduces core loss by a larger amount. Therefore net losses reduced at some voltage when core loss becomes equal to copper loss; the loss has minimum value and efficiency is maximum. Any increase or decrease of voltage from this value increases the loss. Therefore, for each loading, there is an optimum value of voltage for which the loss is minimum. Energy saving is achieved by operating the induction motor at optimum voltage values.
Table:1-Efficiency and power factor of the induction motor with different loads | ||||||||
KW | Pole | Efficiency | Power Factor | |||||
100%load | 75%load | 50%load | 100%load | 75%load | 50%load | |||
5 | 3.7 | 2 4 6 | 83.1 82.0 85.0 | 81.2 82.0 84.3 | 77.3 80.0 81.3 | 0.84 0.88 0.78 | 0.80 0.84 0.69 | 0.70 0.75 0.53 |
10 | 7.5 | 2 4 6 | 84.3 85.5 87.0 | 83.2 84.6 85.5 | 79.4 81.3 82.5 | 0.91 0.85 0.76 | 0.88 0.77 0.68 | 0.80 0.64 0.53 |
25 | 18.5 | 2 4 6 | 88.5 89.5 89.8 | 87.7 89.4 90.0 | 85.0 87.9 89.0 | 0.95 0.89 0.82 | 0.93 0.85 0.78 | 0.86 0.75 0.67 |
50 | 37 | 2 4 6 | 88.5 91.3 91.3 | 87.7 91.0 91.2 | 85.0 89.5 90.2 | 0.92 0.84 0.86 | 0.90 0.81 0.83 | 0.86 0.73 0.77 |
4. EFFECT OF USING OVER SIZE INDUCTION MOTOR Power factor: The power factor of an induction motor depends upon its type, size, RPM and load. Slip ring induction motor have lower power factor than squirrel cage motors of the same size. The power factor of induction motor for different size and at different load is shown in above table. A motor running near to full load has good power factor compared to part load. Power factor deteriorated very much when load below 75% of rated load.
Efficiency: The efficiency of three-phase induction motor varies with type, size and load. It ranges from 85% to 93% in case of squirrel cage motor above 5 HP. It is about 75% in case of smaller motor. The efficiency is less in case of slip ring motors; slow speed motors and motor running at partial loads.
Energy saving: if motor is loaded above 75% of full load, motor has higher efficiency but under load condition the efficiency reduces and increases the losses
Efficiency of Electric Motors
Motor part load efficiency
• Designed for 50-100% load, But is Most efficient at 75% load & Rapid drop below 50% load.
• There is a clear link between the motor’s efficiency and the load. Manufacturers design motors to operate at a 50-100% load and to be most efficient at a 75% load.
But once the load drops below 50% the efficiency decreases rapidly as shown in the figure. Operating motors below 50% of rated loads has a similar, but less significant, impact on the power factor. High motor efficiencies and power factor close to 1 are desirable for an efficient operation and for keeping costs down of the entire plant and not just the motor.
No load current
No load current of induction motor varies from 25% to 60% of full load current. Usually no load power factor of induction motor is around 0.15% lagging or even less no load current of some typical induction motor shown in table below for 2, 4, and 6 poles. Generally as the number of poles increases in induction motor the no load current is also increases, and at the same time full load current of motor is also increases simultaneously. This can visualize from the next table that motor no load current variations.
Load current
To estimate the load current at different loads, we can use the information given in the manufacturers catalogues.
Ø Efficiency of motor at 100%, 75%, 50% of full load.
Ø Power factor of motor at 100%, 75%, 50% of full load.
Ø Motor full load current.
To determine the load current at 100%, 75%, 50% of full load following formula is used.
I75 =0.75*Rated output power*√3*VL*ŋ75*Power factor75
No load current may be determined by no load test of motor or motor test certificate.
Table:2- No load current of different size induction motor | ||||||||||
HP | KW | Full load current | No load current | % of full load current | ||||||
2P | 4P | 6P | 2P | 4P | 6P | 2P | 4P | 6P | ||
5 | 3.7 | 7.37 | 7.13 | 7.76 | 4.5 | 3.8 | 4.5 | 61.06 | 53.30 | 57.99 |
10 | 7.5 | 13.36 | 14.36 | 15.78 | 5.08 | 8.0 | 10.0 | 37.35 | 55.71 | 63.37 |
25 | 18.5 | 30.61 | 32.31 | 34,95 | 8.16 | 13.4 | 16.4 | 26.66 | 41.47 | 46.92 |
50 | 37 | 63.22 | 67.12 | 65.56 | 17.5 | 27.2 | 20.8 | 27.68 | 40.52 | 31.73 |
75 | 55 | - | 97.68 | 102.1 | - | 41.6 | 47.8 | - | 42.59 | 46.83 |
100 | 75 | - | 123.4 | 140.7 | - | 32.8 | 60.0 | - | 26.58 | 42.63 |
5. MOTOR LOAD ESTIMATION TECHNIQUES
Operating efficiency and motor load values must be assumed or based on field measurements and motor nameplate information. The motor load is typically derived from a motor’s part-load input kW measurements as compared to its full-load value (when kW or voltage, amperage, and power factor readings are available), from a voltage compensated amperage ratio, or from an operating speed to full-load slip relationship. Equations used to estimate motor load are summarized below. The kilowatt technique should beused whenever input kilowatt measurements are available. Use the slip technique only when strobe tachometer readings are at hand and kilowatt values are not available. The full-load or synchronous speed for the existing motor may be extracted from the nameplate, whereas speed characteristics for new motors are obtained from manufacturers’ catalogs. The Arizona Department of Commerce Energy Office has recommended against using the slip technique as an indicator of load and suggests that loads be estimated by comparing a motor’s true root-mean-square (rms) amperage draw against its full-load or nameplate value.
Thus, the load on a motor is defined as:
While the amperage of a motor is approximately linear down to 50% load, the relationship is not directly proportional (i.e., at 50% load, current is higher than 50% of full-load current). An improved version of the amperage ratio load estimation technique makes use of a linear interpolation between a motor’s full- and half-load current values. The modified equation, useful values. The modified equation, useful for estimating loads in the 50% to full-load range, is:
The current at 50% load (amps50%) can be found from manufacturer data or Motor Master. The accuracy of the amperage ratio methodology is best for motors with outputs exceeding 10 shaft horsepower (hp). Below 50% load, the amperage curve becomes increasingly nonlinear and is therefore not a good indicator of load.
6. EXPERIMENTAL STUDY
Rating of the motor 1HP,1470 rpm, 1.9Amps, 3ph, class E, 50Hz delta connected Induction Motor. From No Load and Block Rotor Test we have found the following equivalent circuit parameters of induction motor(1HP).
Table3 : No Load test | ||
V0 in volts | I0 in Amps | W0 in |
385 | 1.0 | 192 |
Table4: Block Rotor test | ||
Vsc in volts | Isc in Amps | Wsc in |
145 | 1.9 | 3.12 |
Is=0.58A, Vo=385V, Ro1=86 Ω , Zo1=132Ω, Xo1=100.1 Ω, R2’=8.9 Ω, R=47.1 Ω, I1=0.29A, I1m=0.96A, Ie=0.168A, Ro=2.292 Ω, I0=0.577A, Xo=700 Ω
Block diagram explanation:
The method is used to partial load the induction motor with no effect of efficiency is amperage ratio technique the block diagram shows that the controller used for microprocessor and zero crossing detector as per the amperage ratio technique. The block diagram shows that current is sensed by the current sensor. This current sensed is converted into voltage and its equivalent digital value is converted by ADC and given it to microprocessor. Microprocessor compares the hex value with hex value which are presented in look up table. The look up table got the hex equivalent of the optimum value to which the load in a motor is partially loaded with more efficiency. The difference between the ADC output and look up table output is an error signal this error signal is given which is hex value is given to DAC which converted it into analog and fed it to zero crossing detector. Zero Crossing Detector is the electronics circuit that consists of an operational amplifier with an input voltage at its positive input. Often used in conjunction with other circuit elements its usually function as a simple voltage switch. And fires the ZCD either earlier or later, the zero crossing depending upon the error value. Due to which output of ZCD is regulated and is given to the motor which either output voltage of motor is increased or decreased based on the error signal. Again the new value of current are sensed and the sequence is repeated again until error signal is zero.
Table 5 : No load losses Vs voltage | ||||||||
Voltage(V) | 380 | 375 | 365 | 355 | 345 | 335 | 325 | 315 |
No load loss(W) | 176 | 168 | 152 | 144 | 136 | 128 | 120 | 104 |
Table 6 : Copper loss Vs line current | |||||||||
Current(A) | 1 | 1.1 | 1.2 | 1.3 | 1.4 | 1.5 | 1.7 | 1.8 | 1.9 |
Loss(W) | 86.4 | 104.5 | 124.4 | 146 | 194.4 | 211.18 | 249.6 | 280 | 312 |
Table 7: Voltage Vs load Current | ||||||
Load Current(A) | 1.1 | 1.2 | 1.3 | 1.4 | 1.5 | 1.6-1.9 |
Voltage(V) | 315 | 335 | 355 | 375 | 385 | 385 |
Table7 shows the required voltage for different loading condition of the above motor. For example, consider the load current of 1.3A Copper loss is 146 W (from table6) and the same core loss occurred at 355V is 144W (from table 5). Since the maximum efficiency only occurred at copper loss is equal to core loss, we have selected 355V as an optimum voltage for the above said load.
7. CONCLUSION
The demand supply gap in India
By studying this litracher, it is confirmed that there are significant possibility for energy saving then improve the efficiency and power factor by means of proposed induction motors with reducing the terminal voltage when they operated at no load or partially loaded. In the industrial sector the awareness is increasing towards the need to save energy by use of Partial loaded Induction Motors. The purpose is to obtain information to install an energy saving device in plants using Induction Motors. At no load operation, the saving can be as high as 50%. The saving decreases with the increase of load. Intermittent disconnection of the supply voltage can lead to considerable energy saving if the interruption interval is sufficiently long. Let all of us contribute our efforts for the cause of energy management by promoting the use of Partial loaded Induction Motors with special reference to power electronics.
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