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Improving Motor Performance of Rotating Equipment
Dinesh Nimje, Chhavi Nagpal. Manufacturing and power generation contribute towards the lion’s share of greenhouse gas emissions today (62 per cent). Shutting them down is obviously not an option. This presents us with the unique challenge of saving our environment and our future with minimum impact to technological advancements and the luxuries of modern life.

Since the Industrial Revolution in 18th century, the advancement in science and technology has been exponential. However, it wasnĘt without its drawbacks. The biggest consequence of this sustained and rapid growth was the increase of the global carbon footprint and an unprecedented amount of greenhouse gas emissions. Today, the world is teetering on the brink of an environmental disaster due to the constantly increasing global temperatures. Many environmental studies have been carried to predict the effects of this uncontrolled advancement. One such study has predicted that greenhouse gas emissions will double by 2050. If not controlled today, there may not be a tomorrow for us.

Though most of us understand the risks today, our dependence on technology has made it impossible for us to take a step back. Manufacturing and Power Generation contribute towards the lionĘs share of greenhouse gas emissions today (almost 62 per cent). Shutting them down is obviously not an option. This presents us with the unique challenge of saving our environment and our future with minimum impact to technological advancements and the luxuries of modern life.

As per the Pareto principle, 80 per cent of the effects can be gained through addressing 20 per cent of the major causes. Improving energy efficiency will have a major impact on reduction in CO2 level. In view of this, efforts have been made by various nations and world organisations to formulate regulations, which ensure a sustainable future.

This article will deal with optimisation methods for a major environmental offender, which also shares the distinction of being one of the biggest harbingers of the industrial revolution in its modern form - the Electric Motor. Electric Motors consume the largest share of electrical energy in any manufacturing industry. Hence, even a small increase in motor efficiency can lead to a considerable reduction in energy costs and CO2 levels. Typically, the energy consumption of a motor is nearly 80 per cent of its total operating cost.

Use of Variable Speed Drives with Motors Many applications in the industry require motors to operate at variable load, but are designed for the worst case operating condition, i.e., maximum power requirement. An under-loaded motor not only reduces the efficiency considerably but also reduces the power factor drastically, which deteriorates the power quality of the power supply. This happens because a motor is designed to give its best performance at an optimum operating point. Under variable load conditions this operating point shifts, hence the motor performance deteriorates. This can be avoided by using motor driven by Variable Speed Drives (VSD). A VSD shifts the operating point of a motor as per the load requirement. This helps save energy by running motors at less than 100 per cent output when full power is not needed.

Energy conversion devices are also not 100 per cent efficient. But thanks to advancement in IGBT (Insulated Gate Bipolar Transistor) technology, the present day drive efficiencies are as high as 97 per cent. This can be further improved by using appropriate logic circuits. At full load operation the drive is not advantageous to the motor, but since it remains a part of the circuit, it continues to add losses to the motor-drive set. If proper control circuitry is in place, the drive can be disconnected from the motor during full load operation, further improving the overall efficiency of the motor.

A study conducted on the application of VSD in European Union revealed the promise VSDs holds in reducing the energy consumption. Approximately 35 per cent of the total energy consumption has been reduced by application of VSD, and still potential savings of 100 TWh are expected in near future. Perhaps the most important application of motors in any manufacturing setup is pumps. Pumps constitute a lionĘs share of total energy consumption of manufacturing sector. As per a study on Improving the Energy Efficiency of Pumps, conducted by European Commission in 2001, pumps account for 22 per cent of the total motor and drives energy consumption in industrial sector of EU, equivalent to 176 TWh.

Figure 1 below, shows that the overall efficiency of a VSD operated pump is 1.6 times higher than that of a control valve operated pump.

Let see how by using VSD to control the flow at part load saves energy. Here we will be analysing two situations, one with

50 per cent flow controlled be valve, and the other with 50 per cent flow controlled by VSD. This will demonstrate how much energy saving can be achieved by using a speed control drive.

In Figure 2, the x-axis shows the rate of flow in m3/h and the y-axis shows the Total head in m. The lines H(n1), H(n2) and dotted line parallel to them shows the iso-efficiency lines. The curve Hw represents the system curve. The intersection of isoefficiency line and the system curve gives the operating point of the pump for a given efficiency. The rate of flow (Qa), at 100 per cent flow, is achieved by using valve (H(n2)). The rate of flow (Qb), at 50 per cent flow is achieved by using valve (H(n1)) and AC drive speed control H(n2). The basic equations for determining rate of flow are:

r = density of liquid
g = acceleration
H = head [m]
? = pump efficiency
P = power required
V = Energy required per unit volume

Situation 1:
100% flow
Q (a) = 200 m3

By substituting the value of H(n1) and ? corresponding to Q (a) = 200 m3 in the above equations, we get:
• Required power P = 22.2 kW
• Required energy V = 0.111 kW/m3

Situation 2:
50% flow
Q(b) = 100 m3
Repeating the above procedure and referring the Valve control (VC) and Speed control (SC) charts separately for Q (b) = 100 m3 will give the following results:

VC: Valve Control
SC: Speed control
(From Diagram)
Savings: at 50% flow, speed control requires 0.107 kW/m3 less compared to valve control

A Few Case Studies
1. A Pumping Station:
Let us look at the case study of a Pumping station, where use of drives helped pump twice the amount of water while saving energy. In the nations where river transport is most preferred means of transportation, regulating the water level in streams, lakes, ditches, moats and canals is very important because the water level is of critical importance to the ships. For upgradation of a 50-year-old installation some novel steps were taken. A 690 V motor was used along with a 12-pulse variable speed drive, in combination with a special star/delta transformer, which provided a low THDI (Total Harmonic Distortion of Current). This doubled the pumping capacity from 10 m-/s to 20 m-/s. The VSD not only reduced the energy cost of the plant, but also reduced the stress on power distribution equipment, thanks to lower harmonic distortions on the mains power.

2. Retrofit of Municipality Pump Station: Variable Speed Drives (VSD) installation at a Municipality Pumping station gave multifold advantages when renovating a pump in a pump station. Extended service life for the high-pressure pump, no need for motor-operated valves plus reduced pipe-installation dimensions all contribute to savings of over USD 6955. VSD gives ÂelectricalĘ solution to a mechanical problem. For the pump stations, with the installation of soft-starters it doesnĘt need to install motor-driven valves anymore, which means no pressure surges resulting in expensive burst pipes. In addition, mechanical wear on the shafts and bearings in the pumps is eliminated. Starting current was reduced from 250 A to 150 A. This reduced the monthly electricity bill greatly.

3. A Chemical Process Plant: PVC production is a complex chemical process, which consists of several stages. Pumps, fans, blowers, mixers, mills and centrifuges, controlled by variable speed drives and soft-starters, are used in these processes. The plantĘs boiler room contains two large boilers and three fans, a flue gas fan, a combustion air fan and a flue gas recirculation fan, all of which are controlled by VSDs. The vector brake function of the VSD removes the need for brake choppers and braking resistors to stop the machinery quickly and safely. Soft-starters are used to control the pumps, mills and fans in the PVC production process. The VSD controls the speed of the mixers in the reactors. The reactors are where the particles form and the speed is crucial to ensure that the products have the right properties.

Use of Energy Efficient Motors
Using VSD in the variable load applications is a great way to save energy. This helps in achieving good part load performance of the motor. This however, does not undermine the importance of using energy efficient motors. Though the decision of buying motors is driven by initial cost, energy efficient motors will give a long-term benefit. In view of this, many codes and standards have been formulated by government agencies to guide the manufacturers to adhere to higher efficiencies standards, for example, NEMAĘs Premium efficiency and super premium efficiency motors and IECĘs IE2 & IE3. With the rising global temperatures, these rules are also becoming more and more stringent, compelling motor manufactures and end users to elevate the efficiency standards of the motor. Figure 4 shows the trend in European Union towards adopting higher efficiency motors. IEC 60034-30 specifies efficiency standards for rotating machines. Further, IE4 class motors whose efficiency corresponds to super premium efficiency motors of NEMA. It is intended to be applicable to all types of motors (safe area) with out any exception. This will further enforce the drive towards attaining better efficiency in motors, which will greatly improve the system efficiency.

Salient Design Features of Energy Efficient Motors

• High-pressure aluminum die-cast rotor. More aluminum in rotor.
• Increased rotor bar and end ring section, resulting in lower rotor resistance.
• Superior quality bearings, reduced friction, vibration and rotor dynamics.
• Locked bearing reduces endplay.
• Fan and fan cover designed for maximum cooling and minimum power consumption and quiet operation.
• Smaller fans take less power therefore, Heat producing losses are low. Hence smaller fans can be used due to less cooling requirement.

So, the copper loss of a motor affects the efficiency greatly. Hence higher efficiency motors have higher copper content in their stators, which make the motors costly. Motor Selection for Best Performance Motor Selection plays an important part in the enhancing the motor performance over the operating range. Traditionally, DC motors, and Wound rotor motors are used in applications requiring speed variation; heavy load inertia starting; high starting torque requirements; low starting current requirements; high efficiency at low speed or high power factor. But where DC motors are infamous for commutation problem, the wound rotor motors being costly, increases customersĘ budget. Today, VDF operated motors have replaced these motors.

Variable Torque Application (Speed Range 10 to 100%) For the applications where torque is squarely proportional to the speed like in centrifugal fans, pumps, blowers etc., Motors with IC411 arrangement (with inbuilt fan) are recommended. In such a case de-ration is not required. However, the supply frequency must not increase the base speed, in order to avoid overloading of motor.

Constant Torque Application (Speed Range 10 to 100%) For the applications where torque remains constant over the entire speed range like crane, conveyor, crusher, grinder hammer mill, re-rolling mill, motors provided with external cooling blower with IC416A arrangement is recommended.

Rewinding Motor: Better or Worse? Depending on the size of the motor, re-winding may appear to be a cheaper option than buying a new motor, without taking into account any losses in efficiency. This could be true. But one must ensure that the life-cycle cost of the unit, including the new efficiency of the re-wound unit, is taken into account. Losses in rewound motors can increase up to 20 per cent, which is equivalent to an approximate 1.5 to 2.5 per cent drop in full-load efficiency. A 3 per cent decrease in efficiency is possible for motors older than 15 years that have been rewound more than once.

It's a Choice We Have to Make The most important thing that the industry needs today is not just the means of saving energy, but also the will to do so. Energy saving methods will require initial investment; at times it may even be substantial. But in order to get benefit of using the same, the industry has to look past the initial cost of investment and realise the long-term potential of the energy saving measures for continued sustainability.