Nominal TORQUE, Low Starting Current MotorsS ( Design B).

High TORQUE, LOW STARTING CURRENT MOTORS (DESIGN C).

Good running efficiency & power factor.

High pull out torque

Starting current = 5 times full load current

Starting torque, slip, efficiency are nearly the same as DESIGN A.

Break down torque lower than design A & B.

Low starting current.

High slip.

Low efficiency.  

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Types of Motors  

There are several major classifications of motors in common use, each with specification characteristics that suit it to particular applications.

 MAIN CLASSIFICATIONS

Alternating Current (AC) Motors

Induction Motors (3-phase) are the most widely used motors in industrial and commercial applications. They fall into two sub classifications:

Squirrel cage motors

Wound rotor motors

Single phase Induction motors are used where three phase power is not available, typically in residential and commercial applications. They are used in applications with power requirements below 1 HP. There are several classifications which describe their starting and running modes.

Split Phase

Capacitor Run

Capacitor start

Capacitor start-Capacitor run

Shaded Pole

Universal Motors

Synchronous Motors are commonly used in very large Industrial applications or where exact speed in required.

 Direct Current (DC) Motors

Dc motors are used in applications where precise speed control is required. The manner in which their windings are connected sub classifies them into three groups-

Series

Shunt

Compound  

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Motor Losses

Power Losses in a motor are that portion of the input power that becomes heat rather than driving the load. These losses can be divided into two categories-

Fixed Loses

Variable Losses

Fixed losses are assumed to be constant at all conditions of motor loading from no load to full rated load. This is not exactly true, but it is nearly so, and little significant error is created by this approximation. Fixed losses include magnetic core losses (hysteresis and eddy current) and mechanical friction losses (bearing friction, brush friction, and air friction or windage).

Variable Losses are those that vary with the load on the motor and thus with the motor current. These losses increase as the load on the motor ,and therefore the current drawn by the motor, increase. They are primarily the power lost in the resistance of the motor windings and are often called copper losses, or I2R losses.

Variable losses also include stray load losses such as minor variations in fixed losses with load and speed and other small miscellaneous losses. Variable losses are approximately proportional to the square of the motor load current.

Motor Efficiency is the output of the motor divided by the electrical input to the motor, usually expressed as a percentage .power or work output is input less losses.

  Efficiency(%) = Watts output x 100

                          Watts input

= 746 x HP x 100

   E x I x PF

= Input - Losses x 100

       Input  

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Why Motor Fails

Many Motor failures can be averted ,or at least the useful life of a motor can be extended , if proper preventive measures are taken.

The sources of motor troubles generally falls into one of the following categories-

·         Harsh Environment

·         Improper motor selection/application

·         Inadequate installation

·         Mechanical failure

·         Electrical problems

·         Voltage unbalance

·         Inadequate maintenance

·         Combination of one or more of the above  

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Equipments to read Motor Parameters

The following are the various instruments used for measuring the motor parameters :

·         Power analyzer for monitoring KW, KVA, KVAR, P.F.

·         Digital Ammeter, Voltmeter.

·         Tachometer to measure speed (contact/non contact type)

·         Frequency meter.

·         Tong tester.

·         Analog/Digital Multimeter (Ac/Dc).

·         Temperature Indicator & Thermocouples.

·         Digital Wattmeter  

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Economic Evaluation Methods

The following are the various methods used for the economic evaluation of a motor when it is required to install energy conserving device to improve its performance :

·         Pay back Period (PP).

·         Return On Investment (ROI).

·         Net Present Value (NPV).

·         Benefit Cost Analysis (BCA).

·         Internal Rate of Return (IRR).

The following formulas can be used to evaluate the above mentioned methods :

PP = FC/ ((AES x PEP) - OC).

ROI = ((- FC/EL) + NAS)/EL.

NPV = PV - FC.

BCA = PV/FC.

To Calculate IRR the following equation can be used . The IRR is evaluated by trial and error method using this equation.

NAS x (1- (1+IRR)^-EL) / IRR = FC.

Where

FC : First Cost.

AES : Annual Electricity Saving.

PEP : projected Electricity Price. EL Estimated Life time.

PV : Present Value.

NAS : Net Annual Saving.  

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Features of Energy Efficient Motor

·         Highest Efficiency

·         Lower operating cost

·         Lower demand charge

·         Fewer power factor correction

·         Lower branch-circuit losses

·         Reduced air-conditioned load

·         Savings increase with time

·         Name plate efficiency

·         Interchangeability

·         Confirmation with NEMA standards A Protection and control

·         Cooler and quieter operation

·         Longer insulation life

·         Improved bearing life

·         Less starting thermal stress

·         Greater stall capacity

·         Less susceptible to impaired ventilation

·         Better buy than the old U frame motors -

·         Higher service factors

·         Better suited for energy management systems

·         Thermal margin for speed control  

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 Energy Conservation in Motors

The following formulas can be used to find out % Loading, Motor Losses and Efficiency of the motor :

% Loading of Motor = Actual KW Consumption / Rating of Motor (KW) x 100

The following method has been used to find out motor losses and efficiency

·   Take Designed Efficiency at full load and at 75 % to calculate losses at full load and at 75% load. This data can be obtained from the manufacturer or from the software itself.

Losses at Full Load (L100) = KW of motor x [(1/full load efficiency) - 1]

Losses at 75 % Load (L75)= KW of motor x [(1/75% load efficiency) - 1]

·   Determine variable losses and fixed losses in present % loading of the motor by solving below two equations :

L100 = (%Load) x (%Load) x A + B

L75 = (%Load) x (%Load) x A + B

Where

A - Variable Losses (KW)

B - Fixed Losses (KW)

Total Losses (neglecting windage and frictional losses) = A + B

• Calculate Motor efficiency

Efficiency = Output / input = Output(KW) / (Output(KW) + losses)  

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Energy conservation measures available for an Induction motor are as follows :

·   Replacement

With lower rating (HP) Standard Motor to reduce under loading

With lower rating (HP) energy efficient motors to reduce under loading

Replacing older motors with higher efficiency modern designs. Replacing a standard induction motor with a high-efficiency model has many other advantages. In addition to cutting down electricity costs, the replacement will probably have a longer life because it runs cooler and has better bearings, will need fewer capacitors to boost the motor's power factor and will work better with adjustable speed drives.

With lower rating Multi Speed motors to meet the requirement

Installation of Variable Speed Drive

Incorporation of electronic speed controls for motors driving pumps/fans/compressors requiring variations in throughout on a continuous basis to match production needs. Electronic adjustable speed drives can typically save 14 to 27% of energy.

Delta to star conversion of motors ( with less than 50% loading)

Automatic Delta Star Controllers

Use of Motor Energy Savers / Soft starters

Replacement of V belts by Energy Efficient Flat belts

Replacement of V-belts with "synchronous" belts which have teeth that engage sprocket lugs on the pulley can typically save 5 to 15% of the transmitted energy.

Switch off when not required

Improved maintenance practices

Improved maintenance practice ranges from the simplest task of using clean hands during lubrication, and to the more complex task of replacing windings in a manner which results in no loss in efficiency.

Replacement of oversized motors by appropriate size motor

By knowing the % loading and P.F of compressor motor during full load and off load, it is possible to estimate operating efficiency from motor characteristic curves. If the efficiency is low, the motor may be replaced by a higher efficiency motor after calculating the pay back period.  

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Energy Conservation Analysis

Conversion of Delta connection in to Star Connection

The induction motor with a percentage loading below 50% would operate at lower efficiency in delta mode. This efficiency at low loading can be improved by converting delta connection into star connection. The reported savings due to this conversion varies from around 3% to 10% because the rated output of motor drops to 1/3^rd of delta configuration without affecting performance and the percent loading increases as compared to delta mode. This option does not require any capital investment and is one of the least cost options available for the energy conservation in induction motors.

Though the margin of saving due to this option is low, but as the plant installations normally have hundreds of motors, converting most of the under-loaded motors in the plant would result into considerable savings.

Some motors operate on step loading and some on continuously variable load. The motors which operate on step loading, techno-economic feasibility of Delta-Star Automatic Change-over Switch is to be worked out (e.g. a machine with an induction motor performs three operations in its operating cycle resulting into motor loading of 25%, 40% & 80%; in such cases permanent delta to star conversion is not possible. A n automatic delta-star change-over controller could be installed there. It will connect the motor in star mode in 25% & 40% motor load operations; and in delta mode in 80% load operation). For the applications where starting torque requirement is high but otherwise the load is low, Automatic Delta to star Convertor can give significant energy savings.

The motors which operate on continuously variable load, feasibility of installing Soft-Starter/Energy Saver is to be worked out.

This option of permanent Delta to Star conversion can not be implemented for the loads where starting torque requirement is very high. While implementing permanent Delta to Star conversion, care should be taken to decrease the setting of over load protection relay to 2/3 rd of the delta setting.

Conversion of Standard Motor with Energy Efficient Motor

As the efficiency of standard motor at less loading is low, its operating performance get reduces considerably. If the delta to star change over option is not suitable for improving the efficiency, replacement of existing standard motor with energy efficient motor could be very viable. The conditions which increases viability of installing energy efficient motors are as follows :

Standard motor operating at low load is replaced by a lower rated (HP) energy efficient motor

Operational hours are high (nearly continuous)

Standard motor is old, number of rewindings are more and frequent

The efficiency of the Energy efficient motor is almost constant at all percentage loadings. Due to its flat efficiency characteristics, it maintains efficiency almost constant at all loads. Normally, this option is suitable for the motors with rated capacity below 50 HP. The efficiencies of standard motors above 50 HP rating are almost similar to that of energy efficient motors. In many cases, though the initial cost of energy efficient motor is 15 to 20% higher than the standard motor, the simple payback period is less due to the savings.

Conversion of V-Belts with Efficient Flat Belts

With conventional V-belts the efficiency for power transmission is low as high frictional engagement exists between the lateral wedge surfaces of the belts which causes less power transmission and hence higher power consumption for the same work to be done by the load; but with Flat-belts, this frictional engagement is on the outer pulley diameter only. V-belts contain higher bending cross section and large mass which cause higher bending loss. Also, as each groove of the pulley contains individual V-belt, the tension between the belt and the pulley distributes unevenly which causes unequal wear on the belt. This leads to vibrations and noisy running and hence reduces power transmission further. The consequences could be bearing damage also. This problem can be solved by using energy efficient Flat-belt.

It has been observed that the percentage of energy savings achieved during practical trails fairly match the gain in efficiency represented in above graph. As with the flat belt drive, the frictional engagement and dis-engagement is on the outer pulley diameter, not on the lateral wedge surface as in the case of the V-belt, wear on the belt is less and hence the life of the flat belt drive is higher than V-belt.

Some of the applications where conversion of V-belts with Flat belts is much effective are Compressors, Milling machines, Sliding lathes, Rotary printing presses, Stone crushers, Fans, Generators in Hydroelectric power plants etc.

Using Soft Starters

Soft starters, which have solid state electronic components, are used to control the input voltage according to the torque required by the driven equipment. Thus at almost all the load the motor operates at same efficiency and power factor.

This results in smooth starting of the motors by drawing lower current and thus avoiding the high instantaneous current normally encountered. Starting current and torque are directly related to the voltage applied when starting the motor. By reducing the line voltage when the motor is started, soft starter reduces the starting inrush current and eliminates the high impact or jerk starts that causes mechanical wear and damage. Soft starters are useful in cases where motors operate with high impact loads. Some of the applications are Cranes, Conveyors, Hoists, Compressors, Machine tools, Textile machinery, Food processing machinery etc.  

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Energy Conserving devices for Motors

The following are the various energy conserving devices for motors, also improves the motor performance considerably :

·         Capacitor bank.

·         Variable speed drive.

·         Soft starter/Energy Saver.

·         Automatic Delta - Star Controller.

·         Flat belt with Nylon fiber core.

·         Fluid drive & Fluid coupling.

·         Energy Efficient Motors

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We are constantly adding new features and information on Energy Conservation, so make sure to return.   If you would like to comment or correct any information please e-mail us at seemil_ngp@sancharnet.in.