ENERGY AUDIT OF MOTORS

CONTENTS

• Classification of motors

• Parameters related to motors

• Efficiency of a motor

• Energy conservation in motor

• BEE star rating and labelling

 11.1 CLASSIFICATION OF MOTORS

• Synchronous speed of the motor is approximately the speed that the motor will run under no-load condition and is given by , 

                     Synchronous speed = (120*frequency in Hz) / (number of poles)

• Induction motors always operate at a speed quite lower than that of synchronous speed and the difference between actual speed and synchronous speed is called as slip.

• Further induction motors are also classified as squirrel cage motor and slip ring motor.

• AC motors are widely used compared to the DC motors due to many factors like:

• DC motor are further classified as shunt motor , series motor , compound motor and permanent magnet motor.

• Based on the energy consumption , motors are classified into 3 types:

1. Small motors (<0.75 Kw)

2. Medium motors(consuming 0.75-375kW)

3. Large motors(>375kW) 

• About 9% Small motors are generally used in residential and commercial application like in refrigerator, compressor, fan etc.

• 68% are generally used as medium sized motors and is used as standalone motors.

• Large sized motors are used as high voltage ac motors and consume 23% of total power drawn by motors.

• Based on efficiency motors are classified as IE1, IE2 or IE3 motors where (IE=international efficiency) respectively.

11.2 PARAMETERS RELATED TO MOTORS

• HORSEPOWER

 Horsepower =735.5 watts

• PHASE

It describes the type of power supply, that is either 1-phase or 3-phase but 2-phase supply is widely used.

• CORE

It is the magnetic iron structure of motor’s rotor or stator/stacked iron sheet.

• TORQUE

It is the rotational force which is required to drive the motor.

• ROTOR

It is rotating part of induction motor and it includes shaft, laminated iron and squirrel cage rotor.

• STATOR

It is the stationary part of the motor’s magnetic circuit, it main function is to produce magnetic field in the air gap of the machine.

• INSULATION CLASS

Temperature insulation classes give an indication of the maximum temperature that an insulation material may be exposed to without deterioration.

There are different classes namely class A, class B, class F, class H.

• AIRGAP

It is generally present between the stator and rotor.

We are having 0.2-0.3mm air-gap.

• DESIGN

The design letter indicates the TORQUE-SPEED curve.

• Design A

Low resistance

Low inductance rotor producing low starting torque and high breakdown torque.

Efficiency is high with <3% of slip.

• Design B

High impedance rotor produces high starting torque, low starting current and good performance, hence used in industrial applications.

• Design C

These use 2-cage rotor design with normal starting current and with low slip, used for hard-to-start loads

Design C are specially designed to have high starting torque.

• Design D

It is a high-slip motor which tends to have high starting torque, low starting current but has high slip RPM at full load torque

Here speed N varies with the load since design D has high amount of slip. 

• FRAME SIZE

Most electric motors are built to an industry standard, either US or European.

Part of these standard include physical dimensions like shaft diameter, shaft length , height of the center of the shaft from mounting surface , maximum overall height , width ,length , mounting foot dimensions.

If we consider 286T frame motor, NEMA (national electric manufacturer’s association), this 286T frame is standard for 30hP @1800RPM, 20hP@1200RPM, 15hP@900RPM.

• ENCLOSURE

 It refers to the motor protection from temperature, moisture and contaminants, the proper enclosure must be selected in order for the motor to operate safely. 

Motor can either have open or closed enclosures.

11.3 EFFICIENCY OF A MOTOR

• Standard motors: efficiency is about 83 to 92%.

• Energy-efficient motors: efficiency is about 92 to 94%, with 25% of the reduction losses.

• Losses are basically of 2 types :

                                  1) Fixed losses

                                                 2) Variable losses

• Fixed losses: occur whenever the motor is in use and remain constant.

• Variable losses: just increases with motor load. 

• Core loss: includes hysteresis loss and eddy current loss.

• Windage and friction losses: It generally occurred due to bearing friction and air resistance.by improving bearing selection, airflow and fan design we could reduce the losses.

• Stator losses: this is commonly referred to as I^2R loss, it can be reduced by modifying stator design or by decreasing insulation thickness to increase the volume of the wire in the stator.

 

• Rotor losses: it consists of iron and copper losses.

• During normal operation of induction motors, slip is very small and the magnetic reversals in the rotor core are of the order of 1 or 2 /second.

• Here, the iron losses are small and can be neglected.

     Rotor loss = Cu loss= (slip X input power to rotor)

• Stray-load losses: these are due to the leakage fluxes induced by load currents.

• Both stray-load losses and stator and rotor Cu losses increase with motor load.

11.4 ENERGY CONSERVATION IN MOTORS

• Appropriate loading of motor

              The efficiency of the motor operating below 40% will be very low as shown in figure below.

           

• Different methods to calculate load on a motor are described below:

• DIRECT ELECTRICAL MEASUREMENT:

            Motor loading= (actual input power / input power at rated load)

• SLIP MEASUREMENT :

• Slip is nothing but difference between synchronous speed and actual speed of the rotor and it is expressed in percentage.

• The Ns of the induction motor depends on frequency of the power supply and on the number of the poles for which the motor is wound. 

• Ampere readings: another method to know motor load is comparing load-line amperes with nameplate amperes. This method is suitable for motors of 7.5 HP or more capacity.

• In this method, first method the line amperage with load connected while running; then disconnect the motor from the load and measure and record line amperage.

• Compare them with nameplate amperage as per equation below.

11.4.2 SELECTION OF THE MOTOR

1. Many times, an oversized motor is selected to meet future requirement of the plant which results in lower efficiency, higher motor-controller installation cost, lower power factor and increased operational cost.

2. Motors reduce power factor by storing some energy in magnetic field which creates phase difference between the motor’s peak voltage and peak current, creating reactive power. Large motors users should install capacitors to correct the power factor.

3. When an undesired motor is used, it will operate in overload condition which will result in higher winding temperature, Rise in temperature reduces motor’s life.

       4. Where demand fluctuation is predictable, it is advisable to use a two-speed motor.       For ex: In case of a cooling tower, day-time and night-time ambient conditions are different.     For similar weather-dependent applications use of two-speed motor will save energy.

       5. Select 3-phase motors instead of 1-phase motors because:

• Power factor 

• Higher efficiency of 3-phase efficiency.

• The rating of the motor increases with the number of phases.

11.4.3 ASSESSING MOTOR AND DRIVE-SYSTEM OPERATING CONDITIONS

• MOTOR REWINDING

1. Older U-frame motors have oversized slots. Reduction in copper losses is possible by adding more copper.

2. Motor should be rewound with the same or higher size winding wires; don not opt for smaller size wires in case of non-availability.

3. Reduced number of windings reduces winding resistance and shifts the motor’s peak efficiency towards higher load.

4. Checklist to be followed by the motor-rewind-shop operator.

• Use proper methods of cleaning.

• Install class F for better insulation.

• Use phase insulation between all phase junctions.

• Use tie and blocking methods to ensure mechanical stability.

• Braze rather than crimp connections.

• Use proper lead wire and connection plugs.

• Apply a proper varnish treatment.

5. Measure motor winding resistance after each winding.

6. 15 years or old motors (<100HP size) are to be replaced with new ones instead of rewinding the same.

7. If the rewind cost exceeds 65% of a new purchase of an energy-efficient motor, purchase a new one with better efficiency.

8. Payback will be less than 2 years for a motor running 4000 hours/year at energy cost of Rs.6/unit.

9. Motor rewinding is useful in case of non-standard size motors for which off-the-shelf replacements are not available.

10. The more times a motor is rewound, the more likely the efficiency gets reduced, 2% to 20% drop of efficiency may occur with each rewinding process.

11. Motor rewinding is useful in case of non-standard size motors for which off-the-shelf replacements are not available.

12. The more times a motor is rewound, the more likely the efficiency gets reduced, 2% to 20% drop of efficiency may occur with each rewinding process.

• POWER-FACTOR IMPROVEMENT

• When motors operate near their rated load, power factor is high but for under loaded motors power factor drops significantly. Many utilities imply a penalty if a user’s power factor drops below 90/95%.

• Lower power factor can be corrected by installing capacitors. Power factor can also be improved from 2 to 5% by replacement of standard motors with high efficient motors

• POWER QUALITY

• Power quality involves voltage, frequency and waveform.

• Good power quality can be defined as a steady supply voltage that stays within the prescribed range.

• Possible reasons for voltage imbalance are :

1. Unevenly distributed load on same line

2. Incorrect operation of power-factor corrector

3. Loose and dirty connections

4. Transformer tap

5. Different cable size

6. Faulty lines and circuits

• EFFECTS OF HARMONICS ON AN INDUCTION MOTOR

• High frequency voltage components produce additional losses in an AC motor, which in turn increases operating temperature of the core and the winding surrounding the core.

• When distorted voltage containing 5th and 11th harmonics is applied to 3-phase motor, will attempt to drive the motor in reverse, creating reverse torque.

• In order to compensate this reverse torque, the motor must draw additional fundamental current and thus can cause overheating/tripping of over-current protection devices.

• To minimize effect of harmonics, users should install filtering devices and isolation transformers.

• VARIABLE-FREQUENCY DRIVES

• VFD are used which has ability to precisely match motor output to process requirement.

• VFD has advantages like lower system energy-costs, improved system reliability, fewer maintenance requirements and more effective process control.

• Use of VFD is more used for following energy applications :

1. High annual operating hours.

2. Variable load characteristics as shown in figure 11.5

3. Moderate to high horsepower rating

• Adopting MEPS (minimum energy performance standard): Many countries around the world has adopted MEPS for industrial electric motors. Around 2030, overall 322TWH of electricity will be saved.

• Change of connections: A motor operating <50% loading =50% efficiency in delta connections. In this case, by converting delta to star connection will improve the efficiency by 3 to 10%.

• Use of soft starters: Used in AC motors to reduce load and torque in the power train of the motor during startup.

• Depends upon if motor loading<50% energy saving is achieved

• For motor loading>50%, saving is small and payback period is long.

• Applications like cranes, conveyors, hoists, machine tools etc.

• Use of more copper: Why Cu is used instead of aluminium?

• Cu has lower coefficient of expansion resulting in lower creep.

• Cu is 300% stronger than Al and hence can withstand higher ability.

• Cu has higher melting point to withstand higher temperature.

• Reduce idle and redundant operations : 

• During many situations, motors are running idle.

• Motor controls can be used to adjust motor speeds or turn off the motors.

• Optimize transmission efficiency :

• The motor is a part of system including power supply, control, motor and transmission system.

• Generally a belt is used for power transmission between the motor and load.

• Pulleys of different diameters are used to vary speed of driving equipment.

• Different types of belts are used to connect motor to the load and is shown in figure below :

              11.4.5 What is an energy-efficient motor?

• Energy efficient motor costs 15 to 30% higher than standard motors, depending upon the manufacturer, market competition and quantity of motors to be purchased.

11.4.6 Why should we adopt energy-efficient motors?

• Electric motors are consuming about 40 to 50% of the total generated electrical energy and cost of total electrical energy used by an electric motor is about 120% of its purchase price.

• Energy-efficient motors should be considered for these reasons :

1. When modifications are made to existing installations/processes.

2. When procuring equipment packages.

3. Instead of rewinding failed motors.

4. To replace oversized and under-loaded motors.

5. As part of an energy management or preventive maintenance program.

6. When utility rebates are offered that make high-efficiency motor retrofits even more cost-effective.

• 11.4.7 LIMITATIONS IN ADOPTING ENERGY-EFFICIENT MOTORS

1. The end user should know about the speed and torque requirement.

2. Motors are not sold to end users directly but they are given to manufacturers of fans, compressors, pumps etc.

3. There exists multiple standard like IS, IEC, NEMA, JEM etc.

4. There is lack of awareness among end users about energy-efficient motors.

5. End users demand low investment cost and not low lifecycle cost; hence, OEM’s rarely use premium energy-efficient motors. 

       6.  Use of different measuring units like Hp, kW etc.

• Creates limitations in exchanging motors between countries.

• Again different standards are followed by different countries.

       7.  Uniform testing methods are not available.

       8.  Motor manufacturers misuse “extra”, “high”, “super”, “ultra” or other similar terms         to sell standard motors as energy-efficient motors.

11.4.8 WHEN IS AN ENERGY-EFFICIENT MOTOR COST-EFFECTIVE

• The extra cost of an energy-efficient motor is often quickly repaid in energy savings.

• In industrial applications, EEM are costly since they operate more than 4000 hours a year, given a 2-year simple payback criterion.

• Basically the vendors give 20 to 60% discount based on the number and type of motor purchased.

• Simple payback is defined as the time required for the savings from an investment to equal the initial or incremental cost.

• A simple formula to calculate cost saving on purchase of an energy-efficient motor is generally given as :

• Energy-efficient motors also provide advantages like :

1. Increased reliability and longer manufacturer warranty.

2. Reduced replacement and maintenance costs.

3. When used on large scale, they consume less electricity.

4. Lower power-factor correction is required while operating at low temperature.

5. Reduced losses and power consumption.

6. They have longer insulation and bearing life

7. Noise is reduced.

11.4.9 INDIAN SENERIO FOR ENERGY-EFFICIENT MOTORS

• Presently EFF1 and EFF2 motors are used in India.

• Now it is required for the up gradation and harmonization with IEC60034.

• OEM’s (original equipment manufacturer) are to be persuaded to purchase energy-efficient motors by systematic approach and incentives.

• Table 11.11 shows a quantitative comparison of EFF1 and EFF2 motors of different countries.

11.5 BEE STAR RATING AND LABELLING

• BEE has adopted a star rating and labelling program for three-phase induction motors.

• Guidelines for motor manufacturers available from the websites www.beeindia.com.in are listed below. 

1. Motor manufacturers having three-phase, squirrel-cage 2P, 4P, continuous duty (SI) induction motors with following output ratings are eligible to apply.

 0.75KW, 1.1KW, 1.5KW, 2.2KW, 3.7KW, 5.5KW, 7.5KW, 9.3KW, 11KW and 15KW.

2. The reference Indian standards are IS: 12615-2004, IS: 4029-1967, IS: 325-1996 including all the amendments.

3. EFF2 (as per IS: 12615-2004) with the specified EFF2 (as per IS: 12615-2004) with specified tolerance limits would be the minimum entry level for labelling as per this schedule. 

4. The one-time company registration/security deposit fee is rupees 1 lakh. Registration fee on application for authority to affix labels is 1000 rupees and labelling fee on each piece of qualified product is 5 rupees.