NOTE: Some content may not display correctly, including tables and figures. See PDF for full details.

5. ENERGY PERFORMANCE ASSESSMENT OF MOTORS AND VARIABLE SPEED DRIVES

5.1        Introduction

The two parameters of importance in a motor are effi- ciency and power factor. The efficiencies of induction motors remain almost constant between 50% to 100% loading (Refer figure 5.1). With motors designed to perform this function efficiently; the opportunity for savings with motors rests primarily in their selection and use. When a motor has a higher rating than that required by the equipment, motor operates at part load. In this state, the efficiency of the motor is reduced. Replacement of under loaded motors with smaller motors will allow a fully loaded smaller motor to oper- ate at a higher efficiency. This arrangement is general- ly most economical for larger motors, and only when they are operating at less than one-third to one-half capacity, depending on their size.

5.2        Performance Terms and Definitions

Figure 5.1 Efficiency vs. Loading

5.3  Efficiency Testing

While input power measurements are fairly simple, measurement of output or losses need a labo- rious exercise with extensive testing facilities. The following are the testing standards widely used.

Europe: IEC 60034-2, and the new IEC 61972 US: IEEE 112 - Method B

Japan: JEC 37

Even between these standards the difference in efficiency value is up to 3%.

For simplicity nameplate efficiency rating may be used for calculations if the motor load is in the range of 50 -100 %.

Field Tests for Determining Efficiency

(Note: The following section is a repeat of material provided in the chapter-2 on Electrical Motors in Book-3.)

No Load Test :

The motor is run at rated voltage and frequency without any shaft load. Input power, current, frequency and voltage are noted. The no load P.F. is quite low and hence low PF watt meters are required. From the input power, stator I²R losses under no load are subtracted to give the sum of Friction and Windage (F&W) and core losses. To separate core and F & W losses, test is repeated at variable voltages. It is worthwhile plotting no-load input kW versus Voltage; the intercept is F & W kW loss component.

F&W and core losses = No load power (watts) – (No load current)² x Stator resistance

Stator and Rotor I²R Losses :

The stator winding resistance is directly measured by a bridge or volt amp method. The resis- tance must be corrected to the operating temperature. For modern motors, the operating tem- perature is likely to be in the range of100°C to 120°C and necessary correction should be made. Correction to 75°C may be inaccurate. The correction factor is given as follows :

R₂            235 + t₂

—–  =  ———–  , where, t₁  =  ambient temperature, °C & t₂  =  operating temperature, °C. R₁       235 + t₁

The rotor resistance can be determined from locked rotor test at reduced frequency, but rotor I²R losses are measured from measurement of rotor slip.

Rotor I²R losses = Slip x (Stator Input - Stator I²R Losses - Core Loss)

Accurate measurement of slip is possible by stroboscope or non-contact type tachometer. Slip also must be corrected to operating temperature.

Stray Load Losses :

These losses are difficult to measure with any accuracy. IEEE Standard 112 gives a complicated method, which is rarely used on shop floor. IS and IEC standards take a fixed value as 0.5 % of

output. It must be remarked that actual value of stray losses is likely to be more. IEEE - 112 spec- ifies values from 0.9 % to 1.8 %.

Points for Users :

It must be clear that accurate determination of efficiency is very difficult. The same motor test- ed by different methods and by same methods by different manufacturers can give a difference of 2 %.

Estimation of efficiency in the field can be summarized as follows:

a)      Measure stator resistance and correct to operating temperature. From rated current value, I²R losses arecalculated.

b)      From rated speed and output, rotor I²R losses are calculated

c)      From no load test, core and F & W losses are determined for stray loss

The method is illustrated by the following example :

Example :

Motor Specifications

No load test Data

a)              Calculate iron plus friction and windage losses

b)             Calculate stator resistance at 120°C

235 + t₂ R₂  =  R₁ x  ————

235  +  t₁

c)              Calculate stator copper losses at operating temperature of resistance at 120°C

d)             Calculate full load slip(s) and rotor input assuming rotor losses are slip times rotor input.

e)              Determine the motor input assuming that stray losses are 0.5 % of the motor rated power

f)              Calculate motor full load efficiency and full load power factor

Solution

a)              Let Iron plus friction and windage loss, P_i + fw No load power, Pnl = 1063.74 Watts

Stator Copper loss, P st-30°C (Pst.cu)

= 3 x (16.1 / Ö3)² x 0.264

= 68.43 Watts

Pi + fw = P_nl - Pst.cu

= 1063.74 – 68.43

= 995.3 W

b)             Stator Resistance at 120°C,

120 + 235

R_120°C = 0.264 x —————

30 + 235

= 0.354 ohms per phase

c)              Stator copper losses at full load, Pst.cu 120°C

= 3 x (57 / Ö3)² x 0.354

= 1150.1 Watts

d)             Full load slip

S = (1500 – 1475) / 1500

= 0.0167

Rotor input, Pr    =    P_output/ (1-S)

=    34000 / (1-0.0167)

=    34577.4 Watts

e)              Motor full load input power, P input

= P_r + Pst.cu 120°C + (P_i + fw) + P_stray

= 34577.4 + 1150.1 + 995.3 + (0.005^* x 34000)

= 36892.8 Watts

^*where, stray losses = 0.5% of rated output (assumed)

f)              Motor efficiency at full load

Poutput

Efficiency                   =    ——– x 100 Pinput

34000

=    ——– 36892.8

=    92.2%

=           Pinput

Full Load PF              =    —————–

=    Ö3  x  V  x  I_fl

=          36892.8

=    ——————–

=    Ö3  x  415  x 57

=    0.90

Comments :

a)       The measurement of stray load losses is very difficult and not practical even on test beds.

b)      The actual value of stray loss of motors up to 200 HP is likely to be 1 % to 3 % compared to 0.5 % assumed bystandards.

c)       The value of full load slip taken from the nameplate data is not accurate. Actual measure- ment under full load conditions will give better results.

d)      The friction and windage losses really are part of the shaft output; however, in the above calculation, it is not added to the rated shaft output, before calculating the rotor input power. The error however is minor.

e)       When a motor is rewound, there is a fair chance that the resistance per phase would increase due to winding material quality and the losses would be higher. It would be inter- esting to assess the effect of a nominal 10 % increase in resistance per phase.

5.4        Determining Motor Loading

1.  By Input Power Measurements

• First measure input power Pi with a hand held or in-line power meter Pi = Three-phase power in kW

• Note the rated kW and efficiency from the motor name plate

• The figures of kW mentioned in the name plate is for output conditions. So corresponding input power at full-rated load

Nameplate full rated kW

Pir = ————————————————

hfl

h_fl = Efficiency at full-rated load

P_ir = Input power at full-rated load in kW

• The percentage loading can now be calculated as follows

P_i

Load =  —    x 100%

Pir

Example

The nameplate details of a motor are given as power = 15 kW, efficiency h = 0.9. Using a power meter the actual three phase power drawn is found to be 8 kW. Find out the loading of the motor.

2.  By Line Current Measurements

The line current load estimation method is used when input power cannot be measured and only amperage measurements are possible. The amperage draw of a motor varies approximately lin- early with respect to load, down to about 75% of full load. Below the 75% load point, power factor degrades and the amperage curve becomes increasingly non-linear. In the low load region, current measurements are not a useful indicator of load.However, this method may be used only as a preliminary method just for the purpose of identification of oversizedmotors.

Input load current

% Load = ———————— *100 (Valid up to 75% loading)

Input rated current

3.  Slip Method

In the absence of a power meter, the slip method can be used which requires a tachometer. This method also does not give the exact loading on the motors.

Slip

Load    =    —— *100%

S_s–S_r

Where:

Load = Output power as a % of rated power

Slip = Synchronous speed - Measured speed in rpm

S_s = Synchronous speed in rpm at the operating frequency S_r = Nameplate full-load speed

Example: Slip Load Calculation

Given: Synchronous speed in rpm     = 1500 at 50 HZ operating frequency. (Synchronous speed = 120f/P) f: frequency, P: Number of poles Nameplate full load speed = 1450

Measured speed in rpm          = 1480 Nameplate ratedpower     = 7.5 kW

Determine actual output power.

1500 – 1480

Load  =  —————    *100% = 40% 1500 – 1450

From the above equation, actual output power would be 40% x 7.5 kW = 3 kW

The speed/slip method of determining motor part-load is often favored due to its simplicity and safety advantages. Most motors are constructed such that the shaft is accessible to a tachometer or a strobe light.

The accuracy of the slip method, however, is limited. The largest uncertainty relates to the accuracy with which manufacturers report the nameplate full-load speed. Manufacturers generally round their reported full-loadspeed values to some multiple of 5 rpm. While 5 rpm is but a small percent of the full-load speed and may be considered as insignificant, the slip method relies on the difference between full-load nameplate and synchronous speeds. Given a 40 rpm "correct" slip, a seemingly minor 5 rpm disparity causes a 12% change in calculated load.

Slip also varies inversely with respect to the motor terminal voltage squared. A voltage cor- rection factor can, also, be inserted into the slip load equation. The voltage compensated load can be calculated as shown

Slip

Load = ———————– x 100%

(S_s – S_r)  x  (V_r/V)²

Where:

Load    = Output power as a % of rated power

Slip     = Synchronous speed - Measured speed in rpm S_s = Synchronous speed in rpm

S_r         = Nameplate full-load speed

V         = RMS voltage, mean line to line of 3 phases V_r   = Nameplate rated voltage

5.5        Performance Evaluation of Rewound Motors

Ideally, a comparison should be made of the efficiency before and after a rewinding. A relatively simple procedure for evaluating rewind quality is to keep a log of no-load input current for each motor in the population. This figure increases with poor quality rewinds. A review of the rewind shop's procedure should also provide some indication of the quality of work. When rewinding a motor, if smaller diameter wire is used, the resistance and the I²R losses will increase.

5.6        Format for Data Collection

The motor loading survey can be performed using the format given below:

Motor Field Measurement Format

Company                                                  Date         

General Data

Driven Equipment                                    

Location                                              Process   Department     

Motor Operating Profile:

Motor Name Plate Data                               No of hours of operation

Manufacturer                                             Model                     Serial Number                                                             Type :Squirrel cage/Slp ring  Size (hp/kW)                                              SynchronousSpeed (RPM)                      Full-Load Speed (RPM)                                    Voltage Rating                                                                      Full-Load Amperage        Full-Load Power Factor (%)                          Full-Load Efficiency (%)        Temperature Rise                                       Insulation Class                        

I Shift                              II Shift           III Shift           

Annual Operating Time             hours/year

Type of load

1.Load is quite steady, motor "On" during shift 2.Load starts, stops, but is constant when "On" 3.Load starts, stops, and fluctuates when "On"

Measured Data Supply Voltage By Voltmeter

VRY                          

From Test Certificate

VYB                          

Input Amps By Ammeter

A a                     

V avg             

Stator resistance per phase =

Rewound   □ Yes ,if yes How many

times rewound ?---

□ No

Motor Loading %                                  

A b                    A avg               A c    

Power Factor (PF)                                                                                   Input Power (kW)          

Motor Operating Speed                        RPM At frequency of     

Driven Equipment Operating Speed

                   RPM

Type of Transmission (Direct/Gear/Fluid coupling)

The monitoring format for rewound motor is given below:

5.7        Application of Variable Speed Drives (VSD)

Although there are many methods of varying the speeds of the driven equipment such as hydraulic coupling, gear box, variable pulley etc., the most possible method is one of varying the motor speed itself by varying thefrequency and voltage by a  variable  frequency drive.

5.7.1  Concept of Variable Frequency Drive

The speed of an induction motor is proportional to the frequency of the AC voltage applied to it, as well as the number of poles in the motor stator. This is expressed by the equation:

RPM = (f x 120) / p

Where f is the frequency in Hz, and p is the number of poles in any multiple of 2.

Therefore, if the frequency applied to the motor is changed, the motor speed changes in direct proportion to the frequency change. The control of frequency applied to the motor is the job given to the VSD.

The VSD's basic principle of operation is to convert the electrical system frequency and volt- age to the frequency and voltage required to drive a motor at a speed other than its rated speed. The two most basic functions of a VSD are to provide power conversion from one frequency to another, and to enable control of the output frequency.

VSD Power Conversion

As illustrated by Figure 5.1, there are two basiccomponents, a rectifier and an inverter, to accomplish power conversion.

The rectifier receives the 50-Hz AC voltage and con- verts it to direct current (DC) voltage. A DC bus inside the VSD functions as a "parking

lot"  for  the  DC voltage. The                 Figure 5.1 Components of a Variable Speed Drive

DC bus energizes the inverter, which converts it back to AC voltage again. The inverter can be controlled to produce an output frequency of the proper value for the desired motor shaft speed.

5.7.2  Factors for Successful Implementation of Variable Speed Drives

a)   Load Type for Variable Frequency Drives

The main consideration is whether the variable frequency drive application require a variable torque or constant torque drive. If the equipment being driven is centrifugal, such as a fan or pump, then a variable torque drive willbe more appropriate. Energy savings are usually the pri- mary motivation for installing variable torque drives for centrifugal applications. For example, a fan needs less torque when running at 50% speed than it does when running at full speed. Variable torque operation allows the motor to apply only the torque needed, which results in reduced energy consumption.

Conveyors, positive displacement pumps, punch presses, extruders, and other similar type applications require constant level of torque at all speeds. In which case, constant torque vari- able frequency drives would be more appropriate for the job. A constant torque drive should have an overload current capacity of 150% or more for one minute. Variable torque variable frequency drives need only an overload current capacity of 120% for one minute since cen- trifugal applications rarely exceed the rated current.

If tight process control is needed, then you may need to utilize a sensor less vector, or flux vector variable frequency drive, which allow a high level of accuracy in controlling speed, torque, and positioning.

b)   Motor Information

The following motor information will be needed to select the proper variable frequency drive:

Full Load Amperage Rating. Using a motor's horsepower is an inaccurate way to size vari- able frequency drives.

Speed Range. Generally, a motor should not be run at any speed less than 20% of its specified maximum speed allowed. If it is run at a speed less than this without auxiliary motor cooling, the motor will overheat. Auxiliary motor cooling should be used if the motor must be operated at very slow speeds.

Multiple Motors. To size a variable frequency drive that will control more than one motor, add together the full-load amp ratings of each of the motors. All motors controlled by a single drive must have an equal voltage rating.

c)    Efficiency and Power Factor

The variable frequency drive should have an efficiency rating of 95% or better at full load.

Variable frequency drives should also offer a true system power factor of 0.95 or better across the operational speed range, to save on demand charges, and to protect the equipment (especially motors).

d)   Protection and Power Quality

Motor overload Protection for instantaneous trip and motor over current.

Additional Protection: Over and under voltage, over temperature, ground fault, control or microprocessor fault.These protective circuits should provide an orderly shutdown of the VFD, provide indication of the fault condition, and require a manual reset (except under voltage) before restart. Under voltage from a power loss shall be set to automatically restart after return to normal. The history of the previous three faults shall remain in memory forfuture review.

If a built-up system is required, there should also be externally-operated short circuit protec- tion, door-interlocked fused disconnect and circuit breaker or motor circuit protector (MCP)

To determine if the equipment under consideration is the right choice for a variable speed drive:

The load patterns should be thoroughly studied before exercising the option of VSD. In effect the load should be of a varying nature to demand a VSD ( refer figure 5.3 & 5.4).

Figure 5.3 Example of an excellent variable speed drive candidate

Figure 5.4 Example of a poor variable speed drive candidate

The first step is to identify the number of operating hours of the equipment at various load conditions. This can be done by using a Power analyzer with continuous data storage or by a simple energy meter with periodic reading being taken.

5.7.3  Information needed to Evaluate Energy Savings for Variable Speed Application

1. Method of flow control to which adjustable speed is compared: o     output throttling (pump) or dampers (fan)

• recirculation (pump) or unrestrained flow (fan)
• adjustable-speed coupling (eddy current coupling)
• inlet guide vanes or inlet dampers (fan only)
• two-speed motor.

1. Pump or fan data:

• head v's flow curve for every different type of liquid (pump) or gas (fan) that is handled
• Pump efficiency curves.

1. Process information:

• specific gravity (for pumps) or specific density of products (for fans)
• system resistance head/flow curve
• equipment duty cycle, i.e. flow levels and time duration.

1. Efficiency information on all relevant electrical system apparatus:

• motors, constant and variable speed
• variable speed drives
• gears
• transformers.

If we do not have precise information for all of the above, we can make reasonable assump- tions for points 2 and 4.