1. Planteamiento y objetivos de la tesis 1
1.2. Integración de las EERR en el SEP
1.2.3. Gestionabilidad de las plantas FV: pronóstico de pro-
HP = rpm x 2πx T = rpm x T
33,000 5,250
F.L.T. = Hp x 5250 F.L. rpm
Section 3
Figure 3-13: Motor Speed-Torque - Current Curve (NEMA Design B)
Speed - Torque Relationship Speed - Torque - Current Relationship
Locked Rotor Torque. Locked rotor torque (L.R.T.) is the torque which the motor will develop at rest with rated voltage at rated frequency applied. It is also sometimes known as “starting torque” and is usually expressed as a percentage of full-load torque. The L.R.T. for NEMA design B motors range from 125 to 275% of the F.L.T. value.
Pull-up Torque. Pull-up torque (P.U.T.) is the minimum torque developed during the period of acceleration from locked rotor to the speed at which break down torque occurs.
Breakdown Torque. Breakdown torque (B.D.T.) is the maximum torque the motor will develop with rated voltage applied at rated frequency without an abrupt drop in speed. Breakdown torque is usually expressed as a
percentage of full-load torque. The B.D.T. for NEMA design B motors range from 200 to 300% of the F.L.T. value.
Full - Load Torque. Full-load torque (F.L.T.) is the torque necessary to produce rated horsepower at full-load speed.
Motor Current. In addition to the relationship between speed and torque, the relationship of motor current to these two values is an important application consideration. The speed/torque curve is repeated with the current curve added to demonstrate a typical relationship in Figure 3-13.
Full - Load Current/Amps. The full-load current (F.L.A.) of an induction motor is the steady-state current taken from the power line when the motor is operating at full-load torque with rated voltage and rated frequency applied.
Locked-Rotor Current/Amps. Locked-rotor current (L.R.A.) is the steady-state current of a motor with the rotor locked and with rated voltage applied at rated frequency. NEMA has designated a set of code letters to define locked rotor kilovolt-amperes-per-horsepower (kVA/Hp) and are listed in Table 3-5. This code letter appears on the nameplate of all AC squirrel-cage induction motors; and can be used to estimate in -rush/start-up/L.R.A. current.
The letter designations for locked motor kVA/Hp are based on full voltage and rated frequency at the motor terminals. Starting current for NEMA design B motors range from 400 to 700% of the F.L.A. for a across-the-line (AOL) start.
No - Load Current/Amps. The no-load current N.L.A will range between 30 - 40% of the motors F.L.A. value. The majority of this current is required to establish the magnetic field within the motor.
% SYNCHRONOUS SPEED % SYNCHRONOUS SPEED
Section 3
Table 3-5: Locked-Rotor kVA/Hp Code Letters Designations
Letter Designation KVA/Hp* Letter Designation KVA/Hp* Letter Designation KVA/Hp *
A 0 -3.15 H 6.3 - 7.1 R 14.0 - 16.0
-* The locked-rotor kilovolt-ampere-per-horsepower range includes the lower figure up to, but not including the higher figure. For example, 3.14 is letter “A” and 3.15 is letter “B”.
Note: 1. The maximum motor in-rush current at start-up/Locked Rotor Amps (L.R.A.) can be quickly approximated by multiplying the published full load amps (F.L.A.) value by average numeric value associated with the NEMA code letter printed on the motor nameplated
2. Single-speed motors, starting on Y connection and running on delta connections, are marked with a code letter corresponding to the locked-rotor kVA per horsepower for the delta connection.
3. Dual-voltage motors which have a different locked-rotor kVA per horsepower on the two voltages are marked with the code letter for the voltage giving the highest locked-rotor kVA per horsepower.
4. Motors with 60 and 50 Hertz ratings are marked with a code letter designating the locked rotor kVA per horsepower on 60 hertz.
5. Part-winding-start motors are marked with a code letter designating the locked rotor kVA per horsepower that is based upon the locked rotor current for the full winding of the motor.
Estimating Motor L.R.A./In-Rush Current. The relationship between motor kVA/Hp and L.R.A. for a three phase system is illustrated mathematically as follows:
Note: 1. The same formula can be used for a single phase motors through the elimination of the 1.73, three phase multiplication factor.
2. The L.R. kVA/Hp value is based on the NEMA code letter assignment.
Operating a motor in a locked-rotor condition in excess of 20 seconds can result in insulation failure due to the excessive heat generated in the stator.
Efficiency
Efficiency of a motor is the ratio of its power output to input. It represents the effectiveness in which the motor converts electrical energy to mechanical load, and is illustrated by equation as follows:
All electrical devices heat up in operation due to the losses in the windings, cores, and other machine parts.
Heating represents a loss of energy and reduces efficiency. Enough power must be put into the machine (motor) to overcome these losses, in addition to the power required by the load on the motor. The power input to the motor is always greater than the power output.
The ideal motor would be 100% efficient; however this is a impossible situation as a result of motor losses (see Figure 3-15). The efficiency of electrical submersible motors range from 80 to 90%. The efficiency generally increases with the size of the motor. Motors 10 Hp and larger, generally maintain high efficiency over the load range of 50% to 125%; however, power factor will drop rapidly with decrease in motor load. The efficiency -power factor relationship is illustrated in Figure 3-14.
Efficiency (%) = P output (Hp) x 100
The relationship between efficiency and power factor, as well as economics, favor loading a motor at or near full load. A slight increase in motor efficiency may have a significant impact on power consumption in high use installations.
Motor Losses. Motor losses are categorized as; (1) those which occur while the motor is energized but operating at no load, and (2) additional losses due to the output load. Specific losses are:
1. No load losses: a. Friction windage 2. Load Losses: a. I2R Losses (stator and rotor)
b. Core losses b. Stray Load Losses
The no-load losses and the conductor losses under load can be measured separately; however, the stray load loss requires accurate input-output test equipment for determination. The stray-load loss consists of losses due to harmonic currents and flux in the motor and are difficult to measure directly.
Losses in a typical 50Hp induction motor is shown in Figure 3-15. Motors
operated under VFD/inverter condition will exhibit significantly higher stray load losses under full load conditions.
Section 3
Figure 3-14: Power Factor and Efficiency Variation by Load
Power Factor Variation by Load Efficiency Variation by Load
Figure 3-15: Typical Motor Losses - 50 Hp Induction Motor
Note: The curves indicate a general relationship.
Values will vary with individual motor type and manufacturer.
Section 3 Motor Testing. Motor efficiency is not an absolute or constant for all motors of the same design. Rather, the
efficiencies of a large number of motors will fit a normal “bell curve” distribution. The nominal efficiency which appears on the motor nameplate corresponds to the nominal, or average expected efficiency on the curve. If the efficiency of a specific motor is required, that motor must be factory tested. Most motor manufactures can provide such testing at a additional cost. Efficiency requirements established by NEMA for three-phase surface motors are tabulated in Table 3-6.
Table 3-6: Electric Motor Efficiencies - NEMA Standard, Table 12.6
Hp 12-6B (current) 12-6C (1997) Hp 12-6B (current) 12-6C (1997)
ODP TEFC ODP TEFC ODP TEFC ODP TEFC
1 77.0 72.0 82.5 82.5 30 91.7 91.0 92.4 92.4
1.5 82.5 81.5 84.0 84.0 40 92.4 91.7 93.0 93.0
2 82.5 82.5 84.0 84.0 50 92.4 92.4 93.0 93.0
3 86.5 84.0 86.5 87.5 60 93.0 93.0 93.6 93.6
5 86.5 85.5 87.5 87.5 75 93.6 93.0 94.1 94.1
7.5 88.5 87.5 88.5 89.5 100 93.6 93.6 94.1 94.5
10 88.5 87.5 89.5 89.5 125 93.6 93.6 94.5 94.5
15 90.2 88.5 91.0 91.0 150 94.1 94.1 95.0 95.0
20 91.0 90.2 91.0 91.0 200 94.1 94.5 95.0 95.0
25 91.7 91.0 91.7 92.4 - - - -
-Note: Submersible motors will be slightly less efficient as a result of the compact design.
Basis of Testing. Most motors used in the U.S. are manufactured in accordance NEMA standard MG-1, which incorporates test standard IEEE 112 method B for reporting motor efficiency. Other standards for reporting motor efficiency are IEC 34-2 and JEC37 which are primarily utilized in Europe and Japan respectively. When comparing motors, the efficiency should be calculated on the same basis. A 15 Hp motor tested by IEEE 112 - method B, JEC 37 and IEC 34 - 2 produced efficiency values of 87.4%, 90.1% and 89.2% respectively.
Three Phase Motors (1-200 Hp)
NEMA has designated several specific types of motors, each type having unique speed/torque relationships. The rotor design is the principal factor which distinguishes the various NEMA design types. These designs are described in Figure 3-16 along with some typical applications for each. Only the NEMA design B motors have applicability to submersible motors used in water supply industry. The other NEMA designs are described for completeness, as they are used in wide range of water supply application.
Section 3
Figure 3-16: NEMA Motor Design Performance Characteristics by Design Code
Speed - Torque Performance 1) NEMA Design A&B
• Starting Current: Design A - high to medium (not defined by NEMA)/Design B - low
• Starting Torque: Normal
• Breakdown Torque: Normal
• Full - Load Slip: Low (less than 5%)
• Applications: Low starting torque/variable torque requirements and essentially constant load, such as pumps and fans.
• Expense: Minimal 2) NEMA Design C
• Starting Current: Low
• Starting Torque: High
• Breakdown Torque: Normal
• Full-Load Slip: Low (less than 5%)
• Applications: Hard-to-start loads such as positive displacement pumps and compressors.
• Expense: Moderate 3) NEMA Design D
• Starting Current: Low
• Starting Torque: Very High
• Breakdown Torque: Not Applicable
• Full-Load Slip: High (5-8%; 8-13%)
• Applications: Where a combination of high starting torque and high slip is required. Ideal for high inertia loads and/or for considerable
variations in load; such as; punch presses, shears, cranes, hoists, and elevators.
• Expense: High Rotor Geometry
Section 3 Three-Phase Unbalance
Voltage Unbalance. Unbalanced line voltages applied to a polyphase motor result in unbalanced currents in the stator windings. Even a small percentage of voltage unbalance will result in a larger percentage of current unbalance, thus increasing temperature rise and possibly result in nuisance tripping.
Voltages should be as evenly balanced as can be read on a voltmeter. If voltages are unbalanced, the rated horsepower of the motor should be derated, based upon the percent unbalance as shown in Figure 3-17.
Figure 3-17: Motor Derating for Voltage Unbalance
Note: Motor operated above 5% voltage unbalance is not recommended.
The percent unbalance is calculated as follows:
Example 3-1: Voltage Unbalance Motor Derating Calculation
Given: 100 Hp/3ph motor operating @ 480V, 460V and 440V measured at the motor starter is running with a 4.3%
voltage unbalance (100 x 20/460).
Solution. The rated output of 100 Hp should be derated by .80% (from Figure 3-18) to 80 Hp to reduce the possibility of damage.
% Unbalance = 100 x (Max. Volt Deviation from Avg.) Avg. Voltage
Effects of Unbalanced Voltage on Motor Performance.
• Torques: Unbalanced Voltage results in reduced locked-rotor and breakdown torques for the application.
• Full-Load Speed: Unbalanced voltage results in a slight reduction of full-load speed.
• Current: Locked-rotor current will be unbalanced to the same degree that voltages are unbalanced but locked-rotor KVA will increase only slightly. Full load current at unbalanced voltage will be unbalanced in the order of six to ten times the voltage unbalanced.
• Temperature Rise: A 3.5% voltage unbalance will cause an approximate 25% increase in temperature rise.
Current Unbalance. Current unbalance is typically a result of heavy single-phase loads on the electrical
transmission lines or as a result of open delta secondary transformer connection serving the motor. Excessive current unbalance in three phase motors may cause low output, overload tripping and motor failure if improperly protected.
Two criteria used in determining the acceptable levels of unbalance for submersible pumping applications are; (1) Initial installations should aim for a 5% maximum current unbalance, and unbalance should not exceed 10% for installations that have been in service for 6 months or longer, and (2) Current unbalance should not exceed 5% of service factor load or 10% at rated load for new installations. In order to maintain current unbalance within acceptable levels, voltage unbalance must be maintained within 1-3% line to line. The formula for calculating current unbalance is described as follows:
Example 3-2: Current Unbalance Calculation
Given: 20 Hp/3 ph motor operating @ 230V. Current on each leg was measured at 50, 48 and 52A.
Solution: From the above formula, the current unbalance is 4% (52 - 50/50). When the current unbalance exceeds 2%, as is the case this example, the motor cable leads should be “rolled” to minimize unbalance and determine whether it is mainly caused by the line, or as a result of motor/cable problem.
% Unbalance = 100 x (Max. Current Deviation from Avg.) Avg. Voltage
Section 3
Figure 3-18: Rolled Phases - Current Unbalance Calculation
R = 51 amps
Note: The installation should be left in the hookup 3 configuration
If the unbalanced currents stay with the same line leads when motor leads are rolled, unbalance is in the line.
If the unbalance follows the motor leads, the unbalance is in the cable and motor, and they must be checked for defects.
If unbalance stays with the line leads and is in excess of 3% in the best of the three connections, consult the power company for correction. A typical start-up procedure for submersible pumping system is outlined and illustrated below.
Checking and Correcting Rotation and Current Unbalance
1. Establish correct motor rotation by running in both directions. Change rotation by exchanging any two of the three motor leads. The rotation that gives the most water flow, or produces the greatest pressure is always the correct rotation.
The typical phase designation of motor leads for CCW rotation viewing the shaft end is
• ph 1 or “A” - black • ph 2 or “B” - yellow • ph 3 or “C” - red Note: ph 1, 2 and 3 may not be L 1, L 2 and L 3
2. After correction rotation has been established, check the current in each of the three motor leads and calculate the current unbalance as explained in item 3 below, and described in example 3-1 above.
If the current unbalance is 2% or less, leave the leads as connected.
If the current unbalance is greater than 2%, current readings should be checked on each leg using each of the three possible hook-ups. Roll the motor leads across the starter in the same direction to prevent motor reversal (ie. move each lead one place to the right and move the furthest right lead to the left).
3. To calculate percent of current unbalance:
A. Add the three line amp values together.
B. Divide the sum by three, yielding average current.
C. Pick the amp value which is furthest from the average current (either high or low).
D. Determine the difference between this amp value (furthest from average) and the average.
E. Divide the difference by the average and multiply the result by 100 to determine percent of unbalance.
The rolled phases/current unbalance calculation process is illustrated in Figure 3-18.
Section 3 Single Phase Motors
Single phase (1 ph) motors are most often employed when power requirement range from fractional horsepower (Hp) - 115 to 230V to approximately 3 Hp - 230V. Single phase motor Hp ratings up to 15 Hp are available by special order. In the submersible motor industry, 1 ph motors are categorized as two-wire (2W) or three-wire (3W), and are described as follows:
• 2W motors require no external capacitor to facilitate a motor start and have low starting torque. 2W motors are available in fractional sizes up about 1.5 Hp. The most common 2W motor design used in the submersible pump industry is the split phase type. This design utilizes a built in capacitor in the starting winding with a centrifugal switch, which opens at about 2/3 of full load speed engaging the running winding.
• 3W motors require a external capacitor and switching relay to start, and have higher starting torque compared with 2W designs, but less than 3 phase motors. 3W motors are typically available in sizes ranging from 1/3 to 15 Hp. The capacitor start-induction run motor is the most widely used type 3W motor.
All single phase motors are wired to run in a specified direction at the factory (typically CCW). Some fractional Hp models incorporate built-in overload protection. Overload protection must be built into the starter for larger units.
Power Factor
A motor can be fundamentally described as a electromagnet, power factor (pf) is a measure of the amount of magnetizing current required for the machine to operate.
Power factor (pf) is an important consideration when selecting a motor for a particular application since low pf may result in a pf penalty charges from the utility company. Since the power company must supply kVA, but typical utility metering only kilowatts (kW) used, low motor pf requires additional kVA with low return on kW utilized;
hence, pf penalties. The equation for calculating pf in a three-phase system is listed as follows:
Note: The same equation can be used for a single-phase system with the elimination of the 1.73 term.
This equation represents a numerical method of expressing the phase difference between voltage and current in a motor circuit. The current in an induction motor lags the applied voltage, and only the component that is in phase with the voltage varies with motor power. The relationship expressed in the above equation is shown graphically in Figure 3-19 A, as a vector relationship in which the numerical expression actually the cosine of the angle L.
pf = kW Input = kW 1.73 x V x I kVA
Figure 3-19: Power Factor Illustration