CAPÍTULO 3: DISEÑO DE LA INVESTIGACIÓN
3.2 Metodología
3.2.1 Metodo cualitativo
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• Many frequency inverters use this type of switching pattern
• Also called “sine coded” or “sine weighted” PWM
• Main limitation of Standard PWM is that the maximum output voltage to the motor is limited to 87% of the inverter’s input voltage:
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.
Motor
V = V = (V x √2) Phase-to-Phase
(rms) input voltage:
Vin = (Vpeak / √2)
Maximum phase to earth peak output voltage = V2dc Therefore maximum phase to earth rms output
voltage Vpout = (V2dc / √2)
Therefore, maximum phase-to-phase rms output voltage = (√3 x V )
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“YES!!”
Effect of voltage variation on induction motor characteristics
-20
Percent change in motor performance
Full Load Amps Power Factor Efficiency
Starting and Maximum Torque Starting Amps
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• For a motor operating from a fixed supply frequency (50Hz), to
produce motor nameplate rated shaft torque when the mains supply is -10% below motor nameplate rated voltage results in the following:
21% increase in slip to produce rated torque, 2% decrease in efficiency,
9.5% increase in full load amps, 6 - 7°C temperature rise,
for every +10°C. temperature riserise above rated, the lifelife of the motor is reducedreduced 50%50%
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• Switching pattern modulation index can be increased or third harmonic injection can be used to increase output voltage from standard PWM frequency inverters
• Both methods increase fundamental output voltage however but also increase harmonic voltages = limitations
• Theoretically it is possible to achieve a fundamental output
voltage 95% of the input voltage however this still means motor operates at best in a 5% under voltage condition
• If try to increase output voltage any further increases harmonic frequencies in motor = increased motor temperature + torque ripple + acoustic noise = reduced motor performance and
efficiency
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• In reality what happens if using a standard PWM frequency inverter to control a motor?
• The motor must be derated
It cannot be used for full load output power/torque (e.g. a 15kW motor can only be used for a load which requires maximum 13kW shaft power)
Due to over-design of systems and selection of standard frame size motors this might be OK for some applications but this is a serious limitation of standard PWM – what if you need full flow, full power, full torque for only 5% of the time?
• Frequency inverter supplier relies on fact that motor has over design built in as part of the motor’s service factor (i.e. lifetime)
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• Not all frequency inverters use Standard PWM switching pattern today
• Some frequency inverters have used an
enhanced version of PWM (e.g. Danfoss VVCplus) for many years, others are now introducing this
• Enhanced PWM (e.g. VVCplus) = improved performance
Full motor voltage
Sinusoidal output current
Minimizes motor heating = same motor temp rise as on mains supply
Maximizes inverter efficiency
• Many frequency inverters still use Standard
PWM and rely on the over design in the system, over sizing of the motor, or result in reduced performance or lifetime of the motor
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• Unlike sine weighted PWM, VVC is based on a digital generation of the output voltage.
• Ensures output voltage
reaches rated value of input voltage, motor current is sinusoidal and the motor operates as it does on the mains.
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• Enhanced PWM (e.g. VVC
plus)
• Standard PWM
14%
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• A motor controlled by a frequency inverter using “enhanced”
PWM switching technique:
With rated mains supply voltage applied to the drive input, full rated fundamental output voltage is applied to the motor at rated frequency.
The motor is able to develop its rated power and torque at rated voltage, current, and speed.
The motor operates within its rated temperature rise
allowing full thermal life of the motor to be maintained.
Harmonics in the motor are minimized
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• All frequency inverters are non-linear loads resulting in harmonic currents in the mains supply
• Most frequency inverters have one or two basic solutions to reduce harmonics
DC link reactor OR AC input reactor
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•
Both DC reactors and AC reactors give similar reduction in harmonics
HARMONIC CURRENT ANALYSIS ---Fund. Current 36.22 A
THD 42.51%
RMS current 39.47 A 5’th Harmonic 12.91 A 7’th Harmonic 7.03 A 11’th Harmonic 3.06 A 13’th Harmonic 2.10 A ---HARMONIC CURRENT ANALYSIS ---Fund. Current 36.22 A
THD 42.51%
RMS current 39.47 A 5’th Harmonic 12.91 A 7’th Harmonic 7.03 A 11’th Harmonic 3.06 A 13’th Harmonic 2.10 A
---• DC reactors
HARMONIC CURRENT ANALYSIS ---Fund. Current 36.84 A
THD 43.84%
RMS current 40.22 A 5’th Harmonic 14.71 A 7’th Harmonic 5.74 A 11’th Harmonic 2.66 A 13’th Harmonic 1.34 A ---HARMONIC CURRENT ANALYSIS ---Fund. Current 36.84 A
THD 43.84%
RMS current 40.22 A 5’th Harmonic 14.71 A 7’th Harmonic 5.74 A 11’th Harmonic 2.66 A 13’th Harmonic 1.34 A
---• AC reactors
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• BUT – AC reactors result in a lower DC link voltage and therefore lower voltage available to the motor
• How much lower voltage?
• Example, a 3% AC reactor for an 11kW frequency inverter with a rated current of 25 amps may have an impedance of 1.2mH
• On 60Hz supply this has an impedance XL=2x∏xfxL = 0.45 ohms
• At 25 amps this equates to a voltage drop across reactor of:
V = I x XL = 11.3 volts = 3% of 380V
• Voltage drop across reactor is 90 degrees out of phase with supply voltage so not full 3% drop (Note: DC reactor has no voltage drop)
• But AC chokes also increase diode commutation time
• Typically results reduction in DC link voltage = 0.5 x % inductance
• e.g. 3% AC reactor = 1.5% reduction in DC link voltage, harmonic filter solutions
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• BUT – AC reactors result in a lower DC link voltage and therefore lower voltage available to the motor
• How much lower voltage?
• Example, a 3% AC reactor for an 11kW frequency inverter with a rated current of 25 amps may have an impedance of 1.2mH
• On 60Hz supply this has an impedance XL=2x∏xfxL = 0.45 ohms
• At 25 amps this equates to a voltage drop across reactor of:
V = I x XL = 11.3 volts = 3% of 380V
• Voltage drop across reactor is 90 degrees out of phase with supply voltage so not full 3% drop (Note: DC reactor has no voltage drop)
• But AC chokes also increase diode commutation time
• Typically results reduction in DC link voltage = 0.5 x
% inductance ~ 1 2 3
Statement in inverter supplier’s manual
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4 pole 4kW and 5.5kW motor nominal full load torque
0 5 10 15 20 25 30
0 300 600 900 1200 1500 1800
Speed rpm
Torque Nm
Motor Nominal Torque (4 pole, 4kW) Nm
Motor Nominal Torque (4 pole, 5.5kW) Nm
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Standard PWM
Example of available 4kW and 5.5kW motor shaft torque
0
0 300 600 900 1200 1500 1800
Speed rpm
Torque Nm
Motor Nominal Torque (4 pole, 4kW) Nm
Full torque at full speed/full load not available w ith Standard PWM frequency inverters - typically best max 90% FLT available (especially if AC input reactors used for harmonic reduction)
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Standard PWM
Example of available 4kW and 5.5kW motor shaft torque
0
0 300 600 900 1200 1500 1800
Speed rpm
Torque Nm
Motor Nominal Torque (4 pole, 4kW) Nm Motor Nominal Torque (4 pole, 5.5kW) Nm
Motor Torque available (4 pole, 5.5kW) with Standard PWM Frequency Inverter Nm (Supplier A) Motor Torque available (4 pole, 4kW) with Standard PWM Frequency Inverter Nm (Supplier A)
Full torque at full speed/full load not available w ith Standard PWM frequency inverters - typically best max 90% FLT available (especially if AC input reactors used for harmonic reduction)
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Standard PWM
Centrifugal Pump or Fan Application
Requirement to oversize frequency inverter and motor depending on full flow torque requirement
0
0 300 600 900 1200 1500 1800
Speed rpm
Torque Nm
Motor Nominal Torque (4 pole, 4kW) Nm
Required full flow torque not available w ith standard PWM therefore need to use 5.5kW frequency inverter and motor to give required full flow torque = increased cost
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Enhanced PWM
Example of available 4kW and 5.5kW motor shaft torque
0
0 300 600 900 1200 1500 1800
Speed rpm
Torque Nm
Motor Nominal Torque (4 pole, 4kW) Nm Motor Nominal Torque (4 pole, 5.5kW) Nm
Motor Torque available (4 pole, 4kW) with Enhanced PWM Frequency Inverter Nm (Supplier B) Motor Torque available (4 pole, 5.5kW) with Enhanced PWM Frequency Inverter Nm (Supplier B)
Full torque at full speed/full load is available w ith Enhanced PWM frequency inverters (especially if DC reactors used for harmonic reduction)
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Enhanced PWM
Centrifugal Pump or Fan Application
Same motor as for fixed speed application (4kW) and 4kW frequency inverter required
0
0 300 600 900 1200 1500 1800
Speed rpm
Torque Nm
Required full flow torque is available w ith enhanced PWM therefore can use same 4kW motor as for fixed speed application, w ith 4kW frequency inverter = low est cost + best performance
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• By selecting or specifying a frequency inverter which uses an enhanced PWM switching pattern (e.g. VVCplus) and using DC reactors instead of AC input reactors you can be sure the motor will get it’s nameplate rated voltage when operating at full
speed, full load and therefore be able to provide full motor shaft torque and power without overloading the motor, or reducing it’s lifetime
• This ensures compatibility with the motor and ensures it operates from the frequency inverter, just as it would if connected to the mains supply
• Inverter duty rated motors, oversized (de-rated) motors are NOT required
• For centrifugal pump and fan HVAC applications – enhanced PWM + DC reactors = standard motors can always be used no matter what the speed/flow range
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• Another important factor to consider is whether the motor’s insulation is compatible with the voltage supply from a
frequency inverter
• The voltage supply from a PWM frequency inverter can be very different to the voltage from the mains supply
Motor Insulation – Peak Voltage and Rise Time
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• This is generally NOT a problem with “good” quality standard motors and frequency inverters on mains supplies <500V AC
• It can be a problem on higher voltage mains supplies, with “low cost” motors and on special applications – for these special
precautions can be taken to prevent problems
• However, for 220V/380V/480V mains supplies, using motors
complying with IEC60034-17 and “good” quality frequency inverters, there should be no problems and no need to use inverter rated
motors
• Following slides, explain why potentially there can be problems and how to avoid these problems
Motor Insulation – Peak Voltage and Rise Time
* Following information comes Danfoss internal studies, studies at Dresden University, Germany and from a study into this topic by GAMBICA (UK drives industry association) and REMA (UK motor manufacturers association)
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• It is a fact that motor winding insulation experiences higher voltage stresses when supplied by a frequency inverter than when connected to sinusoidal AC mains supply
• These higher stresses are dependant on motor cable length and are caused by the interaction of the fast rising voltage pulses of the frequency inverter and transmission line effects in the cable
• To ensure compatibility with a motor, it is necessary to ensure the motor terminal peak voltage (voltage and rise time) are below the levels that the motor insulation is immune to
Motor Insulation – Peak Voltage and Rise Time
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•
PWM frequency inverters use fast switching IGBTs to create the PWM voltage waveform
•
A series of square wave voltage pulses are applied to the motor cable
•
The motor draws current and due to the large inductance of the motor this consists of mainly a sinusoidal current waveform at the required frequency of operation
Motor Insulation – Peak Voltage and Rise Time
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• Each pulse of the PWM waveform created by the fast switching IGBTs has a fast rise time (at the output of a typical frequency inverter this could be approx 100 – 300 ns)
• Rise times are so fast that as it travels along length of the
motor cable to the motor it can change the shape of the pulse and may produce voltage overshoot (and change the rise time)
• At the motor, one pulse of the PWM waveform can look very different to the square wave at the frequency inverter output
Motor Insulation – Peak Voltage and Rise Time
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• To understand this effect, due to the fast rising pulse, it is necessary to consider the motor cable as a transmission line
• Transmission line effects can then be considered = pulse travels along the cable and is reflected at the motor like a wave
• A transmission line consists of a long string of
inductor/capacitor sections as shown below (only one phase is considered here):
Motor Insulation – Peak Voltage and Rise Time
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• At each fast rising pulse edge the frequency inverter has to charge the inductance and capacitance of the cable so a pulse of energy is delivered into the cable
• Transmission line theory shows pulse travels at velocity
= [1 / √(LC)] m/s
L and C = inductance (Henries) and capacitance (Farads) per metre
• Velocity of a pulse in typical PVC insulated cable
= approx 1.7 x 108 m/s
= in 100ns the pulse travels 17m
• Different cable types give slightly different velocities, but generally the differences are only small
Motor Insulation – Peak Voltage and Rise Time
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• In this theoretical/ideal explanation:
tr = rise time of pulse at frequency inverter output
tp = time taken for pulse to travel length of cable
tr < tp (which is typical for cable lengths >30m)
• Time t = tr
• Pulse enters cable at t = 0 and rises to DC link voltage Ud in time tr
Motor Insulation – Peak Voltage and Rise Time
• Time t = tr + tp
• Pulse travels from inverter to motor
• When reaches motor it is reflected because motor high frequency impedance is > cable impedance
• Reflection causes pulse to rise towards 2 x original peak = 2 x U
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• Time t = 2tr + 2tp (i.e. after the pulse has traveled to the motor and back to the inverter)
• Reflected pulse returns to inverter
• Because inverter impedance is low pulse is reflected in a negative sense
• Inverter clamps voltage to Ud resulting in negative pulse as it travels back along cable to motor
Motor Insulation – Peak Voltage and Rise Time
• Time t = 2tr + 3tp (i.e. after the pulse has traveled to the motor, back to the inverter and back to the motor)
• Negative pulse is reflected again at the motor and is doubled again = -2 x U
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•
In ideal case of previous 2 slides reflections would cause voltage to oscillate continuously
•