Date post: | 09-Apr-2017 |
Category: |
Engineering |
Upload: | gmw-associates |
View: | 215 times |
Download: | 0 times |
Motor & Drive Systems; January 21, 2016
Accurate Power Conversion Measurements on High Power Motor Drives
Presented by:Ian WalkerGMW [email protected]
Interconnections for the test of a low power AC Motor with Variable Speed Drive
DC or Single Phaseor Three Phase AC Input
Inverter
Motor Load
Speed and Torque Meter
Test Cell
From: Yokogawa
Current and Voltage Transduces provide electrical isolation in testing high power Variable Speed Drives
DC or Single Phaseor Three Phase AC Input
Inverter
Motor Load
Speed and Torque Meter
Test Cell
Current SensorI
itypical i = I/1000
Voltage SensorV
vtypical v = V/100
From: Yokogawa
ππ =πΌπΌππ
π£π£ =ππππβ²
Voltage and Current Waveforms on one phase of a low power, variable speed AC Motor Drive
From: Yokogawa
V
I
Voltage and Current Spectra for the previous Voltage and Current Waveform
From: Yokogawa~200 harmonic
Power MeasurementFor any voltage and current phase pair:
πππ‘π‘π‘π‘π‘π‘π‘π‘π‘π‘ = β1 ππ οΏ½0
ππππ π‘π‘ β πΌπΌ π‘π‘ πππ‘π‘
When Voltage and Current Transducers are used:
ππ π‘π‘ = ππβ² β π£π£ π‘π‘ with ππβ² typically about 100πΌπΌ π‘π‘ = ππ β ππ π‘π‘ with ππ typically about 1000
πππ‘π‘π‘π‘π‘π‘π‘π‘π‘π‘ = ππ β ππβ² β1 ππ οΏ½0
πππ£π£ π‘π‘ β ππ π‘π‘ πππ‘π‘
This calculation provides True Power measurement of any waveform, including all the harmonic content, limited by the bandwidth of the transducers and the processing instrument.
Note also: ππππ (π‘π‘)πππ‘π‘
= 1ππβ²
ππππ (π‘π‘)πππ‘π‘
and ππππ (π‘π‘)πππ‘π‘
= 1ππ
πππΌπΌ (π‘π‘)πππ‘π‘
dramatically reducing the capacitive and inductive coupling from long cables.
The Total Power can also be calculated from the harmonic content
πππ‘π‘π‘π‘π‘π‘π‘π‘π‘π‘ = ππ0 β πΌπΌ0 + ππ1 β πΌπΌ1 β ππππππππ1 + ππ2 β πΌπΌ2 β ππππππππ2 +
ππ3 β πΌπΌ3 β ππππππππ3 + β― + ππππ β πΌπΌππ β ππππππππππ
or
πππ‘π‘π‘π‘π‘π‘π‘π‘π‘π‘ = ππ0 β πΌπΌ0 + οΏ½1
πππ‘π‘ππππππ β πΌπΌππ β ππππππππππ
V0 and I0 are the dc components of Voltage and Current. or
πππ‘π‘π‘π‘π‘π‘π‘π‘π‘π‘ = ππ β ππβ²π£π£0 β ππ0 + ππ β ππβ² οΏ½1
πππ‘π‘πππ£π£ππ β ππππ β ππππππππππ
Voltage and Current Transducers must have accurate amplitude andphase response for all the harmonics that contribute to πππ‘π‘π‘π‘π‘π‘π‘π‘π‘π‘. Any βFalseβ harmonics, π£π£ππ or ππππ , introduced by cross-talk, will generate an error in the πππ‘π‘π‘π‘π‘π‘π‘π‘π‘π‘ if ππππππππππis β 0 and may be significant if ππππ approaches 0 and ππππππππππ is = 1.
Cross Talk Mechanisms
For a variable speed motor drive used in transport applications, the voltage signal rise and fall times can be as short as 50ns, implying significant spectrum components to 6MHz. The wavelength of the electromagnetic radiation at 6MHz is about 50m. In a Motor Drive Test Cell, the dimensions of possible βantennasβ within the signal measurement cables are typically less than 5m so simple capacitance and inductive coupling models, rather than electromagnetic wave coupling, can be used for a simplified analysis.
ππ~ππππ
~3 π₯π₯ 108
6 π₯π₯ 106 ~12
Γ 102~ 50ππ
Capacitive Cross Coupling from the high voltage, high current cables
For π π π΅π΅ = 2ohm, I =500A peak, ππ(t) = 500mA peakπ£π£ π‘π‘ = π π π΅π΅ππ π‘π‘ = 1000mV peak
At low frequency, ~10kHz, capacitive cross-coupling is low with πΆπΆπΆπΆ~100pF and ππππ ~100Vat 10kHz, π§π§πΆπΆ~200kohm.
ππβ² π‘π‘ ~ πππππ§π§πΆπΆ+π π π΅π΅
~ 100ππ200k
~ 0.5mA
π£π£β² π‘π‘ ~π π π΅π΅ππβ² π‘π‘ = 2 π₯π₯ 0.5mA~1mV peak
~0.1% of the βtrueβ current output signal of 1000mV peak.
πππ π + πππΆπΆπππ π + πππΆπΆ
π£π£ π‘π‘ = π π π΅π΅ππ π‘π‘π£π£π£ π‘π‘ = π π π΅π΅πππ£(π‘π‘)
πππ£(π‘π‘)
DCCT 1:1000
ππ(π‘π‘)
V(t) I(t)
πΆπΆπΆπΆ π§π§πΆπΆ = 12πππππΆπΆππ
~ 200k at 10kHz for 100pF
ππ π‘π‘ + πππ£(π‘π‘)
πππ£(π‘π‘)
π π π΅π΅
ππ(π‘π‘)
High impedancecurrent source
πππ£(π‘π‘)
Capacitive Cross Coupling from the high voltage, high current cables
For π π π΅π΅ = 2ohm, I =500A peak, ππ(t) = 500mA peakπ£π£ π‘π‘ = π π π΅π΅ππ π‘π‘ = 1000mV peak
At high frequency, ~1MHz, capacitive cross-coupling is high with πΆπΆπΆπΆ~100pF and ππππ ~100Vat 1MHz, π§π§πΆπΆ~1kohm.
ππβ² π‘π‘ ~ππππ
π§π§πΆπΆ + π π π΅π΅
π£π£β² π‘π‘ ~π π π΅π΅ππβ² π‘π‘ =π π π΅π΅ππππ
(π§π§πΆπΆ + π π π΅π΅) ~2 Γ 100
1k ~100mV peak
~10 % of the βtrueβ current output signal of 1000mV peak.
Note that π£π£β² π‘π‘ on the current signal is approximately in phase with V π‘π‘ .
π£π£ π‘π‘ = π π π΅π΅ππ π‘π‘π£π£π£ π‘π‘ = π π π΅π΅πππ£(π‘π‘)
πππ£(π‘π‘)
DCCT 1:1000
ππ(π‘π‘)
V(t) I(t)
πΆπΆπΆπΆ π§π§πΆπΆ = 12πππππΆπΆππ
~ 200 at 6MHz for 100pF
ππ π‘π‘ + πππ£(π‘π‘)
πππ£(π‘π‘)
π π π΅π΅
ππ(π‘π‘)πππ π + πππΆπΆ
High impedancecurrent source
πππ π + πππΆπΆ
πππ£(π‘π‘)
Capacitive Cross Coupling from the high voltage, high current cables
Capacitive Cross Coupling from the high voltage ππ π‘π‘ on the motor drive cables can be substantially reduced by shielding the drive-to-motor cables in the vicinity of the Current Transducer and signal cables. Self adhesive copper tape can be wrapped around the cables for a length of greater than three Current Transducer Head diameters. The shielding must be grounded at one end only and near the AC Drive so that the shield high frequency current, πππ£(π‘π‘), is returned to the AC drive in a short, low inductance path.
There will still be some capacitive coupling between the Drive-to-Motor cables and signal cables giving rise to a cross coupling current ππβ²β² π‘π‘ with ππβ²β² π‘π‘ << ππβ² π‘π‘ . This can be reduced by shielding the signal cables.
πππ π + πππΆπΆπππ£π£(π‘π‘)
DCCT 1:1000
ππ(π‘π‘)
V(t) I(t)
πΆπΆπ£πΆπΆ
ππ π‘π‘ + πππ£π£(π‘π‘)π π π΅π΅πππ£(π‘π‘)
πππ£π£(π‘π‘)
πππ£π£(π‘π‘)
π£π£ π‘π‘ = π π π΅π΅ππ π‘π‘π£π£π£π£ π‘π‘ = π π π΅π΅πππ£π£(π‘π‘)
πππ π + πππΆπΆ
Capacitive Cross Coupling from the high voltage, high current cables
The Current Transducer to Burden Resister signal cable should be a twisted pair (to reduce magnetic pickup) with the outer shield returned to the AC drive in a short, low inductance path.
πππ π + πππΆπΆπππ π + πππΆπΆ
DCCT 1:1000
ππ(π‘π‘)
V(t) I(t)
π π π΅π΅πππ£(π‘π‘)
πΆπΆπ£πΆπΆ πππ£π£π£(π‘π‘)
π£π£ π‘π‘ = π π π΅π΅ππ(π‘π‘)
πππ£π£π£(π‘π‘)
Capacitive Cross Coupling from the high voltage, high current cables.
Addition of a Common-Mode Choke to a Signal Pair.
Even with a shielding of the drive-to-motor cables and shielded signal cables, there still can be a residual cross-talk, particularly at higher frequencies. A common-mode choke is effective in providing a relatively high impedance to a current on only one line of the signal pair. Single turn, clamp-on, common-mode shunts can have an impedance of about 10ohm at 1MHz and 30ohm at 6MHz to give a reduction of spurious signals at a 2ohm Burden Resistor of between 5 and 15 times. This demonstrates one benefit of using a current source Current Transducer with a remote, low resistance Burden Resistor, π π π΅π΅ .
πππ£π£π£(π‘π‘)
DCCT 1:1000
ππ(π‘π‘)
V(t) I(t)
ππ π‘π‘π£π£ π‘π‘ = π π π΅π΅ππ(π‘π‘)π π π΅π΅
πΆπΆπ£πΆπΆ
πππ π + πππΆπΆπππ π + πππΆπΆ
πππ£(π‘π‘)
πππ£π£π£ π‘π‘
Inductive Cross Coupling from the High Current Motor Drive Cables.
Signal induced in a small βpick-upβ loop at the Burden Resistor.
From π£π£β² = βπ΄π΄π£ππ02
οΏ½π π 2
ππ3 οΏ½πππΌπΌπππ‘π‘
π΄π΄π£~10 Γ 10mm ~ 10β4m2
π π ~100mm ~ 10β1mππ ~1mπππΌπΌπππ‘π‘ ~
400A2ms ~2π₯π₯105 βπ΄π΄ πππ π ππ (ππ~150Hz)
Then the voltage induced in the loop at the Burden Resistor is
π£π£β²~ β 10β4 οΏ½4ππ Γ 10β7
2 οΏ½10β2
1 οΏ½ 2 Γ 105
~ 0.1Β΅V. At low frequencies and with good layout the direct magnetic coupling is small and the developed voltage leads the current I(t) by about 90Β°.
πππ π + πππΆπΆ
βpick-upβ loop area π΄π΄π£
High current loop radius π π
DCCT 1:1000
ππ(π‘π‘)
V(t) I(t)
ππ π‘π‘
2π π ~200mm ππ~1m
10mm
10m
m π£π£ π‘π‘ = π π π΅π΅ππ(π‘π‘)π π π΅π΅
πΆπΆπ£πΆπΆ πππ£π£π£(π‘π‘)
πππ£π£π£(π‘π‘)
πππ£(π‘π‘)
πππ π + πππΆπΆ
Capacitive Cross Coupling from the high voltage, high current cables.
βGroundingβ of high frequency capacitively coupled currents.
To minimize capacitive coupling from the high frequency voltages and for safety, it is often preferable to use shielded Drive to Motor Cables. The parasitic currents πππ π π‘π‘ +πππΆπΆ π‘π‘ are returned by the cable shields. However, to measure the phase currents requires the shields be broken so the parasitic currents return outside the Current Transducer aperture. A separate low impedance return from the Motor to the Drive must be added. This direct return combines with the safety grounds to generate a potentially large βground loopβ in which current can be induced by changing magnetic fields. As far as feasible the loop area should be minimized. High frequency current can be reduced by clamp-on ferrite on the safety ground leads.
πππ π (π‘π‘) + πππΆπΆ π‘π‘ return direct to the AC Drive Safety GroundSafety Ground
Clamp-On Ferrite
ππβ²β²β² π‘π‘ return
DCCT 1:1000
ππ(π‘π‘)
V(t) I(t)
ππ π‘π‘ π£π£ π‘π‘ = π π π΅π΅ππ(π‘π‘)π π π΅π΅
πππ£(π‘π‘)
ππ(π‘π‘)
Safety Ground
Clamp-On Ferrite
Summary
1. In the measurement of power transfer in high power variable speed motor drives a significant source of error in the power calculation can be the generation of βfalseβ current harmonics arising from capacitive coupling to the current channel from the high frequency voltage harmonics.
2. Shield all cables carrying high voltage to reduce capacitive coupling to the Current Transducer and associated signal lines.
3. To minimize the effects of capacitive coupling from the high frequency voltage harmonics, use current source (high impedance) Transducers rather than voltage source Transducers. This is particularly important for the Current Transducers.
4. Locate Transducers near the Drive rather than the Motor to avoid magnetic fields from the Motor generating cross-talk signals in the Transducers.
5. Capacitively induced currents in the cable shields should be explicitly returned to the source common with short, low impedance return leads. Low frequency magnetic fields from the motor currents can be reduced by minimizing the area enclosed by high currents.
6. Grounding for electrical safety compliance should be separately considered from the return of high frequency, capacitively coupled currents.
7. High frequency currents reduced in ground loops can be reduced by clamp on ferrites on the safety ground leads.
References Page
β’ Weston, David A. Electromagnetic Compatibility, Principles and Application. 2nd ed. CRC Press, 2001.
Thank You
Questions?
www.gmw.com650-802-8292