Current Sensing Investigation

End of Internship Presentation

John Zhang

What is Current Sensing?

• To measure current flowing through a conductor

• Typical Approaches:

– Shunt resistor

– Current Transformer

– Hall Effect

Shunt resistor approach:Measure voltage drop across a small resistor.

Step down primary current

Current => Flux => Voltage

Hall Effect Current Sensing

• Physics:

– Lorentz Force

– Uneven Distribution of Charges

– Electric Field & Voltage

• Closed Loop Hall Effect Sensor

– Feedback circuit to drive flux to 0

– IF related to Iin by turn ratio

Why is Current Sensing Important?

• Field Oriented Control: Control of flux-producing (d-axis) and torque-producing(q-axis) currents.

• Sensor quality directly impacts output dqcurrents hence machine torque.

• Also used in sensorless control to determine rotor position

Flux Concentrators (Cores/Toroids)

• Function: Concentrate flux by having a high permeability (B (flux density) = u (permeability) * H (field strength) )

– Permeability of Air ~= 1

– Permeability of 4% SiFe ~= 2000 - 35000

Current in BussBar Magnetic Filed (H) Flux Density (B) Hall Voltage

Ampere’s Law Permeability Hall Effect

Core Properties

• Electrical:

– Permeability (Gain)

– Saturation

– Hysteresis

Desired Linear Relationship

Cores I investigate:

O.D = 31.07 mmI.D = 17.4 mmGap = 5.14mm

Core Saturation

• In saturation, core permeability decreases significantly

• Can cause major torque ripple issues

-2500

-2000

-1500

-1000

-500

0

500

1000

1500

2000

2500

-1.50E-02 -1.00E-02 -5.00E-03 0.00E+00 5.00E-03 1.00E-02 1.50E-02

1400Arms 13MM gap core saturation Actual(red) vs Measured(green)

Core Hysteresis• Generally a nonlinear effect

• For our test results, a delay model can approximate the effect

• More on this later

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

-6.00E-04 -4.00E-04 -2.00E-04 0.00E+00 2.00E-04 4.00E-04 6.00E-04

Hysteresis : Actual (green) and Measured (red)

My Project: Core Evaluations

• Goal: Compare the saturation and hysteresis characteristics of Tape Wound (TW) cores and Stacked Lamination(SL) cores

Note: Comparison is for 38533 PN CoreGap = 5.15 ± 0.4 mmRated current: up to 250 Arms (354 A peak)

Cost Saving:

TW 38533 $0.99Stacked Lam cores $0.348Annual volume ~150,000

Potential Annual Saving from cores ~= 96,300 !

Benchtop Core Comparison

Old Setup:- Larger Center Conductor (0.62 inch d)- Difficult to position cores and sensors => flux to current conversion gain changes due to mechanical vibration

New Setup:- Smaller Center Conductor (0.5 inch d)- Sensor position stable => more consistent gain

Core I.D = 0.68 in

Benchtop Core Comparison

• Main Comparisons between cores:– Hysteresis vs frequency– Hysteresis vs current– Saturation

• Other Considerations:– Sensor Type: Lakeshore Gaussmeter vs Hall Bridge vs

Allegro/Melexsis– Setups

• Center conductor diameter – affects B field• Mechanical – position affects results

– Gain error (calibration)– Variances between cores

Hysteresis vs Freq

• Experiment:

1. Put ~250Arms current through center conductor at 100Hz, 250Hz, 500Hz.

2. Measure gap flux with Lakeshore gaussmeter.

3. Convert gap flux to current measured.

Gain = Irms / Brms.

4. Plot Current Sensing Error vs Current in conductor

Hysteresis vs Freq Results

SLTW

- Spikes due to LEM problem- Differences between cores insignificant at this current level

Hysteresis vs Current Level

- Stacked Lam core slightly worse hysteresis at each current level- At 500Arms @ 500Hz, SL cores have worse hysteresis with max current error ~= 10A.

TW cores have max current error ~= 5A. This current level is x2 rated current for this core!

100 Arms – 500 Hz 200 Arms – 500 Hz 500 Arms – 500 Hz

Core Saturation Comparison (800Arms 500Hz)

TW SL

- SL slightly more error before saturation- SL softer saturation (limited operating range)

- Does not appear to be an issue for the 38533 part at rated current levels- Gap tolerances for SL are significantly tighter- Prudent to further evaluate any additional higher current replacements

Hall Sensor Evaluation

• Compare LEM(closed loop hall effect), Hall bridge, Lakeshore Guassmeter, Allegro and Melexsis

LEMLakeshore

Hall BridgeAllegro

Melexsis

LEM vs Lakeshore Gauss meter vs Hall Bridge

LEM

Lakeshore Hall Bridge

Significant delay in the hall bridge signal output.

Hysteresis appears similar to a delay under steady state conditions.

Difficult to distinguish between sensor delay and core hysteresis.

Lakeshore(L) vs Hall Bridge(R) SL cores

- Current sensing error due to a fixed delay is proportional to freq of operation.

Lakeshore vs Allegro

• Allegro sensor performance close to Lakeshore

LEM

Lakeshore

Allegro

Lakeshore vs Melexsis

• Melexsis slightly less delay than Lakeshore

LEM

Lakeshore

Melexsis

Core Evaluation Conclusion

• PML Core samples are substantially similar during normal operating conditions

• Recommend tooling production candidates with PML

– Requires some further mechanical and electrical evaluation

Further Work

Even with existing current sensors, Iq/Id currents can be quite noisy!• Explore quantization effects and other noise in:

– Position sensor– Current sensors– Rotor harmonics (PMAC)

Currents:20A/Div

Simulation Results

No Theta_e error. No sampling

w/ current quantizer2V encoder output 5V 10-bit ADC

w/ better quantizer2V encoder output3.3V 12-bit ADC

Thank You!

Questions?