Measuring Power Losses in Electric Motors and InvertersHBK ELECTRIC POWER TESTING

Mitch Marks

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Agenda

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1. Introduction

2. Inverter losses

3. Motor Losses

4. Measurement Uncertainty

Acquisition

Software

Sensors

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Each component has losses in energy between states

Losses create heat

Adverse effects

Characterize them so we can • Understand them• Mitigate them• Manage them

Losses

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Conduction Losses • Resistance of the switch• Resistance of the

connections

Inverter Losses

InverterPower source / sink TransmissionElectric machine

Switching Losses• Turn on losses• Turn off

Stray Losses• off state• support circuits

PDC PACPAC = PDC – Pconduction – Pswitching - Pstray

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Inverter Switch operation

TimeTon ToffConduction

Power Loss

Vds

IdsTon Toff

Time

Conduction

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Switch has an “on resistance”

Power loss = I2RswitchOn = Ids*Vds

Number of switches• Switches in series will have higher losses

Temperature• Resistance is a function of temperature

Other resistances in the circuit will cause losses

Conduction Losses Vds

IdsTime

Conduction

TimeConduction

Power Loss

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P = f*(Eon + Eoff) = Ids*Vds• Frequency = f• Energy loss on/off

Losses increase with frequency

Losses increase with turn on/off time• Trise

• Tfall

Switching losses

TimeTon Toff

Power Loss

Vds

IdsTon Toff

Time

Inverter Loss Measurement

Accurately measure DC in & AC out

Estimate On resistance with TC and equation• Calculate losses

Measure Pon & Poff with differential measurement across switch• Scope card @ 250MS/s

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Inverter

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Inverter Switch On and Off Example

Real Example – Load Step

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Inverter Losses

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Power Loss Inverter –Stray loss –Conduction loss –

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Motor Losses

InverterPower source / sink TransmissionElectric machine

Copper Losses• Resistive• Skin effect

Iron Losses• Eddy currents• Hysteresis

Mechanical Losses• Friction• Windage

PAC PmechPmech = PAC – Pcopper – Piron - Pmechloss

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Losses due to resistance of the motor windings

Pcopper = Iph2 * Rwindings

Temperature dependent• Rwinding = R0*(1+α*(T-T0))• As temp increases, resistance increases

Can be reduced by cooling• Super conductor has no resistance

Measured with phase current and temperatures

Copper Losses - Resistive

Motor Circuit equation highlighting the resistive element

Copper Losses - Skin Effect

AC frequency makes the conductor look smaller

Rac = Rdc*k* 𝑓𝑓• K = gauge factor• F = frequency in MHz

Effects high fundamentals and switching frequencies

Taken into effect with resistance used in the copper loss equation

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Frequency Skin Depth Graphic

5 Hz 29.7 mm

50 Hz 9.38 mm

500 Hz 2.97 mm

5 kHz .938 mm

50 kHz .297 mm

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Magnetizing iron requires energy

Losses happen every sine wave

Area of hysteresis loop is the loss

Property of the iron & permeability

Pb = η*f*V *Bmaxn

• η – Steinmetz hysteresis coefficient (material property

• n - Steinmetz exponent (material property)• V – Volume• f – Frequency• B – Max flux density

Iron Losses - Hysteresis

Magnetizing Force H

Flux Density BPositive Saturation

Positive iron magnetization

Negative ironmagnetization

Negative Saturation

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Faraday’s law states “any change in the environment of a coil of wire will cause a voltage to be induced in the coil, regardless of how the magnetic change is produced”• The magnetism of the flux will self induce EMF• Current will flow in the core losses

Thinner laminations have a higher resistance• Results in a smaller current• Laminations must be insulated

Pe = Ke * Bmax2 * f2 * t2 * V

• Ke – eddy current constant• B - Max flux density• f – frequency• t – Thickness• V - Volume

Iron Losses - Eddy Currents

Large eddy Currents

Medium Eddy Currents

Small Eddy Currents

Dec

reas

ing

Loss

es

Dec

reas

ing

Activ

e M

ater

ials

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Internal windage

Bearing Friction

Misalignment

Mechanical losses

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Load changes will result in magnetizing currents that are greater than normal operation

Inrush currents happen even with variable frequency excitation

Dynamic Losses

Motor Loss Measurement

Accurately measure AC in & Mechanical out

Measure copper loss with thermocouples and equations

Measure iron losses thermocouples and advanced equations

Measure mechanical losses with equation or a curve based on measurement

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Electric machine

Example of all losses

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Motor Loss –Copper Loss –Eddy Current loss–Hysteresis loss –Mechanical loss –Stray loss –

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Total Losses

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Measurement Uncertainty and Losses

0

50

100

150

200

250

300

350

400

450

500

P_loss P_loss_mech P_loss_inv P_loss_copper P_loss_inv_cond P_stray_inv P_stray_mot P_Loss_eddy P_loss_friction P_loss_hyst

Thank You

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