designing electrical machines for voltage endurance: … electrical machines for ... – electrical...

Confidential www.edrive-engineering.com Slide 1

Adam Malloy, Juan Gonzalez, Denis Wittich

March 2018

Designing electrical machines for

voltage endurance: the SiC

challenge


─ Engineering service provider specialising in electric power train development

─ Capabilities

– Electrical machine design, analysis and prototyping

– Power electronics design, analysis and prototyping

– ISO26262 embedded software development

– System sizing and optimisation

– Manufacturing engineering

– Benchmarking

– Testing

─ Currently recruiting

– Mechanical design engineer

– Electromagnetic analyst

– Graduate engineer (electrical/mechanical)

eDrive Engineering Services


─ Customer insulation lifetime issue

─ Identifying cause of failure

─ Experimental investigation to resolve immediate issue

─ Next steps and workflow for robust insulation system verification

Overview


─ Motor manufacturer re-qualifying motor performance with new SiC inverter proposed by their end

customer

– PM traction motor for niche high-performance application

– Original target of manufacturer to have 100% carry-over of an existing motor but upgrade inverter from IGBT to

SiC

– Winding: Random-wound wire

– Insulation system: Class N (200°C)

─ Functional tests suggested system performed as expected

─ Constraints at system-level

– Not possible to add/modify inverter components

– Not possible to change cable length

─ Experienced insulation failures during durability design verification testing

Customer problem


─ Measured voltage waveforms of old and new inverter

─ Voltage overshoot identified as one potential cause for early failure

– Supported by evidence* of PDs inside the machine

Identifying cause of failure

* Example image / not from real application


Insulation failures modes: Random-wound vs form-wound windings

a- phase insulation / end-winding insulation

b- ground insulation

c- turn insulation

d- slot corona

e- stress grading

a- phase insulation / end-winding insulation

b- ground insulation

c- turn insulation

d- slot corona

e- stress grading

Random-wound Form-wound

─ 1- phase to phase

─ 2- phase to ground

─ 3- turn to turn


─ Insulation systems types I and II according to IEC 60034-18-4

– Part 4.1 – Insulation systems PD free

– Part 4.2 – Insulation systems PD resistant

Identifying cause of failure: Insulation types


─ Voltage overshoot not necessarily related to SiC devices

─ Most effective solutions are probably implemented at the system level e.g.

– Increase snubbing across each device switching

– Reduce cable length between inverter and motor to avoid transmission line overvoltage effect

– There are established methods to avoid overshoots and reduce the electrical stress produced by the inverter

waveform (filters)

─ However, here the task was finding a solution which the motor manufacturer could implement quickly

Identifying cause of failure: Voltage overshoot


─ Problem investigated using eDrive’s VET rig:

– Reconfigurable power stage to enable customer waveforms to be

replicated

– DC voltage: Up to 1000Vdc in standard configuration

– Switching speed: Up to 50V/nsec

– Switching frequency: Up to 80 kHz

– Temperature: Up to 275 °C

– Suitable for material or component lifetime testing

Identifying cause of failure: Experimental investigation


─ Special probe designed to replicate worst case geometry seen in customers machine

– Non-homogeneous electrical field

– Displaced stator core lamination

─ VET rig power stage tuned to recreate voltage waveform seen in application

─ Corona seen during testing at representative DC voltages

Identifying cause of failure: Experimental investigation


─ As a ‘quick fix’ for the customer different thicknesses of insulation material were investigated

─ CIV characterised for each thickness

─ Regression made and used to recommend best compromise between electrical safety and thermal

performance

Solution: Design investigation

─ Electric stress is represented by electrical field

─ 𝑬 =∆𝑼

𝒅→ 𝑬 =

𝒌𝑽

𝒎𝒎

─ BDV increases with insulation thickness

─ Thermal resistance reduce heat transfer

─ 𝑹𝜽 =𝒅

𝒌∙𝑺→ 𝑹𝜽 =

𝑲

𝑾

─ Heat transfer decreases with insulation thickness


─ Next steps

– Currently responding to customer concerns regarding insulation

system lifetime when implementing SiC devices due to

• Increased switching freq

• Increased dv/dt

– Sensitivity tests on insulation materials being carried out at SiC

and IGBT representative dv/dt

─ IEC 60034-18: Submits the stator or a representation of its

insulation (motorette, formette, pole winding, etc.) to a series

of ageing cycles.

─ Each cycle includes:

– Thermal sub-cycle

– Mechanical sub-cycle

– Environmental sub-cycle

– Electrical sub-cycle with customer’s inverter recreated waveform

– Data is processed with statistical methods as per IEC 60216-3

─ Outcomes:

– Thermal index (TI) and halving interval coefficient (HIC)

Solution: Design investigation


─ Motor manufacturer re-qualifying motor performance with new SiC inverter proposed by their end

customer experienced shorter than expected insulation system lifetime

─ Evidence of PDs found in motor

─ SiC voltage waveform compared with original IGBT

─ Waveforms recreated using eDrive’s VET rig

─ Corona seen during testing at representative DC voltages

─ CIV characterised for a range of thicker insulation materials

─ Recommendation made for best compromise between electrical safety and thermal performance

─ Sensitivity study being performed to investigate the impact of dv/dt on insulation lifetime Precursor to

full IEC 60034-18 testing

─ Automotive future trends

Summary

designing electrical machines for voltage endurance: … electrical machines for ... – electrical...

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