Confidential www.edrive-engineering.com Slide 1
Adam Malloy, Juan Gonzalez, Denis Wittich
March 2018
Designing electrical machines for
voltage endurance: the SiC
challenge
Confidential www.edrive-engineering.com Slide 2
─ 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
Confidential www.edrive-engineering.com Slide 3
─ Customer insulation lifetime issue
─ Identifying cause of failure
─ Experimental investigation to resolve immediate issue
─ Next steps and workflow for robust insulation system verification
Overview
Confidential www.edrive-engineering.com Slide 4
─ 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
Confidential www.edrive-engineering.com Slide 5
─ 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
Confidential www.edrive-engineering.com Slide 6
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
Confidential www.edrive-engineering.com Slide 7
─ 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
Confidential www.edrive-engineering.com Slide 8
─ 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
Confidential www.edrive-engineering.com Slide 9
─ 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
Confidential www.edrive-engineering.com Slide 10
─ 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
Confidential www.edrive-engineering.com Slide 11
─ 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
Confidential www.edrive-engineering.com Slide 12
─ 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
Confidential www.edrive-engineering.com Slide 13
─ 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