Current Pulse Testing on (Starter/Motor/Generator)s from Hyundai/Kia and Buick Hybrids


These tests are performed:

– to gain understanding of and intuition towards these nifty machines.

– to collect data from which motor parameters may be calculated.


Highly engineered, power dense, mass produced, cantaloupe-sized electric machines have started to appear on the hybrid vehicle scene.  The two examined here are interesting for the following reasons:

– small (<20kg, 40lbs)

– discrete and not part of a giant casting containing other stuff

– liquid cooled

– cheap ($80-$160)

– designed with large rotor bearings to take high radial loads, because they are coupled to the engine through the serpentine belt.

Because of the above reasons, the motors are better suited for stoopid things like fighting robots and gokarts than components pulled from a hybrid transaxle.  They are standalone units that can quickly be bolted to a frame and belted in to a mechanical system.  (If only the manufacturers published a datasheet for them!)


gen 2 Prius Inverter.  Bus capacitance of approximately 1.2 mF.

Bayley’s HV ECU leftover from battlebots

Nucleo F446RE development board

bench power supplies for logic power and battery trickle charging

Mike’s 20V (6S10?P) A123 pack

the derpbike A123 pack, configured as 40V (12S 30P).  “The stiffest 40 Volts in this neck of the woods”

test setup

The motors tested are:

Hybrid Starter Generator from (Hyundai Sonata Hybrid/Kia Optima Hybrid)

ornl hyundai kia hsg[1]

Apart from its small size, alternator-style packaging, and cooling loop, the HSG is a fairly typical-looking IPM in the context of hybrid vehicle motors.

eAssist Electric Motor Generator from Buick Lacrosse Hybrid

2012-Buick-LaCrosse-eAssist-Motor-Generator BUICK-LACROSSE-EASSIST-5[6]

The eAssist is particularly interesting as it is an induction motor.  Consumer hybrid/electric vehicle traction motors are almost exclusively interior permanent magnet (IPM) motors.  The notable exception to this trend is Tesla, who uses induction motors in their models S and X.  See [4] for GM’s justification for choosing this machine.  The size, housing construction, and cooling loop is similar to that of the HSG.  One notable difference is that the eAssist stator is bar wound, making the end turns very compact.

Both machines come with alternator pulleys for 7-groove “K-series” belts.


The bridge idles with all three PWMs centered at 50% duty cycle.   When the test starts, the phases are set as follows:

A: 0% – low side switch conducting

B: 100% – high side switch conducting

C: 0% – low side switch conducting

for approximately 100ms or until the system reaches steady state.  Then, the PWMs are all reset to 50% duty cycle.  The switching frequency is a constant 5kHz.

In all of the following ‘scope captures, the channels are as follows:

1 – trigger, Nucleo DAC output

2 – phase B current out of the Prius inverter connector and into a resistor divider.  The scaling is 307A/V.  Positive current is defined as current exiting that phase from the inverter.

3 – phase B voltage.  Scaling is correct, measured with respect to Bus Minus

4 – Inverter Bus Voltage: Bus Plus – Bus Minus.

The bus power source is varied three times, and the test is run for both motors.  On the HSG, care is taken to align the rotor’s D-axis with the stator axis that is to receive a positive current step/pulse.  This is to avoid accidental rotor demagnetization.

Bus Power Source Variant 1:  65V via weenie bench supply

Device Under Test: eAssist


Device Under Test: HSG


same capture, longer timebase:


Bus Power Source Variant 2:  Mike’s 20V red toolbox battery


Bus is precharged with the weenie bench supply.

Device Under Test: eAssist


Device Under Test: HSG


Bus Power Source Variant 3:  40V derpbike battery

derpbike battery

The bus is precharged with the weenie bench supply before plugging in the giant battery shown above

Device Under Test: eAssist

Prius inverter hardware overcurrent protection is tripped at about 4ms


same capture as above, shorter timebase


Device Under Test: HSG



**Sources and more information on these machines

** HSG:

[1] pages 13-15 of this  [ORNL Benchmarking State-of-the-Art Technologies Presentation – May 14, 2013

]( [2] pages 104-118 of [ORNL Power Electronics and Electric Machinery Program annual report – October 2011

]( [3] pages 28-33 of ORNL Power Electronics and Electric Machinery Program annual report – November 2013


[4] [Development of General Motors’ eAssist Powertrain – April 16, 2012

]( [5] [GM BAS+ (eAssist) Motor Generator Components and Operation

]( [6] motor trend article with some cutaway pictures