Download Motor Drive Characterization for Short-Term High Power Density: Methods, Challenges, and Practical Aspects ,

Motor Drive Characterization for Short-term
High Power Density: Methods, Challenges and
Practical Aspects
Veysel T. Buyukdegirmenci
Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
Power Affiliates Program Presentation
May 09, 2014
This research is primarily supported by the Grainger Center for Electric Machinery and
Electromechanics and Rolls-Royce.
Outline
•
•
•
•
•
Motivation
Electrical Limitations
Thermal Limitations
Self-Limiting Control
Conclusions
2
Motivation
• Traction Drives
– High instantaneous peaks
– Repetitive rapid acceleration
and deceleration
• Aircraft Actuators
– More-electric aircraft
– Intermittent loads
– High power during turbulence
• Launcher Catapults
– 120 MJ in less than 3 seconds.
3
Electrical Limitations on Drives
• Series dc motors have
high stall torque
– Control allows similar
characteristics in ac drives
• V/f drives reduce voltage
below base speed
– Voltage headroom
• Voltage headroom can be
used for high torque
• How can we use this
voltage headroom?
4
Electrical Limitations on Drives
• Torque is maximized for bus
voltage
• Flux control strategy is
derived
– Critical speed defined by
voltage headroom
FOC
• Below critical speed:
– Rated flux
• Above critical speed:
– Reduced flux to optimize
torque
DTC
5
Electrical Limitations on Drives
• Developed strategies:
– Up to 2.5× breakdown torque
– 6× rated torque.
• Field-Oriented Control
(FOC):
– Higher stall torque
• Direct Torque Control
(DTC):
– Easier to implement
Does not consider thermal
or mechanical limitations
6
Machine Thermal Characteristic
• For how long is peak
power permissible?
• How does this vary with
power level?
• Time rating:
– Time required for a cold
machine to reach maximum
temperature
• Focus on end windings
Experimental characteristic at stall
with ac excitation
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Thermal Response
1: Transient
Subtransient
• Mode 3:
2:
Transition
Adiabatic,
i2t to endmodel
– Heat
1st-order
transfer
black-box
space
Linear
temperature
– Convective
End
space
heats
cooling
uprise
Conductor
heat
capacity
– Difficult
• Frame
to model
to
ambient
Experimental characteristic at stall
with ac excitation
8
Thermal Impedance Approach
• Thermal impedance used for rating, not protection
– Step response due to 1 Watt of power loss
• Losses isolated through decoupled testing (ac, dc)
Ac test
Dc test
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Results Refined
Time
Ratings
Loss
Capability
Current
Capability
30 sec.
(Mode 1)
24 p.u.
4.9 p.u.
1 min.
(Mode 2)
17 p.u.
4.1 p.u.
10 min.
(Mode 2)
4 p.u.
2 p.u.
60 min.
(Mode 3)
2 p.u.
1.4 p.u.
Test machine is a 1.5 hp 1.15 SF
ODP Induction Machine
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Case Study:
• Ratings refer to peak
capabilities in automotive
duty
• Drives are under
significant power cycles
– During acceleration
– During regeneration
• Traction system sized based
on peak torque is oversized
significantly
• Energy-based design
strategy
Temperature Rise (°C)
Automotive Duty Ratings Revisited
11
Potential Size Reductions
• Energy-based sizing
– Assuming a mileage of 100,000 miles
– Using Arrhenius chemical rate
Driving
Schedule
Size
Reduction
Line-fed
Rating
Peak
Automotive
Duty Rating
Maximum
Temperature
UDDS
70%
1.5 hp
67.1 hp
189.2°C
HWYFET
59%
1.5 hp
61.5 hp
195.5°C
NYCC
66%
1.5 hp
69.5 hp
166.9°C
EUC
65%
1.5 hp
64.4 hp
171.0°C
EUDC
60%
1.5 hp
54.0 hp
197.7°C
12
Self-Limiting Control Methodology
• Beyond 1 p.u. current permissible for limited time
• What happens after maximum temperature is reached?
• Common devices turn off the system.
– Not desirable
• Reduce the current to continuous rating
– Works, but…
13
A Safe Mode of Operation
• But…
– Cannot use full capacity
– Temperature drops rapidly
– Slowly reach steady state
HOT
HOW?
COLD
COLD
HOT COLD
14
A Safe Mode of Peak Operation
• Cooling rate is higher
when end space is cold
• Pin = Pout
– Keep the hot spot at a fixed
temperature
– Gradually back down
current as end space heats
up
• Control law:
15
Safe Overload Margins
• A new thermal protection scheme for peak duty drives
• A map for various initial internal temperatures
• Below the surface:
– End winding cools down
• On the surface:
– End winding temperature is fixed
• Above the surface
– Overheat
16
Implementation and Results
17
Extended Time Ratings
•
Tests at 80°C end winding
temperature with 5 different
initial internal temperatures
•
Roughly half hour further
overload capability
•
Tests for different max. end
winding temperatures and
similar initial internal
temperatures
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Conclusions
•
•
•
•
•
•
Electromagnetic limitations are investigated, up to 6 times rated torque is
measured under reduced voltage.
Time ratings are refined and impacts of stator and rotor related heating are
decoupled.
30 second rating of 5 p.u. current is recorded.
Energy-based and lifetime-based rating strategies are proposed for peak
duty applications with specific service time expectancies.
70% potential rating reduction in automotive duty
Self-limiting control is proposed to fully exploit peak capabilities of motor
drives.
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Questions and Comments
20
Energy-based Drive Rating
• Peaks cause subtransient
stress on drives
• Define a service factor as
overhead temperature rise
under peaks
• Tradeoff on stator heat
capacity and service
factor
21
Inverter Model Validation
• IGBT inverter model is
developed
• It uses IGBT thermal
impedances
• Validated for three
junction temperature
rises
– 10°C, 15°C and 20°C
• Error is less than 10%
Time ratings vs. power loss
BSM100GB60DLC Infineon IGBT
23
Extended Time Ratings
• Reducing the switching frequency from 20 kHz to 10 kHz
allowed an order of magnitude longer time ratings.
Inverter time rating as a function of current
24
PCM in Electric Machines
• Enthalpy of Fusion
– Heat dissipation without
temperature rise
• Possible Implementations
– As conductors
– As external heat storage
• Around end windings
• Inside end windings
28
PCM for External Heat Storage
Outside the end windings
Implementation
On the major
heat flux path
LP model
Time Rating Comparison
29
PCM for External Heat Storage
Inside the end windings
Implementation
outside from the
heat flux path
LP model
Time Rating Comparison
30
Implementation Challenges
• Need real time
temperature estimates
– 9th-order system
implemented
– Parameters fit from
experimental data
• Least squares
• Real time loss estimation
– RFO drives (Tested IFOC)
– SFO drive (Tested DTC)
32
Initialization Challenges
• Wrong initialization is
hazardous
• Luenberger state observer
– Mitigate hazards
– Error converges to zero
asymptotically
– Three surface measurements
• Available measurements
(easy and nonintrusive)
–
–
–
–
Frame Surface
End Caps
End Space
Back Iron Surface
34