Massachusetts Institute of Technology

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Massachusetts Institute of Technology Department
of Electrical Engineering and Computer
Science 6.131 Power Electronics
Laboratory Lecture 24 November 10, 2005
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Coils are wound on a toroidal core ( a magnetic
circuit)
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Axial View Magnets are interacting with stator
current
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Magnetic materials exhibit hysteresis For core
materials you want a narrow curve But for PM
materials you want a wide curve
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Hard permanent magnet materials have B-H curves
like this Remanent Flux density can be as high
as 1.4 T Incremental permeability is like free
space
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Magnet Characteristic Geometry Combination
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This graphic shows that calculation
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Nomenclature Two more views of the machine
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This is a cut from the radial direction
(section BB)
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Here is a cut through the machine (section
AA) Winding goes around the core looking at 1
turn
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Voltage is induced by motion and magnetic
field Induction is Voltage induction rule Note
magnets must agree!
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Single Phase Equivalent Circuit of the PM
machine Ea is induced (speed)
voltage Inductance and resistance are as
expected This is just one phase of three
Voltage relates to flux
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PM Brushless DC Motor is a synchronous PM machine
with an inverter
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Induced voltages are
Assume we drive with balanced currents
Then converted power is
Torque must be
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Now look at it from the torque point of view
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Terminal Currents look like this
So torque is, in terms of DC side current
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Rectified back voltage is max of all six
line-line voltages
Va
Vc
Vb
ltEbgt
Vab
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Average Rectified Back Voltage is
Power is simply
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So from the DC terminals this thing looks like
the DC machine
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Magnets must match (north-north, south-south) for
the two rotor disks. Looking at them they should
look like this
End A
End B
Keyway
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Need to sense position Use a disk that looks
like this
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Position sensor looks at the disk 1white,
0black
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Some care is required in connecting to the
position sensor
Vcc GND Channel 1 Channel2 (you need to figure
out which of these is count and which is zero)
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Control Logic Replace open loop with position
measurement