Massachusetts Institute of Technology Department of Electrical Engineering

Massachusetts Institute of Technology Department of Electrical Engineering and Computer Science 6. 131 Power Electronics Laboratory Lecture 24 November 10, 2005 6. 131 Lecture 24 1

November 10, 2005 6. 131 Lecture 24 2

November 10, 2005 6. 131 Lecture 24 3

November 10, 2005 6. 131 Lecture 24 4

November 10, 2005 6. 131 Lecture 24 5

Coils are wound on a toroidal core ( a magnetic circuit) November 10, 2005 6. 131 Lecture 24 6

Axial View: Magnets are interacting with stator current November 10, 2005 6. 131 Lecture 24 7

Magnetic materials exhibit hysteresis For core materials you want a narrow curve But for PM materials you want a wide curve November 10, 2005 6. 131 Lecture 24 8

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 November 10, 2005 6. 131 Lecture 24 9

Magnet Characteristic Geometry Combination November 10, 2005 6. 131 Lecture 24 10

This graphic shows that calculation November 10, 2005 6. 131 Lecture 24 11

Nomenclature: Two more views of the machine November 10, 2005 6. 131 Lecture 24 12

This is a ‘cut’ from the radial direction (section BB) November 10, 2005 6. 131 Lecture 24 13

Here is a cut through the machine (section AA) Winding goes around the core: looking at 1 turn November 10, 2005 6. 131 Lecture 24 14

Voltage is induced by motion and magnetic field Induction is: Voltage induction rule: Note magnets must agree! November 10, 2005 6. 131 Lecture 24 15

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: November 10, 2005 6. 131 Lecture 24 16

PM Brushless DC Motor is a synchronous PM machine with an inverter: November 10, 2005 6. 131 Lecture 24 17

Induced voltages are: Assume we drive with balanced currents: Then converted power is: Torque must be: November 10, 2005 6. 131 Lecture 24 18

Now look at it from the torque point of view: November 10, 2005 6. 131 Lecture 24 19

Terminal Currents look like this: So torque is, in terms of DC side current: November 10, 2005 6. 131 Lecture 24 20

Rectified back voltage is max of all six line-line voltages Va Vab November 10, 2005 Vc Vb <Eb> 6. 131 Lecture 24 21

Average Rectified Back Voltage is: Power is simply: November 10, 2005 6. 131 Lecture 24 22

So from the DC terminals thing looks like the DC machine: November 10, 2005 6. 131 Lecture 24 23

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 November 10, 2005 6. 131 Lecture 24 24

Need to sense position: Use a disk that looks like this November 10, 2005 6. 131 Lecture 24 25

Position sensor looks at the disk: 1=‘white, 0=‘black’ November 10, 2005 6. 131 Lecture 24 26

Some care is required in connecting to the position sensor Vcc GND Channel 1 Channel 2 (you need to figure out which of these is ‘count’ and which is ‘zero’) November 10, 2005 6. 131 Lecture 24 27

Control Logic: Replace open loop with position measurement November 10, 2005 6. 131 Lecture 24 28