deliberation and following
design review, modifications were made to the carriage design and
copper contacts to provide better response to the contact
surfaces. Final development proceeded with the arrival of the main
components and design decisions regarding improvements to the
Motor, and Controls
the stepper motor, driver, and linear rail (comprising our linear rail
system, or LRS), work began with trying to get the components
integrated and operational. Extend-retract buttons were incorporated
into the system, and, ultimately, the LRS was set up to change
positions linearly every 1.375" (the approximate distance from one
completed tap change to
the next). The maximum speed of the LRS is 2 in./sec.
A1 and A2.
IDEA drive with the stepper motor (left). Working
to get the
drive and motor synchronized and operational.
A3 and A4. Left
image: setup showing the extend-retract buttons
(left arrow) and on/off switch (right arrow)
control system. Right image: manipulating the carriage along the
using the extend and retract buttons.
Like many things on the project,
the controls underwent several design changes to reach their final
state. The white breadboard was replaced with a smaller and more
practical solderless unit. A 7-1/2" by 5-1/2" opening was created
in the top of the cart and a box, 1-3/4" deep, was constructed to house
the controls and fitted to the opening. A 1/4" thick clear acrylic
held down with four screws, completes the control interface. The power
ON/OFF switch is located immediately below the battery pack. To the
right of the power switch are the extend and retract buttons that move
the tap changer in
either direction along the track.
Figs. A5, A6, and A7.
Control box under construction (left). Two
views of completed control system for the phase-shifting transformer
made to the carriage design from the previous page. The plates were
laser cut in both clear and dark acrylic material, before the clear
acrylic was selected for final design. Wave springs were
introduced into the brush holder to provide reaction for each resistor
bridge. The carriage was then assembled with four long machine screws
(3-1/2" by 1/4")
comprising the four upright posts connecting the top and bottom plates.
The carriage was then mated to the LRS for testing of the mechanical
aspects of the
resistor bridge was redesigned to
improved contact between the brushes and copper contacts.
Set of three
brushes incorporated into carriage bottom plate by way of an earlier
Fig. B2. Wave spring used
to generate contact force between brushes and contacts.
Figs. B3, B4, and B5.
carriage (inverted) mated to LRS (left). Close up of carriage from side
and above at an angle (right).
Figs. B6 and B7.
Assembled carriage traveling along LRS: side and top views.
Figs. B8 and B9. Side
(left) and head-on close views of carriage.
bridge, the central component of tap changer, was designed for optimum
contact between the brushes and copper contacts. The constituent parts
of the resistor bridge--the brushes and resistors--were selected based
on the design specifications of the tap changer and the operating
parameters of the transformers.
Figs. C1, C2, and C3.
brush (left), resistor (with wires attached), and brush and resistor
joined (second from left). The
carriage will hold nine carbon
brushes and six resistors. The brush dimensions (not including the
wire) are 1-1/4" long by 6/10" by 5/10". The size
of the brushes is important for meeting our 80 A/in-squared current
density goal. Each of the
wire wound resistors is approximately 1-1/3" long and 4/10" wide (not
including the connecting wires). The resistor rating was dictated by
the maximum voltage and amperage flowing through the tap changer, as
well as the expected speed of carriage movement.
Figs. C4 and C5.
and intact views of a later design of the resistor bridge cartridge.
Figs. C6 and C7. Assembled
cartridge top view (left) and side view.
C8 and C9. Three
cartridges inserted into carriage bottom plate: top view (left) and
bottom view. One wave spring (Fig. B2)
is inserted under the head of each of the 1/4-28 x 1/2 socket head cap
screws that hold the cartridge to the acrylic bottom plate to provide
downward contact force.
C10 and C11. Views
front and back of cartridges inserted into the carriage bottom plate.
C12 and C13. Views
top (left) and underside (bottom) of cartridges fully assembled around
carbon brushes and integrated into carriage bottom plate. All three
cartridges were manufactured from ABS plastic in the University of
Idaho's 3-d printer.
Note the 2-ohm resistors (two per cartridge), completing the resistor
and Base Plate
final design of the copper contacts specifies a flat contact surface. A
prototype was made
of each design: one with a threaded exterior and another with a
threaded interior. The two prototypes were then test-fitted on one of
the test pieces of acrylic. The decision was then made to go with the
contacts with the threaded exterior. Each contact will be fastened to
the acrylic contact board using 5/16" brass hex machine nuts holding a
5/16" ring terminal in place.
Figs. D1 and D2.
prototype models (left), and top of copper contact used in the final
the base plate on the manual mill (left). Drilling holes in side of
Figs. D5 and D6.
the center holes (left) and the half-inch holes (right) on the Haas
base plate began as a piece
of clear acrylic with dimensions
18.150" x 6" x 0.708". It is important that the plate
holes and the contacts be precise. In that regard,
plate drawing and dimensions were
entered into MasterCam and the board drilled on a Haas mill. Center
holes were drilled first, followed by the 5/16"
thru-holes, and then the 1/2" counterbores.
fluid running over the board during the process is a mixture
water and coolant. Prior to drilling, the board was shaped on a manual
mill by having its edges faced with a 1" end mill.
Base plate with
first step in shaping the
thirty-three copper contacts was to write a program in MasterCam.
program was entered into the Haas CNC
lathe. Five separate CNC lathe tools were used to cut and shape each
contact. After machining was completed on the Haas lathe, the contact
was taken to the Logan lathe where shaping of the contact face was
completed using an aluminum holder made specifically for the purpose of
holding each contact in the same position with respect to the Logan
facing tool. The finished contacts extrude 0.025" above the surface of
the acrylic base plate.
Figs. D8 and D9. Shaping a
copper contact on the Haas CNC lathe (left) and the Logan lathe.
Fig. D10. Base plate with
contacts in place.
ensure all contacts were the
same height with regards the carriage,
the base plate with copper contacts attached was mounted on the Haas
mill. A 1/2" end mill traveled up and down each row, removing 0.002"
with each pass, except for the final pass when only 0.001" was removed
(0.013" removed in all) until all contact surfaces were on the same
plane with respect to the
z-axis. As a final touch, the copper contacts were removed one at a
the board to a Logan lathe to have residual burrs removed and the
contact surfaces made
Figs. D11 and D12.
plate contacts undergoing end milling process on the Haas mill (left).
Base plate after finishing work on the Logan lathe. Brass hex nuts
underneath the board hold the contacts to the board.
After end milling, the copper
contacts were of varying heights with respect to each other, owing to
slight variance in the thickness of the acrylic base plate. As a
precaution, each contact was chemically etched with its own
identification number so that in the event more than one contact
was separated from the base plate at the same time they could be
restored to their original positions, thereby maintaining an even
contact surface across the base plate.
Fig. D13. Bottom of copper
contact chemically etched with a unique identification number.
hex-head cap screws, 2-1/2"
by 3/8", were used to secure each of the shelves. The shelves are 3/4"
thick MDF. Sharp corners were rounded using a router with a 3/16"
radius corner bit.
A decision was
changer directly to the transformer through the top shelf. To
that end a 5" by 15-1/4" opening was made
in the top shelf to accommodate the wires for
all thirty-three contacts. The hole is centered 23-1/2 inches from the
leading edge of the table (toward the back of the top shelf) to create
a workspace around the
front of the tap changer.
Figs. E1 and E2.
supporting platforms on the cart (left), and rectangular opening
cutting the opening
through the top shelf the next
was to construct the support structure for the LRS and carriage. Two
pieces of acrylic, each approximately 11/16" thick, 6" wide, and 5-1/2"
the main support structure. Two more acrylic pieces, each
1-1/4" tall, cap off these
structures and form a clamp on either end of the
reinforcement pieces (two per end) will be
fitted to each end piece,
completing the support structure for the tap changer.
Figs. E3 and E4.
Base plate (left) positioned
over hole, and partially-completed
support system holding up the LRS and carriage.
Figs. E5 and E6. Two views of
tap changer (still under construction) as of March 27, 2012.
E7. A second rectangular
opening, 10" by 1", was cut and centered behind the tap changer to
accommodate the wires coming out of the top of the carriage. A wire
leading from each of the three resistor bridges passes down through
this aperture and connects with the exciter transformer below. A 1"
dia. hole was drilled in cart top near the stepper motor to accommodate
the wires between the motor and the controls.
E8 and E9. Left: finishing touches on the cart included
installing 1/8" thick clear acrylic panels to the sides and front and
adding a hinged door to the back. Right: after coats of wood stain and
polyurethane seal the cart is ready to have the transformers and tap
re-installed for display at Expo.
for testing required wiring the transformers to
the new tap changer. To minimize "wire clutter," the wires leading
to the exciter transformer were trimmed and new ring terminals fitted
to the trimmed ends. Also a linkage connecting
the windings on the
transformer was replaced with a solid bar, for use when the transformer
isn't connected to the tap changer. Finally, four 1/2" holes were
drilled in the bottom plank so that the transformers could be securely
bolted to the cart.
F1 (TL), F2 (TR), F3 (BL), and F4 (BR). Top
connecting linkage atop the exciter transformer. Top right: drilling
the replacement bar. Bottom left: comparisons
between old and new connecting pieces. Bottom right: new bar
place on the exciter
transformer. The dimensions of the bar are 14" by 1" by 1/3".
Figs. F5 and F6.
Wires leading to the partially-wired base plate as seen from below the
base plate (left)
and above the base plate looking down.
Figs. F7 and F8.
Transformer wiring after wires had been trimmed.
the Tap Changer
finishing touches were being applied to various components, testing
began on the mechanical and electrical properties of the
partially-integrated tap changer.
Figs. G1 and G2. April 5,
2012: testing of the
partially-assembled tap changer using a digital multi meter (DMM).
10, 2012, all of the components of the tap changer were fully
integrated in preparation for a test of the complete system. The tap
changer was initially tested for its mechanical capabilities in the
Gauss Johnson Senior Design Suite. This test consisted of using the two
control buttons to repeatedly move the tap changer back and forth
across the contacts of the base plate. Adjustments were made until the
team was satisfied as to the tap changer's mechanical capabilities.
Click on the link below to see a demonstration of the tap changer's
Later that afternoon the tap changer was moved to the University of
Idaho's Power Laboratory in the same building for the testing of its
electrical phase-changing capabilities. SEL engineer Doug Taylor was
present to supervise and direct this test and to oversee the proper
wiring of the transformers.
Figs. G3 and G4.
April 10, 2012:
wiring and power connections for testing the tap changer in the
University of Idaho's power laboratory.
Fig. G5. Another angle of test
setup. The power analyzer is in the lower left quadrant of the picture.
41B Power Harmonics Analyzer was
used to test the output voltage on one line during each successive tap
change. The testing started at the 230 V tap and moved across the board
in successive steps all the way to the 23 V tap, then back
under a 1.8 amp load current. The power analyzer measured the voltage
added in the series transformer between the input (source) side and the
output side. Results were called out during the test
from visual reading of the analyzer and recorded by hand
contemporaneous with the testing.
Click on the links below to view the power lab tests:
Test Part One
Test Part Two
Test Part Three
The test results, showing
approximately 25 volts at maximum phase shift (25 V corresponding to 10
degrees of phase change), appeared to demonstrate the phase-changing
capability of the tap changer. Also, the results were consistent in
both directions of travel.
Fig. G6. Results from first
of tap changer.
On April 20, 2012, SEL engineer Doug Taylor performed a second test on
the tap changer in the UI Power Laboratory. This time the measuring
instrument was an SEL 487-E
Station Phasor Measurement Unit. The tap changer was having difficulty
from tap to tap, a problem which was remedied in time for Expo. Because
of this latter problem only a few of the taps rendered useful
data on this second electrical test. However, the data
collected showed a voltage phase shift as well as a smooth
current transition when moving
the seventh to eighth tap windings, providing further evidence that the
tap changer works as intended.
Figs. G7 and G8. Relay used to
gather data (left). Connections between relay and the transformers
beneath the tap changer.
Figs. G9 and G10. April 20 test
results showing phase shift (left) and smooth current
transition (right) (test results courtesy of SEL).
Day (March 6,
Figs. H1,H2, and H3.
Carriage and LRS mated to
mock up of
support structure (left), front view, and side view (right).
Expo (April 27, 2012)
Figs. I1, and
I2. PSTTC booth at the University of Idaho
Engineering Expo in the SUB Ballroom.
Figs. I3 and I4.
Two more views of Expo display.
Side view of tap changer.