Ch 3 - MyWeb at WIT

Elec467 Power Machines &
Electric Machines by Hubert, Chapter 3
Topics: Polarity, Transformer types including
Auto, Buck-Boost, Instrument, and ThreePhase
Transformer Polarity Marks
Nameplate information
XXX kVA is the apparent power
 Voltage ratings for high and low side are no-load values.
The symbol between the values indicate how the
voltages are related:
 Long dash (—) …from different windings
 Slant (/) …from same winding
 240/120 is a 240 V winding with a center tap
Cross (X) …connect windings in series or parallel. Not
used in wye-connected winding.
 240X120 two part winding connected in series for 240 or
parallel for 120.
Wye (Y) …a wye-connect winding
Nameplate data entry
to see a tutorial on entry of nameplate
data in a specialized power transformer
Autotransformer types
More autotransformers
Variac autotransformer
Our handy-dandy dial up a voltage machine.
How to make an autotransformer
from an ordinary transformer
A short is placed between the positive side of the primary to the negative side of the
secondary. Because the currents generated by the regular transformer configuration will
still occur; this short completes the current path for I2. The voltage source still feeds the
positive terminal on the primary side thus keeping the current flow through the primary
loop the same. The current flow through the secondary loop remains the same but both
currents flow in the primary in opposing direction; thus canceling the counter emf voltage
and the mutual flux created by the source in the primary coil.
Example 3.2 current flow
A dirty secret about motors
When starting a motor, if the torque is
not enough to start the motor, it will burn
out. This low torque problem is caused
by low voltages.
 This problem is solved using a specially
designed Buck-Boost transformer that
adds or subtracts ≈ 10% to the line
voltage (aka utilization voltage).
Buck-Boost wiring diagram
Parallel Operation
In (a) for transformer that are wired in parallel but not matched, a
circulating current will develop who direction is given by the stronger
of the two transformers. Obviously the current generated from the EA
induced voltage dominates and the current flows as seen in the
dashed lines. In (b) when the load switch is closed, the same current
exists and either adds to one coils current or substracts.
Transformer in-rush current
When a load switch is closed, there is a current released that has a
transient response (also called in-rush current) and a steady-state
The magnitude of the transient response dies off over a few cycles to the
steady state level.
The initial magnitude depends on the magnitude and phase angle of the
voltage wave at the instant the switch is closed and the magnitude and
direction of the residual flux.
If there is not residual magnetism and the voltage wave is at maximum
value at the close of the switch then the current will be limited to the
rated current.
If the voltage wave is at or near 0 Volts and the buildup of current is
additive with the residual flux causing magnetic saturation of the iron
(reducing counter emf) this permits a very high in-rush current.
Inductive load increase the in-rush while resistive and capacitive loads
decrease the in-rush.
Exciting current harmonics
Connections for 3-Phase
Various methods of connecting three-phase lines use banks of
transformers to be connected in either wye, delta, delta-wye,
wye-delta configurations. The bottom configuration is an open
Common 3-phase equations
Wye connections:
 Vline = √3 Vphase
 Iline = Iphase
Delta connections:
 Vline = Vphase
 Iline = √3 Iphase
S = √3 Vline Iline
Phasor diagram of delta-delta
Physical arrangement of 3 Phase
Figure 3-14 (a) shell type, (b) core type
Instrument transformers

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