Ch 3 - MyWeb at WIT

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Transcript Ch 3 - MyWeb at WIT

Elec467 Power Machines & Transformers Electric Machines by Hubert, Chapter 3 Topics: Polarity, Transformer types including Auto, Buck-Boost, Instrument, and Three Phase

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  Visit

to see a tutorial on entry of nameplate data in a specialized power transformer software.

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 I 2 . 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 E A 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 response. 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 delta.

Common 3-phase equations    Wye connections:   V line I line = √3 V = I phase phase Delta connections:   V line I line = V phase = √3 I phase S = √3 V line I line

Phasor diagram of delta-delta

Physical arrangement of 3 Phase Figure 3-14 (a) shell type, (b) core type

Instrument transformers