Unit 26: 3-Phase Circuits

How much greater is the hp rating of 3-phase motors and the kVA rating of 3-phase transformers than those of single-phase with similar frame size?

150%

In a wye connection, phase current and line current are ______________.

Equal I(Line) = I(Phase)

In a balanced 3-phase system, the conductors need to be about _______ the size of conductors for a single-phase, 2-wire system of the same kVA rating.

75%

How many times does power fall to 0 during a 3-phase cycle?

0

In a delta connection, during some period of time, current will flow through ______________ lines only, but other times, current will flow from 2 lines to the 3rd.

2 There is always more than 1 path for current to flow.

Phase voltage

Voltage measured across single winding, or phase

In a delta connection, what must be used to find the sum of the currents? Why?

Vector addition because the phases are 120º out of phase with each other.

Delta connection

- 3 separate inductive loads. - 3 hots

In a wye connection, line voltage is higher than phase voltage by a factor of ______________.

1.732 E(Line) = E(Phase) x 1.732

In a delta connection, line current is higher than phase current by a factor of ______________.

1.732 I(Line) = I(Phase) x 1.732

In a wye connection, the vector sum of the 3 voltages is ______________.

208V

Wye connection

A method of connecting the windings on a 3-phase transformer in which one end of all three windings is connected to a common neutral center point. - 3 hot, 1 neutral - "Star connection"

Line and phase voltages are ______________ in a wye connection.

Different

In a delta connection, line voltage and phase voltage are ______________.

Equal E(Line) = E(Phase)

How is 3-phase voltage produced?

Three separate coils are placed 120° apart and 3 voltages 120° out of phase with each other are then produced when the magnetic field produced cuts through the 3 coils.

Is the power delivered to a load the same at any instant when being supplied by 3-phase power?

Yes