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8.1 Introduction

Question How do we connect loads to a three-phase system? We connect three-phase loads to a three-phase system in either a delta or star configuration. We connect single phase loads between a phase and neutral or between different phases. In most cases, these loads are three-phase balanced loads. This includes three-phase motors and furnaces.

Single-phase loads include lamps and various types of single-phase receivers. In this situation, it is better to split the single-phase loads between the phases to avoid an unbalanced system. In each phase, an imbalance might result in destabilizing currents that would affect the system's stability.

Next, we look at the three-phase loads that connects in either both star and delta configurations. We also verify that, for balanced loads the power output for these connections is always similar. We express the power of a three-phase system as:

$P = 3 \cdot U_{C} \cdot I_{L} \cdot cos φ (8.1)$

where P is the active power of the three-phase load, $U_{C}$ is the composite voltage, $I_{L}$ is the line current and cosφ is the power factor of the load.

练习题

What are the two possible configurations for connecting three-phase loads to a three-phase system?

A. Series and parallel configurations

B. Delta and star configurations

C. Wye and zigzag configurations

D. Primary and secondary configurations

In the three-phase active power formula $P = 3 \cdot U_{C} \cdot I_{L} \cdot cos φ$ , what does $I_{L}$ represent?

A. The phase current

B. The composite voltage

C. The line current

D. The power factor

Which of the following are valid ways to connect single-phase loads in a three-phase system?

A. Between a phase and neutral

B. Between different phases

C. In a delta configuration only

D. In a star configuration only

Three-phase motors and furnaces are examples of three-phase balanced loads.

Phase imbalance in a three-phase system can result in destabilizing currents that affect system stability.

For balanced loads, the power output is different for star and delta configurations.

The active power of a three-phase system is expressed as $P = 3 \cdot U_{C} \cdot I_{L} \cdot cos φ$ , where $cos φ$ represents the ___ of the load.

To avoid an unbalanced system, single-phase loads should be ___ between the phases.

Why is it important to distribute single-phase loads evenly across the phases of a three-phase system?

A three-phase motor is connected in star configuration with line voltage $V_{L} = 400 V$ and line current $I_{L} = 10 A$ . If the power factor is $cos φ = 0.8$ , what is the relationship between line current and phase current in this star connection?

A. Line current equals phase current (

I_{L} = I_{P}

)

B. Line current is

3

times phase current

C. Line voltage equals phase voltage

D. Line current is

3

times line voltage

A three-phase transformer has its secondary winding connected in star configuration with an accessible neutral. If a three-phase balanced motor is connected to this secondary, which statement correctly describes the relationship between the transformer secondary and the load configuration?

A. The motor must be connected in delta configuration since the transformer secondary is in star

B. The motor can be connected in either star or delta configuration, and the power output will be similar for balanced loads

C. The motor must be connected in star configuration to match the transformer secondary

D. The transformer secondary configuration determines the power factor of the motor load

In a star-connected three-phase load, the line current equals the phase current ( $I_{L} = I_{P}$ ), while in a delta-connected load, the line voltage equals the phase voltage ( $V_{L} = V_{P}$ ).

Which of the following statements correctly describe the relationship between three-phase transformer connections and load connections?

A. Both transformer windings and three-phase loads can be connected in star or delta configurations

B. A star-connected secondary provides a neutral point that can be used for connecting single-phase loads

C. Delta connections are commonly used for motors because they equalize the voltage across each phase

D. The power factor of a balanced load depends on whether it is connected in star or delta

E. Single-phase loads should be distributed evenly across phases to avoid system imbalance

In the three-phase active power formula $P = 3 \cdot U_{C} \cdot I_{L} \cdot cos φ$ , the variable $I_{L}$ represents the ___ current, which equals the phase current in a star connection but is $3$ times the phase current in a delta connection.

A Dyn transformer has its primary connected in delta and secondary in star with accessible neutral. Explain how this configuration supports both three-phase balanced loads and single-phase loads.

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8.1 Introduction

8.1 Introduction

练习题

What are the two possible configurations for connecting three-phase loads to a three-phase system?

In the three-phase active power formula P=3⋅UC​⋅IL​⋅cosφ, what does IL​ represent?

Which of the following are valid ways to connect single-phase loads in a three-phase system?

Three-phase motors and furnaces are examples of three-phase balanced loads.

Phase imbalance in a three-phase system can result in destabilizing currents that affect system stability.

For balanced loads, the power output is different for star and delta configurations.

The active power of a three-phase system is expressed as P=3⋅UC​⋅IL​⋅cosφ, where cosφ represents the ___ of the load.

To avoid an unbalanced system, single-phase loads should be ___ between the phases.

Why is it important to distribute single-phase loads evenly across the phases of a three-phase system?

A three-phase motor is connected in star configuration with line voltage VL​=400V and line current IL​=10A. If the power factor is cosφ=0.8, what is the relationship between line current and phase current in this star connection?

A three-phase transformer has its secondary winding connected in star configuration with an accessible neutral. If a three-phase balanced motor is connected to this secondary, which statement correctly describes the relationship between the transformer secondary and the load configuration?

In a star-connected three-phase load, the line current equals the phase current (IL​=IP​), while in a delta-connected load, the line voltage equals the phase voltage (VL​=VP​).

Which of the following statements correctly describe the relationship between three-phase transformer connections and load connections?

In the three-phase active power formula P=3⋅UC​⋅IL​⋅cosφ, the variable IL​ represents the ___ current, which equals the phase current in a star connection but is 3​ times the phase current in a delta connection.

A Dyn transformer has its primary connected in delta and secondary in star with accessible neutral. Explain how this configuration supports both three-phase balanced loads and single-phase loads.

In the three-phase active power formula $P = 3 \cdot U_{C} \cdot I_{L} \cdot cos φ$ , what does $I_{L}$ represent?

The active power of a three-phase system is expressed as $P = 3 \cdot U_{C} \cdot I_{L} \cdot cos φ$ , where $cos φ$ represents the ___ of the load.

A three-phase motor is connected in star configuration with line voltage $V_{L} = 400 V$ and line current $I_{L} = 10 A$ . If the power factor is $cos φ = 0.8$ , what is the relationship between line current and phase current in this star connection?

In a star-connected three-phase load, the line current equals the phase current ( $I_{L} = I_{P}$ ), while in a delta-connected load, the line voltage equals the phase voltage ( $V_{L} = V_{P}$ ).

In the three-phase active power formula $P = 3 \cdot U_{C} \cdot I_{L} \cdot cos φ$ , the variable $I_{L}$ represents the ___ current, which equals the phase current in a star connection but is $3$ times the phase current in a delta connection.