What causes the voltage to drop when there is an increase in load on electrical machines such as transformers and motors?

When the load on motors such as transformers and motors increases, the voltage drop (voltage drop) is usually due to several reasons:

Line resistance

Reason

• Increased current: As the load increases, the current flowing through the power line increases.

• Ohm's Law: According to Ohm's Law (V=IR), an increase in current leads to an increase in voltage drop. here

• V is the voltage drop,

• I is for current,

• R is the resistance of the wire

Explain

Because there is a certain resistance in the power line, when the current passes through the wire, it will produce a voltage drop. This voltage drop is proportional to the current and is proportional to the resistance of the wire.

In the case of increased load, the current increases, resulting in an increase in voltage drop, which decreases the voltage at the load end.

Transformer internal resistance

Reason

Transformer internal resistance: The transformer itself has a certain internal resistance (including winding resistance and leakage reactance), when the load increases, the current flowing through the transformer increases, resulting in an increase in the voltage drop at both ends of the transformer.

Explain

The internal resistance of the transformer will cause a voltage drop, especially in the case of heavy loads, this voltage drop will be more obvious.When the load increases, the transformer needs to transfer more current, and the internal resistance of the transformer causes a voltage drop, which reduces the voltage at the load end.

Motor starting

Reason

• Starting current: The motor consumes a large amount of current at the moment of starting, which is called starting current.

• Starting current causes voltage drop: The starting current is much larger than the current when the motor is running normally, so the voltage drop is more significant at the moment of starting.

Explain

When the motor is started, because the torque needs to overcome the static friction force, it needs a large starting current.

This larger starting current creates a larger voltage drop in the power lines and transformers, which causes the voltage to drop.

System stability

Reason

• Insufficient system capacity: If the total system capacity is insufficient to handle a sudden increase in load, the voltage will drop.

• Insufficient regulation capacity: If the system lacks sufficient regulation capacity to maintain voltage stability, the voltage will drop as the load increases.

Explain

In a grid system, if the total capacity is not enough to support the simultaneous operation of all the loads, the system will not be able to provide sufficient voltage when the load increases.

In addition, if the regulation capacity of the system is insufficient, such as there is not enough reactive power compensation device, the voltage regulation capacity is limited, and the voltage will decrease when the load increases.

Reactive power

Reason

• Increased reactive power demand: When the load increases, especially the induction motor load, the reactive power demand also increases.

• Reactive power causes voltage drop: Reactive power also causes voltage drop during transmission.

Explain

Devices such as induction motors require reactive power to establish magnetic fields when operating, which leads to increased reactive power requirements in the system.

Reactive power will also produce voltage drop during transmission, especially in the case of insufficient reactive power compensation in the grid, the voltage drop will be more obvious.

System design

Reason

• Unreasonable design: If the system is not designed to fully consider the increase in load, it may lead to voltage drop.

• Improper equipment selection: If the selected equipment (such as transformers, wires, etc.) capacity is insufficient, the voltage will drop when the load increases.

Explain

When designing electrical systems, you need to take into account the maximum load conditions that can occur, and ensure that the system has sufficient capacity and margin to cope with the increase in load.

If the equipment is not properly selected, such as the cross section of the wire is too small or the transformer capacity is insufficient, the voltage drop will be caused when the load is increased.

Sum up

When the load of motors such as transformers and motors increases, the voltage drop is mainly due to the combination of factors such as line resistance, transformer internal resistance, motor starting current, insufficient system capacity, increased reactive power demand, and unreasonable system design. In order to reduce the influence of voltage drop, we can take measures such as increasing the cross section of conductor, selecting transformer with appropriate capacity, rational design system and strengthening reactive power compensation.