# Open circuit armature voltage of dc machine?

• theone
In summary, open circuit armature voltage refers to the voltage measured across the armature terminals of a DC machine when it is not connected to any external load. It can be calculated using the formula V = E - IaRa and is important in determining the speed of the machine and detecting any faults. Factors that can affect it include the strength of the magnetic field and the condition of the machine's components, and it can be controlled by adjusting the field current.
theone

## Homework Statement

The book says that "the induced voltage E_a can be found by measuring the open circuit voltage at the armature terminals"

## The Attempt at a Solution

But isn't there a voltage drop across the resistor so that they can't be equal? or is this ignored

#### Attachments

• question2.png
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There will only be a voltage drop over the resistor if current
is drawn from the circuit. The voltmeter draws very little current
so no voltage is lost over the resistor and the full induced voltage
is measured. If the current is zero then the IR product is zero.

NascentOxygen

## 1. What is open circuit armature voltage?

Open circuit armature voltage refers to the voltage measured across the armature terminals of a DC machine when it is not connected to any external load. In other words, it is the voltage generated by the armature winding when there is no current flowing through it.

## 2. How is open circuit armature voltage calculated?

The open circuit armature voltage can be calculated using the formula V = E - IaRa, where V is the open circuit voltage, E is the generated voltage, Ia is the armature current, and Ra is the armature resistance. This formula takes into account the voltage drop caused by the armature resistance.

## 3. Why is open circuit armature voltage important?

Open circuit armature voltage is an important parameter in DC machines as it directly affects the speed of the machine. A higher open circuit voltage means a higher generated voltage, which in turn leads to a higher speed. It is also used to determine the condition of the armature winding and to detect any faults or abnormalities in the machine.

## 4. What factors can affect the open circuit armature voltage?

The open circuit armature voltage can be affected by various factors such as the strength of the magnetic field, the speed of the machine, the number of turns in the armature winding, and the condition of the brushes and commutator. Any changes in these factors can lead to a change in the open circuit voltage.

## 5. How can the open circuit armature voltage be controlled?

The open circuit armature voltage can be controlled by adjusting the field current in the DC machine. By increasing or decreasing the field current, the strength of the magnetic field and therefore the generated voltage can be changed. This can be done manually or through automatic voltage regulation techniques.

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