Why field rheostat is kept in minimum position while armature rheostat at maximum position?
2 Answers
The reason why the field rheostat is kept in the minimum position while the armature rheostat is at the maximum position during the starting of a DC motor is based on ensuring proper control over the current, particularly to avoid excessive inrush currents and to achieve smooth startup.
### 1. **Field Rheostat at Minimum Position:**
- **Purpose: To produce maximum field flux.**
- In a DC motor, the field rheostat controls the strength of the magnetic field by varying the current through the field windings. When the field rheostat is in the minimum position, it offers very little resistance in the field circuit, allowing maximum current to flow through the field winding.
- **Stronger Magnetic Field:** This results in a strong magnetic field, which is necessary during startup because it ensures higher back EMF. The back electromotive force (EMF) opposes the applied voltage and limits the current flowing through the armature. Without a strong magnetic field, the back EMF would be low, and the armature current could increase to dangerous levels.
### 2. **Armature Rheostat at Maximum Position:**
- **Purpose: To limit the armature current.**
- At the time of starting, the motor is at rest, meaning that the armature is not moving. Without rotation, there is no back EMF generated, as back EMF is proportional to the speed of the motor.
- **Controlling High Inrush Current:** If the full supply voltage is applied across the armature at the time of startup (when back EMF is zero), the armature current can be extremely high due to the low resistance of the armature winding (which is typically very small). This excessive current could damage the motor.
- By setting the armature rheostat to its maximum position, additional resistance is added to the armature circuit, limiting the current drawn from the supply. As the motor accelerates and starts to generate back EMF, this resistance is gradually reduced to allow normal operation.
### Summary:
- **Field Rheostat (minimum position)**: Ensures maximum magnetic field strength, leading to higher back EMF to control current during startup.
- **Armature Rheostat (maximum position)**: Limits the starting current to prevent excessive inrush that could damage the armature windings. As the motor gains speed, this rheostat is adjusted to reduce resistance and allow more current for normal operation.
This method provides a controlled and safe way to start a DC motor.
### 1. **Field Rheostat at Minimum Position:**
- **Purpose: To produce maximum field flux.**
- In a DC motor, the field rheostat controls the strength of the magnetic field by varying the current through the field windings. When the field rheostat is in the minimum position, it offers very little resistance in the field circuit, allowing maximum current to flow through the field winding.
- **Stronger Magnetic Field:** This results in a strong magnetic field, which is necessary during startup because it ensures higher back EMF. The back electromotive force (EMF) opposes the applied voltage and limits the current flowing through the armature. Without a strong magnetic field, the back EMF would be low, and the armature current could increase to dangerous levels.
### 2. **Armature Rheostat at Maximum Position:**
- **Purpose: To limit the armature current.**
- At the time of starting, the motor is at rest, meaning that the armature is not moving. Without rotation, there is no back EMF generated, as back EMF is proportional to the speed of the motor.
- **Controlling High Inrush Current:** If the full supply voltage is applied across the armature at the time of startup (when back EMF is zero), the armature current can be extremely high due to the low resistance of the armature winding (which is typically very small). This excessive current could damage the motor.
- By setting the armature rheostat to its maximum position, additional resistance is added to the armature circuit, limiting the current drawn from the supply. As the motor accelerates and starts to generate back EMF, this resistance is gradually reduced to allow normal operation.
### Summary:
- **Field Rheostat (minimum position)**: Ensures maximum magnetic field strength, leading to higher back EMF to control current during startup.
- **Armature Rheostat (maximum position)**: Limits the starting current to prevent excessive inrush that could damage the armature windings. As the motor gains speed, this rheostat is adjusted to reduce resistance and allow more current for normal operation.
This method provides a controlled and safe way to start a DC motor.