The Impact of Reversed Supply Voltage on the Direction of Rotation of a DC Shunt Motor
Understanding the Effects of Reversed Supply Voltage
In the field of electrical machinery, particularly with DC shunt motors, it is important to understand how the reversal of supply voltage impacts their operation. When the supply voltage is reversed, the effects on the motor’s direction of rotation can vary based on the type of field winding used (shunt, series, or compound).
Firstly, if a DC shunt motor is operated as a generator and the supply voltage is reversed, it will not function as a generator until the field connections are also reversed. This ensures that the magnetic field continues to support the generator operation.
Complexities with Compound and Differentially Compounded Generators
Compound and Differentially Compounded Generators: These types of generators require special attention. For a compound generator, if you do not reverse the series field connection, the generator will operate differentially compounded instead of cumulatively compounded. To change the electrical polarity, the field must be re-flashed.
Severe Overloading: Severe overloading of a differentially compounded shunt generator can cause heavy current in the series field to reverse the magnetic polarity of the field. This results in a polarity flip of the generator, which may require re-flashing the field to restore the original polarity.
Exploring the Reasons for Reversed Motor Direction
Motors do not inherently need to reverse direction when the supply voltage is reversed. However, reversing the supply voltage can affect the generator output. All modern equipment is designed and tested to handle such situations, but for generators, contact the manufacturer for specific details regarding the rotation direction.
How Residual Field Affects the Motor
A key component in starting a DC shunt generator is the residual field, which results from previous magnetization of the field winding. This initial small field is crucial to initiate the self-excited process. If the field winding is not reversed, the residual field will not immediately change, but the output voltage polarity will. A small reversed voltage will be produced, which will cause the field winding current to oppose the original field current, leading to demagnetization and loss of output voltage.
To address this issue, a small external DC current can be manually applied to the field winding. This ensures the field remains magnetized in the correct direction, maintaining generator operation.
References: Chapman, S. J. (2011). Electric Machinery Fundamentals - 5th Edition. Chapter 8