The correct answer is D. All of the above methods.
A universal motor is a type of electric motor that can run on both AC and DC power. It is a very versatile motor and is used in a variety of applications, including vacuum cleaners, power tools, and food mixers.
The speed of a universal motor can be controlled by varying the field flux, connecting a rheostat in series, or applying variable voltage by means of a silicon controlled rectifier (SCR).
Varying the field flux is the most common method of controlling the speed of a universal motor. This is done by using a tapped field winding. The tapped field winding allows the amount of current flowing through the field winding to be varied. This, in turn, varies the strength of the magnetic field produced by the field winding. The stronger the magnetic field, the faster the motor will run.
Connecting a rheostat in series with the motor is another way to control the speed of a universal motor. The rheostat is a variable resistor that can be used to control the amount of current flowing through the motor. The more current that flows through the motor, the faster it will run.
Applying variable voltage by means of an SCR is the most efficient way to control the speed of a universal motor. The SCR is a semiconductor device that can be used to control the amount of voltage that is applied to the motor. The less voltage that is applied to the motor, the slower it will run.
All of the above methods can be used to control the speed of a universal motor. The best method to use will depend on the specific application.