Universal Motor Speed Control in Sewing Machines

The speed of a universal motor, commonly found in sewing machines, is adjusted by altering the electrical conditions that affect its operational speed. The primary method used for this purpose is:

Varying the Resistance

Adding or changing resistance in the motor's circuit directly impacts the voltage supplied to the motor windings and the current flowing through them. This manipulation affects the motor's speed.

• Mechanism: A variable resistor (rheostat) is typically connected in series with the universal motor.
• Effect: By increasing the resistance, the voltage drop across the resistor increases, reducing the voltage across the motor terminals. This lower voltage results in a slower motor speed. Conversely, decreasing the resistance allows more voltage to reach the motor, increasing its speed.
• Application: This method provides a straightforward way to achieve variable speed control suitable for sewing machine applications.

Other Options Analysis

• Friction: Friction is a force opposing motion and is not a method for controlling the electrical speed of a motor.
• Tapping the Field: While changing the number of field turns (field tapping) can alter motor speed by changing magnetic flux, it's a less common and less precise method for the variable speed control needed in sewing machines compared to resistance variation.
• Centrifugal Mechanism: Centrifugal mechanisms are usually employed for speed limitation or starting/stopping switches (like in ceiling fans) rather than continuous speed control.

Therefore, varying the resistance is the standard method for controlling the speed of a universal motor in sewing machines.