In a motor speed control using TRIAC and UJT, the potentiometer controls:

Explanation:
The correct answer is: 3) the frequency of the pulses from UJT.

In a motor speed control circuit that uses a TRIAC (Triode for Alternating Current) and UJT (Unijunction Transistor), the potentiometer is typically used to control the firing angle of the TRIAC by adjusting the timing of the pulses generated by the UJT. Here's how it works:

1. UJT Pulse Generation:

The UJT is used as a relaxation oscillator, generating pulses at a certain frequency. The frequency of these pulses is determined by the characteristics of the UJT and the RC timing components in the circuit.

2. Firing Angle Control:

The pulses generated by the UJT are used to trigger the TRIAC. By adjusting the resistance of the potentiometer, you can change the timing of the pulses. When the pulses trigger the TRIAC, it conducts and allows current to flow to the motor.

3. Speed Control:

By altering the timing of when the TRIAC is triggered, you effectively control the phase angle at which the TRIAC conducts. This, in turn, controls the amount of power delivered to the motor, which affects its speed. The greater the delay in triggering the TRIAC, the lower the average voltage supplied to the motor, and thus the slower the motor will run.

So, the potentiometer adjusts the timing or frequency of the pulses generated by the UJT, which, in turn, controls the firing angle of the TRIAC and consequently the speed of the motor. It does not directly control the thyristor gate current or the gate voltage; rather, it indirectly influences these parameters through its impact on pulse timing.