Closed Loop Field Current Control - Phase Controlled Rectifiers and Bridge Inverters

TL;DR

This video explains how to control the speed of a separately excited DC motor by varying the field current, and the relationship between field current, speed, torque, and power.

Transcript

Click the bell icon to get latest videos from Ekeeda Hello friends in this video we are going to see the second part of a separately excited DC motor that is the constant horsepower region or you can say failed current control how you can vary fill current to control the speed of the motor that we are going to see in this video now this diagram if ... Read More

Key Insights

• 🧑‍🦼 Varying the field current in a separately excited DC motor controls the speed by inversely affecting torque.
• 🎮 The closed-loop field current control system uses a comparator, speed controller, and alpha control unit to adjust the firing angle of the thyristor and change the field current.
• 🐎 Increasing the field current decreases the speed, while decreasing the field current increases the speed.

Questions & Answers

Q: How does varying the field current affect the speed of a separately excited DC motor?

Varying the field current in the constant horsepower region of a separately excited DC motor inversely affects the speed. When the field current decreases, the speed increases due to the decrease in torque.

Q: How is the speed of the motor controlled in the closed-loop field current control system?

The speed of the motor is controlled by comparing the reference speed (gradually accelerated by the ramp unit) with the actual speed measured by the tacho generator. If there is a difference, the speed controller adjusts the firing angle of the thyristor to vary the field current, consequently changing the motor speed.

Q: When should the field current be increased or decreased in the closed-loop field current control system?

If the actual speed is higher than the required speed, the field current needs to be increased to decrease the speed. On the other hand, if the actual speed is lower than the required speed, the field current should be decreased to increase the speed.

Q: In what situations would closed-loop field current control be used?

Closed-loop field current control is specifically used when the speed of a separately excited DC motor needs to be increased beyond the base speed, as increasing the armature voltage is not sufficient.

Summary & Key Takeaways

• The video demonstrates the constant horsepower region of a separately excited DC motor, where the field current is varied to control the motor speed.

• Unlike closed-loop armature voltage control, in this region, the armature voltage is kept constant, and the field current is changed.

• By decreasing the field current, the speed of the motor increases and the torque decreases, keeping the power constant.