DC series motor at no load condition gives infinite speed ideally. Practically it will damage all the armature circuit. Thus, as the load is reduced speed of the motor will go on increasing rapidly. So, speed control is very poor in series motor.

The correct answer is a series motor.

Explanation:

A series motor is a type of DC motor where the field winding and armature winding are connected in series. This means that the current flowing through the field winding is the same as the current flowing through the armature winding.

Speed control in a DC motor is typically achieved by varying the field current or the armature voltage. However, in a series motor, changing the field current also changes the armature current, which can lead to poor speed control.

Here are the reasons why a series motor has the poorest speed control:

1. Speed-Torque Characteristics:
- A series motor has a non-linear speed-torque characteristic. As the load on the motor increases, the speed decreases.
- This means that even a small change in load can cause a significant change in speed.
- The speed-torque characteristic of a series motor makes it difficult to maintain a constant speed under varying load conditions.

2. High Starting Torque:
- A series motor has a high starting torque, which means it can accelerate heavy loads quickly.
- However, this high starting torque also leads to poor speed control.
- When the load is suddenly reduced, the motor can speed up rapidly, potentially causing damage.

3. Lack of Speed Regulation:
- Due to the non-linear speed-torque characteristic, a series motor lacks speed regulation.
- Speed regulation refers to the ability of a motor to maintain a constant speed regardless of load variations.
- In a series motor, the speed is highly dependent on the load, and small changes in load can cause significant changes in speed.

4. Risk of Overspeeding:
- A series motor is prone to overspeeding if the load is suddenly removed.
- Since the speed is inversely proportional to the torque, removing the load can cause the speed to increase excessively.
- This overspeeding can lead to mechanical failure or damage to the motor.

In summary, a series motor has the poorest speed control due to its non-linear speed-torque characteristics, lack of speed regulation, high starting torque, and the risk of overspeeding.

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