Starting torque and fully load torque are two important characteristics of electric motors. The starting torque is the torque produced by the motor when it is first started, while the fully load torque is the torque produced when the motor is operating at full load. The ratio of starting torque to fully load torque is an indicator of the motor's ability to deliver high torque at startup compared to its performance under full load conditions.

In the case of shunt motors, the ratio of starting torque to fully load torque is least compared to other types of motors such as series motors, cumulatively compounded, and differentially compounded motors. This means that shunt motors have the least ability to deliver high torque at startup relative to their performance under full load conditions.

There are several factors that contribute to this characteristic of shunt motors:

1. Field Winding Configuration:
- In shunt motors, the field winding is connected in parallel with the armature winding, allowing the field current to remain relatively constant during operation.
- This configuration results in a relatively constant magnetic field strength, which limits the ability of the motor to produce high starting torque.

2. Flux Density:
- The flux density in the air gap of a shunt motor is relatively constant, as the field current remains constant.
- This limits the ability of the motor to generate high starting torque, as the torque produced is directly proportional to the product of the magnetic flux and armature current.

3. Armature Reaction:
- In shunt motors, the armature reaction is relatively weak compared to other types of motors.
- This weak armature reaction results in a smaller distortion of the magnetic field, which further limits the motor's ability to produce high starting torque.

4. Speed Regulation:
- Shunt motors have better speed regulation compared to series motors and compounded motors.
- This means that shunt motors are designed to maintain a relatively constant speed under varying load conditions, rather than providing high starting torque.

In conclusion, the ratio of starting torque to fully load torque is least in the case of shunt motors due to their field winding configuration, relatively constant flux density, weak armature reaction, and emphasis on speed regulation rather than high starting torque.