The squirrel cage motor is the only one without any connection to the armature.

Not Suitable DC Machine

Introduction:
DC machines are electrical machines that convert direct current electrical energy into mechanical energy. They are widely used in various applications like electric motors, generators, and battery chargers. Among various types of DC machines, some are suitable for DC machines, while others are not. In this question, we need to identify the motor that is not suitable for use as a DC machine.

Squirrel Cage Motor:
The squirrel cage motor is a type of AC induction motor that is not suitable for use as a DC machine. The reason behind this is that the squirrel cage motor is designed to work on AC supply, and it has no commutator and brushes. The commutator and brushes are essential components of a DC machine that enable the conversion of electrical energy into mechanical energy. In a squirrel cage motor, the stator's rotating magnetic field induces a current in the rotor, which produces a torque that drives the motor. However, when a DC voltage is applied to the squirrel cage motor, it will not rotate because it is not designed to operate on DC supply.

Suitable DC Machines:
The following are the types of DC machines that are suitable for use as a DC machine:

1. Permanent Magnet Motor: A permanent magnet motor is a type of DC motor that uses permanent magnets to produce a magnetic field. It is simple in construction, reliable, and efficient.

2. Series Motor: A series motor is a type of DC motor that has its armature and field windings connected in series. It is suited for applications that require high starting torque, like electric traction.

3. Synchronous Motor: A synchronous motor is a type of AC motor that can also operate as a DC motor. It has a rotor that rotates at the same speed as the rotating magnetic field in the stator. It is used in applications that require constant speed, like in clocks and synchronous generators.

Conclusion:
In conclusion, the squirrel cage motor is not suitable for use as a DC machine because it is designed to operate on AC supply and does not have commutator and brushes. On the other hand, permanent magnet motors, series motors, and synchronous motors are suitable for use as DC machines.