Maximum torque in DC series motor is limited by commutation. The commutation process is responsible for changing the direction of current flow in the armature windings as the rotor rotates. The commutation process is essential to maintain the torque production and efficiency of the DC motor. The following are the reasons why commutation limits the maximum torque in DC series motor:

1. Commutation Effect on Armature Reaction: During commutation, the brushes make contact with the commutator segments, which causes arcing and sparking. These phenomena cause a distortion in the magnetic field produced by the armature windings, known as armature reaction. The armature reaction opposes the field produced by the stator windings, which reduces the torque production of the motor.

2. Commutation Effect on Motor Efficiency: The arcing and sparking during commutation cause energy loss in the form of heat, which reduces the overall efficiency of the motor. The heat generated during commutation can cause damage to the motor's insulation and reduce its lifespan.

3. Commutation Effect on Motor Speed: The commutation process also affects the speed of the motor. The timing of the commutation must be precise to maintain the motor's speed and torque production. If the commutation is not timed correctly, it can cause the motor to slow down or stop.

Therefore, the maximum torque in DC series motor is limited by commutation due to its effect on armature reaction, motor efficiency, and motor speed. To improve the torque production and efficiency of the motor, proper commutation timing and maintenance of the brushes and commutator are necessary.