Cogging Torque in Permanent Magnet DC Machine

Cogging torque refers to the phenomenon of a pulsating torque that occurs in the permanent magnet DC (PMDC) machine. It is a torque that opposes the motion of the rotor when the machine is not energized. However, in a PMDC machine, the cogging torque is absent. Let's explore the reasons behind this.

1. Magnetic Nature of Rotor
The rotor in a PMDC machine is made up of permanent magnets, which have a strong magnetic field. The permanent magnets are usually made of materials like neodymium iron boron (NdFeB) or samarium cobalt (SmCo). These magnets have a high remanence, coercivity, and energy product, which enables them to generate a strong magnetic field.

2. Non-Magnetic Nature of Rotor
Unlike the rotor in other types of machines, the rotor in a PMDC machine does not have any magnetic properties. It is a non-magnetic component that acts as a support structure for the permanent magnets. The non-magnetic nature of the rotor eliminates the possibility of any cogging torque being generated.

3. Absence of AC Supply
Cogging torque is primarily observed in machines that operate on AC supply, such as induction motors. In these machines, the cogging torque arises due to the interaction between the stator and rotor magnetic fields. However, PMDC machines operate on direct current (DC) supply, and the absence of alternating magnetic fields prevents the occurrence of cogging torque.

Conclusion
In conclusion, the absence of cogging torque in a permanent magnet DC machine is primarily due to the non-magnetic nature of the rotor, which supports the permanent magnets. Additionally, the absence of AC supply in PMDC machines eliminates the interaction between magnetic fields that would generate cogging torque. These factors contribute to the smooth operation of PMDC machines without the occurrence of cogging torque.