Residual magnetism refers to the phenomenon where a magnetic material retains some level of magnetization even after the external magnetic field is removed. This residual magnetism can occur in materials like iron or steel, which are commonly used in the construction of electromagnets, transformers, and motors.
When it comes to AC (alternating current) motors, residual magnetism can have both positive and negative effects on motor starting, depending on the circumstances:
Positive Effect: In some cases, residual magnetism can actually aid in the starting of an AC motor. When an AC motor is initially energized, the presence of residual magnetism in its core can provide a small amount of starting torque. This can help the motor overcome the initial inertia and start rotating more easily, requiring less current from the power supply. This effect can be particularly noticeable in single-phase induction motors, which tend to have lower starting torque.
Negative Effect: On the other hand, excessive residual magnetism or an improperly aligned rotor can lead to a phenomenon known as "sticking" or "cogging." This occurs when the rotor's magnetic poles align with the stator's poles due to the residual magnetism, making it difficult for the motor to start smoothly. The motor may experience jerky or uneven rotation, and it might draw higher current during starting, which could potentially trip overload protections or lead to increased wear and tear on the motor and associated equipment.
To mitigate the effects of residual magnetism on AC motor starting, various techniques can be employed:
Manual Assistance: In some cases, a gentle push or external force can help break the rotor free from the "sticking" position and initiate smooth rotation.
Design and Construction: Motors can be designed and constructed in ways that minimize the impact of residual magnetism, such as using skewed rotor slots or specific pole shapes.
Soft Starters and Variable Frequency Drives (VFDs): These devices provide controlled starting voltage and frequency to the motor, allowing for smoother and more controlled acceleration, which can help mitigate the effects of residual magnetism.
Rotor Position Detection: Sensors or encoders can be used to detect the position of the rotor and ensure that it is properly aligned with the stator's magnetic field before initiating starting.
In summary, residual magnetism in AC motors can influence the motor's starting behavior, both positively by providing some initial torque and negatively by causing sticking or cogging. Proper design, construction, and control techniques can help manage these effects and ensure smooth and reliable motor starting.