Wound Rotor Motors: On Rotor Winding Throw-Out Failure

The Chinese language carries distinctive nuance; identical terminology delivers divergent connotations across distinct scenarios. Take the term "throw-out" (Shuai Bao) as an example. In daily vernacular, it means evading responsibilities or abandoning others, and is further extended to describe couples breaking up amid irreconcilable disputes. More frequently, however, this expression applies to motor fault diagnosis of wound rotor motors.

In electrical machinery terminology, rotor winding throw-out refers to a specific malfunction exclusive to wound rotor motors: radial outward deformation of the rotor winding overhang triggered by overspeed conditions. Through professional analysis, we observe clear rotational speed limits for such motors. Faults are more prevalent in 6-pole and higher-pole variants, which feature relatively low rated rotational speeds. Some manufacturers also produce 4-pole wound rotor motors, yet their fabrication procedures demand far greater complexity, mandating overspeed reliability verification for the entire rotor winding assembly.

Production testing and validation data demonstrate that rotors with preformed hard windings exhibit superior resistance to throw-out deformation compared to those with flexible soft windings. Moreover, rigorous fixing, banding, varnish impregnation and curing treatments applied to winding overhangs serve as decisive protective measures. Naturally, installing dedicated overspeed limiting devices on operational motors completely eliminates this failure risk.

Root Cause of Throw-Out Failure: Centrifugal Effect

Any object executing circular motion inherently tends to travel along the tangent of its orbit due to inertia. When the resultant external force acting upon it vanishes or fails to supply sufficient centripetal force to sustain circular motion, the object drifts progressively away from the rotation center - this physical phenomenon is defined as centrifugal motion.

During motor operation, every particle on the rotor revolves circumferentially around the motor shaft's central axis. Per the mathematical correlation between rotational velocity and centrifugal force, centrifugal magnitude rises proportionally with rotational speed.

Centrifugal principles underpin numerous commonplace applications: washing machine spin dryers, cotton candy makers, centrifugal governors, centrifugal test rigs, centrifugal dryers, centrifugal separators, automatic coin sorting machines, as well as athletic events such as discus and hammer throw.

Like all physical phenomena, centrifugal force bears both merits and hazards, and its adverse effects can induce destructive accidents. For an automobile navigating a horizontal road bend, centripetal force originates from static friction between tires and pavement. If the vehicle rounds the bend at excessive speed, the required centripetal force exceeds the maximum static friction threshold, prompting centrifugal skidding and traffic collisions. This explains the mandatory speed caps enforced at highway curve sections. High-speed rotating abrasive grinding wheels and flywheels also frequently shatter and eject sharp fragments at high velocity due to insufficient material strength or internal cracks, posing severe injury risks to personnel.

Definition of Centrifugal Force

Centrifugal force is a fictitious inertial force that appears to push rotating bodies away from their axis of rotation. Within Newtonian mechanics, the term historically denotes two separate concepts: an inertial force observable within non-inertial reference frames, acting as the counterbalance to centripetal force. In Lagrangian mechanics, centrifugal force may also describe a generalized force under specific generalized coordinate systems.

In conventional contexts, centrifugal force possesses no physical reality; it is merely a computational construct that preserves the validity of Newton's motion laws within rotating non-inertial reference frames. Centrifugal force does not exist in inertial reference frames - inertial forces such as centrifugal force must be introduced exclusively for non-inertial frames to maintain compliance with Newton's physical rules.

To illustrate this principle: imagine a disk rotating at angular velocity ω, with a block of mass m secured to the disk's central rotation axis by a cord of length r, with zero frictional resistance. The block rotates synchronously with the disk, held in orbit solely by cord tension. To an observer rotating alongside the disk, the block appears stationary. Newton's laws dictate that a stationary object must bear a net force of zero, yet the block only receives unidirectional cord tension, creating an apparent contradiction. This conflict arises because Newton's laws exclusively hold true within inertial reference frames, whereas the rotating observer occupies a non-inertial frame. To reconcile this discrepancy and retain Newton's equations for rotating frames, a fictitious inertial force - centrifugal force - is introduced.

Centrifugal force matches cord tension in magnitude yet acts in the opposing radial direction. With centrifugal force factored in, the rotating observer perceives the block subject to two balanced forces: inward cord tension and outward centrifugal force of identical magnitude, yielding a net force of zero. The block therefore remains stationary, fully consistent with Newton's laws of motion.