The side derived from the stator is called the primary, and the side derived from the rotor is called the secondary. In practical applications, the primary and secondary are manufactured with different lengths to ensure that the coupling between the primary and secondary remains constant within the required stroke range. Linear motors can have either a short primary and a long secondary, or a long primary and a short secondary. Considering manufacturing costs and operating expenses, a short primary and a long secondary are generally used currently.
The working principle of a linear motor is similar to that of a rotary motor. Taking a linear induction motor as an example: when an AC power supply is applied to the primary winding, a traveling wave magnetic field is generated in the air gap. Under the cutting of the traveling wave magnetic field, the secondary winding induces an electromotive force and generates a current. This current interacts with the magnetic field in the air gap to produce an electromagnetic thrust. If the primary winding is fixed, the secondary winding moves linearly under the action of the thrust; conversely, the primary winding moves linearly.
The principle of a linear motor is not complicated. Imagine cutting a rotating induction motor along its radius and flattening it; this becomes a linear induction motor (see figure). In a linear motor, the part equivalent to the stator of a rotating motor is called the primary; the part equivalent to the rotor is called the secondary. When alternating current is passed through the primary, the secondary moves linearly along the primary under the influence of electromagnetic force. In this case, the primary needs to be very long, extending to the required position, while the secondary does not need to be that long. In practice, a linear motor can have either a very long primary or a very long secondary; it can have either a fixed primary and a moving secondary, or a fixed secondary and a moving primary.
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