For motor products, especially three-phase motors, the connection of the lead wires is a crucial aspect. First, there's the connection between the motor lead wires and the winding itself; second, the connection between the lead wires and the terminals; and finally, the connection between the lead wires and the terminal block via the terminals.
The reliability of the welding between the lead wire and the main wire is an important factor. During the welding process, the insulation cleaning of the enameled wire, the conformity of the solder, and the subsequent wrapping and binding must all meet the requirements. The connection position between the lead wire and the terminal must ensure a reliable connection between the terminal and the lead wire, and at the same time, it must not damage the conductor of the lead wire.
When discussing the connection between terminal blocks and lead wires, most manufacturers rely on the most basic method: the lead wires are connected to the terminals in a relatively free state. For large-sized motors, due to the larger diameter of the lead wires and the enamel impregnation process along with some leads, connecting the lead wires to the terminal block is particularly difficult, resulting in an unsightly and unreliable connection. Reliable motor manufacturers will focus on the design of the terminal block and improvements to the connectors to ensure that the lead wires are not damaged during the connection process and that the connection is aesthetically pleasing.
Further Knowledge – How to Estimate and Select Motor Cable Specifications
When designing a specific control system, it is often necessary to calculate the load size to select the cable specifications. On the one hand, we can determine it based on the motor nameplate, or we can calculate it using some formulas. In fact, there is a simple empirical algorithm that can easily estimate the load, although it is not absolutely accurate, it is sufficient to serve as a basis for selecting cable specifications.
Three-phase asynchronous motors generally have two connection methods: star connection and delta connection. Except for smaller power motors (no more than 3kW for industrial frequency motors and no more than 45kW for variable frequency motors, excluding hoisting and metallurgical motors), most motors are delta connected. Taking a delta connected motor as an example, for a 7.5kW three-phase asynchronous motor powered by 380V, its rated operating current per phase is approximately twice its power (slightly less for higher power motors with the same number of poles, and slightly more for motors with the same power but a larger number of poles), i.e., 15-16A. The actual calculated current may be slightly less than this, and we can select the cable specifications based on this current.
How do we choose the appropriate cable cross-sectional area based on the current? Let's assume we choose a copper core cable. For example, if we're wiring a 4-pole 15kW wound-rotor motor, its rated current is 30A. Based on experience, 1 square millimeter of copper wire can carry 4-6A of current; let's take the midpoint of 5A. Therefore, the cable cross-sectional area should be 30/5 = 6 square millimeters. Standard cables are available in 6 square millimeter and 10 square millimeter sizes. To ensure reliability, we recommend choosing a 10 square millimeter cable to avoid lead failures due to overload, overheating, or aging. However, in practice, a 6 square millimeter cable might be chosen. This depends on the power consumption of the load. If the power consumption is less than the rated power, this choice is acceptable. If the power consumption is close to the rated power, then a 10 square millimeter cable is recommended.
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