Electric Cable Composition and Motor Starting Solutions

Electric Cables: Components and Types

Cable Constitution

• Power Conductor: Material (copper or aluminum) responsible for carrying the electric current.
• Insulator: The envelope surrounding the electric conductor, made of insulating material.
• Cover: The outer envelope providing mechanical protection.
• Filling: Material separating the insulation and the cover.

Types of Electrical Cables

Cables are categorized based on the number of electrical conductors and their nominal voltage rating.

Based on Number of Conductors:

• Unipolar: Formed by a single insulated and covered electrical conductor.
• Multipolar: Contains multiple conductors, each with its own insulation, sharing the same outer cover.

Based on Nominal Voltage:

This refers to the maximum voltage the insulation can withstand; this value must be higher than the nominal voltage of the receiver.

Mounting Aids for Electrical Panels

These components facilitate the mechanical assembly and subsequent electrical wiring of automation equipment inside an electrical panel.

• Uprights: Used to fasten perforated panels or racks via screws.
• Perforated Panels or Racks: These are attached to the uprights where electric automation devices are placed.

Problems Associated with Starting a Three-Phase Asynchronous Motor

High Starting Current and Torque

The starting current can reach very high values, typically 5 to 7 times its rated value. This results in a starting torque exerted on the motor shaft at the instant of connection, with torque values between 1.5 and 3 times the nominal torque.

Consequences

These high starting currents lead to thermal effects due to the Joule effect, which limits the machine's useful life. Additionally, they cause voltage drops in the mains supply.

Current and Torque Limitation Methods

It is advisable to limit the current and torque. This can be modified in two primary ways:

1. Acting on the effective value of the voltage applied to the motor.
2. Acting on the combined resistance of the motor.

Starting Devices Based on Variation Method

Starting by Varying the Stator Voltage:

• Autotransformer starting.
• Star-Delta (Star-Triangle) starting.
• Softstarters.

Starting by Varying the Combined Resistance:

• Resistance starters (for stator).
• Starting with rotor resistance.