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1. Structure and working principle of electromagnetic relays. Electromagnetic relays are the most widely used type, having been applied since the earliest times. Its structure and working principle are generally the same as those of a contactor. It consists of an electromagnetic system, a contact system, and a release spring, etc. The principle of an electromagnetic relay is shown in Figure 1. Since relays are used in control circuits, the current flowing through the contacts is relatively low (usually below 5A), so an arc extinguishing device is not required. String 8 String 2 Figure 1 Schematic diagram of an electromagnetic relay: 1 – Core; 2 – Rotating corner; 3 – Release spring; 4 – Adjustment nut; 5 – Armature; 6 – Moving contact; 7 – Stationary contact; 8 – Non-magnetic gasket; 9 – Coil. Common types of electromagnetic relays include voltage relays, intermediate relays, and current relays. The graphic and textual symbols of the electromagnetic relay are shown in Figure 2. Figure 2: Graphic and textual symbols of electromagnetic relays. 2. Characteristics of electromagnetic relays: The main characteristic of a relay is its input-output characteristic, also known as the relay characteristic; the curve representing this characteristic is shown in Figure 3. Until the relay input X increases from zero to X2, the relay output Y is zero. When the input value X increases to X2, the relay closes, and the output value is Y1 ； If X continues to increase, Y remains unchanged. When X decreases to X1, the relay releases and the output value changes from Y1 to zero; if X continues to decrease, the Y value remains zero. String 9 String 7 Figure 3 Relay characteristic curve. In Figure 3, X2 is referred to as the relay engagement value; for the relay to engage, the input value must be equal to or greater than X2 ； X 1 is referred to as the relay release value; for the relay to be released, the input value must be equal to or less than X 1. Kf = X1/X2 is called the return coefficient of the relay, and it is one of the important parameters of the relay. The Kf value is adjustable. For String 1, ordinary relays require a low return coefficient; the Kf value should be between 0.1 and 0.4. This ensures that when the relay is activated, large fluctuations in the input signal do not cause incorrect operation ； Under-voltage relays require a high return coefficient, with a Kf value of over 0.6. Suppose a relay has a Kf value of 0.66, and its operating voltage is 90% of the rated voltage; then when the voltage drops to 50% of the rated value, the relay releases, thereby providing under-voltage protection. Another important parameter for string 5 is the suction time and release time. The holding time refers to the time required from when the coil receives an electrical signal until the armature is fully attracted ； Release time refers to the time required from the de-energization of the coil to the complete release of the armature. The closing and releasing times of ordinary relays are 0.05–0.15 seconds, while those of fast relays are 0.005–0.05 seconds; these values affect the operating frequency of the relay.