Thread Content
The last edit to this post was made by wang_qia on 2016-10-8 at 20:53. ==Selection of power supply equipment 1. For 【instant-action】【overcurrent circuit breakers】, the 【setting current should be higher than the peak currents in the distribution circuit】, typically set at 【1.2 times】 the peak current of the circuit load. 4. The 【setting current】 of the short-time delay overcurrent release for distribution circuit breakers should be set so as to avoid the 【peak load current】 that occurs temporarily in the distribution lines; it is generally set at 1.2 times or more of the peak load current. The short delay ensures the 【selectivity】 of the protection device’s operation; the short delay trip times are available in three values: 0.5 (0.2), 0.4, and 0.6 (in seconds). The setting current of the [long-time delay overcurrent trip device] for power distribution is greater than the calculated current of the circuit ； A value equal to 【1.1 times】 the calculated current of the circuit. Note: The 【setting current】 is not the 【rated current】. The setting current is a 【standard value】 used by relay protection systems to compare with the actual current in order to determine when to trigger a trip ； (My personal opinion: The setting current is a current value that is calculated manually; it represents the calculated value of the current in the circuit. Once the actual current reaches this calculated value, the relay protection device triggers a trip action.) This calculated value is derived from the calculations of the circuit and the capacity of the power grid. ) Refer to the explanations in [GB14048]. For example, for the three-stage protection of the DW45 circuit breaker, the default value for the 【long-time delay protection setting current】 at factory is usually 1 time the 【rated current】 ； 【Short-circuit short delay setting current】: By default, it is 8 times the 【rated current】, with a timing duration of 0.4 seconds ； The [short-circuit instantaneous setting current] is set by default to [12 times] the [rated current]. ======I am a fancy divider line ============================= Baidu reference: Operating time and setting current for thermal elements. The motor starts in star-delta mode, allowing for easy adjustment of the starting time ； Take the square root of the capacity, multiply by two; add four units of seconds to the result. Motor starts in star-delta configuration; overload protection via thermal element ； The setting current for the phase current is capacity multiplied by eight divided by seven. Note: (1) The QX3 and QX4 series are automatic star-delta starters, consisting of three AC contactors, a three-phase thermal relay, and a time relay; they are equipped with a start button and a stop button. Before using the starter, the time relay and thermal relay must be properly adjusted; both of these tasks are carried out at the site where the starter is installed. Most electricians only know the capacity of motors, but not their normal starting time or rated current. The operating time of a time relay is the start-up time of the motor (the time from startup until the speed reaches the rated value), and this value can be calculated using a mnemonic formula. (2) When adjusting the time relay, do not connect the motor for operation at this stage; instead, test whether the operating time of the time relay matches the start-up time of the motor being controlled. If there is a discrepancy, the operating time of the time relay should be fine-tuned again, and testing should be conducted once more. However, the interval between the two tests must be at least 90 seconds to ensure automatic reset of the bimetallic time relay. (3) Adjustment of the thermal relay: Since the thermal element in the thermal relay of the QX series starters is connected in series in the motor’s phase current circuit, and the motor is connected in a triangular configuration during operation, the phase current while the motor is running is 1/√3 times the line current (i.e., the rated current). Therefore, the setting current value of the thermal element in a thermal relay should be calculated using the formula in the mnemonic: \"capacity multiplied by eight divided by seven\". Based on the calculated value, adjust the setting current knob of the thermal relay to the corresponding scale – around the midpoint scale. If the calculated value falls outside the rated current range of the thermal element in the thermal relay – that is, it is greater than or less than the upper or lower limit values indicated on the scale of the adjustment mechanism – then it is necessary to replace it with an appropriate thermal relay or select a suitable thermal element. ======I am a fancy divider line ============================ Automatic control instruments