A known three-phase motor capacity, and its rated current Formula (c): kilovolt capacity divided by the number of points to take factor 76.Description: 

 (1) formula applies to any of the three-phase voltage calculation of motor rated current. 

By the formula and that formula may be the same voltage capacity , rated current of different motor is not the same,that is not the same as a few kV voltage,removal of the same capacity,from "quotient" is clearly not the same as, Quotient is not the same by the same coefficient to 0.76, from the current value is not the same

If the above formula is called generic formula, the calculation can be derived 220,380,660,3.6 kV voltage motor rated current special calculation formula, the special calculation formula calculated by a dedicated three-phase motor on one rated current, Kilowatt of capacity with the current multiple of amperes of direct relations, save  as capacity divided by the number of kilovolt , the number of quotient take by factor 0.76 .220 three-phase motor, kW 3.5 amps. Commonly used 380 motor, 2 amps one kilowatts. 660 low-voltage motors, kW 1.2 amps. 3000 V high voltage motors, 1 Amps 4 kilowatts. 6000 V high voltage motors, 1 amps 8 kilowatts.

  (2)the use of formula c, the capacity of units is kW, voltage units is kV, current units is A, must take note of this point.

  (3) factor of 0.76 in c formula is to consider the motor power factor and efficiency of the integrated value calculated. Power factor is 0.85, efficiency is not 0.9, this comparison for two values of more than tens of kW motor, the motor frequently used the following seems to 10kW greater. This was calculated using the formula c motor nameplate rated motor current and the numerical mark a margin of error, the error on the following 10kW motor rated current in accordance with the first switch, contactors, such as the impact of small wires.

 (4) the use of formula calculation skills. 380V calculation formula used by the motor rated current, the first motor with a few 0.38kV then remove 0.76, 2 to take the quotient of capacity (kW) number. In case of 6kV larger capacity motor, the capacity of several kW is also just a few multiples of 6kV, the capacity divided by the number of kilovolt, quotient multiplied by the coefficient 0.76.

 (5) error. In the coefficient c by the formula 0.76 in which the motor power factor to 0.85, while the calculated efficiency of 0.9, so that different power factor, efficiency of the motor rated current on the existence of error. C derived from the formula of the five special formula, capacity (kW) and current (A) the multiplier is the voltage (kV) to remove the number of coefficients to 0.76. Simple and easy-specific mental arithmetic formula, it should be noted that the error will increase. Generally the larger the number of kilowatts, calculated on the nameplate current slightly larger than some; and a few smaller kW, calculated current is slightly smaller than those on the nameplate. In this regard, in the calculation of current, when the current of a few dozen to more than a security or safety, then do not count after the decimal point. Four and five homes can be nothing, just take an integer, it is simple and without affecting the utility. For as long as the smaller current is also considered to be a decimal. 

  Second, Order to the capacity of current demand without a nameplate capacity of no-load motor current estimate of its rated capacity

Formula:NO nameplate motor capacity, measured no-load current value,take by ten, divided by eight, near by the number of kilowatt-level

Description:formula is not the three-phase asynchronous motor nameplate

, do not know the number of kilowatt capacity may be by measuring the motor no-load current value, to estimate the number of motor kW capacity method.

  Third, the known cage motor capacity, operators seek stars - Delta starter (QX3, QX4 series) of the action of time and heat setting current formula components: Star Delta motor starter, setting a good start-up time; capacity multiplied by the second prescription, plus four units plot a few seconds. Star Delta starter motor, thermal overload protection device; tuning current phase current capacity taked by eight,divided by seven.

  Note: (1) QX3, QX4 series automatic star - triangle starter from 3 AC contactor, a relay and a three-phase thermal time relay of the outside with a start-up button and a stop button. before use of starter, to deal with time relay and thermal relay adjustment, these two tasks are installed in the start-up scene. Electrical capacity of most electric motors only, and do not know the normal start-up time motor, motor rated current. Time relay time is the action of the motor start-up time (from start to achieve the rated speed of the time), this time to operator numerical formula available. 

2) time relay adjustment, not to access the motor to operate, test time relay time of the action with the motor under the control of the start-up time line. If not, it should be time to fine-tune the action of time relay, and then test. However, the interval between two tests of at least more than in the 90s in order to ensure bi-metallic relay automatically reset the time.

(3) the adjustment of thermal relay, as the QX Series starter in the heat of hot electrical components in series in the motor phase current circuit, while the motor is then run into a triangle, then the motor running current is the phase line current (that is, rated current) of 1 / √ 3 times. Therefore, the thermal relay of thermal components of the whole application will be current formula of "capacity by 8 divided by 7" basis. According to the calculated value of heat relay current setting of the adjustment knob to the appropriate scale - scale about the center line. If the calculated value of thermal relay thermal component is not rated current range, that is greater than or less than the regulation of body marking scale high-limit or low limit values should be appropriate to replace the thermal relay, or select the appropriate components of the heat.

Four known asynchronous motor capacity, and its no-load current operator

Formula: Motor no-load current, with a capacity of about discount for each purchase order; new tickets a few very large, very old small number of multi-kilowatt.

  Description: (1) induction motor no-load run-time, will be a three-phase winding by the current, known as the no-load current. Most of the no-load current used to generate the rotating magnetic field, known as the no-load excitation current, is the no-load reactive current component. There are a small fraction of no-load no-load motor current for the production of a variety of run-time power loss (such as friction, wear and tear, Core  loss  and so on), this part is no-load current of the active component, the percentage is very small, negligible. Therefore, the no-load current can be considered that are reactive current. From this point of view, it is the smaller the better, so that the motor power factor increased, the power supply is good. If the no-load current, If the no-load current, and stator windings of the wire because it is a certain area , Current is a certain .then allowed to flow through wire active current can only be reduced, the motor can drive the load will be reduced, efforts to reduce motor with the load is too large, winding, it is easy to heat. However, the no-load current can not be too small, or to affect the motor performance of the other. Generally small motor rated current no-load current of about 30% to 70%, large and medium-sized motor rated current no-load current of about 20% ~ 40%. Specific to a certain Motors is the number of no-load current in the motor nameplate or product brochure, there is generally no mark. Electrician often need to know how much this value is as a value to determine the quality of motor repair, can use or not. 

 (2) formula is the calculation of the fast motor no-load current value of specific formula, which is derived from a large number of test data. It is consistent with the "no-load current of motor rated current is usually 1 / 3." At the same time, it is consistent with the practical experience: "The no-load motor current, no more than a few kilowatts of capacity to use" principle (referring to old-fashioned maintenance, small-capacity motor). Formula "about the capacity of discount for each purchase order" refers to the general no-load motor current value is the number of motor kW rated capacity of 0.8 times. Medium, 4 or 6 pole motors of no-load current, motor capacity is 0.8 times the number of kilowatt; new series, high-capacity, very few of the two small electric motors, the no-load current calculation by the "new big six very few discount "; on the old, old series, a smaller capacity, very few large motor of more than 8 poles, and its no-load current, by" a very small number of multi-kilowatt ", that is similar to no-load current equivalent to the capacity of a few kW , but is generally less than a few kilowatts. Calculated using formula of no-load motor current, motor specification count value of the mark, measured values have a certain margin of error, but the count value formula can meet the electrical needs of day-to-day work.