Estimating the current in blinking devices or motors – an old electrician relies on this trick

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This post was last edited by yunrun on 2018-9-18 at 11:42. During electrical maintenance work, it is often necessary to estimate the current of equipment or electrical appliances. In actual electrical wiring, cabling, and repair tasks, many experienced electricians can determine the corresponding current value just by looking at the power rating of the equipment or appliance; the speed at which they do this is astonishing, and it makes many novice electricians very envious. If you ask these experienced electricians how they manage to calculate such values so quickly… Veteran craftsmen usually shrug and say: Experience! With enough practice, you’ll understand it naturally! This post shares this method for estimating motor current along with relevant experience. So is it really experience? Can beginners in electrical work also quickly estimate the current value of equipment or electrical appliances? Even just a glance tells you about the current; the answer is yes! Today we will take a look at quick methods for estimating current values; learning this will benefit you for life. Current estimation: yunrun.com.cn/tech/2191.html http://yunrun.com.cn/upload/201809/16/201809160205538833.png 1. There are generally two methods for calculating the corresponding current value based on power. ① Calculate the current value using formulas. The formulas for calculating current values differ for three-phase and single-phase electrical equipment: ◆ Formula for calculating current in three-phase equipment: Note: P represents power, in kW units ； U is the voltage, 380V ； COSφ is the power factor, with a typical value ranging from 0.6 to 0.8 ； η is the efficiency, typically taking a value of 0.9. ◆Formula for calculating current in single-phase electrical equipment: a. For resistive or electrically heated loads: P=U×I; therefore, I=P/U. b. For motor-type loads: P=U×I×COSφ×η. Note: P represents power, with the unit being kW ； U is the voltage, 220V ； COSφ is the power factor, which can range from 0.6 to 0.8 ； η is the efficiency, typically taking a value of 0.9. ②Common current estimation methods used by experienced electricians ◆ Current estimation method for three-phase 380V motors: a) For motors with power > 3kW, the estimated current = power (kW) × 2A/kW; b) For motors with power ≤ 3kW, the estimated current = power (kW) × 2.5A/kW. ◆ Current estimation method for three-phase 380V heating equipment: Estimated current = power (kW) × 1.5A/kW. ◆ Current estimation method for three-phase 660V heating equipment: Estimated current = power (kW) × 1.2A/kW. ◆ Current estimation method for three-phase 3000V heating equipment: Estimated current = power (kW) ÷ 4kW/A. ◆ Current estimation method for three-phase 6000V heating equipment: Estimated current = power (kW) ÷ 8kW/A. ◆ Current estimation method for single-phase 220V equipment: Estimated current = power (kW) × 4.5A/kW. ◆ Current estimation method for equipment using two phase wires at 380V: Estimated current = power (kW) × 2.5A/kW. For example, the wiring of a welding machine. 2. Provide examples. Example: How to calculate the operating current of a 5.5KW three-phase AC motor? ①Calculate using the formula; take COSφ as 0.85 during calculation ； With an efficiency of 0.9, the current is 10.9A. The specific calculation is as follows: ② Empirical estimation: Based on experience, the estimated current for a motor with a power rating of more than 3 kW is equal to power (in kW) × 2 A/kW; thus, the estimated current for a 5.5 kW motor is 11 A. The two values are very similar; in my opinion, the estimation method is the fastest and most practical to use. This is also the secret behind how experienced electricians can quickly estimate the operating current simply by taking a look at the power of an electrical appliance or device! Summary: The above is the method for quickly estimating the operating current based on the power of electrical appliances; it is also the trick that experienced electricians use to determine the current simply by looking at the power of the equipment. Of course, there are more than one way to make such estimates. If you have a better method for estimating current, feel free to leave a comment below so we can exchange ideas and learn from each other!