Introduction
In power work, contact with electric motors is inevitable. The rated current and operating current of a motor directly affect its operating parameters and efficiency. Calculating a motor's rated current is basic knowledge for electrical technicians, but some technicians are not comfortable performing these calculations. The methods below show several simple approaches.
Basic Formulas
1. Three-phase motor rated current formula:
P = 1.732 * U * I * cos(phi) (power factor: resistive load = 1, inductive load ≈ 0.7–0.85, P in watts)
2. Single-phase motor rated current formula:
P = U * I * cos(phi)
Selection of a protective circuit breaker should be based on the load current. Typically the breaker rating is chosen about 20–30% higher than the load current.
General Calculation Notes
3. General calculation formula for three-phase: P = 1.732 * U * I * power factor * efficiency. For single-phase, do not multiply by 1.732. Protective circuit breakers are commonly sized at 1.2–1.5 times the total rated current.
Experience-based rules by nominal voltage:
- 380 V: about 2 A per kW
- 660 V: about 1.2 A per kW
- 3000 V: about 1 A per 4 kW
- 6000 V: about 1 A per 8 kW
Experience-based rules by power:
- Above 3 kW: current ≈ 2 * power (power in kW, current in A)
- 3 kW and below: current ≈ 2.5 * power
Example Calculation
Calculate the rated current of a three-phase 15 kW motor.
1. Empirical formula: I = 15 * 2 = 30 A.
2. Using the formula: P = 1.732 * U * I * power factor * efficiency. Assume power factor = 0.9 and efficiency = 0.9, U = 380 V, P = 15000 W. Substituting gives I ≈ 28.1 A.
Concept of Rated Current
1. In a three-phase four-wire AC supply where the line voltage is 380 V and the phase voltage is 220 V, the line voltage equals sqrt(3) times the phase voltage.
2. For a motor, the voltage across one winding is the phase voltage; the conductor-to-conductor voltage is the line voltage. The current in one winding is the phase current; the conductor current is the line current.
3. In a star (wye) connection: line current = phase current; line voltage = sqrt(3) * phase voltage. The three winding end terminals are connected together and the winding voltage is the phase voltage (for example, 220 V).
4. In a delta connection: line current = sqrt(3) * phase current; line voltage = phase voltage. Each winding is directly connected to the line voltage, and the conductor current is the vector sum of the three winding currents.
5. A clamp ammeter clamped on any one of the A, B, or C phase conductors measures the line current.
Supplementary Motor Basics
1. Rated voltage: the voltage at which the motor is intended to operate under normal conditions.
2. Rated current: the current when the motor operates normally at the rated voltage.
3. Rated power: the motor power when operating normally at rated voltage and rated current.
4. Difference between rated and actual power: The rated voltage is specified for the optimal operating condition and is fixed, with an allowable tolerance of about ±10%. Actual power and current vary with the load. A larger driven load increases actual power and current; a smaller load reduces them. If actual power and current exceed the rated values, the motor may overheat and be damaged. If actual power and current are significantly below rated values, the motor and system may be oversized.
