Operating characteristics of three-phase asynchronous motors

The working characteristics of three-phase asynchronous motors The working characteristics of asynchronous motors refer to the relationship between the rotation speed N, stator current I1, power factor cosφ1, electromagnetic torque T, efficiency η, etc. of the motor and the output power P2 under the condition of rated voltage and rated frequency operation, I .e. when U1 = UN,f = fN, n, I1, cosφ1, t, n = f(P2). Since the asynchronous motor is an AC motor, it is necessary to consider the power factor for the power grid. At the same time, due to the unilateral excitation, the excitation current and the load current coexist in the stator winding, so the stator current should be paid attention. The rotor current is generally not directly measurable, so that these characteristics are not necessary for the output power. Like the DC motor, after being familiar with the working characteristics of the asynchronous motor, it can make it complete the mission entrusted by the drag system.

Analysis of 1. working characteristics

1. Speed characteristics

The curve n = f(P2) of the speed change when the output power of the asynchronous motor changes under the rated voltage and rated frequency is called the speed characteristic.

When the motor is unloaded, the output power P2 ≈ 0, in which case I2 = 0. The above relationship shows that the slip rate s is almost proportional to I2, so s ≈ 0, and the speed is close to the synchronous speed, I .e. n = n1. When the load increases, the speed will drop slightly, and the rotor electric E2s will increase, so the rotor current I2 will increase to produce a larger point of electromagnetic torque and load torque balance. Therefore, with the increase of output power P2, the slip S also increases, and the speed decreases slightly. In order to ensure the high efficiency of the motor, in general asynchronous motors, the rotor copper loss is very small, and the slip at rated load is 1.5 ~ 5% (small number corresponds to large-capacity motor). The corresponding speed characteristic is a slightly downward slope curve, and shunt DC motor speed characteristics are very similar.

2. Stator current characteristics

Asynchronous motor in rated voltage and rated frequency, the output power changes, the stator current change curve I1 = f(P2), known as the stator current characteristics. When no load, the rotor current, the stator current is almost all excitation current. As the load increases, the rotor speed decreases, the rotor current increases, and the stator current and magnetic potential also increase, offsetting the magnetic potential generated by the rotor current to maintain the balance of the magnetic potential. The stator current increases almost in direct proportion to P2.

3. Power factor characteristics

Asynchronous motor in rated voltage and rated frequency, when the output power changes, the stator power factor change curve cosφ1 = f(P2), known as the power factor characteristics. Since the total impedance obtained by the asynchronous motor equivalent circuit is inductive, the asynchronous motor is equivalent to an inductive impedance for the power supply, and its power factor is always lagging, and it must absorb inductive reactive power from the power grid. When there is no load, the stator current I1 is basically the excitation current, which is mainly used for reactive excitation, so the power factor is very low, 0.1~0.2. When the load increases, the active component of the rotor current increases, and the active component of the stator current increases, which can improve the power factor. The power factor reaches a maximum near the rated load. Since the slip rate s of the motor is very small and changes very little after no load to the rated load, the rotor power factor angle is almost unchanged. However, when the load exceeds the rated value, the value of s becomes larger, so φ2 becomes larger, and the reactive component in the rotor power supply increases, so that the power factor of the motor stator decreases again.

4. Electromagnetic torque characteristics

When the output power of the asynchronous motor changes under the rated voltage and rated frequency, the change curve of the electromagnetic torque T = f(P2) is called the electromagnetic torque characteristic.

Because the output power P2 = T2Ω, the load of the asynchronous motor does not exceed the rated value. The speed and angular velocity Ω change very little, and the no-load torque T0 can be considered basically unchanged, so the electromagnetic torque characteristic is approximately a straight line with a slope.

5. Efficiency characteristics

Asynchronous motor in rated voltage and rated frequency, when the output power changes, the efficiency of the change curve η = f(P2), known as the efficiency characteristics. The losses in asynchronous motors can also be divided into two parts: constant loss PFe, PΩ and variable loss PCu, Pδ. As the output power P2 increases, the variable loss increases more slowly, so the efficiency rises more quickly. Like the efficiency characteristics of a DC motor, the efficiency of an asynchronous motor reaches its maximum when the variable loss is equal to the constant loss. Then the load continues to increase, the variable loss increases quickly, and the efficiency decreases. For small and medium-sized asynchronous motors, the maximum efficiency occurs when there is about 3/4 of the rated load. The larger the motor capacity, the higher the efficiency.