Classification and characteristics of AC traction motors

2021-07-26 11:17

Under the condition that the rated frequency and rated voltage remain unchanged, the natural working characteristics of AC traction motor refer to the change relationship between the given motor speed n, electromagnetic torque T, stator power factor cosφ1 and output power P2. The characteristic refers to the change relationship between the motor and the rotational speed and the electromagnetic torque. Commonly used AC traction motors have the following categories:1. Single-phase series commutator motorThe input voltage is single-phase low frequency AC, usually only series excitation is used, and its natural operation characteristics are basically the same as those of direct (pulse) current series excitation motors. It has a large number of poles and the commutation problem is not fully solved. Only some European countries use it under the low-frequency traction grid.2. Three-phase asynchronous motor(1) Speed characteristic n=f(P2). In the stable running speed range, when P2 increases, the electromagnetic power Pem and rotor winding copper loss PCu2 also increase, and PCu2 increases slightly faster than Pem, that is, the slip ratio s=PCu2/Pem increases slightly. Therefore, the rotational speed characteristic is an approximate straight line with a slight decrease, which shows that the rotational speed characteristic of the asynchronous motor is hard.(2) Torque characteristic T=f (P2). From T=CTΦI2=T0+P2/Ω, it can be seen that the three-phase asynchronous motor is the same as the direct (pulse) separately excited motor, and the torque characteristic is an approximate straight line showing an upward trend.(3) Stator power factor characteristic cosφ1=f(P2). The stator current at no-load is mainly the excitation component, so it is very low at no-load. With the increase of P2, the active component of the rotor current increases, and the corresponding active component of the stator current also increases, and cosφ1 rises quickly. When P2 increases to the rated load, due to the significant increase in slip s, the rotor leakage reactance sX02 increases, and the reactive component relative to the stator current will also increase, but cosφ1 gradually decreases. Therefore, the stator power factor characteristics are curved lines.(4) Mechanical characteristics n=f(T). In the stable operation range, when T increases, the active component I2cosφ2 of the rotor current increases, indicating that the slip s is slightly increased, and the mechanical characteristics are an approximate straight line with a slight decrease. Therefore, the mechanical characteristics of the asynchronous motor are hard.3. Three-phase synchronous motor(1) Speed characteristic n=f(P2) and mechanical characteristic n=f(T). Since the synchronous motor is an AC/DC bilateral excitation motor, its speed only depends on the armature current frequency and the number of pole pairs of the motor, and has nothing to do with P2 and T. The speed characteristics and mechanical characteristics are level absolute hard characteristics.(2) Torque characteristic T=f (P2). According to the knowledge of "Electrical Engineering", the electromagnetic power Pem increases with the increase of the output power P2. Because the rotor synchronous angular velocity Ωr remains unchanged, the torque characteristic rises linearly.(3) Stator power factor characteristic cosφ1=f(P2). According to the knowledge of "Electrical Science", a synchronous motor in an overexcited state will draw capacitive current from the power source to improve cosφ1. When no-load, it is mainly DC excitation. When the excitation is small, the power factor cosφ1 can be higher than that of the asynchronous motor. With the increase of P2, the cosφ1 can be made higher when the excitation is larger.