电磁转差离合器原理与异步电动机很相似，机械特性也相似，但理想空载点的转速为异步电动机转速 n 而不是同步转速 n 1 。励磁电流 I f 越大，磁场增多，改变 I f 的大小好象改变异步电动 机电源电压的大小一样：若转速相同，则 I f 越大，电磁转矩 T 也越大；若转矩相同，则 I f 越大， 转速越高。改变励磁电流 I f ，就可以调节负载的转速。
电磁转差离合器设备简单，控制方便，可平滑调速，这是它的优点。但是，由于其机械特性较软，转速稳定性较差，调速范围较低。低速时，效率也较低。适合于通风机和泵类负载，与异步电动机降压调速相似。
电磁转差离合器与异步电动机装成一体时，即同一个机壳时，称为滑差电机或电磁调速异步电动机。
4.7 绕线式异步电动机双馈调速及串级调速
所谓双馈，是指绕线式异步电动机的定、转子三相绕组分别接到两个独立的三相对称电源，其中定子绕组的电源为固定频率的工业电源，而转子电源电压的幅值、频率和相位则需按运行要求分别进行调节。
o 双馈调速的基本原理
绕线式异步电动机双馈调速系统不仅能调节电动机的转速，还能改变电动机定子边的功率因数。
异步电动机双馈调速系统的组成
双馈电机转子回路要求能提供可控幅值、频率及相位的电源。实现方法：采用晶闸管组成的交交变频器或交直变频器。
异步电动机转子感应电动势的频率是随转速的变化而变化的，因此，要求在任何转速下，变频器输出的电压 应与转子感应电动势同频率。实现方式：他控式、自控式。
他控式：由独立的频率可控正弦波发生器发出控制信号，去控制交交变频器，使之产生转差频率电压 。这种运行方式，虽然电机称异步电机，实际上相当于转子加交流励磁的同步电机运行。类似于同步电动机，在恒转速下，电动机存在着最大电磁转矩，当负载转矩超过此值时，电动机将发生失步现象。此外，运行中，也会出现转子振荡问题。
起动异步电动机时，先将转子绕组接到起动电阻 R 上，以减小起动电流，增大起动转矩。待电动机转速上升到设计值时，再将交交变频器投入运行。这是由于受交交变频器输出频率范围小的限制，不可能在电机转速为零时投入变频器。
串级调速的基本原理
基本思路：把异步电机转子感应电动势变为直流电动势，同时把转子外加电压也变为直流量，也能满足同频率的要求，即大家的频率都为零。
4.8 Speed control of three-phase induction motors
In this chapter we have been the basic control equipment that is used to start and stop induction motors. However, some industrial drives require a motor to function at various torques and speeds, both in forward and reverse. In additional to operating as a motor, the machine often has to function for brief periods as a generator or brake. In electric locomotives, for example, the motor may run clockwise or counter-clockwise, and the torque may act either with or against the direction of rotation. In other words, the speed and torque may be positive or negative .
Methods of speed control are distinguished as to the main effect exercised on the motor: 1) from the stator side and 2) from the rotor side.
The following methods of speed control from the stator side are used:
a) variation of the impressed voltage,
b) change-over of the number of pairs of poles,
c) variation of the power circuit frequency.
From the rotor side the speed may be controlled:
a) by varying active resistance in the rotor circuit,
b) by introducing into the rotor circuit an additional e.m.f. of the same frequency as the fundamental e.m.f. of the rotor.
To effect the latter method of speed control an additional or auxiliary electrical machine or several such machines are required. A unit consisting of a regulated induction motor and of one or more additional electrical machines connected to it electrically or mechanically is called a cascade. Commutator machines are commonly used as the additional electrical machines.
Speed control of induction motors by change in the primary voltage is of secondary importance, since it does not allow to accomplish control within wide limits and is not economical. Various special motor designs have also been suggested, as for instance, a design with a double rotor, etc.
Vocabulary
1. field control
2. armature resistance control
3. series and shunt armature resistance control
4. excitation winding
5. fundamental speed equation
6. stepless control of speed
7. differential compound motor
8. arcing
9. appreciable power loss
10. diversion
11. semiconductor
12. offset
13. plugging
14. deenergized
15. regenerative braking
16. mechanical braking
17. jogging
18. homogeneous
19. symmetrical
20. lagging
21. leakage
22. overshooting
23. undershooting
24. quadratic
25. conjugate
26. damped oscillation
Reading Comprehension
1. Field control as a method of speed control to obtain speeds above basic speed has the following advantages over other speed-control methods:
(1) field control is relatively inexpensive and simple to accomplish, both manually and automatically;
(2) it is relatively efficient in terms of motor performance, since the field circuit loss is only 3 to 5 per cent of the total power drawn by the motor;
(3) within limits, field control does not affect speed regulation in the cases of shunt, compound, and series motors ;
(4) it provides relatively smooth, stepless control of speed.
2. For any no-load speed setting below the basic speed , armature resistance control will produce a sharp drop in speed with the application of load, resulting in poor speed regulation.
3. In the case of motors of higher horsepower, however, torque, good speed regulation , and smooth, stepless speed control are all extremely important considerations
4. If the armature voltage supplied from the variable dc source is zero, the motor develops zero torque and is at a standstill . If the armature voltage is increased slightly, in accordance with the fundamental speed equation, the motor starts and turns at a slow speed with a minimum of acceleration.
本文关键字：电动机 变频器基础，变频技术 - 变频器基础
