Control of motors and transformers

It has been said that the operation of non-linear loads such as motors and transformers requires a complete sine wave voltage. Otherwise, the motor or transformer may become hot or buzz. Many readers sent emails on this issue, asking for more knowledge in this area. Next, I will give you an analysis on this issue. A very interesting and easily confused issue in smart home control is "speed" control. And I also found that it is easy for us to associate "dimming" with "speed" control. So let’s start with the difference between dimming and speed control! We all know that an incandescent lamp is a resistive load (although it also contains a small amount of inductance, the inductance is insignificant compared to its resistance, so we regard it as a purely resistive load). In this regard, electrical knowledge tells us that a purely resistive load is a load that does not contain inductance and capacitance. Then we compare an incandescent lamp to a large resistor. Many X10 dimmer switches are designed to treat an incandescent lamp as a load that is neither inductive nor capacitive. The device that can drive a purely resistive load is a thyristor, but the thyristor cannot change the amplitude of the sine wave. We can use a large variable resistor or adjustable voltage transformer to change the amplitude of a complete sine wave (as shown in Figure 1). The variable resistor dimming method was once used for stage dimming, but that was already in the first half of the twentieth century. Using that method to dim the light is not only inefficient but also expensive. Today we have solid state devices, thyristors. It controls the brightness of the light not by changing the amplitude of the sine wave, but by turning on and off the sine wave output to the light very quickly. For example, if you have a switch that turns on and off fast enough, in the sine wave In each half-wave of the wave, you switch once, allowing part of the sine wave to flow through the lamp. As a replacement for the variable resistor, the thyristor changes the power output to the load by changing the position of the sine wave. As shown in Figure 2, the two-cycle sine wave in the picture is the output waveform of the X10 dimmer switch. You will definitely notice that a small part of the first half wave is cut off. When the X10 dimmer switch is turned to "brightest", the brightness of the light is not 100%, but 96%. If you are very careful, you will find that when you install the X10 dimmer switch, the maximum brightness ratio of the light It turns out to be a little darker. Where does the 4% sine wave go? This is also something that mechanical switches cannot do. Let me talk about where the 4% sine wave goes under normal circumstances. In a house, there are usually only two wires in the wall switch box, but they are not the neutral wire and the live wire. The neutral wire is usually directly connected to the lamp, and then a wire is led from the lamp to the wall switch box. Wire it to the wall switch box, so that the two wires in the switch box are the live wire and the wire from the lamp. In order to adapt to this situation, many models of X10 dimmer switches are designed to be two-wire and pass through the filament. To provide energy for it (as shown in Figure 3). Since the incandescent lamp is a "linear" load, it will provide a good operating current for the X10 dimmer switch when the incandescent lamp is not lit. Enough current is passed to maintain the normal operation of the X10 dimmer switch. When the light is "full on", the X10 dimmer switch will reserve 4% of the power for itself. Loss of power and failure. This is where the 4% cut off sine wave in Figure 2 goes. At the same time, this is why the two-wire X10 dimmer switch has a minimum load power limit. If the lamp power is 400W or 500W. It will provide enough power for the X10 dimmer switch. Of course, a 40W or 60W lamp will also provide it with enough current. However, if a small power lamp (5W or 10W) ​​is used, the X10 dimmer switch will not. It will "die" because the maintaining current is too small! There is another very useful place for the X10 power carrier signal to propagate in this very clean area (as shown in Figure 4). There will be two very steep edges in each cycle. This edge is also when the "switch (thyristor)" is turned on. The load changes from no power to suddenly having power very quickly. A lot of electrical "noise" will be generated at the edge. Therefore, when designing the X10 dimmer switch, the small end of the sine wave will be left to provide a clean space for the X10 carrier signal . For switches (with neutral input), theoretically there is no need to reserve the 4% energy for it (see Figure 5). Because there are neutral and live wires supplying power to it at the same time, maintaining its operating current no longer needs to pass through the filament. Despite this, X10 engineers left 1.1mS of clean space for the X10 signal to propagate. Representatives of inductive loads are motors and transformers. What will happen if the X10 dimmer switch is used to control inductive loads? Since the output of the X10 dimmer switch is a non-sinusoidal wave, this will cause great "opinions" in the motor and transformer. The input power supply when designing inductive loads is designed to be a clean, smooth sine wave. They don't want their power supply to be "chopped to pieces". If you use an X10 dimmer switch to control inductive loads (like motors or transformers), they will get hot or hum. First, let’s talk about electric motors. A standard AC motor relies on a rotating magnetic field to drive the rotor to rotate.