📚 Electric Motors: How They Work and What Affects Their Speed | 电动机的工作原理与影响因素
Electric motors are everywhere – in household appliances, electric vehicles, and even tiny vibration motors in phones. For IGCSE Edexcel Science students, understanding the simple DC motor is a key part of the magnetism and electromagnetism topic. This article explains the motor effect, the construction and operation of a DC motor, the crucial role of the split‑ring commutator, and the factors that determine how fast a motor spins.
电动机无处不在——家用电器、电动汽车、甚至手机里的微型振动马达都有它的身影。对于学习 IGCSE Edexcel 科学课程的同学来说，掌握简单直流电动机是电磁学部分的关键内容。本文将讲解电动机效应、直流电动机的结构与工作过程、换向器的关键作用，以及影响电动机转速的主要因素。
1. The Motor Effect | 电动机效应
When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is called the motor effect. The force arises because the magnetic field created by the current interacts with the external magnetic field from permanent magnets.
当通电导线置于磁场中时，它会受到一个力的作用，这被称为电动机效应。这个力的产生是由于电流产生的磁场与永磁体提供的外部磁场之间发生相互作用。
The force is maximum when the current is perpendicular to the magnetic field. If the current is parallel to the field, no force acts on the conductor.
当电流方向与磁场方向垂直时，力达到最大。如果电流平行于磁场方向，则导线不受力。
F = B I L
Where F is the force in newtons, B is the magnetic flux density in tesla, I is the current in amperes, and L is the length of the conductor inside the magnetic field in metres.
其中 F 表示力（牛顿），B 表示磁通量密度（特斯拉），I 表示电流（安培），L 表示处在磁场中的导线长度（米）。
2. Fleming's Left‑Hand Rule | 弗莱明左手定则
To predict the direction of the force, we use Fleming's left‑hand rule. Hold your thumb, forefinger, and middle finger of your left hand at right angles to each other.
为了判断力的方向，我们使用弗莱明左手定则。将左手的拇指、食指和中指相互垂直张开。
- Forefinger points in the direction of the magnetic field (from north to south).
- Middle finger points in the direction of the conventional current (from positive to negative).
- Thumb then shows the direction of the thrust (force) on the conductor.
- 食指指向磁场方向（从北极到南极）。
- 中指指向常规电流方向（从正极到负极）。
- 拇指则指示导线所受推力（力）的方向。
This rule is essential for understanding which way the coil will rotate in a motor.
这条定则对于理解电动机中线圈的旋转方向至关重要。
3. From a Single Wire to a Rectangular Coil | 从单根导线到矩形线圈
In a practical motor, a single straight wire is replaced by a rectangular coil of wire. The coil is free to rotate about an axis. Each side of the coil that is perpendicular to the magnetic field experiences a force, but in opposite directions because the current flows in opposite directions on the two sides.
在实际电动机中，单根直导线被矩形线圈取代。线圈可以绕一根轴自由转动。线圈中垂直于磁场的两条边都会受到力的作用，但因为两边的电流方向相反，所以受力方向也相反。
These two opposite forces create a turning effect (a couple), which makes the coil spin. The coil continues to rotate until it reaches the vertical position.
这两个方向相反的力形成一个力矩（力偶），使线圈转动。线圈会一直旋转，直到到达竖直位置。
4. The Need for a Split‑Ring Commutator | 换向器的必要性
Once the coil reaches the vertical position, the forces are still acting but they are now pulling the coil apart rather than rotating it further. If the current direction remained unchanged, the coil would stop rotating and possibly oscillate briefly.
一旦线圈到达竖直位置，力虽然仍然存在，但此时它们将线圈向外拉扯，而不会继续推动它旋转。如果电流方向不变，线圈就会停止转动，并可能短暂地来回摆动。
To keep the motor spinning continuously, the current must be reversed every half turn. This is achieved by a split‑ring commutator. The commutator consists of a conducting ring split into two halves, each connected to one end of the coil. As the coil rotates, the two halves swap their connections to the brushes (graphite contacts), thereby reversing the current direction through the coil exactly when the coil passes the vertical position.
为了让电动机持续旋转，必须在每转半圈时改变电流方向。这个任务由换向器（split‑ring commutator）完成。换向器由分成两半的导电环构成，每一半分别与线圈的一端相连。当线圈旋转时，这两片半环交替接触电刷（石墨触点），从而在线圈刚好经过竖直位置时，反转线圈中的电流方向。
5. The Role of Brushes | 电刷的作用
Brushes are usually made of carbon (graphite) and press lightly against the commutator. They provide sliding electrical contacts, allowing current to flow from the external circuit into the rotating coil without tangling the wires.
电刷通常由碳（石墨）制成，轻轻压在换向器上。它们提供滑动电接触，使电流能从外部电路流入旋转的线圈，而不致缠绕导线。
Graphite is a good choice because it conducts electricity, reduces wear on the commutator, and can withstand some friction without overheating.
石墨是很好的材料，因为它既能导电，又能减少换向器的磨损，并且能承受一定的摩擦而不致过热。
6. How a DC Motor Works – Step by Step | 直流电动机工作步骤
Let us trace one complete rotation.
- Step 1: The coil is horizontal. Current flows, both sides experience forces in opposite directions, producing maximum turning effect. The coil begins to spin.
- Step 2: As the coil approaches the vertical position, the turning effect decreases. When it becomes vertical, the forces still act but produce no rotation.
- Step 3: Momentum carries the coil past the vertical point. At the same instant, the commutator swaps the connections, reversing the current in the coil.
- Step 4: The forces reverse on each side, so the coil continues to rotate in the same direction. The motor keeps spinning.
让我们追踪一整圈的过程。
- 步骤一：线圈处于水平位置。电流通过，两边受到反向力，产生最大力矩，线圈开始旋转。
- 步骤二：线圈接近竖直位置时，力矩减小。到达竖直时，力依然存在但不产生转动效果。
- 步骤三：惯性带着线圈冲过竖直点。与此同时，换向器完成换向，线圈中的电流方向反转。
- 步骤四：每边的受力方向也随之反转，因此线圈继续沿原方向旋转。电动机持续运转。
7. Factors Affecting the Speed of a Motor | 影响电动机转速的因素
Several factors determine how fast the coil spins. They can all be explained using the motor effect equation.
有多个因素决定线圈转速的快慢，这些都可以用电动机效应方程来解释。
| Factor | Effect on speed | Reason |
|---|---|---|
| Increasing the current (I) | Increases speed | Larger force (F = BIL) gives a larger turning effect. |
| Using a stronger magnetic field (B) | Increases speed | Greater flux density increases the force. |
| More turns on the coil (L effectively increases) | Increases speed | A coil with N turns multiplies the force approximately N times. |
| Larger area of coil | Increases speed | Gives a greater turning moment for the same force. |
| Adding a soft iron core | Increases speed | Concentrates the magnetic field lines, effectively increasing B. |
| 影响因素 | 对速度的影响 | 原因 |
|---|---|---|
| 增大电流 (I) | 转速增大 | 力更大 (F = BIL)，产生更大的力矩。 |
| 使用更强的磁场 (B) | 转速增大 | 更高的磁通量密度增大了力。 |
| 线圈匝数更多 (等效增加 L) | 转速增大 | N 匝线圈的力约放大 N 倍。 |
| 线圈面积更大 | 转速增大 | 相同的力产生更大的转动力矩。 |
| 添加软铁芯 | 转速增大 | 会聚磁感线，有效增大 B。 |
8. Practical Investigation: Varying the Current | 实验探究：改变电流
In the IGCSE laboratory, you may investigate how changing the current affects the motor's rotation speed. The motor is connected in series with an ammeter and a variable resistor (rheostat). By changing the resistance, you alter the current through the motor. The speed can be estimated by counting the number of rotations in a fixed time or by using a stroboscope / light gate.
在 IGCSE 实验室中，你可能会探究改变电流如何影响电动机的转速。电动机与电流表和可变电阻（变阻器）串联。改变电阻即可改变通过电动机的电流。转速可以通过记录固定时间内的旋转圈数，或使用频闪仪/光门来估算。
from F = BIL, until other factors such as friction and air resistance become significant.
典型的实验结果显示，转速近似与电流成正比，这与 F = BIL 的预期一致，直到摩擦和空气阻力等因素变得显著。
9. The Difference Between DC and AC Motors | 直流与交流电动机的区别
A simple DC motor uses a split‑ring commutator to keep the coil rotating in one direction. An AC motor, by contrast, often has slip rings (continuous rings) and relies on alternating current to naturally reverse the current at the mains frequency. This article focuses on the DC motor as required by the Edexcel IGCSE specification (3.6.6).
简单的直流电动机使用换向器使线圈沿一个方向连续旋转。而交流电动机通常使用滑环（连续的环），依靠交流电在主电源频率下自然换向。本文根据 Edexcel IGCSE 大纲（3.6.6）的要求，重点讲解直流电动机。
10. Common Mistakes and Key Exam Tips | 常见错误与考试要点
Students often confuse the split‑ring commutator with slip rings. Remember: a commutator reverses the current every half turn; slip rings maintain the same connection and are used in AC generators. Also, never say that the commutator 'stops' the current – it reverses it. And always apply Fleming's left‑hand rule correctly by checking the direction of conventional current (positive to negative), not electron flow.
学生经常混淆换向器和滑环。记住：换向器每半圈反转电流方向；滑环保持连接不变，用于交流发电机。另外，不要说换向器“切断”电流——它是反转电流。同时，应用弗莱明左手定则时务必检查常规电流方向（正到负），而不是电子流动方向。
When explaining the turning effect, clearly state that the forces on the two sides of the coil are in opposite directions because the currents are opposite. This creates a couple.
在解释力矩时，请明确指出线圈两边电流方向相反，因此受力方向也相反，从而形成力偶。
11. Real‑World Applications | 实际应用
The principle of the DC motor is used in battery‑powered drills, electric toothbrushes, toy cars, and cooling fans in computers. Larger versions are found in electric vehicles and industrial machinery. Although many modern motors are brushless, the classic brushed DC motor remains an excellent model for understanding electromagnetic rotation.
直流电动机原理广泛应用于电池驱动的电钻、电动牙刷、玩具汽车和电脑散热风扇。大型直流电动机还用于电动汽车和工业机械。尽管许多现代电机已经无刷化，但经典的有刷直流电动机仍然是理解电磁旋转的绝佳模型。
12. Summary Checklist | 总结清单
- Motor effect: a current-carrying wire in a magnetic field experiences a force.
- Fleming's left‑hand rule gives the direction of force.
- A rectangular coil in a magnetic field experiences a couple and rotates.
- The split‑ring commutator reverses current every half turn, ensuring continuous rotation.
- Speed increases with higher current, stronger field, more coil turns, larger coil area, and a soft iron core.
- Brushes and commutator enable sliding electrical contact and current reversal.
- 电动机效应：通电导线在磁场中受到力的作用。
- 弗莱明左手定则确定力的方向。
- 磁场中的矩形线圈受到力偶作用而旋转。
- 换向器每半圈反转电流方向，确保线圈连续旋转。
- 增大电流、增强磁场、增加线圈匝数、增大线圈面积以及加入软铁芯都可提高转速。
- 电刷和换向器实现滑动电接触和电流换向。
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