5.1.1　Advantage of Electric Motors

Text（课文）

Let us note first of all that the mechanical torque of the electric motor results from the action of a magnetic induction flux on an electric current.In an electric motor，the flux imposes the dimensions of the magnetic material（iron）；the current imposes the diameter of the wire（generally made of copper）which constitute the coil-windings.

Thus，electric-motor dimensions depend largely on the torque characteristics which we want to obtain.The motor power is equal to the product of the torque by the rotation speed，such that at a given power，the size of the motor is small as its rotation speed is high.

Whatever its nature is—DC motor or AC motor—the electric motor presents a certain number of advantages:

It can provide a torque to all speeds and in particular when stationary.This property permits the elimination of the clutch in the torque transmission chain with the help of a suitable supply control voltage for a DC motor，or with help of the inverter in the case of an AC motor.

It can support short loads，and provide significant overtorques，about 2 to 4 times'the nominal torque，during the acceleration or starting period.

It presents a high thermal inertia；the maximum power is higher than the nominal continuous output.The continuous power/maximum power depends on the size of the motor and its cooling mode.In general，for motors used in electric traction，this ratio is about 1.3.

New words and expressions（单词和短语）

1.New words（单词）

2.Expressions（短语）

Notes to the text（难点解析）

★1.Thus，electric motor dimensions depend largely on the torque characteristics which we want to obtain.因此，电动机的尺寸很大程度上取决于我们所需的转矩特性。

★2.It can provide a torque to all speeds and in particular when stationary.它都可以在任意转速下输出转矩，特别是在静止状态起动时能够提供较大转矩。

Exercises（练习）

◆1.Translate the following passages（expressions）into Chinese（英译汉）

（1）Such that at a given power，the size of the motor is small as its rotation speed is high.

（2）The continuous power/maximum power depends on the size of the motor and its cooling mode.

（3）DC motor

（4）AC motor

★2.Translate the following passages（expressions）into English（汉译英）

（1）机械转矩

（2）额定转矩

★3.Directions:Answer the following questions briefly according to the text

What are the advantages of the electric motor？

Reading material（阅读材料）

In a DC motor the rotational speed is limited:

mechanically by removing risks on binder windings and commutated segments；

electrically by the commutation of the current between the commutator segments.In practice the electric motors used in road-vehicle drive train systems have unit powers lower than 50-60 kW and their supply voltage most of the time remains lower than 200 VDC.

Under these conditions it is possible to manufacture engines with a DC motor，which turn at 5，000 r/min.Motors with AC current，because of the absence of a collector，can reach rotation speeds higher than those of the DC motor:10，000 r/min and a perfectly feasible speed for the powers concerned in conventional road vehicles.As such，the specific power of an AC motor is higher than that of a DC motor.