How DC machines work

DC motor

DC motors are often built into inexpensive products that are produced in large quantities. In this post you will learn how a DC motor works and how you can calculate it.

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DC motor definition

A DC motor is, as the name suggests, with DC voltage provided. We find this in cordless tools, for example. Of course there are also engines that come with AC voltage are driven. These are called Three-phase motors. If you want to understand exactly how these work, just have a look at ours Video to do this.

DC motor structure

Let's go through again which components make up a DC motor.

The important components are the stator, the rotatable rotor, the commutator and the Carbon brushes.

The rotor is supplied with power via the sliding contacts. There arises a Magnetic field. In our video "Coil and Induction" we explain why and how this happens. This magnetic field interacts with the magnetic field of the stator. The rule is that unequal Magnetic poles attract each other and the same magnetic poles repel each other, whereby forces act on the rotor and cause it to rotate.

A force acts until the unequal magnetic poles of the rotor and the stator face each other. The polarity of the rotor's magnetic field must then be reversed. This is done through the Commutator bars. The inertia The rotary movement allows the rotor to pass through the dead center position, so that the commutator continues to rotate and the next lamella is supplied with power.

The direction of the current is reversed in the rotor and thus also the magnetic field of the rotor. Attractive and repulsive forces act on the rotor again, causing the rotor to continue rotating.

Permanent magnet direct current motor

The Stator field is either through a permanently excited magnet or generated by an electrically excited coil. In the case of an electrically excited stator field, you should be aware that the DC machine can have either one or two circuits.

In which Shunt, Series connection and Double-wound motor there is only one circuit that supplies power to the stator and rotor strange excited two. The first is used to generate the excitation field in the stator and the second to generate the torque in the rotor.

Separately excited DC motor

"Foreign" simply means that the excitation field has an external or separate circuit. The power of the motor is proportional to the product of the current strength of the rotor and the Excitation field strength:

Now we know how the DC motor works. Let's look at another interesting aspect of the DC machine at. Any DC machine can also be used as a generator be used. In this case, the generator takes up mechanical power and converts it into electrical power. One example of this is the induction brake in electric cars. This feeds electrical energy back into the battery with every braking process and, unlike a mechanical brake, does not wear out.

DC motor calculation

Next we want to deal with the determination of the direct current. In order to be able to calculate a DC motor we have to have a electrical equivalent circuit diagram create. This should take into account that when a voltage is applied to the Anchor clamps, a current flows through the armature winding. The fact that the rotation of the armature and the associated change in area of ​​the coil induce a voltage in it, which is dependent on the motor structure, the excitation field strength and the speed of the motor. And that the armature current is exposed to resistance from the copper windings.

stands for the Clamping voltage, for the Armature current, for the induced voltage and for the resistance in anchor.

The induced voltage is the product of a Machine constant, the magnetic flux the excitation field and the speed .

The following applies to the terminal voltage U:

DC motor generator

A valuable property of electric motors is that they are also called generator can be used. If this is the case, the armature current is negative and the machine outputs power in order to generate electrical power. In the case of motor operation, the armature current is positive and the machine absorbs energy in order to convert this into mechanical power. If the armature current is 0, then the motor is in Neutral.

You can also see which company it is based on the voltages. If the clamping voltage is less than that induced voltage, the machine runs in Generator operationIf it is bigger, it is in the Engine operation. In no-load operation, the clamping voltage is equal to the induced voltage.

The formula of the induced voltage

shows us that this depends on the engine speed. At idle we have a speed of the height n zero. If the speed is now less than that Idle speed the engine mode is again present (n n0).

The electrical power of the DC motor results from the formula:

If we multiply the formula of the terminal voltage by the armature current, we get the formula for the electrical power:

U times is the one electrical powerthat is fed to the motor: P electric. is the Power dissipationthat is lost due to the resistance in the wire: P loss. is the power that is converted into mechanical power: P mechanical.

For the torque we have the formula:


it follows:

and one sets for the clamping voltage a,

so it results: .

This shows that the torque is proportional to the armature current.

With increasing torque, i.e. increasing load, the armature current increases proportionally with it.

DC motor summary

Let's recap what we now know about the DC motor. The DC voltage supplies the DC motor. The stator creates a static field in which a rotatably mounted rotor is placed. This is supplied with electricity via the commutator bars, which ensure a constantly changing magnetic field. The magnetic fields of the rotor and the stator interact with each other, whereby a torque acts on the rotor. Furthermore, every DC machine can also be operated as a generator.