DC motor

A DC machine, and direct current motor, a commutator motor or commutator, a rotary electric machine which is operated with direct current or DC power generated. Depending on the direction of the power flow ( an electric motor, the electric power is supplied and removed mechanical energy) between the DC motor and the DC generator distinguished ( an electric generator ). DC machines have good starting behavior and good controllability.

Characteristic of the conventional direct-current machine is a mechanical inverter, a commutator which ( Rev. ) is referred to, and is mounted on the axle of the rotating machine. It is used in motor mode as Rev. generated in the rotor and a speed-dependent alternating current. In the generator mode, he directed supplied by the rotor alternating voltage and produces a pulsating DC voltage. In some applications, the DC machine can be operated as a motor or as a generator.

A special form of the DC machine without commutator constitutes Unipolarmaschine

• 2.3.1 coreless machines
• 2.3.2 Brushless DC Machine

• 2.4.1 armature reaction
• 2.4.2 reverse voltage

History

Benefiting from the development of the first galvanic elements in the first half of the 19th century, the first electromechanical energy converters were DC machines. In 1832 the Frenchman Hippolyte Pixii built the first DC Generator. Antonio Pacinotti built around 1860, a DC motor with commutator vielteiligem. Friedrich von Hefner - Alteneck in 1872 he developed the drum armature which to the dynamo-electric principle opened the possibility of self-excitation and industrial use in the area of ​​large-scale engineering with the work of Werner von Siemens.

In the following decades, lost the DC machine, due to the development of the three- phase alternating current, the large machine in importance. In particular, the synchronous machines, and for maintenance of the induction motor drive systems triggered DC machine as in many applications. Through the good controllability of the DC machine with separate excitation, this is due to the separate control of the current through both the armature winding and through the rotor winding, the DC machine has retained some importance especially in the area of highly dynamic drive systems. These include drive motors in machine tools with precise speed and torque control. In small power range, as for example in model railways, particularly the permanent-magnet DC motor of the simple structure is common for.

Construction

The machine has a stationary part, the stator. It consists of a yoke in the form of a hollow cylinder. Attached to it are salient poles. The main poles and in larger machines also commutating poles. The stator is not laminated, but is made of solid material, as there is no alternating magnetic field acts and thus no eddy currents occur. The Hauptpolkern carrying the main pole excitation coil or, unless the machine is excited by permanent magnets. This permanent magnets generate the necessary Hauptpolfluss. In the pole pieces sitting in larger machines the compensation winding.

The rotatably mounted part of the DC machine is called rotor or conventional machines also anchor. The rotor is laminated, as otherwise large eddy current losses would occur due to the magnetic reversals occur. Most DC motors are designed as internal rotor: The rotor is the inner part of the outer stator. With external rotor is reversed. For smaller machines, the stator may consist of permanent magnets, these are called permanently excited DC machine.

One or more coils on the armatures (stator) placed in a magnetic field, a torque is produced by the Lorentz force. The windings of the armature are connected thereto via a commutator ( Rev. ). The sliding contacts on the commutator ( brushes or brushes ) are arranged so that they change so during the rotation of the polarity of the armature windings that getting those coils are traversed by current corresponding direction that move transversely to the exciter field.

The brushes are made of a material which provides a low-wear good contact (often self-lubricating graphite, partially mixed with copper powder, with small motors for cassette tape recorders also precious metal brushes are used, see carbon brush ).

By reversing the principle (anchor is moved ), we obtain a generator. To use as a generator to produce electricity only permanently - or separately-excited designs are used. The commutator allow the transformation of the generated alternating current to direct current.

The general motor behavior is determined by the field strength of the field winding and the properties of the armature winding ( winding number, number of poles ).

DC Permanent Magnet Machine

The stator magnetic field with smaller engines (toys, actuators, fan and radiator fans in motor vehicles) often produced by permanent magnets. These magnets have become more and more powerful with the development of DC motors and allow today the construction of engines, as any in their performance with electrical excitation into nothingness. However, the cost of the permanent magnets are often higher than those of a field winding on larger engines.

Permanent magnet motors have, as well as separately excited machines, very high inrush currents. Your performance is explained in the mathematical foundations. Permanent-magnet machines have the advantage, that for generating the magnetic field energy is not required. This improves the efficiency especially at low overall performance. The disadvantage is that the field weakening is impossible and thus the possible rotational speed range is smaller.

Electrically excited DC machine

If the stator field generated by an electromagnet, one speaks of electrical excitation. If the field winding from the armature circuit independently, it is called external excitation. The rotor and stator winding connected to one another it may be:

Series-wound machine

The series motor is also called the main circuit machine with him excitation winding and armature winding are connected in series. The excitation winding, therefore, has to be low in contrast to the shunt motor. At a power supply with AC excitation field and to change both of the armature current according to its direction each half cycle, so that the motor can also be used with AC voltage. , The iron core of the stator, this must, however, in order to avoid eddy currents, consisting of a laminated core.

Single-phase series motors are in older electric locomotives (hence the frequency 16.7 Hz compromise in traction power network) to find and in tram railcars. The term universal motor universal motor or drive them to many household appliances such as vacuum cleaners, food processors, drills and other electric hand tools. The starter of large-volume combustion engines, such as by truck, are series-wound motors.

To a series motor to operate (eg the electric brakes of trams ) as a generator, its excitation winding needs to be reversed, otherwise the generated, flowing through the field winding current raises the exciting field on.

The torque of a series-wound machine is strongly dependent on speed ( series circuit behavior ). At low engine speeds, the back-emf of the armature winding is low. Therefore, a very large current flows through the armature, and thus for the series circuit and through the excitation winding. As a result, can be applied by the machine at standstill and at low speed, a very large torque. Because the back-emf in the circuit increases with the speed, the current decreases by the same. The excitation field is weakened in comparison to a standstill and thus the torque of the engine.

Have series wound motors, especially for AC voltage operation (" universal motor ", eg in vacuum cleaners ), a much lower inrush current as a shunt or permanent magnet motors. However, they provide short duration and high starting torque. That is why they are used in starters, trams and electric locomotives, where they are extremely overloaded in the short operation.

For operation with AC power which is a pulsating ( double mains frequency) performance. Therefore, the torque is pulsating, so that for large motors elastic elements must be interposed. This also applies to single-phase synchronous machine.

The terminals of the armature is denoted by A1 and A2. The excitation winding with D1 and D2 In the illustrated circuit, the motor rotates clockwise (clockwise), recognizable by the arrow drawn in the armature.

Shunt machine

In the shunt machine excitation and armature winding are connected in parallel. The exciter current is limited only by the ohmic resistance of the exciter winding, having a high number of turns and the inductance. An AC voltage operation is hardly possible, since the pathogen to the armature current would lag far. The speed of large shunt machine is almost independent of torque, making them particularly suitable for constant-speed drives with variable load moment, such as conveyor belts and hoists. In all of these areas but also the induction machine is penetrated.

Shunt motors can go in case of interruption of the excitation circuit, since the collapse of the excitation field, the speed and power consumption for the same supply voltage rises dramatically.

Shunt motors as a generator (eg for braking) work if an auxiliary voltage source or a residual magnetization ensure that at the start of the braking process excitation is present. With increasing excitation or speed, the generated voltage increases - it's the tension that makes counteracts the supplying power even when the engine operation and a constant speed. It is therefore also referred to as back-emf. The increase of the back EMF to the energization, ie, at shunt-wound motors with the supply voltage, causing their speed depends little on the voltage, as long as no magnetic saturation occurs. With decreasing voltage, the speed rigidity decreases. In a separately excited DC motor with independent -Powered, constant excitation field, however, the idle speed is proportional to the armature voltage.

The terminals of the armature is denoted by A1 and A2, that of the excitation winding E1 and E2. In the illustrated circuit, the motor rotates clockwise (clockwise), recognizable by the arrow drawn in the armature.

The maximum achievable torque is limited by the allowable armature current, this is mainly dependent on the assumptions cooling measures. Large shunt machines in mills are forced ventilation, even at low speeds to high armature current, and thus enable a high torque.

If a shunt machine suddenly applied to its nominal operating voltage flows through the armature, a very high inrush current, which can cause the protective devices. Large machines must therefore be approached with a lower voltage. Characterized the characteristic curve is shifted in parallel at a low rotational speeds, so that it intersects the axis of the moment in a region outside of the overload. The starting torque and the armature current at standstill are then limited. Together with the consequent increase of the driving speed can the voltage be increased. Alternatively can be used for starting resistors in the armature circuit, thereby the characteristic flat, so that they in turn intersects the axis in a region outside of the overload. The disadvantage of this method is the power loss in the resistance, it must then also be cooled actively.

Composite machine

The compound motor (also composite or compound engine called ) combines the properties of the secondary and the series motor in a machine. He has a series circuit and a shunt winding. Depending on the design of the compound motor has different operating characteristics. With proper compounding it has a slightly lower torque than an equivalent series motor. His speed drops under load then something more than that from a corresponding shunt motor. At idle it does not go through. If the compound motor überkompoundiert, he has mainly series circuit behavior, ie, a high starting torque, but an unstable speed. In Unterkompoundierung he has predominantly shunt behavior, ie high speed stability, but less torque. The compound motor is used because of its same speed-torque behavior for driving, for example, by pressing and punching.

Separately excited machine

Here armature and field winding composed of two distinct and separate adjustable DC choppers are fed. While in the shunt machine, the excitation voltage is equal to the armature voltage, you can boost for separately-excited machines by reducing the field current, it is called in this case from a field weakening of the magnetic flux Φ, the speed n of the rated speed nN addition. In the right graph, this corresponds to the range of n / NN is greater than 1, where it is at the same time a reduction of the torque M. On the other hand, can be reduced regardless of the excitation current in the machine, the separately excited armature voltage UA. This results in constant torque to a power reduction and simultaneous speed reduction.

The separate control and controllability of the armature and field windings, different characteristic fields below the red dashed line ( torque curve ) by the type of control can be achieved. Advantages such as high torque at standstill or low speed are given. Therefore, in particular separately excited DC machines like the DB Class 420 played in the field of highly dynamic drive systems, such as machine tools or electric urban rapid transit train set up in the 1980 years into a significant role. DC machines were increasingly replaced in subsequent years in these applications due to the availability of electronic frequency converters in combination with three-phase machines, which offer the same benefits for highly dynamic drives with less maintenance in combination.

Separately excited DC machines were previously used in Leonardsatz, the formerly single variable speed drive for high performance, which consisted of a three-phase asynchronous motor, each a separately excited DC generator and DC motor.

Special designs

Coreless machines

Small machines to about 100 watts with permanent magnets can also be constructed with a hollow rotor. The rotor is ironless, self-supporting wound and impregnated with synthetic resin. The stator, a permanent magnet in this case is inside the rotor. The outboard motor housing made of iron is the necessary conclusion for the magnetic flux of the stator. The electrical structure corresponds to the first illustration. Through the ironless rotor design, the engine no cogging torque is formed, it can be turned completely free. Since, in contrast to all other motors in operation, no iron parts must be re-magnetized, this engine is free from iron losses and achieved at high speeds higher efficiencies. In particular, but his moment of rotational inertia is less, so therefore highly dynamic, lightweight actuators can be realized. Equipped with coreless motor model railway vehicles are characterized by smooth running and good low-speed handling characteristics. A disadvantage is the large air gap in excitation circuit which has a reduced magnetic flux density. The self-supporting construction provides high technological requirements, since the centrifugal forces must be included and a subsequent balancing of the anchor through material removal is not possible. Necessary impact resistance limits the anchor size.

The disk motor has a similar structure, but the rotor winding is not in the form of a cylinder, but as a disk.

Brushless direct-current machine

Disadvantage of DC machines are sparks that arise ( " brush fire " ) at the brushes. The brush fire is the main cause of high-frequency interference, which feeds back the motor is in operation in the pipe network and interfere with other electrical appliances. It also limits the maximum rotational speed, since the brushes are hot at high speeds and wear out very quickly. Furthermore, cause high speeds even higher induced voltages that can lead up to the revolving brush fire.

With the development of electronic small permanent magnet synchronous motors can be operated so that they can be similarly described from the outside as a DC machine. These engines with electronic inverter were promoted particularly in the English language as a brushless direct current (BLDC ) in German translated brushless DC machine. The machine is also referred to as EC - motor (EC for electronically commutated ). Structurally, these engines undamped permanent-magnet synchronous machines are identical and can be used in applications that have sufficient inherent damping, also be driven as a synchronous machine.

Special Effects

Armature reaction

Since the armature current is removed, is also formed by this magnetic field. This reinforces the main field on the one side of the conductor and weakens it at the other. Overall, this leads to shifts of the neutral area in which the polarity of the current to be switched, somewhat belatedly, that is, in the generator mode in the rotational direction and in motor operation against the direction of rotation. However, since not the commutator adjusts (ie always perpendicular switches to the pole shoes and not perpendicular to the "effective" field lines ), located at the time of switching (commutation ) nor an induced voltage at the carbon brushes, and it comes to sparking the brush fire. In systems that require a uniform torque and are used only for one direction ( eg strong fan), the brush fire can be reduced by the brush holder is rotated slightly fitted and in normal operating condition but switches perpendicular to the effective field lines. However, this requires an adjustment in operation and is rarely performed today for reasons of cost. Instead, in large machines commutating and compensating windings are used, which virtually " turn back " the field lines in the ideal location.

Counter voltage

The armature rotates within the stator motor. After the generator principle, a voltage is induced in the coils as well as when the engine is operating. This induced voltage is polarized as the operating voltage, and therefore counteracts the rotor current. It is referred to as offset voltage or back-emf. It is an important parameter of engines, with their help can be in about the idle speed permanent-magnet motors determine.

Of the armature current causes a voltage drop across the ohmic resistance of the anchor (copper), this voltage drop thus increases with the load on the engine ( increased power consumption ) and results in a drop in the engine speed. For large separately excited motors, this speed reduction is very low.

The back EMF is strictly a linear function of the rotational speed of the armature and the strength of the excitation. The back EMF can be used by control circuitry to stabilize the rotational speed permanent-magnet motors exactly; this is applied eg cassette tape recorders.

The back EMF makes at reversing the current ( terminal voltage < EMF) from the engine a generator, it can be used for braking and energy recovery ( regenerative braking ).

With the engine stopped there is no counter voltage. Therefore, foreign - and permanent-magnet DC motors have a high starting power - the resistance of the rotor coil is relatively small and thus the current very large at the moment when. Without limiting the starting current large motors or the supply system are possibly overloaded, we therefore used in series with the armature starting resistors, which are short-circuited gradually after the run. The series wound motors for trams were previously approached via travel switch ( resistance levels ), today this lower losses on switching regulator ( chopper operation ) is achieved. In electric locomotives was used transformers with tap changers at which smaller variable transformers from stage to stage " dangled ". Again, you used today instead power electronics ( IGBT switches )

Mathematical Foundations

Under Default consumer - counting arrow system ( as assumed, for example, the Ohm's law ) applies

Substituting the current constant over time requires follows

Also taking into account the law of induction, it becomes

This equation can be interpreted as follows: For a constant and small in practice, the induced voltage is slightly smaller than. Thus, at constant torque n is approximately proportional to the armature voltage. In the area so that the rotational speed of the armature voltage controllable. One speaks of the armature setting range. In the event, and we speak of type point. Type is above the point at constant armature voltage, a speed increase with a decrease of the magnetic flux to a reduction in the excitation current store ( field weakening ). Here, however, some constraints must be observed. The speed must not exceed a maximum value permitted. Because of the effect of the Lorentz force is considered and therefore the permissible torque M is proportional with less.

It is

The equations of the mechanical system with the assumption that the excitation circuit is not saturated:

It is

Some equations written differently:

Here is: