Wound rotor motor

The slip-ring motor is an electric motor of the type three-phase asynchronous. He differs from those engines usually performed with squirrel-cage rotor, characterized in that the rotor winding is not shorted, but is guided through slip rings to the outside.

The development of the slipring motor goes back to work of Mikhail Ossipowitsch Doliwo - Dobrowolski at the AEG from 1890 and 1891. Doliwo - Dobrovolsky recognized as a result of developments in the squirrel cage that the low starting torque can be improved by increasing the resistance in the rotor circuit. It was led by slip rings, the contacts of the rotor winding to the outside and manually, depending on engine speed and torque load, switched via switch different resistors. The then known as slip-ring rotor motor engine was patented in Switzerland and England, in Germany succeeded no patent is granted.

Slip-ring motors are used for many decades there as driving machines where high starting torques are required combined with a low starting current. They are increasingly being replaced in practical applications while operating with electronic frequency converters conventional asynchronous motors with squirrel -cage rotor without slip rings, as can be achieved by the variable frequency and thus controllable rotating field with a conventional squirrel cage high starting torques and thereby the disadvantage of the slip rings and their wear is avoided.

• 5.1 advantages
• 5.2 disadvantages

• 6.1 Examples of use

Basics

The slip ring motor is basically a form of rotary transformer, three-phase alternating current transformer with revolving secondary winding, dar. Either the secondary winding by connecting with resistors to be influenced from the outside short-circuit winding, or the rotor winding is supplied with an auxiliary voltage variable frequency. The former allows the start-up and speed points with simple means by starting resistors in the form of the slip-ring motor, the latter which was technically difficult to realize in earlier times without electronic drive but a more efficient speed setting.

A variant represents the resistance of the rotor, which is constructed similar to a squirrel-cage rotor, whose resistance is but artificially increases in short-circuit rotor and thus similar start-up behavior like a slip-ring motor with fixed, external resistors has.

Construction

The stator of the slip-ring motor is the same design as the stator of the squirrel-cage induction motor. On the rotor shaft are the core and the slip rings. Depending on the size of the motor either a ridge wave or a cylindrical shaft used. On the shaft is the core stack, in which there are groove-shaped grooves shrunk. The rotor winding is inserted in the slots of the rotor core. The leading to the slip rings coil ends are secured as well as in commutator motors with a bandage against centrifugal forces.

The rotor winding has a smaller conductor cross-section than in the squirrel cage rotor and a corresponding number of turns, so that the entire copper surface is approximately the same for both types. For this reason, the induced voltage and the effective resistance is substantially larger than is the case in the cage rotor. The current is reduced, allowing the transfer of slip rings and carbon brushes is possible. On phase network, the teeth of rotor and stator may not be exactly above each other at a standstill, because otherwise the engine will not start. For this reason, rotor and stator must have different Nutenzahlen.

To allow easier access to carbon brushes, slip rings are often housed in a separate housing from the motor housing. This allows for easy maintenance of the slip rings and the brushes. In addition, the brush abrasion (soot) is kept away from the actual engine. Maintenance of slip ring apparatus basically comprises the exchange shutdown state brushes and cleaning.

The rotor winding is a three-phase winding in the rule. The windings are usually rare in star, delta-connected. The star point of the windings is connected in the interior of the rotor. In some motors, the star point is conducted via a fourth slip ring outwards. This star point connection is denoted by Q. The winding ends are connected to slip rings on which lie as a current collector brushes. The three-stranded rotor winding has the connection designations K, L, M.

In addition to the three-phase rotor winding there are also slip-ring rotor with two-phase rotor winding. This design is chosen in order to save costs when starter. Instead of three starting resistors, this one requires only two starting resistors. This type of winding is used in particular like machines with great performance. The strands of the two-phase windings are connected in a V-connection. The two-phase rotor winding has the connections K, L. Here, too, the interconnection within the winding. Sometimes the center of the rotor windings is fed through a slip ring to the outside. This connector is labeled with Q. In any case, the slip-ring is made with at least three slip rings.

On the functioning of the machine, it has no effect if the rotor winding is three-phase or two-phase. However, the rotor and the stator must have the same number of poles; rotor and stator have different numbers of poles, no torque is generated.

About the slip rings of the slip-ring motor can be started by means of power resistors. The start-up circuit is electrically isolated from the supply mains. Among other things, at Carousel drives, variable resistors were common, which consisted of different stages immersed in an electrolyte electrodes. The liquid is heated at the same time was a simple and safe method of heat dissipation, the change in the depth of immersion of the electrodes was carried out for example via a pedal.

There are two types of slip rings:

• Slip rings with permanent contact brushes
• Slip ring with brush lifting.

Slip rings with permanent carbon brushes

The slip rings are usually made of a copper / tin / nickel alloy and are provided with a radial arrangement usually with a spiral groove. Such continuously contacted with carbon brush slip rings are used for smaller engines. The carbon brushes are held by the brush holder in the respective position.

Slip ring with brush lifting

Slip ring with brush lifting device are active only during the start-up phase. You have a score-free and smooth surface and are made of stainless steel. They are found mainly in engines with an output of 20 kW. Due to its complicated mechanics they lose more and more important.

Via the slip rings, the armature windings are accessible from outside and can be short-circuited through resistors defined. ( Has booted ) After the engine has reached its rated speed, the rotor windings are shorted initially via the slip ring short-circuiting device, then lifted the brushes from the brush lifting of the slip rings. In the rotor circuit, no additional losses occur more and the wear of carbon brushes and slip rings is limited to the startup. By lifting the carbon brushes which contact resistance between the slip rings and carbon brushes are avoided. In addition, the friction between the carbon brush and eliminates slip rings, this leads to a slight increase in efficiency.

Operation

Slip-ring motors are induction motors and act like squirrel-cage motors. At standstill, the rotor and stator act like a transformer. The stator rotating field causes in the windings of the rotor, a flux change. Thus, a voltage is induced in the rotor winding, it is called rotor standstill voltage. The rotor standstill voltage is indicated on the motor nameplate. The height of the rotor standstill voltage is measured from the line voltage for which the motor is independent.

The rotor standstill voltage can be measured at the open slip rings. Use can also determine if the rotor is wound three-phase or two-phase:

The voltages of the three-phase winding between all three slip rings of equal size. In two-phase winding, the voltage between the terminals K and Q and terminals L and Q are equal. Between the terminals K and L, the voltage √ 2 times as large as that between the other terminals.

If the rotor terminals short-circuited, calls induced in the rotor voltage in the rotor winding produces a current flow. This current flowing in the rotor windings consists of three mutually phase -shifted AC currents. The induced phase current generates the rotor rotating field. The rotor rotating field causes together with the stator rotating field torque. The rotor speed box retains regardless of rotor speed always the same position to the stator rotating field. Therefore, the slip-ring motors can not fall out of step.

Performance

With short-circuited rotor connections the performance of the slipring motor of the squirrel-cage induction motor. The torque and speed characteristic is the same as that of the squirrel-cage induction motor. The inrush current is about 6 times as large as the rated current. The starting torque is approximately 1.5 times as large as the rated torque.

With short-circuited slip rings acts in the rotor circuit mainly the reactance of the rotor winding. This reactance induces a phase shift between the induced armature voltage and the armature current. Due to this phase shift, the rotor rotating field moves. This displacement of the rotor rotating field causes the poles of the rotor rotating field lie just below the poles of the stator rotating field. This means that only a force in the direction of the wave is then applied. Since, however, the effective resistance of the coil is also present, the phase shift between the armature voltage and armature current is a little smaller than 90 °, this creates a small torque.

When the rotor turns in the rotating field direction, the frequency of the rotor current decreases. Due to the lowering of the rotor frequency the reactance of the rotor winding decreases with the inductance.

The ohmic resistance remains constant, the phase shift between voltage and current decreases and the unfavorable position of the rotor poles with the stator poles is improved. The smaller the phase shift between the rotor voltage and rotor current is, the greater the torque.

With increasing speed, the induced voltage in the rotor decreases, which lowers the rotor current and the torque. , The torque decreases when the reduction of the induced voltage is predominant. The torque increases when the reduction of the phase shift is predominant.

If resistors are connected in the rotor circuit, the phase shift between the armature voltage and the armature current is reduced. Thus, the starting current is considerably reduced. Another beneficial aspect is that by switching resistors in the rotor circuit, the starting torque of the motor is greater than with short-circuited rotor circuit. In addition, the tipping point shifts to lower speeds. The displacement is the larger, the greater the resistance. The slip ring motor starting resistors shows with better starting characteristics than the squirrel cage induction motor with rotor current displacement.

With slip -ring motors which occurs at rated speed rated slip is 3 to 8%. By increasing the slip induced rotor voltage is increased and thereby increases the armature current. At the same time increases at larger slip the rotor frequency and the engine is from a larger torque. With increasing slip, however, deteriorates the efficiency of the engine.

By the interposition of resistors in the rotor circuit, the slip increases. This is because the rotor also in addition, the power consumption of the intermediate resistors must be provided by the higher induction action of the rotating field. These adjustable resistors, the slip is a continuous change in a wide range speed control accessible. However, the speed control by means of resistors in the rotor circuit requires a motor load with constant torque. Since the speed reduction leads by means of resistors in continuous operation at high current heat losses, it is uneconomical for large motor performance. The force generated by the slip in the rotor power loss is called rotor power loss or even slip power. Through a cascade can this slip power recover. For this purpose, a second slip ring motor is mechanically coupled to the first motor and powered by the rotor winding. A further possibility is the coupling via an inverter.

Pros and Cons

Benefits

• High starting torque
• Low start-up current
• Speed ​​control with simple means
• Possible influence on the rotor circuit
• Soft start possible with large load

Disadvantages

• Maintenance intensive
• Long start-up phase
• Ill-suited for short-time operation
• Lower efficiency than induction motors, especially at low speeds

Areas of application

Slip-ring motors are used everywhere, where their benefits outweigh ( high starting torque combined with low start-up current ). This is the case particularly in the high power range. Especially in a weak power system are slipring in certain cases a cost-effective drive solution over squirrel-cage motors with frequency converters. This is especially true if the work machine the motor oppose large moments of inertia or counter moments. In weak networks, large inverters can adversely affect the sinusoidal shape of the mains voltage, also provides the slip-ring motors an alternative.

The advantages of the slipring motor outweigh benefits from 630 kW and supply voltages from 6000 volts. For all other applications, the combination was able to prevail in squirrel-cage induction motor and frequency converter.

Examples of use

• Drive large machine tools
• Drive from stone crushing machines
• Drive mills
• Large water pumps and fans
• Hoists
• Large cranes
• Drives with full load or heavy running

In agricultural buildings slip ring motors must not be used because of the increased risk of fire.

For a long time the slipring is the dominant driving force for speed- controlled drives. On cranes he was for many years the most widely used drive motor. Due to the ever cheaper and better and better converters of the slipring motor was pushed out of its dominant position in specialty areas where his good start characteristics are useful. The achievable with simple technical means good starting characteristics are the reason that the wound-rotor motor is also still used today in specialized areas.

Legal regulations and other rules

• EN 60 034 Part 1 General requirements for rotating electrical machines
• EN 60 034 Part 8 Terminal markings and direction of rotation for electrical machines
• DIN IEC 34 Part 7 types rotating electrical machines
• EN 60034-12 startup behavior, single-speed electric machines on voltage > 660 volts
• EN 60034-5 protection of rotating electrical machines
• EN 60034-6 types of cooling for rotating electrical machines