Slip ring motor

Small slip ring motor with 2.9 kW power and flat belt pulley from the 1930s

The same motor with exposed slip rings. Also on display: Switch (White), CEE DC plug (red) and Adjustable- start resistor (black)

The slip ring motor is an electric motor of the three-phase asynchronous machine type . It differs from those usually with squirrel-cage rotor motors carried by the fact that the rotor winding is not shorted, but via slip rings is guided to the outside. The development of the slip ring motor goes back to the work of Michail Ossipowitsch Doliwo-Dobrowolski at AEG in 1890 and 1891. As a result of the developments on squirrel cage rotors, Doliwo-Dobrowolski recognized that the low starting torque can be improved by increasing the resistance in the rotor circuit. For this purpose, the contacts of the rotor winding were led to the outside via slip rings and, depending on the engine speed and torque load , different resistors were switched on manually via step switches . The motor, known at the time as slip ring armature motor, was patented in Switzerland and England, but no patent was granted in Germany. Slip-ring motors have been used as drive machines for many decades where high starting torques are required with low starting current. In practical applications, they are increasingly being replaced by conventional asynchronous motors operated with electronic frequency converters with squirrel-cage rotors without slip rings, since the variable frequency and the controllable rotating field also enable high starting torques to be achieved with a conventional squirrel-cage rotor, thereby avoiding the disadvantage of slip rings and their wear and tear.

Basics

The slip ring motor is basically a form of rotary transformer , a three-phase alternating current transformer with a rotatably mounted secondary winding. Either the secondary winding is connected to a short-circuit winding that can be influenced from the outside, or an auxiliary voltage of variable frequency is applied to the rotor winding. The former allows starting up and speed setting with simple means through starting resistors in the form of the slip ring motor, the latter a more efficient speed setting, which in earlier times was technically difficult to achieve without electronic frequency converters. A variant is the resistance rotor, which is constructed in a similar way to a squirrel-cage rotor, but whose resistance in the squirrel-cage rotor is artificially increased and thus has a start-up behavior similar to that of a slip-ring motor with fixed, external resistors.

construction

Connection of resistors for start-up, at nominal speed on short circuit

Schematic structure with slip rings and winding

Slip rings of an electric motor

The stator of the slip ring motor has the same structure as the stator of the squirrel cage motor . The laminated core and slip rings are located on the rotor shaft . Depending on the size of the motor, either a ribbed shaft or a cylindrical shaft is used. The laminated core, in which there are groove-shaped grooves , is shrunk onto the shaft . The rotor winding is inserted into the slots in the laminated rotor core . The coil ends leading to the slip rings are secured against centrifugal forces with a bandage, as is the case with commutator motors .

The rotor winding has a smaller conductor cross-section than the squirrel cage rotor and accordingly has a large number of turns, so that the total copper area is roughly the same for both types. For this reason, the induced voltage and the effective resistance are significantly greater than is the case with a squirrel cage. The current is lower, which enables transmission via slip rings and carbon brushes . On the single-phase network, the teeth of the rotor and stator must not be exactly on top of each other when the motor is at a standstill, otherwise the motor cannot start. For this reason, the rotor and stator must have different numbers of slots.

To allow easier access to the carbon brushes, the slip rings are often housed in a housing that is separate from the motor housing. This enables easy maintenance of the slip rings and the carbon brushes. In addition, the brush abrasion ( soot ) is kept away from the actual motor. The maintenance of the slip ring apparatus essentially comprises replacing brushes that have been shut down and cleaning.

The rotor winding is usually a three-phase winding. The windings are mostly connected in star , less often in delta . The star point of the windings is connected inside the rotor. With some motors, the star point is routed to the outside via a fourth slip ring. This star point connection is referred to as Q. The winding ends are connected to slip rings to which carbon brushes are used as current collectors. The three-strand rotor winding has the connection designations K, L, M.

In addition to the three-phase rotor winding, there are also slip ring rotors with two-phase rotor windings. This design is chosen in order to save costs on the starter. Instead of three starting resistors , you only need two starting resistors. This type of winding is particularly popular with machines with high performance . The strands of the two-phase winding are connected in a V connection. The two-phase rotor winding has the connections K, L. Here, too, the interconnection takes place within the winding. Sometimes the center point of the rotor windings is also led to the outside via a slip ring. This connection is labeled Q. In any case, the slip ring rotor is made with at least three slip rings .

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

The slip ring motor can be started using power resistors. The starting circuit is isolated from the supply network . In the case of carousel drives, among other things, variable resistors were also common, which consisted of electrodes immersed in an electrolyte to different lengths. The heated liquid was at the same time a simple and safe method of heat dissipation; the immersion depth of the electrodes was changed using a pedal , for example .

There are two types of slip rings:

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

Slip rings with permanent carbon brushes

The slip rings usually consist of a copper / tin / nickel - alloy and are provided in a radial arrangement usually with a spiral groove. Such slip rings, which are permanently contacted with carbon brushes , are used in smaller motors. The brush holders hold the carbon brushes in their respective positions.

Slip rings with brush lifting device

Functional principle of a brush lifting device with a short circuit of the rotor

Slip rings with a brush lifting device are only active during the start-up phase. They have a smooth, grooveless surface and are made of stainless steel. They are mainly found in engines with an output of 20 kW or more. Due to their complicated mechanics, they are becoming increasingly less important.

The armature windings are accessible from the outside via the slip rings and can be short-circuited in a defined manner using resistors. After the motor has reached its nominal speed (has run up), the rotor windings are first short-circuited via the slip ring short-circuiting device, then the carbon brushes are lifted off the slip rings by the brush lifting device. Additional losses no longer occur in the rotor circuit and the wear and tear of carbon brushes and slip rings is limited to acceleration. By lifting the carbon brushes, the contact resistance between the slip rings and the carbon brushes is avoided. In addition, there is no friction between carbon brushes and slip rings, which leads to a slight increase in efficiency.

Mode of action

Two-phase rotor circuit of the slip ring motor

Example: nameplate of
a slip ring motor

Slip ring motors are induction motors and act like squirrel cage motors . When stationary, the rotor and stator act like a transformer . The stator rotating field causes a change in flux in the windings of the rotor . This induces a voltage in the rotor winding; it is called the rotor standstill voltage. The rotor standstill voltage is specified on the motor nameplate. The amount of the rotor standstill voltage is independent of the line voltage for which the motor is dimensioned.

The rotor standstill voltage can be measured on the open slip rings. You can also determine whether the rotor is wound three-phase or two-phase:

The voltages of the three-phase winding are the same between all three slip rings. In the two-phase winding, the voltages between terminals K and Q and between terminals L and Q are the same. The voltage between terminals K and L is √2 times greater than that between the other terminals.

If the rotor connections are short-circuited, the electrical voltage induced in the rotor causes a current to flow in the rotor winding. This current flowing in the rotor windings consists of three alternating currents out of phase with each other . The induced three-phase current generates the rotor's rotating field. The rotor rotating field together with the stator rotating field produces a torque. The rotor rotating field always maintains the same position relative to the stator rotating field regardless of the rotor speed. Therefore, the slip ring motor cannot fall out of step.

Operating behavior

If the rotor connections are short-circuited, the operating behavior of the slip-ring motor corresponds to that of a squirrel-cage motor. The torque or speed characteristic is exactly like that of the squirrel cage motor. The starting current is about 6 times as large as the rated current . The starting torque is about 1.5 times as large as the nominal torque.

With short-circuited slip rings it is mainly the reactance from the rotor winding that acts in the rotor circuit. This reactance causes a phase shift between the induced rotor voltage and the rotor current. This phase shift shifts the rotor rotating field. This shift in the rotor rotating field causes the poles of the rotor rotating field to lie just below the poles of the stator rotating field. As a result, only one force is then exerted in the direction of the shaft. However, since the effective resistance of the winding is also present at the same time, the phase shift between the rotor voltage and the rotor current is a little less than 90 °, which results in a small torque .

When the rotor rotates in the rotating field direction, the frequency of the rotor current drops . When the rotor frequency drops, the reactance of the rotor winding (the inductance ) drops . ${\displaystyle f}$ ${\displaystyle X_{L}}$ ${\displaystyle L}$

${\displaystyle X_{L}=2\pi \cdot f\cdot L}$

Torque as a function of the power resistances, ${\displaystyle R_{4}>R_{3}>R_{2}>R_{1}}$

If the ohmic resistance remains constant, the phase shift between voltage and current becomes smaller and the unfavorable position of the rotor poles in relation to the stator poles is improved. The smaller the phase shift between the rotor voltage and the rotor current, the greater the torque.

As the speed increases, the voltage induced in the rotor decreases, so the rotor current and the torque decrease. The torque decreases when the reduction in the induced voltage predominates. The torque increases when the decrease in phase shift predominates.

If resistors are switched into the rotor circuit, the phase shift between the rotor voltage and the rotor current is reduced. This significantly reduces the starting current. Another useful effect is that switching on resistors in the rotor circuit increases the starting torque of the motor than if the rotor circuit is short-circuited. In addition, the tipping point shifts towards lower speeds. The greater the resistance, the greater the shift. The slip ring motor with starting resistors shows better starting properties than the squirrel cage motor with a current displacement rotor .

In slip ring motors, the nominal slip that occurs at nominal speed is 3 to 8%. By increasing the slip, the induced rotor voltage is increased and this increases the rotor current. At the same time, the rotor frequency increases with greater slip and the motor outputs a greater torque. However, as the slip increases, the efficiency of the motor deteriorates .

The interposition of resistors in the rotor circuit increases the slip . This is due to the fact that, due to the higher induction effect of the rotating field on the rotor, the power requirement of the interconnected resistors must also be provided. If these resistances are infinitely variable, the change in the slip enables infinitely variable speed control within wide limits. The speed control by means of resistors in the rotor circuit, however, requires a constant torque on the motor. Since the speed reduction by means of resistors leads to high current heat losses in continuous operation , it is uneconomical with high engine outputs. The power loss resulting from the slip in the rotor is called the rotor power loss or also the slip power. This slip performance can be recovered through a cascade connection. For this, a second slip-ring motor is mechanically coupled to the first motor and fed via the rotor winding. Another possibility is the coupling via an inverter.

Advantages and disadvantages

advantages

• High starting torque
• Low starting current
• Speed ​​control with simple means
• Influence on the circle of runners is possible
• Soft start-up possible with heavy loads

disadvantage

• Maintenance intensive
• Long start-up phase
• Badly suited for short-term use
• Lower efficiency than squirrel cage motors, especially at low speeds

Areas of application

Slip ring motors are used wherever their advantages (high starting torque combined with low starting current) predominate. This is particularly the case in the high-performance sector. Especially in weak power grids , slip ring rotors are in certain cases a cost-effective drive solution compared to squirrel-cage motors with frequency converters . This is especially true when the work machines oppose the motor with large moments of inertia or counter-torques. In weak networks, large converters can have a detrimental effect on the sinusoidal shape of the network voltage ; here, too, the slip-ring motor offers an alternative. The advantages of the slip ring motor outweigh the power from 630  kilowatts and operating voltages from 6000  volts . In all other applications, the combination of squirrel cage motor and frequency converter prevailed.

Application examples

• Drive large machine tools
• Drive of stone breaking machines
• Driving mills
• Drive of cable cars and drag lifts
• large water pumps and fans
• Hoists
• large crane systems
• Drives with full load or heavy running

Slip-ring motors must not be used in agricultural facilities because of the increased risk of fire .

For a long time, the slip ring rotor was the dominant drive motor for speed-controlled drives. For many years it was the most popular drive motor in crane systems. Due to the ever cheaper and improving converters, the slip ring motor was pushed from its dominant position into special areas where its good starting properties are useful. The good starting properties that can be achieved with simple technical means are the reason why the slip ring motor is still used in special areas today.

Statutory provisions and other regulations

• EN 60 034 Part 1 General provisions for rotating electrical machines
• EN 60 034 part 8 Terminal designations and direction of rotation for electrical machines
• DIN IEC 34 Part 7 Types of rotating electrical machines
• EN 60034-12 Start-up behavior of single-speed electrical machines at voltage> 660 volts
• EN 60034-5 Degrees of protection of rotating electrical machines
• EN 60034-6 Types of cooling, rotating electrical machines

literature

• Hans Günter Boy, Horst Flachmann, Otto Mai: The master's degree in electrical machines and control technology. 4th edition. Vogel Buchverlag, Würzburg 1983, ISBN 3-8023-0725-9 .
• Detlev Roseburg: Electrical machines and drives. Fachbuchverlag Leipzig in Carl Hanser Verlag, 1999, ISBN 3-446-21004-0 .

Individual evidence

1. ^ Günter Springer: Electrical engineering. 18th edition. Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .
2. ↑ Michail Ossipowitsch Doliwo-Dobrowolski: Improvement in the Regulation of the Speed ​​and Power af Alternating Current Motors. , British Patent No. 20,425, January 31, 1891.
3. ↑ Michail Ossipowitsch Doliwo-Dobrowolski: AC power machine operated by currents of different phases, with a device for regulating the speed and traction. Swiss Patent No. 3062, December 19, 1890.
4. ↑ VEM Motors GmbH (ed.): Three-phase motors with slip ring drives for cranes, crushers and other applications. Online (accessed October 4, 2012; PDF file; 161 kB)
5. ↑ ^{a b} Ekbert Hering, Alois Vogt, Klaus Bressler: Manual of electrical systems and machines. Springer-Verlag, Berlin Heidelberg New York 1999, ISBN 3-540-65184-5 .
6. ^ ^{A b} Franz Moeller, Paul Vaske (ed.): Electrical machines and converters. Part 1 structure, mode of operation and operating behavior, 11th revised edition, BG Teubner, Stuttgart 1970.
7. ↑ Hans-Otto Seinsch: Fundamentals of electrical machines and drives . 3rd revised and expanded edition, Springer Fachmedien Wiesbaden, Wiesbaden 1993, ISBN 978-3-519-06164-9 .
8. ↑ Germar Müller, Bernd Ponick: Basics of electrical machines . 9th edition, Wiley-VCH Verlag GmbH & Co KGaA., Weinheim 2006, ISBN 3-527-40524-0 .
9. ↑ ^{a b} Ernst Hörnemann, Heinrich Hübscher: Electrical engineering specialist training in industrial electronics. 1st edition. Westermann Schulbuchverlag GmbH, Braunschweig, 1998, ISBN 3-14-221730-4 .
10. ^ Paul Vaske, Johann Heinrich Riggert: Electrical machines and converters. Part 2 Calculation of electrical machines, 8th revised edition, BG Teubner, Stuttgart 1974, ISBN 3-519-16402-7 .
11. ↑ Historical chain carousel at the BRN / Dresden 2007
12. ↑ Mechanical structure of the slip ring motors ABB ( Memento of October 29, 2010 in the Internet Archive ) (last accessed on October 4, 2012).
13. ↑ ^{a b c d} Electrical engineering examination book. Europa-Lehrmittel Verlag, 1970.
14. ↑ ^{a b} Wilhelm Biscan: The heavy current technology. Publishing house for architecture, technology and trade Carl Scholtze, Leipzig 1907.
15. ↑ ^{a b c d} A. Senner: Electrical engineering. 4th edition. Verlag Europa-Lehrmittel, 1965.
16. ^ ^{A b} Hanskarl Eckardt: Basic features of electrical machines. BG Teubner, Stuttgart 1982, ISBN 3-519-06113-9 .
17. ↑ Dieter Brockers: Lexicon resistances . Gino Else GmbH Electrotechnical Factory, 1998.
18. ↑ U. Winter: Safe operation of slip ring motors - static and dynamic processes on asynchronous motors with slip ring rotors and starters. VEM Sachsenwerk GmbH, Dresden.
19. ↑ Menzel Motoren (Ed.): Modular slip ring motors . Online (accessed August 17, 2015).