Current displacement rotor
The current displacement rotor is a special type of squirrel cage rotor , it is installed in three-phase asynchronous motors . Thanks to their special design, three-phase motors with current displacement rotors have better starting properties than other squirrel cage motors.
Basics
When starting a three-phase asynchronous motor, the starting current should be low, but the starting torque should be high. The effective resistance of the rotor must be high so that the starting current is low. A high starting torque is achieved when the phase shift between the rotor current and the rotor voltage is low. In order to achieve this, the rotor must be designed in such a way that the effective resistance of the rotor winding is much greater than its reactance . However, a high rotor resistance has the consequence that the slip increases after the motor has started up . At the same time, the efficiency of the engine deteriorates.
After the run-up, the rotor frequency f L and thus also the inductive reactance X L are reduced according to equation ( ). Thus, only the active resistance component is relevant. So that the disadvantages described (large slip, poor efficiency) do not occur, the effective resistance must also be smaller after startup . The required properties are not provided by round bar rotors (squirrel cage rotors) or by resistance rotors .
Structure and mode of operation
In order for a runner to have a high effective resistance during start-up, which becomes small again after run-up, it must have a special construction. Most standard motors have a double rod rotor. For this purpose, two electrically conductively connected (either directly or at the end faces) rotor bars are arranged one above the other in the rotor slots , through which an alternating current then flows during operation . When starting, this current also has a high frequency due to the high rotor frequency .
This rotor current induced by the stator rotating field generates a magnetic stray field around each rotor bar , which is very inhomogeneous. The two stray fields act like inductive reactances in the rotor's alternating current circuit and induce electrical voltages in the respective rotor bars . According to Lenz's rule , these voltages strive to reduce the alternating currents that cause them in each rotor bar.
The alternating magnetic field around the lower rotor bar is stronger because the magnetic field lines have a shorter air path on the one hand and they can close in the iron package on the other . The counter-voltage induced by the alternating field, thus the current-reducing effect of the induced voltage, is therefore greater in the lower arm than in the upper arm. Due to the semi-closed iron slot typical of three-phase asynchronous machines, the current is displaced towards the slot opening, i.e. towards the outer edge of the rotor, i.e. towards the air gap ( skin effect ).
For this reason, the current displacement rotor has almost only the smaller slot cross-section of the upper rotor bar available when it starts up. As a result, the current flow in the lower armature is much lower than in the upper armature. The magnetic flux density is lower on the outer edge of the rotor than on the inside. Due to the inhomogeneous stray field, the reactance X L is also not the same over the entire cross section of the rotor bars. It increases sharply from the rotor edge to the rotor center, which is why the rotor current almost only flows in the upper part of the rotor rod when the motor starts up.
Due to the current density in the rotor bar and the large slot resistance, the current displacement rotor has a high resistance when it starts, and the starting current is therefore smaller. Due to the large effective resistance of the rotor, there is less phase shift in the rotor circuit. For this reason, the current displacement rotor has a greater starting torque than the round rod rotor.
When running up, the rotor frequency decreases and the leakage flux decreases; this reduces the current displacement. In nominal operation, the current can again use the entire slot cross-section of both rotor bars. This greatly reduces the effective resistance of the rotor, which results in a smaller tilting slip . The rotor losses are now low, which results in good efficiency . Thus, a good output can be achieved. Due to its steep characteristic curve, the motor runs at a high nominal speed and now also has less slip. This effect can also be achieved with other groove shapes.
There are the following groove shapes:
- High bar groove ➁
- Keyway ➂
- Double groove ➃
- Double bar groove ➄
- Drop rod groove ➅
also a variety of combinations.
Depending on their groove shape, the runners are called wedge runners, gob runners, high runners, double groove runners or high groove runners. For smaller motors up to shaft height 315, die-cast rotors are used, which offer a relatively free design option for the rotor groove shape. For larger motors (shaft height> 315), deep slots or double cages are used to achieve a balanced motor characteristic curve.
The size of the current displacement can be influenced by the shape of the grooves . At the same time, this influences the size of the rotor resistance. By designing the rotor cage accordingly, the spread can be minimized and the overturning moment increased. As a result, the motor can briefly output a multiple of its nominal torque . This eliminates the need for the motor to be oversized in certain applications.
The double cage armature rotor is a special design of the current displacement rotor . Here, two individual rods are arranged one above the other, which are galvanically separated from one another. The motor thus consists of two cages, an outer cage and an inner cage. The cages have different resistances due to the appropriate choice of material ( copper , aluminum , brass ) and the dimensioning of the conductor cross-sections.
The inner cage is designed to have a small electrical resistance and the outer cage to have a high electrical resistance. This construction method makes the effect even more pronounced than with the simple current displacement anchor. When the motor starts, the current almost only flows in the high-resistance outer cage. This greatly reduces the starting current and increases the starting torque. After the start-up, the current is distributed to both cages according to their effective resistances. The low-resistance inner cage now determines the operating behavior of the motor.
Operating behavior
The shape of the grooves has a significant influence on the start-up behavior of the motor. Depending on the slot shape of the rotor, there are different run-up curves for the motor. However, the respective runner designation (double rod runner, double groove runner, etc.) says little about the exact shape of the corresponding run-up curve.
In three-phase asynchronous motors with a current displacement rotor, the starting torque is approximately twice as high as the nominal torque. Due to the increased rotor resistance, the breakdown torque shifts towards low speeds. The breakdown torque in current displacement rotors is around two to three times as high as the nominal torque. The starting current is about three times as high as the rated current, which is why even larger motors with current displacement rotors must be started using special starting methods.
Due to the large groove cross-section, current displacement rotors have a greater spread than round rod rotors. As a result, they are somewhat less efficient and have a lower power factor than motors with round rod rotors. Current displacement rotors are less suitable than circular rod rotors for operation with frequency converters . Nevertheless, three-phase asynchronous motors with current displacement rotors have significantly better starting properties than motors with round bar rotors.
Advantages and disadvantages
- advantages
- good starting properties
- high tightening torque
- lower starting current than round bar runner
- high overturning moment
- low maintenance
- disadvantage
- poorer efficiency than round bar runner
- poorly suited for operation with a frequency converter
- low power factor
Areas of application
Current displacement rotors are used in three-phase asynchronous motors when constant power is required from the electric motor over a wide range . Especially in the small and medium power range, where the use of slip-ring motors does not make sense, three-phase asynchronous motors with current displacement rotors always have a great advantage over three-phase asynchronous motors with round bar rotors .
Application examples
- Machining
- Center winder
- Traction vehicles
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 60034-5 Degrees of protection of rotating electrical machines
- EN 60034-6 Types of cooling, rotating electrical machines
literature
- Detlev Roseburg: Electrical machines and drives. Fachbuchverlag Leipzig in Carl Hanser Verlag, 1999, ISBN 3-446-21004-0
- Rolf Fischer: Electrical machines . 12th edition. Carl Hanser Verlag, Munich / Vienna 2004, ISBN 3-446-22693-1
Individual evidence
- ↑ a b c d e f A. Senner: Electrical engineering . 4th edition. Verlag Europa-Lehrmittel, 1965.
- ↑ a b c d e f Ernst Hörnemann, Heinrich Hübscher: Electrical engineering specialist training in industrial electronics. 1st edition. Westermann Schulbuchverlag, Braunschweig, 1998, ISBN 3-14-221730-4 .
- ↑ a b c d Günter Springer: Electrical engineering. 18th edition, Verlag Europa-Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9 .
- ↑ Hanskarl Eckardt: Basic features of electrical machines. BG Teubner, Stuttgart 1982, ISBN 3-519-06113-9 .
- ^ A b c d e 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.
- ↑ Günter Boy, Horst Flachmann, Otto Mai: The master's examination in electrical machines and control technology. 4th edition. Vogel Buchverlag, Würzburg 1983, ISBN 3-8023-0725-9 .
- ^ H. Greiner: Start-up and electrodynamic deceleration in squirrel cage motors. (PDF) Danfoss Bauer GmbH; Retrieved July 18, 2016.
- ^ A b 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 .
- ↑ Seifert, Thomas Fladerer: Electric motor design for special requirements . ( Memento of March 31, 2007 in the Internet Archive ) (PDF)