Occurrence severity

The starting severity is a constant that is used in drive technology to characterize the starting behavior of a driven machine. This constant is used to dimension resistance starters for slip-ring motors . In addition, knowledge of this constant is necessary in order to select the right drive machine and to determine the required starting method for the respective electric motor.

Basics

In order for an electric motor to drive a machine, it must develop a torque that is sufficiently large to overcome the "counter torque" of the machine to be driven. The torque required to drive a machine is made up of the load torque and the moment of inertia . While the moment of inertia mainly has an effect during the start-up phase of the machine, the load moment is noticeable over the entire speed range.

Typical load moments are:

• Practically constant load torque
• Load torque increasing linearly with speed
• Torque increasing quadratically with speed
• Torque decreasing reciprocally proportional to the speed

Source:

If the engine and starter are not matched to one another, the engine cannot generate sufficient torque to reach the rated speed. It is particularly critical when the engine has a saddle torque. Since the saddle torque is the smallest torque of the motor, the motor gets stuck at a saddle speed. To prevent this, starters and prime movers are dimensioned according to the severity of the start.

calculation

The starting severity is defined as the ratio of the average starting torque to the nominal torque . ${\displaystyle f}$ ${\displaystyle M_{M}}$ ${\displaystyle M_{n}}$

${\displaystyle f={\frac {M_{\text{M}}}{M_{n}}}}$

Based on the nominal current and the mean starting current , the starting severity can be determined using the formula. ${\displaystyle I_{n}}$ ${\displaystyle I_{M}}$

${\displaystyle f={\frac {I_{\text{M}}}{I_{n}}}}$

The starting severity can also be determined in relation to the maximum starting torque , but the values ​​for are then somewhat higher than in the calculation in which the mean torque is used. ${\displaystyle f}$ ${\displaystyle {M_{\text{max}}}}$ ${\displaystyle f}$

${\displaystyle f={\frac {M_{\text{max}}}{M_{n}}}}$

The starter is dimensioned according to the determined constant . ${\displaystyle f}$

Categories

The starting severity of a drive can be divided into 4 categories, half-load starting, fan starting, full-load starting and heavy starting .

Half-load start-up is characteristic of machines that either develop only a low load torque or in which the load torque increases with the square of the speed. The prerequisite is that these machines have a low moment of inertia. In the case of half-load start-up, the starting severity has a value of 0.7. The value increases to 1.0 if the maximum torque is taken into account for the calculation. This is the case with centrifugal pumps.

When the fan starts up, the drive machine must rotate against a high moment of inertia. However, only a small load torque has to be managed. The starting severity for the fan start is 1.0. If the maximum torque is taken into account, the value increases to 1.4. This applies, for example, to exhaust fans.

The full-load start-up can be subdivided once again into an approximate full-load start and a full-load start based on the severity of the start. Full load start applies to machines that start under load. The starting severity for the approximate full load start is 1.25 and for the full load start 1.4. Taking into account the maximum torque, this results in a value of 1.7 when starting at full load.

The approximate full-load start-up applies to raw mills , coal mills and cement mills . The full load start is characteristic of drives for winches and hoists or for piston machines such as B. piston pumps or compressors.

The heavy starting is the greatest load on the drive machine. In the case of heavy starting, the starting severity reaches a value of 2.0; based on the maximum torque, the value is 2.5. Centrifuges or crushers are machines with heavy starting.

• Overview of the typical characteristics
• 

  Half load start

• 

  Fan start-up

• 

  Full load start

• 

  Heavy starting

Individual evidence

1. ↑ Wladimir Schuisky: Electric motors. Their properties and their use for drives, Springer Wien GmbH, Vienna 1951, pp. 140-145.
2. ↑ ^{a b c d} 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, pp. 134-135.
3. ^ Wilhelm Lehmann: The electrical engineering and the electromorical drives. Springer Verlag Berlin Heidelberg GmbH, Berlin Heidelberg 1948, pp. 248–249.
4. ↑ Helmut Greiner: Starting, braking, positioning with three-phase motors. Danfoss Bauer GmbH, Esslingen 2001.
5. ↑ HR RISG: electrical engineering for the practitioner. 1st edition, Verlag Elektrotechnik Walter Liechti, Aarau (Switzerland) 1990, ISBN 3-905214-11-3 .
6. ↑ Starter for slip-ring motors Calculation of the starting severity ( Memento from December 12, 2009 in the Internet Archive ) (accessed via the web archive on July 25, 2016).
7. ↑ Three-phase-rotor-starter starter difficulty  ( page can no longer be called up , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. .  
8. ↑ ^{a b c d e f g h} Gino Else (Ed.): Small starter glossary. Online ( Memento from March 4, 2016 in the Internet Archive ) (PDF; 865 kB) (accessed via the web archive on July 25, 2016).
9. ^ ^{A b c} Hans-Ulrich Giersch, Hans Harthus, Norbert Vogelsang: Electrical machines testing, standardization, power electronics . 5th edition, BG Teubner / GWV Fachverlage GmbH, Wiesbaden, 2003, ISBN 3-519-46821-2 , p. 288.
10. ^ ^{A b} Wilhelm Hille, Otto Schneider, Klaus Großmann, Knud Lensch: Elektro-Fachkunde 2. Energietechnik, 3rd edition, BG Teubner Verlag, Stuttgart 1991, ISBN 978-3-519-26806-2 , pp. 183-184.