Electricity: motive power systems – Induction motor systems – Braking
Patent
1996-09-25
1998-10-20
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Braking
318337, 318365, 318366, 318367, H02P 318
Patent
active
058251545
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a phase control method of stopping a drive smoothly, in particular a pump drive, in accordance with the phase angle between the voltage and the current of the drive motor.
BACKGROUND INFORMATION
There are known phase control methods of stopping and starting a drive smoothly as a function of the phase angle between the motor voltage and the motor current. Smooth stopping makes special demands on the control system. When a pump drive is shut off, it causes knocking in pipeline systems which is also known as "water hammering" and is caused by the abrupt changes in flow. This not only damages the pipeline system but is also disturbing because of the acoustic noise.
An abrupt change in flow occurs especially when the drive of a rotary pump, for example, is shut down directly by opening the respective motor contactor. The following relationship holds between the rpm and flow and between the change in rpm and the change in pressure: the flow is directly proportional to pump rpm, and the change in pressure is directly proportional to the square of the change in rpm.
As FIG. 1 illustrates, in the case of a direct shutdown of a pump drive with a three-phase induction motor, the drive is stopped within a very short period of time because of the load torque curve Pum in which the pump has a high counter-torque. FIG. 1 shows the moment M plotted against the rpm n. Mom is the motor torque, Pum is the load torque, Bm is the accelerating torque, Kim is the breakdown torque and N is the nominal working point. Because of the high counter-torque Pum of the pump, there is a rapid change in rpm n, resulting in an abrupt change in flow and pressure because of the above-mentioned relationships. This leads to a pressure surge known as a water hammer. Any check valves will exacerbate the problem.
FIG. 2 shows a plot of motor current I, motor voltage u and rpm n over time in a direct disconnect.
In the past, various methods have been used as countermeasures.
Mounting an inertial weight on the pump drive makes it possible to prevent the drive from stopping immediately when there is a shutdown. The stored energy of the inertial weight leads to delayed stopping of the pump, so the change in pressure and the change in flow are slowed down. This method is complex mechanically, and additional power is needed for operation.
By using frequency converters, the pump motor can decelerate from its rated speed N according to an rpm ramp. This method is comparatively expensive, especially for higher-powered pumps.
So-called smooth starting devices that operate according to the principle of a 3-phase a.c. power controller are used for turning pump motors on and off. In addition to smooth starting, these devices also permit smooth stopping. Through phase control, the motor terminal voltage is not disconnected suddenly but instead is ramped down, so the motor also stops smoothly.
There are different types of smooth stopping. The simplest type consists of increasing the phase control angle linearly from 0.degree. until the motor has stopped. As FIG. 3 illustrates, if the reduced motor torque drops below the load torque, there is a rapid change in rpm, which in turn causes water hammering in most cases. Another method consists of measuring the motor terminal voltage and ramping it down when stopping. However, additional hardware is needed to measure the motor terminal voltage. One problem here is that the voltage is phase-controlled and furthermore it must be supplied in a potential-free form for control. In a simple solution a time lag that can have a negative effect on the control response occurs due to the measurement and "smoothing" of the motor terminal voltage.
The present phase control method of smoothly stopping is based on the principle of a known smooth starting device according to FIG. 4, as is described in German Published Patent Application 4,005,679. A 3-phase induction motor Mt is connected to the three phases A, B and C of a 3-phase system via a thyristor circuit of anti-para
REFERENCES:
patent: 4833386 (1989-05-01), Unsworth
patent: 4994951 (1991-02-01), Iwasa et al.
patent: 5008608 (1991-04-01), Unsworth et al.
Runggaldier Diethard
Seitz Johann
Lockett Kim
Shoop Jr. William M.
Siemens Aktiengesellschaft
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