Method of controlling an electric motor driving a centrifugal pump

Abstract: A method for controlling an electric motor (2) driving a centrifugal pump (3) having a diameter ratio D₁/D₂ less than approximately 1/2 and a varying fluid flow-through (Q), the motor (2) being connected to a supply mains (R,S,T) through a power-control device (1). An electrical measuring signal (I) is produced, being proportional to the current drawn by the motor (2) or by the power-control device (1), and based on the known characteristics of the motor (2) and the pump (3), the electrical measuring signal (I) is processed so as to produce a control signal (f), being used as an input signal to the power-control device (1). The electrical measuring signal (I) is processed in such a manner, that the control signal (f) causes the delivery pressure (H) of the pump (3) to be substantially constant over a large variation interval for the fluid flow-through (Q).

Description: TECHNICAL FIELD The present invention relates to a method according to the preamble of claim 1 for controlling an electric motor driving a centrifugal pump. The diameter ratio D 1 /D 2 is the ratio between the impeller diameters at the fluid inlet to and the fluid outlet from the centrifugal pump. BACKGROUND ART Conventionally, such a motor-pump combination is controlled with a view to producing a constant pressure by measuring the output pressure and increasing the rotational speed of the motor, when the pressure decreases, and decreasing the rotational speed of the motor, when the pressure rises. Normally, the pressure is measured by means of a differential pressure sensor, producing one signal when the pressure is too low and another signal when the pressure is too high, and no signal when the pressure is correct. Then, these signals are used for regulating the rotational speed of the motor, thus achieving a substantially constant delivery pressure from the pump. The primary disadvantage with this arrangement is that the differential pressure sensor has a certain hysteresis, so that the regulating function does not occur continously, but in steps, causing a certain variation in the delivery pressure of the pump. EP-A-0 226 858 discloses a method according to the preamble of claim 1 in which the measurement comprises the measuring of at least two parameters, namely speed of rotation and current consumption. DE-A-38 24 057 discloses a method by which several measuring values are taken from the electricl power supply to the motor for generating a parameter which corresponds to the characteristic line for pressure versus flow. The generation of this parameter necessitates that the measuring values comprise the current, the effective power and a certain frequency corresponding to the slip of the motor. There is no mention of a possibility of using the current alone directly for controlling the motor via the power control device, without complex computation and frequency analysis. DISCLOSURE OF THE INVENTION On the basis of this prior art it is the aim of the present invention to provide a method according to the preamble of claim 1 which is substantially simplified compared to the above known methods. This is achieved by the features according to the characterizing clause of claim 1. The current measurement is relatively easy to perform and there is no need to introduce sensors in the pump construction to be able to measure the speed of rotation and accordingly, the method can be performed on a standard motor-pump construction without complex modification thereof. The present invention is based on the realization that in a pump-motor combination with a diameter ratio D 1 /D 2 less than 1/2, and in which the delivery pressure of the pump is maintained constant, there exists a unique correlation between the current drawn by the motor and the rotational speed of the motor. Normally, the motor is a three-phase asynchronous motor, the rotational speed of which is controlled by means of a frequency converter. If the asynchronous motor is replaced by a synchronous motor, which like the asynchronous motor may be constructed with a so-called wet-running rotor, and in which the rotor can be in the form of a permanent-magnet rotor, the characteristic properties of the synchronous motor produce a substantially improved regulating characteristic with the above-mentioned regulating based on the current drawn, and the efficiency is also improved. If a frequency converter is used for the regulating function, the usual known starting problems for synchronous motors are easily solved by always letting the frequency converter start with a low frequency, i.e. a kind of "gentle start". Alternatively, the motor can e.g. be a shunt motor for alternating or direct current, and the controlling of the rotational speed can be achieved by controlling a shunt-winding resistance. BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed portion of the present description, the invention will be explained in more detail with reference to the exemplary embodiments shown in the drawing, in which fig. 1 shows a motor-pump arrangement being controlled in a known manner by a frequency converter, and fig. 2 shows a motor-pump arrangement being controlled by means of the method according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a motor 2, connected to a supply mains marked RST through a frequency converter 1. The motor 2 drives a centrifugal pump 3, the output pressure of which is monitored by means of a differential pressure sensor 4, in a known manner controlling a servo-motor 5, with which the frequency of the frequency converter 1 is controlled. When the pressure is too low, the differential pressure sensor 4 produces a signal to the servo-motor 5 to increase the frequency of the frequency converter 1, while when the pressure is too high, the differential pressure sensor produces a signal to the servo-motor to decrease the frequency of the frequency converter. The differential pressure sensor 4 has a certain hysteresis, so that the frequency of the frequency converter 1 is held constant within a certain pressure interval about the desired pressure. Correspondingly, figure 2 shows a frequency converter 1, a motor 2 and a centrifugal pump 3, but in this case, the frequency of the frequency converter 1 is controlled by means of a control device 7, receiving an electrical measuring signal either I or I', being proportional to the current I drawn by the frequency converter 1 and the current I' drawn by the motor 2 respectively. Based on this signal, the control device 7 produces a frequency-control signal f, that is a function of the measuring signal I. In connection with this configuration, the transfer function for the control device 7 is preferably F(I) = F 0 + K I , in which f 0 and K are constants that may be determined from the characteristics of the motor 2 and the centrifugal pump 3. When the current I or the current I' rises, corresponding to an increase in the fluid flow Q through the centrifugal pump 3, the control device 7 will adjust the frequency f of the frequency converter 1 upwardly, so that the increase in the rotational speed of the motor 2 will compensate for the decrease in pressure H, when the fluid flow through the pump 3 increases. With a decreasing current I, the control device 7 will adjust the frequency f for the frequency converter 1 downwardly, so that the decrease in the fluid flow Q corresponding to the decrease in the current I, and the consequent increase in the pressure H, will be compensated by a reduction of the rotational speed of the motor. It is conceivable that the method according to the invention may be used in connection with other types of motors 2 with associated power controllers 1, as it is sufficient to have a basic principle, according to which there will be produced an electrical measuring signal I, that is proportional to the current drawn by either the motor or the power-control device, and that this measuring signal I is processed so as to produce a control signal f for the power-control device 1 in such a manner, that the control signal f ensures a substantially constant output pressure from the pump. The desired transfer function (F(I) = f 0 + K I or F(I) = R 0 + kI) for the control device 7 can be achieved by means of a self-balancing bridge circuit, in which an unbalance in the bridge causes a change in the control signal f until balance is achieved, and in which one branch of the bridge circuit comprises a first element that is proportional to the control signal f, and a second element that is proportional to I (-K I) or I (-kI), so that balance is achieved for f-K I = f 0 , i.e. f = f 0 + K I, or f-kI = R 0 , i.e. f = R 0 + kI. In those cases, in which the pump is connected to a pipe system or the like having a certain flow resistance, it may be an advantage that the delivery pressure of the pump increases with increasing fluid flow, thus compensating to some extent for the flow resistance in the pipe system. Thus, a dimensioning criterium could be that the pressure at the first consumer be maintained substantially constant.

Claims: Method of controlling an electric motor (2) driving a centrifugal pump (3) having a varying fluid flow-through (Q), said motor (2) being connected to a supply mains (R,S,T) through a power-control device (1), by which method, for causing the delivery pressure (H) of the pump (3) to be substantially constant over a large interval of variation for the fluid flow-through (Q), a measurement is made on the electrical supply system for the motor and based on the known characteristics of the motor (2) and the pump (3), the measurement is processed so as to produce a control signal (f), used as an input signal to the power-control device (1), characterized in that the centrifugal pump (3) has a diameter ratio D 1 /D 2 less than approximately 1/2, and that the measurement only comprises the measurement of one parameter (I) being proportional to the current drawn by the motor (2) or by the power-control device (1). Method according to claim 1, characterized in that the power-control device (1) is a frequency converter and the motor (2) is a three-phase synchronous motor. Method according to claim 1, characterized in that the power-control device (1) is a frequency converter and the motor (2) is a three-phase asynchronous motor. Method according to claim 2 or 3, characterized in that the control signal (f) is a frequency-adjustment signal and that f = F(I). Method according to claim 4, characterized in that F(I) = f 0 + K I . Method according to claim 1, characterized in that the motor (2) is a shunt motor for alternating or direct current, and that the power-control device (1) operates by controlling a shunt-winding resistance. Method according to claim 6, characterized in that the control signal (f) is an adjustment signal for the shunt-winding resistance, and that f = F(I) . Method according to claim 2, characterized in that F(I) = R 0 + k · I . Method according to any one of the preceding claims, characterized in that the control signal (f) is produced by means of a self-balancing bridge circuit, in which an unbalance causes a change of the control signal (f) until balance is achieved, and in which a bridge-circuit branch comprises a first element being proportional to the control signal (f) and a second element being proportional to the current (I) or the square root of the current ( I ).

Inventors: RASMUSSEN LEIF, RASMUSSEN, LEIF
Applicants: SMEDEGAARD AS, T. SMEDEGAARD A/S
Application year: 1993
Main classification: F04D15
IPC codes: F04D15