METHOD FOR REDUCING REGENERATED ENERGY AND REVERSAL STRESS IN A RECIPROCATING LOAD POWERED BY AN ELECTRIC MOTOR BY MODULATING MOTOR SPEED USING A VARIABLE FREQUENCY DRIVE AND VARIABLE FREQUENCY DRIVE PROVIDED FOR PERFORMING THE METHOD

Abstract

The present disclosure refers to a method for controlling the speed of a reciprocating load motor, wherein the motor speed is a function of two input variables, namely a user defined speed set-point and a load dependent input variable. According to the disclosure, the load dependent input variable is a function of the motor current. The disclosure also refers to a variable frequency drive for controlling the speed of a reciprocating load motor, wherein the drive is programmed to perform the presently described method.

Claims

1. A method for controlling the motor speed of a reciprocating load motor, wherein the motor speed of the motor is a function of two input variables, namely a user defined speed set-point and a load dependent input variable, wherein the load dependent input variable is a function of the motor current.

2. The method according to claim 1, wherein a variable frequency drive for controlling motor speed is provided for performing the method.

3. The method according to claim 2, wherein the method is implemented as a control algorithm in the variable frequency drive.

4. The method according to claim 1, wherein motor torque is kept positive during a complete cycle of the reciprocating load.

5. The method according to claim 3, wherein the control algorithm uses only measured reciprocating load motor current value as input.

6. The method according to claim 2, wherein the motor current is measured by at least one current sensor in the variable frequency drive.

7. The method according to claim 1, wherein the reciprocating load motor speed variation is proportional to the inertia accumulated in the load driven by the motor, in particular the inertia accumulated in a shaft, crankshaft and/or a gear box.

8. The method according to claim 1, wherein reciprocating load motor torque is calculated from measured current flowing through the reciprocating load motor.

9. The method according to claim 2, wherein the variable frequency drive only comprises current sensors as sensors.

10. A variable frequency drive for controlling the speed of a reciprocating load motor, wherein the converter is programmed to perform the method according to claim 1 and/or that no accumulated waste heat is generated by the motor during a reciprocating load stroke and hence does not have to be dissipated by dedicated brake resistors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Further details and advantages of the invention are described with reference to the figures. The figures show:

[0029] FIGS. 1, 2: schematic views of an application employing the presently described method;

[0030] FIG. 3: graphs indicating the performance of state of the art applications;

[0031] FIG. 4: graphs indicating the performance of an application employing the presently described invention; and

[0032] FIG. 5: graphs of motor torque and speed split into sections of half strokes.

DETAILED DESCRIPTION

[0033] FIGS. 1 and 2 show schematic views of an application employing the presently described invention. The application may comprise a number of electrical and mechanical components. The application comprises a variable frequency drive which may be a power converter, a motor, a gearbox and a rod pump as a reciprocating load driven by the motor. The motor, the gearbox and the rod pump are coupled mechanically such that power can be transmitted from the motor to the rod pump.

[0034] The variable frequency drive provides an output signal to the motor for driving said motor. The application has the reciprocating load motor speed control integrated into the variable frequency drive. The reciprocating load motor speed control receives an input variable indicating the motor current or load, which is also indicative of the motor torque at any given time. A second input variables received by the drive is a user defined speed set-point which may be input by a user of the application. Based on the two input variables a reciprocating load motor speed control algorithm is provided at the drive for calculating the final speed reference, used by the variable frequency drive to output signals to the motor.

[0035] As both, the user defined speed set-point and the actual motor current, or load, are considered as input variables by the drive, the drive may control the motor torque to be kept positive during a complete load cycle of the reciprocating load, thereby avoiding an input of regenerated energy from the load to the motor and the drive.

[0036] The motor current indicative of the motor torque may be measured by at least one or exactly one current sensor, provided at or as part of the variable frequency drive.

[0037] It is possible to carry out the present invention by only measuring current values. Accordingly, the variable frequency drive may only comprise current sensors or only one current sensor, and no other sensor equipment.

[0038] FIG. 3 shows graphs indicating the performance of known state of the art applications, in which an electric motor is driving a reciprocating load. As indicated by the horizontal line in the central motor torque vs. time graph, negative torque may occur, wherein the reciprocating load effectively drives the motor, thereby creating regenerative power that may charge the DC-link of the variable frequency drive. In order to handle these reverse conditions, resistor braking is provided in applications known from the art.

[0039] FIG. 4 shows graphs indicating the performance of an application employing the presently described method and/or variable frequency drive. The top speed reference vs. time graph shows that while the known solutions provide for a fixed speed reference, the present invention provides a load dependent and therefore varying speed reference. The load dependent speed reference varies as a function of the torque from the reciprocating load to the motor.

[0040] The central motor torque vs. time graph shows that, according to the invention, no negative torque, i.e. torque from the reciprocating load to the motor is present anymore. Hence, the motor transmits net positive torque to the reciprocating load at all stages of its stroke, the corresponding curve is above the horizontal zero line.

[0041] FIG. 5 shows graphs split into sections with focus on motor torque. The four sections Sector A, Sector B, Sector C, Sector D, are half stroke torque data of the reciprocating load to the motor.

[0042] In Sector A and Sector C a decreasing curve of torque and energy to drive the motor is required. The motor is speeding up at the end of Sector A and Sector C while the motor torque is kept positive to avoid regenerative energy, which would normally require brake resistors, which again is wasted heat and energy.

[0043] In Sector B and Sector D, an increasing curve of torque and energy to drive the motor is required. The motor is slowing down at the end of Sector B and Sector D to have a speed that when going into next sector, will not generate a negative torque which would normally require brake resistors and would be wasted heat and energy.