GB2109590A

GB2109590A - Motor control system

Info

Publication number: GB2109590A
Application number: GB08134088A
Authority: GB
Inventors: John Arthur Bauly
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-11-12
Filing date: 1981-11-12
Publication date: 1983-06-02
Also published as: GB2109590B

Abstract

In a motor control system, e.g. for a motor used to drive a pump whose speed is varied by adjusting the frequency of the output of speed controller (1), the parameters of the supply to the motor and the operating conditions thereof are monitored, and detectors such as (6) and (7) which do the monitoring apply their outputs to a processor (3). For example the detector (6) monitors power factor and the detector (7) monitors motor speed. The processor (3) supplies a control to the controller (1) which adjusts the voltage so as to maximise the efficiency of the motor. <IMAGE>

Description

SPECIFICATION Motor control system This invention relates to variable speed controllers for electric motors.

Such controllers for electric motors, e.g.

induction motors, achieve the required motor speed by varying one of the parameters of the power supply, e.g. frequency in the case of an induction motor. The other parameters of the supply are also varied, normally in a predetermined way. Thus for an induction motor the voltage might be varied in proportion to the frequency. In practice, as the speed varies the motor does not operate at best efficiency, and generally the efficiencies obtained are substantially less than the best values obtainable at each speed.

An object of the invention is to enable this loss of efficiency to be overcome.

According to the invention there is provided a variable speed controller for an electric motor, which includes first control means for adjusting at least one of the parameters of the electrical supply to give the desired motor speed, monitoring means associated with the motor or the power supplied thereto for monitoring the conditions under which the motor is operating, the monitored condition or conditions being related to the degree of efficiency of operation of the motor, and second control means under control of said monitoring means and arranged to so modify at least one of the parameters of the electrical supply to the motor that the operational efficiency thereof is enhanced.

Thus the loss of efficiency can be substantially recovered from by adjusting one or more of the electrical parameters of the supply e.g. by adjusting its voltage. Such adjustments would usually have to be done continuously because of variations in the motor speed needed and also to variations in the motor's load. As will be seen there are several ways in which an automatic adjustment can be achieved. Thus for an induction motor it is possible to monitor the power factor and to adjust the voltage until the power factor of the supply to the motor approaches unity. Another method is to monitor the motor's speed and the supply frequency, or the motor's slip and to adjust the applied voltage to the motor so that the slip is always held to preset values.For an induction motor the best efficiency correlates approximately to a high power factor and also to a particuiar slip value.

More complex adjustment is also possible, for example by monitoring both slip and power factor, and possibly other parameters, and from there computing the values of the electrical parameters needed for the most efficient running.

Embodiments of the invention will now be described with reference to the accompanying drawing, which shows block diagrams of two embodiments of the invention.

In the arrangement of Fig. 1, we have a motor speed controller 1 to which the alternating current supply is applied, and which also has inputs from a demand frequency unit 2 and a processor unit 3. The speed at which the driven device, e.g. a pump, is to be driven is signalled to the unit 2 via its input connection 4, and this unit applies an appropriate control to the controller 2.

The output voltage and frequency from the controller 1 is applied to the motor 5, which drive the load. Associated with the motor's input is a sense unit 6 and associated with its output side is another sense unit 7. The unit 6 senses from the supply to the motor the power factor, the current or the efficiency is a result of measurements performed on that supply. In the case of power factor measurement, this involves measuring both the current and the voltage and determining power factor therefrom. Since power factor gives an indication of effiency, the result of the power factor determination can be used to assess efficiency. The unit 7 senses speed using a tachometer with frequency measurement, power using torque and speed measurement, or torque using transducers.The outputs of the sense units 6 and 7 thus indicate the characteristics of the supply to the motor and the way in which it is operating. In some cases it may be adequate to have only one sense unit, i.e. 6 or 7. The processor 3 is so programmed that for any given combination of motor speed and load it determines a control signal which adjusts the supply in a manner appropriate to increasing efficiency.

Thus in general if the motor speed is to be reduced, with an induction motor this involves reducing the frequency of the AC applied to the motor. Now without adjusting the voltage it is found that efficiency falls off, often quite seriously. Hence in this case voltage is also reduced until the results of the sensing by the two sense units 6 and 7 together indicate that efficiency is improved. Thus the controller varies the frequency, and also adjusts the voltage of the AC supply in such a way as to maximise efficiency.

In the circuit of Fig. 2, which is similar in many respects to that of Fig. 1, we have a feedback from the output of the controller 10 to a comparator 11 therein of the parameters of that supply, such as power factor or slip. The comparator 11 compares this with a signal applied to the comparator's other input, and which represents the value of power factor or slip needed for maximum efficiency. The error signal thus produced is applied to a processor 12, whose output controls the controller in accordance with an input 13 via which the intended speed is notified to the controller 10.

Hence the controller alters its output in such a way as to maximise efficiency.

In addition to, or in place of the above it is possible to use two sense units 14 and 15, which are similar to the sense units 6 and 7 in Fig. 1.

The unit 14 responds to power factor and unit 1 5 responds to actual speed of the motor. The associated processor 1 6 derives from the values of 14 to 1 5 an indication as to how the supply is to be adjusted to maximise efficiency.

Claims

1. A variable speed controller for an electric motor, which includes first control means for adjusting at least one of the parameters of the electrical supply to give the desired motor speed, monitoring means associated with the motor or the power supplied thereto for monitoring the conditions under which the motor is operating, the monitored condition or conditions being relates to the degree of efficiency of operation of the motor, and second control means under control of said monitoring means and arranged to so modify at least one of the parameters of the electrical supply to the motor that the operational efficiency thereof is enhanced.

2. A variable speed controller as claimed in claim 1, in which the power factor of the voltage supplied is sensed from that supply and compared with the motor speed, or slip or torque, or with predetermined values of power factor, the result of the comparison being used to effect the required adjustment of the motor's supply.

3. A variable speed controller for an electric motor substantially as described with reference to Fig. 1 or Fig. 2 of the accompanying drawing. New Claim Filed on 10 Dec 1982 Superseded claims New Claim:

4. A variable speed controller for an electric motor, which includes control means for adjusting at least one of the parameters of the electrical supply to give the desired motor speed, said control means having an input over which there is received a signal indicative of the wanted motor speed, a first monitoring means associated with the input to the motor for monitoring the motor's input in respect of the power supply's power factor, current value or efficiency, a second monitoring means associated with the motor's output for monitoring its output in respect of its speed or torque, the results of said monitoring by both said monitoring means being representative of the degree of efficiency under which the motor is operating, a processor to which the outputs of said monitoring means are applied, which processor produces an output indicative of the efficiency of operation, the output from said processor so influencing the control means so as to cause it to vary the parameters of the supply to the motor as to improve the efficiency, e.g. to reduce the voltage as well as the frequency when the speed of the motor is to be reduced.