GB2171863A - Speed control of A.C. motors - Google Patents

Abstract

A variable speed motor control system eg for an overhead crane or hoist comprises a pair of two- position switches 7 each of which can be manually actuated so as to switch from a normal state to either one of two actuated states and which assumes the normal state automatically when the switch is released. A control circuit 6 is responsive to actuation of the switch to apply inputs to an inverter 3 which inputs control the motor speed. The control circuit is responsive to actuation of the switch to a first one of the actuated states to accelerate the motor 1 to and thereafter maintain the motor at a predetermined minimum speed, or, where the motor speed is already at or above the predetermined minimum speed when the switch is actuated to the first actuated state, to maintain the motor speed constant. The control circuit is responsive to actuation of the switch to the second actuated state to accelerate the motor, and is responsive to the switch assuming its normal state to decelerate the motor to rest. The control system may be connected between a standard variable frequency or other drive and an operator's pendant. The pair of two-position switches can be provided in a single pendant so as to provide variable speed operation in two directions. <IMAGE>

Description

SPECIFICATION Variable speed motor control system The present invention relates to a variable speed motor control system and in particular to a control system which is suitable for controlling a motor driving for example an overhead crane or hoist.

Overhead cranes are generally controlled by electric motors in response to actuation of control switches on a pendant suspended from the crane itself. Thus the user simply takes hold of the pendant, presses the appropriate switches to obtain the desired movement of the crane and then simply releases the pendant. It is of course necessary for safety reasons to ensure that if the pendant is ever released the crane control motors are automatically braked. It is also highly desirable to make it possible for the user to control the switches on the pendant with one hand, thereby leaving his other hand free to for example guide a load supported by the cane.

It is also desirable particularly with large overhead cranes to be able to vary the speed at which the crane moves so that loads can be transported a significant distance at a reasonable speed but manoeuvred into a final position at low speed.

Prior art crane control systems can be considered as falling into two general types. The first type comprises multiple pole motors. Selective switching of the poles enables various speeds to be achieved, a doubling of motor speed being produced generally by each switching step. Unfortunately multiple pole motors are relatively expensive and separate wiring is required to each set of poles. Several switch control contactors are generally required and a large multiple position switch is needed in the pendant. The other type of system is the soft-start/variable frequency invertor system. A soft-start controller or variable frequency invertor drive enables a continuously variable speed to be obtained although the soft-start device has the disadvantage that the torque available from a given motor is reduced.In both cases a potentiometer is required to give continuously variable soeed and it is difficult to arrange a potentiometer for simple one-handed operation. In addition relatively complicated wiring is required to the pendant. To avoid the need for a potentiometer a multiple position switch (typically a fiveposition switch) can be used but this restricts the range of speeds which can be selected and requires a large expensive pendant with a lot of relatively complex wiring.

It is an object of the present invention to provide an improved variable speed motor control system.

According to the present invention, there is provided a variable speed motor control system comprising at least one two-position switch which can be manually actuated so as to switch from a normal state to either one of two actuated states and which assumes the normal state automatically when the switch is released, and a control circuit responsive to actuation of the switch to apply inputs to a drive system which inputs control the motor speed, wherein the control circuit comprises first means responsive to actuation of the switch to a first one of the actuated states to accelerate the motor to and thereafter maintain the motor at a predetermined minimum speed, and, when the motor speed is already at or above the predetermined minimum speed when the switch is actuated to the first actuated state, to maintain thr motor speed constant, second means responsive to actuation of the switch to the second actuated state to accelerate the motor, and third means responsive to the switch assuming its normal state to decelerate the motor to rest.

Preferably the drive system is an inverter.

Preferably means are provided to adjust the rate of acceleration and deceleration to match the mechanical characteristics of the motor and its load.

The variable speed motor control system may be connected between a standard variable frequency or other drive and an opera tor's pendant. The two-position switch can be of standard form. Only four wires are required to connect a pair of two-position switches provided in a single pendant so as to produce variable speed operation in two directions.

The supply used to the switches can be standard mains voltage as used in prior art crane control circuits. The use of such voltages is advantageous as firstly maintenance personnel are familar with control systems operating at such voltages and system using such voltages are very tolerant to electrical noise. Electrical noise can be a severe problem in environments where large electric motors are being turned on and off.

Preferably the mains voltage inputs to the operator's pendant are electrically isolated from the control circuit by for example optical couplers.

A system according to the invention can be used with standard industrial induction motors which are relatively cheap when compared with multiple pole motors.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of a system in accordance with the invention; Figure 2 illustrates the variation of motor speed with time given particular actuations of the control switches of the system illustrated in Fig. 1; and Figure 3 illustrates in detail the logic circuitry of practical embodiment of the control circuit of Fig. 1.

Referring to Fig. 1, a standard industrial induction motor 1 has its output shaft mechanically coupled to for examp,le a hoist by a mechanical coupling represented by lines 2.

The induction motor 1 is controlled by a standard AC variable frequnecy inverter drive 3.

One example of such an inverter drive is the CFM 237 marketed by Cableform Inc. of Zion Cross Roads, Troy, Virginia. The inverter drive receives speed controlling inputs 4 and direction of rotation controlling inputs 5, The speed inputs are effectively a single potential which in a conventional system would be derived from the wiper of a potentiometer. The direction inputs are simple forward and reverse signals.

The inputs to the inverter drive 3 are derived from a control circuit 6 which is responsive to a pair of two-position switches 7 which in practice would be mounted in a pendant. The circuit 6 comprises an input section 8 which receives typically 110 volts signals from the switches 7, an isolation section 9 and an output section 10. The isolation section 9 isolates the large AC voltages in section 8 from solid state logic level signals in section 10. An output 11 from the section 10 controls a mechanical brake which will typically be arranged to automatically stop the motor in the event of loss of electrical control.

The output section 10 comprises means for adjusting the "crawl" or minimum motor speed, the rate of acceleration of the motor and trhe rate of deceleration of the motor.

Referring now to Fig. 2, the response of the motor to actuation of the upper two position switch A will be described. It will be seen that the upper contacts of swich A provide "A" and the lower contacts provide output "A accel". As the operator depresses the switch the upper contacts close first and a further depression of the switch is required to close the lower contacts. The switch A controls movement of the motor in one direction and the switch B simply controls movement in the opposite direction. The response to actuation of two-position switch B will not be described as it is equivalent to the result of actuation of switch A.

With reference to Fig. 2, the motor is initially at rest and the operator then depresses the button so as to close the upper contacts but leave the lower contacts open. The motor accelerates at a constant rate until time t at which time a minimum or crawl speed is achieved. That crawl speed is maintained until time t2 when the operator presses harder on the switch A and closes the lower set of contacts. The motor then accelerates at a constant rate until time t3 when the operator lifts his finger partially from switch A so that the lower contacts open but the upper contacts remain closed. The motor speed achieved at time t3 is then maintained. At time t4 the operator lifts his finger completely off the switch A and the motor then decelerates at a constant rate until it comes to rest.

The rate of acceleration up to the crawl speed and deceleration to rest is generally selected to match the maximum acceleration and deceleration possible given the mechanical inertia of the system controlled by the motor.

The acceleration from the crawl speed (between time t2 and time t3) can be selected so as to suit the particular requirements of the operator using the system.

Thus with the described system the operator can easily achieve continuously variable speed control using one hand as only one button has to be pressed and the button has only two possible positions. Furthermore as soon as the button is released the motor is automatically braked at a predetermined rate.

The system is therefore fully fiexible and yet safe.

Referring now to Fig. 3 details of the circuit 6 of Fig. 1 are illustrated. Inputs Al, A2, B1 and B2 respectively receive the "A", "A accel", "B" and "B accel" inputs from the operator's pendant. The mains neutral supply is connected to input N. These inputs are applied as shown to three identical opto-coupler integrated circuits IC1, lC2 and 1C3. The output of ICI indicates that button A has been pressed. The output of IC2 indicates that button B has been pressed. The output of IC3 indicates that whichever button has been pressed its second set of contacts have been closed so as to demand an acceleration. The opto-couplers isolate the mains voltages appearing on inputs Al to B2 from the solid state logic of the rest of the circuit.

The operation of the logic circuit will be apparent from the standard logic notation used. Briefly however the outputs comprise an analogue voltage appearing at output V2 representative of the speed demanded of the motor, output V3 is connected to ground, output SF indicates forward rotation and output SR indicates reverse rotation. The analogue voltage on output V2 is provided by a digital to analogue converter IC7. The rate of acceleration above the basic crawl speed is determined by a variable resistor VR1 and its associated integrated circuit IC6A. The rate of deceleration which will generally be greater than the rate of acceleration is controlled by an integrated circuit IC6B, fine tuning of the rate of deceleration being achieved by adjusting variable resistor VR2. Switches SW1 and SW2 enable the matching of the output of IC6B to the particular inverter being used.

The minimum crawl speed is adjustable by the variable resistor VR3. An auxiliary output relay RL1 could be used to for example control a mechanical brake fixed to the motor. It will be appreciated that the device is compatible with armature type brakes and threephase brakes.

Claims (4)

1. A variable speed motor control system comprising at least one two-position switch which can be manually actuated so as to switch from a normal state to either one of two actuated states and which assumes the normal state automatically when the switch is released, and a control circuit responsive to actuation of the switch to apply inputs to a drive system which inputs control the motor speed, wherein the control circuit comprises first means responsive to actuation of the switch to a first one of the actuated states to accelerate the motor to and thereafter maintain the motor at a predetermined minimum speed, and, when the motor speed is already at or above the predetermined minimum speed when the switch is actuated to the first actuated state, to maintain the motor speed constant, second means responsive to actuation of the switch to the second actuated state to accelerate the motor, and third means responsive to the switch assuming its normal state to decelerate the motor to rest.

2. A control system according to claim 1, comprising fourth means to adjust the rate of acceleration and deceleration to match the mechanical characteristics of the motor and its load.

3. A control system according to claim 1, connected between a standard variable frequency drive and an operator's pendant, wherein the electrical supply to the switches is standard mains voltage electrically isolated from the control circuit by optical couplers.

4. A variable speed motor control system substantially as hereinbefore described with reference to the accompanying drawings.