GB2249849A

GB2249849A - Motor speed control

Info

Publication number: GB2249849A
Application number: GB9105704A
Authority: GB
Inventors: James B Cockcroft
Original assignee: Sunbeam Corp
Current assignee: Sunbeam Corp
Priority date: 1990-11-15
Filing date: 1991-03-18
Publication date: 1992-05-20
Also published as: GB9105704D0; CA2038257A1; DE4114168A1

Abstract

A speed control circuit for a permanent magnet motor 20 utilizing full wave rectified current and including an SCR 30 controlled by a circuit responsive to the counter- or back-EMF of the motor which is compared to a reference voltage produced in a transistor circuit, the resulting level of conduction of the transistor 58 determining the time taken to charge an RC network which controls the firing of the SCR. The portion of back emf fed to the transistor is selectively adjustable by means of speed-setting potentiometer 44. <IMAGE>

Description

MOTOR SPEED CONTROL THIS invention relates to a variable speed control for

a permanent magnet electric motor and more specifically relates to a feedback type of variable speed control for the motor in an electric food mixer.

In the field of electric household mixers, it has always been regarded as important to provide variable speed controls for such mixers. The many types of mixing chores to which a household mixer is applied require beater speeds from several hundred rpms to over 1000, depending on whether one might be mixing cookie dough or beating egg whites. More recently, domestic food mixers have been sold with various alternative mixing implements such as whippers or dough hooks, the latter requiring substantial power at low speeds.

The simplest type of speed control still used on some mixers involves a tapped field and a series wound electric motor. The principal problem with such an approach is the fact that the lower speed windings for the series motor provide less power when under load conditions. Substantial power is often required at lower speeds. Accordingly, the tapped field type of speed control is generally regarded as unsatisfactory for mixers that are to be operated under substantial power at low speeds.

is Another type of speed control commonly used on electric food mixers is a governor type control using a mechanical centrifugal governor or tachometer feedback which may be adjusted to control the speed of the mixer over a wide range of speeds. This type of governor control is reasonably expensive but has the advantage of being able to provide high power at low speeds. There are also available more sophisticated electronic controls which utilize pulse generating tachometers associated with the motor to sense speed accurately and adjust power to compensate for variations of the actual speed from the required speed. These more sophisticated electronic controls are fairly expensive and are unsuitable for lower priced electric mixers.

There have also been available electronic controls which utilize the counter electromagnetic voltage produced by the motor to sense whether the motor is operating at the proper speed and make suitable adjustments to control the speed accurately in the face of a varying load.

While the general theory of operation of such feedback controls is well known, there has been considerable difficulty in providing operating circuits which are simple enough to be competitively priced and at the same time serve the desired function. Various problems have been encountered which render such controls generally unsatisfactory. They are sensitive to line voltage changes, have a tendency to provide cyclical swings in speed, sometimes called "coggingm, which is objectionable to the user since it at least creates the impression that the mixer is unable to settle down and operate at the selected speed level. It would, therefore, be desirable to provide a feedback control utilizing the counter EMF of the motor to accurately control the speed under varying load conditions utilizing a simple circuit having little filtering of the control signal.

The present invention involves a feedback control for a permanent magnet or equivalent motor operating on rectified unfiltered AC current which is controlled by an SCR. The gate of the SCR is regulated by a signal which is inversely proportional to the feedback voltage and which is applied to an RC circuit associated with the gate of the SCR. A comparison is made between a reference voltage and a signal proportional to the back EMF by means of a transistor circuit in which the base of the transistor is connected to a voltage divider which is set according to the speed desired and thus provides the voltage which is a predetermined portion of the back EMF. The emitter of the transistor is connected to a zener diode which provides a reference voltage to control the conduction of the transis- tor depending on magnitude of the portion of the feedback voltage applied to the base. By tying the emitter of the transistor to ground through a zener diode and a resistor, a speed stability is achieved which is reasonably insensitive to variations of line voltage. In addition, the resistor in circuit with the transistor permits the circuit to be designed with the constant slope on the speed torque curve which improves the speed stability and the control sensitivity.

It is an object of the present invention to provide an improved variable speed control for use with a permanent magnet motor.

It is another object of the present invention to provide an improved feedback control circuit for varying the speed of a permanent magnet motor utilizing the back or counter EMF produced during the portion of the cycle in which the power to the motor is shut off.

It is another object of the present invention to provide a speed control circuit for a permanent magnet motor utilizing full-wave rectified AC current and an SCR controlled by a circuit responsive to the counter EMF of the motor during the period in which the SCR is shut of f 1 5 and including a voltage divider which may be set f or a desired speed to produce a signal proportional to the counter EMF to be compared with a standard f or regulating the charging of an RC circuit associated with the gate circuit of the SCR.

It is still another object of the present invention to provide a speed control circuit f or a permanent magnet motor utilizing full-wave rectified AC current controlled by an SCR in which the counter EMP of the motor is compared to a reference voltage in a transistor circuit having means in the emitter circuit of the transistor to regulate the slope of the speed torque curve for the motor and its control circuit.

Further objects and advantages of the instant is invention will become apparent to one skilled in the art as the following description proceeds, and the features of novelty which characterize the invention will be pointed out in the claims annexed to and forming a part of the specification.

Fig. 1 is a schematic diagram of the speed control circuit embodying my invention; Figs. 2a and 2b are diagrams of the voltage across the motor and SCR of the control circuit embodying my invention under conditions of light and heavy load; Figs. 3a and 3b are diagrams of the voltage across the permanent magnet motor under light and heavy loads, respectively; Figs. 4a and 4b are diagrams of the voltage across the gate and cathode of the SCR in the motor control circuit under the same load conditions as Figs. 2 and 3; Figs. 5a and 5b are diagrams of the voltages across the condenser in the gate circuit of the SCR which controls the firing of the SCR under the same load condi tions as Figs. 2 - 4; is Figs. 6a and 6b are diagrams of the voltage across the base of the transistor in the motor control circuit under the same load conditions as in Figs. 2 - 5; and Figs. 7a and 7b are diagrams of the voltage across the emitter of the transistor control circuit to ground under the same load conditions as in Figs. 3 - 6.

Referring to Fig. 1 of the drawings, there is shown a schematic diagram of the control circuit embodying my invention. The speed control circuit of the instant invention is applicable to a permanent magnet type motor operating on AC current. The motor is shown in Fig. 1 schematically and designated by reference numeral 20.

The motor control circuit is designated generally by reference numeral 22 and includes a full-wave rectifier 24 connected to a line cord 26. The output of the full wave rectifier 24 is connected at one output terminal by a conductor 28 to an SCR 30 and the other side of the recti fier output is connected through a conductor 32 through a line switch 34 and by conductor 36 to the motor 20. The other terminal of the motor 20 is connected by a lead 38 to the cathode 30a of the SCR 30. The SCR 30 also has an anode 30b to which the lead 28 connects and a gate 30c to which a lead 40 connects to one terminal of a resistance 42, capacitor 54, and diode So.

In order to regulate the speed of the motor 20, there is provided in the circuit 22 a potentiometer 44 having one end connected to the lead 36 by lead 46 with the other end connected by lead 48 to an adjustable resistor 50. Also connected in series with potentiometer 44 and resistor 50 is a resistor 52. Connected between resistor 52 and the lead 40 of the gate of the SCR circuit is a capacitor 54.

The purpose of the resistances 50, 52 and potentiometer 44 is to provide a portion of the motor feed back voltage to use in controlling the operation of the SCR 30. Interconnected between the potentiometer 44 and the resistance 50 is a lead 56 which connects to a transistor 58 at its base terminal 5sa. The transistor 58 is further provided with a collector terminal 58b which is connected by a lead 60 to a resistance 62 which in turn is connected by lead 64 to the input lead 28. The transistor 58 is further provided with an emitter terminal 58c which is connected to a resistance 66 and a zener diode 68 and lead 70 which is connected to the other input lead 36. Connect- ed in parallel with the zener diode 68 and the resistance 66 is a capacitor 72. The resistance 62 and the resistance 42 are interconnected by lead 74 which also connects to one terminal of a capacitor 76, the other terminal of which is connected to the base terminal 58a of the transistor 58.

Thus, the capacitor 76 is connected across the collector terminal 58b and base terminal 58a of the transistor 58.

The motor back EMF signal is supplied tothe resistance 52 by a lead 78 connected to the motor lead 38.

Interconnecting the lead 78 to the lead 40 of the gate of the SCR 30 is a diode 80. There is also a diode 82 inter connecting the other motor lead 36 to the lead 78. To explain the operation of the motor control circuit forming my invention a review will first be -made of the functions performed by the various elements of the circuit described above and then reference will be made to the curves forming Figs. 2 through 7 of the drawings to explain the dynamic operation of the circuit and its functions in controlling speed at various load-levels.

In a preferred embodiment of my circuit, the SCR 30 comprised a model T-106D1 silicon controlled rectifier which requires a positive gate to cathode voltage of on the order of.25 to 1.0 volts to trigger the firing of the SCR 30. In the gate circuit of the SCR 30, there is an RC network comprising the resistance 42 and the capacitor 54.

In my preferred embodiment, the resistor 42 had a value of 22k ohms and the capacitor 54 was a.22 microfarad capacitance. The power for charging the RC network on the gate circuit for the SCR is supplied through the resistance 62 which is a 30k ohm resistance. The rate at which the capacitor 54 is charged depends to a large extent on the conduction through the transistor 58. Thus the transistor 58 determines how soon in the cycle the SCR begins to conduct. If the transistor 58 diminishes conducting, the capacitor 54 will charge more rapidly.

The diode 80 allows the back EMF from the motor to provide voltage through the load resistor 42 to the collector of transistor 58 when the unfiltered full-wave voltage on buss 28 is below the motor back EMF This maintains transistor 58 operational for both short and long delay angles, i.e., near zero crossing and improves stability of the ramp voltages in Figs. 4 and 5.

In general terms the circuit 22 may be described as a speed control circuit which utilizes a back EMF signal indicative of motor speed to control on each half cycle the point at which the SCR fires. The sensing of the back EMF is performed on each half cycle prior to the firing of the SCR. Since the back EMF voltage is quite high, on the order of 100 volts, the network including the resistances 44, 50 and 52 are used as a voltage divider to pick off a portion of the feed back signal which is then compared to a reference signal appearing across the resistor 66 and the zener 68. The resistance 52 is a fixed 130k ohms resistance, whereas the resistance 50 is an adjustable resistance having a maximum of 250k ohms. Resistance 50 is adjustable so that the low speed setting of the speed control 22 may be accomplished accurately as a factory setting. The resistance of the potentiometer 44 may be varied from 0 at the high speed setting to a maximum of 50k ohms at the low speed setting. At the low speed setting of the potentiometer 44, about a fifth of the voltage across the network of resistances 46, 50 and 52 would appear across the base of the transistor 58. With a low speed setting on the potentiometer 44, the voltage produced by the back EMF would produce a higher voltage on the transistor base 58a causing the transistor 58 to conduct earlier in the cycle and, therefore, increase the time for the current through the resistor 62 to charge the RC network including the resistor 42 and the capacitor 54. On the high speed setting one would get a smaller voltage on the base of the transistor 58. Therefore, the transistor 58 would begin conducting later in - the cycle causing the capacitor 54 to charge to the firing voltage of the SCR 30 earlier in the cycle. With the earlier firing of the SCR more power would be delivered to the motor 20.

In considering the function of the transistor 58, it should be noted that one of the problems frequently 15 encountered in the less complex type feed back circuits is the lack of stability and the sensitivity of the circuits to variations in line voltage. Through the use of the zener 68 to provide a fixed voltage reference of about 5.1 volts and the resistance 66 to vary the reference in a 20 predetermined manner, we have achieved the object of increased stability and lack of voltage sensitivity. if the resistance 66 were made larger, the regulation of the speed for various changes in the load would in theory be more precise. However, it has been found that the precise 25 regulation produces an undesirable instability which is eliminated by the use of a 3k ohm resistance 66 to provide slope to the speed torque curve to achieve greater stability. While this results in some speed changes with variations in load, this relatively small change is acceptable to achieve the desired stability.

The capacitor 72 in parallel with the zener 68 and resistor 66 filters the commutation noise and any sudden changes in the reference voltage to again add to the stability of the circuit in controlling the speed of the motor 20. The capacitor 76 is similar in function in that it reduces the gain of high frequency transient and keeps noise from de-stabilzing the control 22 and serves to bypass the transients from the collector to the base circuit. capacitor 72 IS 1.0 microfarad and capacitor 76 is.01 microfarad.

The capacitor 54 is a.22 microfarad capacitor, and along with the 22K ohm resistor 42, forms the RC network which controls the firing of the SCR 30. The resistor 42 not only controls the rate at which the capacitor 54 is charged but also serves to limit the current from the motor 20 through the transistor 58 after the SCR 30 has fired.

The diode 80 limits the negative voltage on the capacitor 54 to 1 volt when the cathode of the SCR is at a higher voltage than the collector on the transistor 58. The diode 82 is for the purpose of commutating the inductive energy in the motor 20 such that it is used as motor output power rather than as destructive high voltage energy.

Turning now to the curves shown in Figs. 2 through 7, we are better able to understand the manner in which the control circuit 22 functions. The curves of Figs. 2a and 2b show the voltage across the output of the full-wave rectifier during conditions of light load and heavy load. This voltage has been shown as Vs on the schematic diagram of Fig. 1 and in the diagrams of Figs 2a and 2b. Under both load conditions the voltage is approximately the same with the arc shaped curves being spaced by periods of substantially constant voltage which represents the result of the back EMF of the motor 20. In the curves of Fig. 3 we note the changes in the f iring of the SCR as the load is increased from the- light load to the heavy load. Figs. 3a and 3b represent the voltage across the motor as the load is varied. Under the light load condition, the SCR f ires past the peak of the input voltage curve, whereas, in the heavy load condition, the SCR 30 fires as the rising voltage approaches about 150 volts.

- 10 The curves of Figs. 4a and 4b represent the gate to cathode voltage on the SCR 30, or the voltage across the terminals 30a to 30c. To best understand the manner in which the back EMF affects the charging of the capacitor 54 and the firing of the SCR 30, reference should be made to the curves of Figs. 6 and 7. Fig. 6 shows the voltage on the base of the transistor 58 which is principally a result of the back EMF of the motor 20 attenuated through the resistance network 44, 50 and 52. Under light load conditions the back EMF is greatest and decreases slightly as the load is increased from Fig. 6a to 6b. Figs. Sa and 5b show the voltage to the collector terminal 58b which is essentially the charging voltage for the RC network, including the resistor 42 and the condenser 54. As this charging voltage, as shown in Figs. -5a and 5b, increases with load, the SCR 30 fires earlier and earlier in each half cycle of power, the firing point being shown by the curves of Figs. 3a and 3b. Figs. 4a and 4b show the voltage across the gate 30c and cathode 30a of the SCR 30 which voltage determines when firing occurs. The f iring point is affected by the charge on the capacitor 54 as well as the back EMF which, as shown by Figs, 3a, and 3b is reduced as load increases.

In considering the change in the voltage at the base of 58a of the transistor 58 in going from a light load to a heavy load, we note that the higher voltage in Fig. 6a as compared to Fig. 6b results in conduction in the transistor 58 that lowers the charging voltage at the collector terminal 58b thereby delaying the point as shown in Fig. -4 when the gate to cathode voltage for SCR 30 reaches a level required for firing.In Fig. 4b, the charging completes rapidly and the SCR 30 fires early in the half cycle.

In claiming the speed control circuit 22, reference is made to the charging circuit which is connected to the RC network that controls the f iring of the SCR in i i - 11 series with the motor across the full wave power supply. This charging circuit includes the transistor 58, and the voltage divider comprised of the resistances 44, 50 and 52.

The speed control circuit 22 described above provides excellent speed control for motors in domestic appliances, particularly mixers, giving good control under varying loads at speeds from 500 to 1000 revolutions per minute for mixing elements driven through gear reductions of on the order of 20 to 1.

is - 12

Claims

1. A speed control circuit for a motor operated appliance of the type utilizing a permanent magnet motor comprising an SCR connected in series with said motor, rectifier means for supplying unfiltered full wave rectified power to said motor and SCR, a voltage divider connected across said motor, said SCR having a gate, said voltage divider including a selectively variable resistance and an output terminal to provide a voltage which is a selected portion of the back EMF produced by said motor, an RC network including a resistor and a capacitor connected to said gate to supply a signal for f iring said SCR, a charging circuit connected across said motor and SCR and to said RC network, said charging circuit including amplifier means controlled by said voltage divider and a reference voltage to vary the rate of charging said RC network in an inverse relation to said back EMF produced by said motor.

2. The speed control circuit of claim 1 wherein said amplifier comprises a transistor connected to the output terminal of said voltage divider and said reference voltage being provided by a zener diode.

3. The speed control circuit of claim 2 wherein said transistor has a base and emitter connected between said output terminal and said zener diode.

4. The speed control circuit of claim 2 wherein said variable resistance is manually adjustable to a plurality of motor speed settings, and a resistance in series with said transistor and said zener diode to reduce the sensitivity of said control circuit by causing said circuit to vary motor speed inversely with load at each said speed setting.

1 i t i

5. The speed control circuit of claim 3 wherein said transistor includes a collector connected to said RC network.

6. The speed control circuit of claim 5 includ ing a diode connect between the cathode and gate terminals of said SCR to conduct said back EMF to said collector to maintain conduction of said transistor when the power supply voltage applied to said transistor falls below said back EMF

7. The speed control circuit of claim 2 wherein said amplifier is connected across the output of said rectifier means, said transistor including a collector connected to the anode of said SCR and having an emitter connected through a resistor and a zener diode to said output of said rectifier means, said transistor having a base connected to said voltage divider output terminal.

8. Any novel feature or combination of features disclosed therein-