1. Field of the Invention
The invention concerns a rotary speed control circuit for feeding a dc universal motor, in particular for driving a washing machine.
In modern washing technology the motor must cover a wide range of rotary speeds, from slow washing speeds up to high spin speeds for properly removing water from the laundry.
2. Discussion of the Prior Art
The foregoing can be achieved on the one hand by field weakening or attenuation of the motor, by the field winding of the motor being switched over from "long field" to "short field", as is known for example from DE 43 05 477 Al. In the long field mode the motor operates at a washing speed of about 300 motor revolutions per minute while in the short field mode the motor operates at a spin speed of for example 12000 motor revolutions per minute.
The motor speed can be regulated with an electronic regulating device which operates a power switch, for example an IGBT (Insulated Gate Bipolar Transistor) for regulation of the motor voltage, in which case a tachogenerator of the regulating device, which is carried on the armature shaft of the motor, indicates the actual value. The on-off ratio of the power switch determines the mean dc voltage applied to the motor and thus controls the rotary speed.
On the other hand, as is also described in DE 43 05 477 Al, for the purposes of increasing the range of rotary speed, the smoothing capacitance connected to a rectifier taking the mains ac voltage can be formed by two capacitors which are connected in series with each other, in such a way that, to double the operating voltage, the electronic regulating device switches the junction of the capacitors to a tapping of the bridge rectifier so that the positive mains half-waves charge up one capacitor and the negative mains half-waves charge the other capacitor.
In the state of the art shown in FIG. 1, as is generally conventional practice in relation to rotary speed control circuits for the operation of a dc universal motor, operation is conducted in accordance with the topology of a low-setting device. The ac voltage feeding the rotary speed control circuit ME is rectified by way of a rectifier GR for feeding the dc universal motor UM. The rotary speed control circuit ME regulates by way of a regulating device R the motor voltage in accordance with the actual signals from a tachogenerator T carried on the armature shaft A, for conformity with the desired reference or target rotary speed. Besides the rotating armature A with commutator and tachogenerator T the dc universal motor UM has the stationary field winding F. The motor voltage is derived from the rectified mains voltage in accordance with the switch-on ratio of the power switch TR1 which is arranged in the topology of a low-setting device. During the on time of the power switch TR1 the full rectified mains voltage is applied to the motor while in the off time of the power switch TR1 the motor current is commutated to the commutation diode D1 and the voltage applied to the motor is thus equal to zero. The motor voltage can be regulated by way of the switch-on ratio of the power switch TR1 between zero to a maximum of the rectified mains voltage, and therewith also the rotary speed of the motor. A continuous motor current is afforded with an advantageously selected high cycle frequency for the power switch TR1 above the audible range (&gt;16 kHz).
In order to enlarge the range of rotary speed, in accordance with the state of the art to achieve high spin speeds, for example over 10,000 revolutions per minute, the field winding F is switched over at the motor with the effect of a field weakening or attenuation action by way of the electrical contact k2, for example by way of the coil K2.
A dc universal motor with a field switching arrangement of that kind for field weakening at the motor is expensive. Doubling the operating voltage by means of two series-connected capacitors in accordance with DE 43 05 477 was also not satisfactory.