This invention pertains to speed regulators for direct current DC motors and, more particularly, is concerned with open loop speed regulators for DC motors.
DC motors find numerous applications because of their intrinsic variable speed characteristics and capabilities which offer very high speeds and small size.
The rotating member of a DC motor is named the armature and the stationary member is named the field. The armature has windings and the field can have either windings or permanent magnets.
Some applications have a need for constant speed regardless of torque. A general statement about DC motors is that with an increase in torque, speed will drop and current will increase, assuming a constant input voltage. The amount each parameter varies depends on the type of motor. For a motor with the armature and field winding connected in series the drop in speed will be more pronounced than the increase in current. For motors with shunt connected windings or permanent magnet fields the opposite is true, the speed will be more nearly constant while there is a marked increase in current. There will be some drop in speed however, and this amount may be undesirable in critical applications. For this reason, a number of constant speed controls have been devised over the years.
Speed regulating systems may be classified as either closed loop or open loop. Closed loop systems derive a signal from the actual speed of the motor with a tachometer, for example, and use the signal in a feedback loop.
An open loop system does not measure speed directly but measures some other parameter. In some open loop systems the measured parameter is current.
A well known example of an open loop motor regulating system includes a resistor in series with the input of the motor. The voltage across the resistor corresponds to motor current and is directed to a control circuit. The resistor voltage influences a control circuit which supplies the input voltage to the motor. A change in resistor voltage indicates a change in torque and indirectly indicates a change in speed. In response to the resistor voltage the control circuit adjusts the voltage to the motor thereby supplying the right amount of power required to maintain a constant speed over variations in torque.
The series resistor causes I.sup.2 R power losses particularly when during high torque conditions because current is high. These losses cause heat build-up and a need for a larger power supply capability.
It will be seen that a speed regulator according to the present invention does not require a resistor in series with the motor and is thereby more efficient.