A wide variety of devices have been used in the past for controlling the speed of an electric motor ranging from a simple variable resistor connected in series between the motor and a power source to rather sophisticated control devices. The commonly used variable resistor is not suitable for use with a shaded pole motor, often used with laboratory equipment, because it reduces the starting torque such that the motor may not start at low speeds.
One known method for controlling motor speed is to switch a bidirectional semiconductor device, such as a triac, on and off to control the delivery of the power source to the motor. See, for example, U.S. Pat. No. 3,780,366 to Henderson; U.S. Pat. No. 3,523,234 to Turtle; U.S. Pat. No. 3,541,412 to Worth; U.S. Pat. No. 3,596,158 to Waltour; U.S. Pat. No. 3,678,360 to Minarik et al.; U.S. Pat. No. 3,742,337 to Digneffe; U.S. Pat. No. 3,803,468 to Soeda; and U.S. Pat. Nos. 3,875,485 and 3,848,167 to Hornung. Such devices could be used with shaded pole motors; however, the devices shown in these patents are either very complicated and expensive or utilized some feedback mechanism coupled to the motor speed for adjusting the speed of a motor and are just not practical for replacing simple variable resistor controls.
Accordingly, it is an object of the present invention to make a motor speed control circuit which utilizes a bidirectional switch to control the power supplied to the motor in a simple, inexpensive manner and which is suitable for use with shaded pole motors.
It is also known in the art that the speed of a motor is directly related to the number of energy pulses supplied to it over a period of time. See, U.S. Patent No. 4,104,570 to Hamby et al; U.S. Pat. No. 3,243,677 to Cannalte et al.; and U.S. Pat. No. 3,559,017 to Dinger. Cannalte et al. switch a D.C. voltage source on and off in a controlled manner to vary the speed of a motor. Dinger utilizes a complicated counting circuit to supply the proper number of pulses to the motor which matches the number of pulses which should be supplied to obtain a certain motor speed. Hamby et al. supply a number of pulses to six thyristors to adjust the motor speed or phase of firing signals from desired values. These devices are not suitable for replacing the variable resistor used to control shaded pole motors.
Therefore, it is a further object of this invention to provide for a device which controls the speed of a motor directly by supplying a specific number of complete A.C. signal cycles to the motor in a simple and inexpensive manner.