Conveyor speed control ciruit for a conveyor oven

A speed control circuit for controlling the speed at which an AC electric motor including a field coil drives a conveyor to move products through a conveyor oven, comprising:

• a rectifier coupled in series between an AC power source and the coil to provide a rectified current to the coil;
• a shunt circuit configured to introduce a DC voltage into the coil by shunting the rectifier with a variable-resistance device, the speed of the conveyor depending upon the resistance of the variable-resistance device;
• a temperature-sensing device configured to generate a control signal when a temperature within the conveyor oven drops below a predetermined value;
• and
• a speed-reducing circuit responsive to the control signal to introduce an additional resistance into the shunt circuit when the temperature at the point within the conveyor oven drops below the predetermined value, thereby decreasing the speed of the conveyor.

A conveyor oven, comprising:

• an oven housing;
• a plurality of heating elements mounted within the oven housing, wherein at least one heating element is turned on and off in response to a temperature within the oven housing;
• a conveyor configured to move products through the oven housing adjacent the heating elements, whereby the products are exposed to the plurality of heating elements;
• an AC electric motor mechanically coupled to the conveyor to move the conveyor at different speeds in response to the temperature within the oven housing;
• a temperature-sensing device configured to generate a temperature signal based upon the temperature within the oven housing;
• and
• a control circuit coupled to the at least one heating element, the motor, and the temperature-sensing device, wherein the control circuit is configured to step down and step up the speed of the conveyor and to turn on and off the at least one heating element when the sensed temperature drops below and rises above a predetermined value, respectively.