Cycling on the overload indicates a condition that must be corrected. The overload protector is sensitive to three sources of heat: • • Excess amperage flow (usually an electrical problem related to the start winding and start components). • • Excess motor heat (usually resulting from dry or badly worn bearings). • • Excess compressor heat (low on oil or compressor cooling is not working). Tripping the overload protector is a warning that the motor is in danger of a burnout. It is important to locate and repair the cause of an overload. Two-Wire Overload Connections The terminals on the overload protector are numbered 1, 2, and 3. The contacts are located between numbers 1 and 2. A small resistance heater wire is located between numbers 2 and 3. The resistance heater makes the overload react more quickly to overheating. See Figure 25‑12. Connections to the overload are limited to Terminals 1 and 3, which simplifies making elec- trical connections and prevents bypassing the heater. The heater is internally connected in series with the contacts. One side of the supply voltage is connected to Terminal 1 so the current must flow through the contacts and the small heater to reach Terminal 3. The most common form of motor protection uses a “snap-acting” bimetallic disc to make and break a set of contacts. See Figure 25‑11. The snap action is obtained by fusing together two thin, circular discs of different metals (bimetallic), usually steel on top and copper on the bottom. The bimetallic disc is anchored at the center. Since copper expands faster than steel, a rise in temper- ature causes the disc to expand. The outer edge curls (bows). The movement of the bowed edge opens a set of stationary contacts. When the bimetallic disc cools, it snaps back to its original (closed) position. Excessive heat from any source will affect the overload and cause it to open the circuit supplying power to the motor. Cycling on the Overload If an overload condition is not corrected, the bimetallic disc will trip repeatedly, a malfunction known as cycling on the overload. After each trip, more and more time is required for the bimetallic disc to cool. Eventually, the bimetallic disc could require up to an hour to cool and reset. Cycling on the overload is easy to hear. The motor makes a noise described as “hmmm...click.” The humming noise is made by the motor trying to start, and the sharp “click” is made when the bimetallic disc snaps open. Closed Bimetal disc Solder Screw Open Bimetal disc Solder Screw Goodheart-Willcox Publisher Figure 25‑11. A bimetallic disc of steel and copper is used in many overloads. In normal operation, the disc holds the contacts in the closed position. When sufficiently heated by an overload, the differing expansion rates of the metals cause the disc to curl in one direction. This opens the contacts and breaks the motor circuit. When the motor cools sufficiently, the disc returns to its normal shape, closing the contacts again. 492 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc.