Circuit Protection Strategies

The electric motors, transformers and controllers used in home and professional grade appliances are often subjected to mechanical overloads, overheating, stalls, lost neutral, severe overvoltages, humidity and other damaging factors. Raychem´s polymeric positive temperature coefficient (PPTC) devices of the LVR/LVRL series are designed for operation at line voltages of 120/240 VAC, and can help appliance designers prevent safety and fire hazards, as well as reduce warranty return and replacement costs resulting from motor failure.

Protecting increasingly sophisticated and complex control boards from misconnection, power surges or short circuit damage is of particular concern to the equipment manufacturer. Although appliance transformers, their enclosures and connections are capable of withstanding higher voltage transients, the use of sensitive solid-state devices on the board necessitates improved overcurrent, overtemperature and overvoltage control.

Coordinating overcurrent and overvoltage protection can also help designers comply with safety agency requirements, minimize component count and improve equipment reliability. A metal oxide varistor (MOV) overvoltage protection device, used in a coordinated circuit protection strategy with a line-voltage rated PPTC overcurrent device helps manufacturers meet IEC-6100-4-5, the global standard for voltage and current test conditions for equipment connected to AC Mains.

Technology Comparison
When a fault occurs in a motor, transformer or power supply, some of the components may begin to overheat. There are several circuit protection schemes that can be used to help guard against damage caused by these fault conditions and the resultant overtemperature damage, including thermal fuses, current fuses, and circuit breakers. However, in many appliance applications resettable devices such as PPTC devices, CPTC devices and bimetal breakers are the preferred solution. These devices do not generally require replacement after a fault event, and allow the circuit to return to the normal operating condition after the power has been removed and/or the overcurrent condition is eliminated.

CPTC devices help provide resettable protection; however, their application may be limited due to their relatively high operating temperature, high resistance and large size. The composition of the CPTC device tends to be brittle, which makes it vulnerable to damage from shock, vibration, and the thermal stress of heating and cooling found in many appliance applications.

Traditional bimetal circuit breakers, although widely used to help protect the electric motors found in appliances, do not latch and require additional action to interrupt their on-off cycle. The cycling nature of the bimetal circuit breaker has several disadvantages. Among those are material fatigue and a tendency to damage contacts, spark or weld shut. If the device “fails closed” it can cause overcurrent damage to the motor as well as sensitive follow-on electronics. Potential noise or “chatter” and electromagnetic interference (EMI) can make bimetal circuit breakers incompatible with advanced electronic control systems.

Motor Protection Strategy
Intermittent operation motors, such as those used in blenders and food processors, are usually designed to operate for a limited time. In general, operating these products for longer than the designed maximum limit usually results in stalling, overheating, and ultimately failure. Fault conditions arise when the power is held on, either because of contact failure or customer misuse. To prevent overheating, the circuit protection device used must “trip” quickly, but not sooner than intended, to avoid creating a nuisance condition for the user. The design challenge is to create a protection scheme that effectively protects the motor without nuisance tripping.

Nuisance tripping is often caused by inrush currents associated with certain electrical components found on motorized equipment. The major advantage of the PPTC device is that it can be specified with a trip current substantially below the normal operating current of the motor, but with a time-to-trip that is several times longer than a full system operating cycle, to avoid nuisance tripping.

Coordinating Protection
In the past, control board designs often used no overcurrent protection on the primary or secondary side, relying on the transformer to sink sufficient heat to prevent control board damage in the event of a fault condition. Coordinating overcurrent and overvoltage protection at the AC Mains input can help designers comply with safety agency requirements and minimize component count and cost.

The MOV device's high current-handling and energy absorption capability, fast response and lowcost make it suitable for overvoltage protection in power supplies, control board transformers and electric motors. The PPTC overcurrent protection device of the LVR series is also rated at 240 VAC, permitting maximum intermittent voltages of up to 265 VAC and can be installed with the MOV device in the AC Mains input lines.

Unlike a single-use current fuse, the resettable PPTC device helps protect against damage from conditions where faults may cause a rise in temperature with only a slight increase in current draw. When installed on the primary side of the circuit, in proximity to potential heat-generating components such as magnetics, field-effect transistors (FET), or power resistors, the PPTC device helps provide both overcurrent and overtemperature protection with a single installed component.

Certain Mains overload conditions may cause the MOV device to remain in a clamped state where it will continue to conduct current. This may eventually result in an overtemperature failure of the device. While not directly applicable to passing IEC 61000-4-5 tests, placing the PPTC device in close thermal proximity to the MOV device can help protect the MOV device in extended overload conditions – by transferring heat to the PPTC device. This causes the PPTC device to trip faster, limiting the current through the MOV device.

This application of the PPTC device allows designers to leverage the temperature response of the PPTC device and replace other thermal protection devices in the circuit. Not only does the PPTC device perform dual functions in this case, it also provides a fully resettable solution. Because the device resets after the fault is cleared and power to the circuit is removed, maintenance or replacement are not normally required. This helps reduce warranty and service costs and improves customer satisfaction.

The PPTC and MOV devices chosen for a particular application depend on the IEC-61000-4-5 class rating for the equipment as well as the operating conditions of the equipment itself. When selecting a PPTC device, the primary consideration is to match the hold current rating of the device to the primary current drawn by the electrical equipment under normal operating conditions.

PPTC Devices
Coordinating overcurrent, overtemperature and overvoltage protection can help designers minimize component count and reduce warranty returns resulting from failed motors and control board transformers.

The low resistance, fast time-to-trip, low profile, and resettable functionality of the PPTC device helps circuit designers provide a safe and dependable product and comply with regulatory agency requirements.

Raychem PolySwitch devices are qualified for and widely used in appliance designs, compliant with the UL-1434 standard, CSA and TÜV approved, RoHS-compliant, and compatible with lead-free solders and high-volume assembly processes.


Additional Information
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