Switches & Relays: Powerful Protection (March 2007)
March 1, 2007
Although generally reliable, the electric motors used in home and professional-grade appliances are subjected to mechanical overloads, overheating, stalls, lost neutral, severe over-voltage conditions, humidity, and other damaging factors. Appliance designers employ a variety of circuit protection techniques to help prevent safety and fire hazards, as well as reduce warranty return and replacement costs that may result from motor failure.
The latest generation of PPTC (polymeric positive temperature coefficient) devices includes components that are rated for line voltages of 120 VAC and 240 VAC and can be used in parallel for increased current capacity. Their low cost, resettable functionality, and latching attributes make PPTC devices a reliable, cost-effective circuit protection solution for transformers and for the intermittent and continuous-operation electric motors used in appliances.
Protecting an electronic circuit from damage due to excessive current or heat is the primary function of many circuit protection technologies. In the past, this protection took the form of a fuse or fusible link. In today’s electric motor applications, resettable devices such as PPTC devices, CPTC (ceramic, positive-temperature coefficient) devices, bimetal circuit breakers and thermostats are the preferred solution. These devices help protect the motor or transformer from over-current damage caused by an electrical short, overloaded circuit, or customer misuse.
PPTC devices help protect against damage caused by both over-current surges and over-temperature faults, they offer low resistance, and are compatibly sized with fuse solutions. Like traditional fuses, they limit the flow of dangerously high current during fault conditions. The PPTC device, however, resets itself after power to the circuit is removed. This leaves the protected equipment fully functional once the fault condition has been removed, and eliminates the need for fuse replacement.
Comparison with CPTCCeramic PTC (CPTC) devices help provide resettable protection; however, their application is 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.
Fig. 1 and Fig. 2 show the results of comparison testing of CPTC and PPTC devices performed by Raychem Circuit Protection. The PolySwitch™ PPTC devices were compared to CPTC devices as primary protection elements using two identical transformers. The PPTC and the CPTC devices were selected to have the same hold current. In this test, a fault was created with a secondary short while current, coil temperature, and time-to-trip were measured. As shown in Fig. 1, the PPTC device reacted more quickly, and at a lower temperature.
Compared to the CPTC device, which had a surface temperature of about 75 DegC to 185 DegC, the PPTC device offers a lower surface temperature (about 100 DegC to 120 DegC) in the tripped state. The PPTC device also has lower resistance in the circuit, is lower in capacitance, and is less frequency dependent.
Thermal images illustrate the difference in surface temperatures of the CPTC and PPTC devices in Fig. 2. In this comparison of a 220 VAC trip, the CPTC device reached a maximum temperature of 184.5 DegC, whereas the PPTC device reached a maximum temperature of 118.9 DegC.
Comparison with bimetalBimetal circuit breakers, although widely used to help protect the electric motors found in appliances, do not latch, and they require additional action to interrupt their on-off cycle. The bimetal strip is constructed of two different metals bonded together. When the bimetal’s current rating is exceeded, heat generated by the excessive current causes the bimetal strip to bend and open a set of contacts to stop current flow. With no current flowing, the device returns to its normal shape, closing the contacts so current flow may resume. In the case of a stall, the bimetal circuit breaker continues to cycle until power is removed.
The cycling nature of this device has several disadvantages. Among those are material fatigue and a tendency to burn contacts, spark or weld shut. If the device “fails closed,” it can cause over-current damage to the motor, as well as sensitive follow-on electronics. Potential noise, or chatter, and electro-magnetic interference (EMI), can also make bimetal circuit breakers incompatible with advanced, electronic-control systems.
Recent testing by Raychem Circuit Protection compared the thermal and electrical characteristics of a common bimetal thermal protector and the PolySwitch LVR PPTC device, each installed on an icemaker motor. The protection devices were coupled to the motor winding, and the motor shaft was locked during the test period. The voltage, current, temperatures of winding/core, and the temperature of the PPTC device and the bimetal protector were recorded during the test.
Fig. 3 and Fig. 4 illustrate the results of the two tests. In the test using a bimetal circuit breaker, the motor winding reached a temperature of approximately 129 DegC at 60 min. This was significantly higher than the test that used a PPTC protection device, where the motor winding reached a temperature of 44 DegC within the same time frame.
Intermittent operationIntermittent 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. Developing a protection scheme that effectively protects the motor without nuisance tripping is the design challenge.
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.
Fig. 5 shows how a PPTC device can be installed in a motor circuit to help protect against damage from over-current or over-temperature events. When the device is enclosed within the motor housing, it reacts to the current flowing in the motor, as well as any temperature rise that may occur during a fault condition.
Continuous operationContinuous-operation motors, such as those used in refrigerators and air-conditioning equipment, are designed to optimize size and cost. Since they often drive fans, some airflow can be diverted through the motor to allow operation under more stress than would otherwise be possible. As a result, the stall current of fan motors is usually only two times the run current, compared to a ratio of three-to-four times run current that is common in other applications. This complicates finding and sizing a fuse that will open reliably if the fan becomes blocked, yet not blow from an inrush when the motor is first switched on.
As noted in the discussion on intermittent-operation motors, PPTC devices offer advantages in motor protection schemes. By altering their characteristics as the motor’s vulnerability changes with temperature, they can provide a slower response when appropriate.
In applications where a fan is driven, both the PPTC device and the motor can benefit from being placed in the air stream. With this method, the trip current of the PPTC device will be greatly increased because the airflow tends to prevent it from reaching its trip temperature. However, if the fan stalls for any reason, the cooling effect of the airflow ceases, causing the overrated motor to heat up quickly. This condition causes the PPTC device to trip and limit current flowing to the motor.
Unlike a single-use fuse, the PPTC device helps prevent damage where faults may cause a rise in temperature with only a slight increase in current draw - providing both over-current and over-temperature protection with a single installed component.
SummaryNew generation PPTC devices are qualified for, and widely used in appliance designs, compliant with the UL 1434 standard, and are compatible with lead-free solders and high-volume assembly processes. Their low resistance, fast time-to-trip, low profile, and resettable functionality help circuit designers provide a safe and dependable product, comply with regulatory agency requirements, and reduce warranty repair costs.
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