Power Electronics

Specifying Resistors for Motor Control Applications

Thin film, thick film, wirewound and solidcomposition resistors offer different thermal dissipation and surge-withstand characteristics, which impact their abilities to handle overcurrent and overvoltage conditions. power surge instead.

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Motor controllers used to power, stop and control the speed of a variety of motors must deal with a wide variety of surges due to switching events. When a motor starts, the current draw during the speed ramp-up is excessive, and during this relatively short period of time, the energy surge can cause failure in resistors or other circuit components. If capacitors are used to store energy for motor starting, the current-limiting resistors see excessive loads during the charge/discharge cycles.

Power surge is a common cause of failure in resistors in motor control circuits. Many designers underestimate the potential for damage to resistors and other components resulting from the effects of power surge. Additionally, many engineers will design their circuits for steady-state conditions without realizing that their “worst case” may indeed be power surge instead.

The first step in making sure power surges do not cause resistor failure is to distinguish a surge condition from short-time overload. Short-time overload occurs when an overpower event has a duration of 0.5 seconds or more. This causes the resistive component to heat up, but it also gives the substrate to which the component is mounted time to dissipate the heat generated by the increased current.

In contrast, “surge” occurs over such a short period of time (less than 0.5 s) that the substrate has no chance to help dissipate the heat and it must be completely absorbed by the resistive element. Common causes of surge are capacitor charge/discharge (less than 1 ms) and motor startup (typically less than 0.5 s).

Overcurrent Versus Overvoltage Surges

The two types of power surge conditions that can occur in motor control circuits are overcurrent surges and overvoltage surges. Overcurrent surges are the more common of the two, especially in components with resistance values of 250 Ω or less. As noted previously, in an overcurrent surge condition, the resistive material must absorb the heat generated by the surge.

The temperature rise of that material is expressed in the following equation:

Thus, the key to specifying a resistor that withstands overcurrent surge is ensuring that the maximum temperature of the resistive material does not exceed the design limits of the material. In order to calculate this limit, one needs to know the specific heat (heat capacity) of the given resistor material, as well as its mass.

The temperature rise of a resistor will be inversely proportional to its mass (i.e. the larger the resistor, the slower the temperature rise). Both the specific heat and maximum temperature are characteristics of the resistor material. Calculated maximum temperatures (TMAX) are typical for designing wirewound resistors, but less useful for designing film resistors where the failure point is usually a small point of localized heating.

Unlike overcurrent surges, high-voltage surges are of primary concern in circuits with resistance values that are greater than 250 Ω. High-voltage surges affect resistors in various ways. Under extreme voltage stress (calculated by volts per linear distance), thick film resistive elements can exhibit deterioration due to voltage-induced conduction from normally nonconductive materials in the film. For this reason, thick film resistors typically have a maximum specified voltage to avoid potential deterioration. Wire-resistive components are not generally subject to this deterioration. Hence, wirewound resistors do not typically specify maximum voltages.

When discussing surge in power-management applications, resistor manufacturers typically determine the success or failure of a component under test based on comparison of the component's resistance shift to a selected test limit. It is important for users of the resistors to pay close attention to this specification in order to determine if it matches expectations with regard to their particular application.

For example, vendors may specify failure of the resistor if the resistance shifts more than ±0.25%. Other manufacturers allow as much as ±1% shift. The consumer should scrutinize the product specification or query the manufacturer's technical support staff to make accurate comparisons and understand the impact of surge events on critical components.

Surge Versus Resistor Type

Surge conditions affect different resistor technologies in various ways. Thus, some consideration must be given to the surge characteristics of each technology when deciding whether or not to specify it in a given circuit. The four main resistor technologies are thin film, thick film, wirewound and solid-composition resistors.

Thin film resistors consist of an extremely thin layer of resistive material (often measured in atomic layers) deposited on a ceramic or silicon substrate. While this technology offers extreme precision and stability (as demonstrated by their tight tolerances and low temperature coefficients of resistivity), thin film resistors have relatively limited surge capabilities due to the low mass of resistive material.

Thick film resistors are comprised of ceramic-based materials combined with metallic particles, which have a higher TMAX value than polymer systems. For example, IRC's tantalum-based MetalGlaze process involves firing the thick film resistive material to a solid ceramic core at 1000°C, thus providing a rugged, stable base for the resistors to withstand surge conditions. By tailoring the glaze formulation and altering production processes, it is possible to optimize the resistor's performance under harsh conditions, including power surges.

As evidenced by the differences in surge-withstand capabilities between thin film and thick film technologies, the mass of a resistor's film element is critical in determining the device's surge survivability. The mass of the resistor's film element is directly proportional to its thickness times the surface area of the component. Thus, thick film technology, being approximately three to 10 times the thickness of thin film technology, offers a substantial increase in surge capability over thin film resistors — one to two orders-of-magnitude difference based on the difference in mass.

Another important aspect of surge capability is the geometric shape of the resistor itself. Given a consistent film thickness, the mass of the resistive material on a flat-chip resistor is primarily determined by its surface area (length × width). A cylindrical resistor, however, has significantly more surface area (defined as π × length × width), which in turn provides higher film mass and ultimately a three times greater surge performance than flat chips. In addition, a cylindrical resistor's increased surface area allows more heat dissipation, thus enabling the resistor to operate at lower temperatures during surge conditions.

How a resistor is trimmed also can significantly impact surge capability. Typically, cylindrical resistors are trimmed or adjusted to achieve the desired resistance value — usually by having a laser etch away some resistive material along a spiral path in the element. Unfortunately, this can create a potential failure point where the current path is crowded around the laser's trim kerf, which is the gap left by the laser trimmer. Eliminating this potential failure point increases the surge-withstand capability of the untrimmed part by a factor of three to five times over trimmed cylindrical resistors.

Fig. 1 depicts the affect of surges on the trimmed and untrimmed cylindrical resistors. As the duration of the surge lengthens, the power-handling capacity of the resistors decreases.

Wirewound resistive components are often specified for the high surge energy requirements of motor control applications. In almost all cases, the mass of the wire element exceeds the mass of either thick or thin film technology, enabling higher surge ratings. Further, the metallic resistance wire construction eliminates the voltage stress concern of films. The energy dissipated in a surge event is often measured in Joules (J; watts × seconds). Thick film parts are generally in the range of < 0.01 J to 3 J, while large circuit board wirewounds (10 W) can be designed to take up to 200 J. Even larger wirewound dynamic braking resistors can be designed to withstand even higher surges.

Wirewound resistors are constructed with metallic wire wrapped around a cylindrical substrate, usually fiberglass or a ceramic rod. Fiberglass is a lower-cost material, but it provides little thermal benefit in overload situations. Ceramic, while more expensive, acts as a heatsink and becomes more effective as the surge event lengthens.

The surge capability of a wirewound resistor can be raised by increasing the mass of the resistance element. This is achieved by selecting a larger diameter wire, which has a lower resistance value, and winding it to a longer length to achieve the desired resistance. The surge capability can be further increased by using the Ayrton-Perry noninductive winding technique. This method uses two layers of wire, wound in opposite directions, to reduce the inductance. This technique has the added benefit of increasing the resistance wire mass, further increasing the surge-handling capability.

After the resistance wire is wound on the fiber or ceramic core, it is encapsulated with a molded shell or is conformally coated. The encapsulation protects the resistor from moisture and physical damage. Molded bodies are frequently used in 1-W and 2-W parts. Silicone-based conformal coatings are capable of operating at higher temperatures and are well suited for high-energy surge parts. Vitreous enamel coatings provide an impervious shield against moisture and other harsh environments, and are capable of the highest operating temperatures. Figs. 2, 3 and 4 show IRC resistors with molded, conformal and vitreous enamel coatings, respectively.

The TMAX and maximum surge rating of a resistor are bounded by the characteristics of the resistance wire alloy and the maximum operating temperature of the encapsulation. IRC's AS Series of resistors incorporate a ceramic core and a silicone-based conformal coat to provide a robust component for the surge and overload requirements found in motor controls. When wound noninductively, the AS Series becomes “NAS,” further increasing the surge ratings. Typical surge ratings for the NAS-5, NAS-7 and NAS-10 are shown in Fig. 5. IRC can engineer the parts to withstand even higher levels of surge energy.

Carbon-Comp Resistors

Another resistor technology used for surge applications is the solid composition-type device — historically called a “carbon-comp” resistor, although contemporary components may use other materials with or without carbon. These devices use a solid core that combines particles of carbon, or other conductive materials, with a ceramic material that, when fired at high temperature, fuses into a resistor core that conducts current and dissipates heat throughout the full cross-section, rather than through a limited resistance element on the surface of an insulating substrate.

Because of their solid ceramic core, solid composition resistors exhibit surge-withstand characteristics that may exceed that of either film or wirewound resistors. These components are usually available at resistance tolerances of 5% to 10% due to difficulties in finely adjusting the resistance value.

When specifying resistors for motor control circuits, it is important to take into account the impact of ultrashort duration surge conditions. Various resistive technologies, thin film, thick film, wirewound and carbon comp, all have different strengths as well as limitations.

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