The increasing power density of industrial machinery increases the risk of spurious tripping, overheating and catastrophic failures, which can lead to the shutdown of the entire production line. To reduce these risks while meeting efficiency requirements, designers need resistors that can handle a variety of problems. Some resistors must be able to limit surge or fault events, some must be able to dissipate regenerated energy, and others need to provide reliable heat dissipation in a compact enclosure.
In short, the selection of suitable resistors has become a key step in designing reliable industrial motor drive systems.
This paper focuses on the challenges faced by industrial machinery designers and the advantages of the corresponding resistor technology, followed by representative resistors in Ohmate's extensive product line, which designers can use to address in common braking and transient protection scenarios.
Impulse energy absorption for surge current limiting and surge protection
Industrial motor drives often expose resistors to transient high energy events. A good example is the precharge phase of a VFD. When this phase is energized, the DC bus capacitor presents a state of near short circuit to the power supply, generating steep inrush current peak. If there is no current limiting resistor in the precharge circuit, this spike can trip the upstream protection or damage the IGBT of the drive.
Similar high energy pulse demands occur in fault energy absorption, crowbar circuit, and power supply protection levels. In all these cases, the resistor must absorb short but large energy pulses without mechanical degradation and be able to repeat this process over a number of operating cycles.
Ohmate's PulsA series ceramic composite resistors are specifically designed for this purpose. Its non-inductive block ceramic structure allows energy to be distributed evenly across the body, reducing the risk of lead fatigue that could cause damage to conventional wound resistors. This non-inductive construction also helps to reduce spurious voltage spikes during fast current transients, which are very useful in protective circuits where the switch edge may be steep.
Series A covers resistance values from 1.0 Ohm to 15 k Ohm, continuous power ratings from 2.0 W to 5.5 W, impulse voltage ratings from 1000 V to 2500 V, and single-pulse energy capacity from 250 J to 2800 J. This range allows designers to select and match the bus voltage and energy characteristics of a particular protective circuit.
For example, the 3.3 Ω AY33GKE (Figure 1) limits the peak surge current on a typical 600 VDC bus to approximately 180 A (I=V/R), depending on system impedance and capacitance. This current is high enough to quickly charge the capacitor bank and low enough to protect the upstream contactor and IGBT. The 2000 V impulse voltage rating provides a margin well above the standard industrial bus voltage, while the 1400 J monopulse energy rating provides sufficient margin for typical charging cycles.
Ohmite Ay33GKE Resistor Picture
Figure 1: The AY33GKE resistor uses a body ceramic structure to absorb up to 1400 J of monopulse energy. Image source: Ohmite)
It is noted that the continuous power rating of the AY33GKE is only 4.5 W, but this is sufficient for the target transient application. For example, once the VFD precharge cycle is completed, the resistor is bypassed and no energy dissipation is required.
Low inductance dynamic brake in compact drive housing
When the VFD decelerates the motor, the motor acts as a generator and feeds the regenerated energy back to the DC bus. The chopper circuit shunts this energy to the brake resistor at high frequency making and breaking currents. If the brake resistor has significant parasitic inductance, these fast current transitions will produce voltage spikes that may damage the chopper IGBT. At the same time, modern control cabinets are becoming smaller and smaller, leaving less and less physical space for the bulky convection cooling resistance box.
TAP800 series thick film planar resistors solve these two problems. Its resistance elements are constructed on a high alumina ceramic substrate and the bottom metallization allows efficient heat transfer. The flat profile transfers the heat directly to the chassis or to the cold plate, enabling high power dynamic braking in enclosures where conventional convection cooling resistors cannot be installed. This planar configuration also minimizes parasitic inductance and capacitance, resulting in stable performance when subjected to high frequency pulse loads.
The TAP800 series covers a range of resistances from 1 Ω to 10 k Ω with a continuous rating of 800 W for all models with appropriate heat dissipation.
A typical example is TAP800K390E (Figure 2). It has a resistance value of 390 Ω and a continuous power dissipation rating of 800 W when mounted on liquid or air cooled radiators. The critical specification for dynamic braking is its inductance of 80 NH, which ensures that high-speed IGBT switches do not induce disruptive voltage transients at both ends of the chopper circuit.
Image of Ohmate TAP800K390E Thick Film Flat Resistor
Figure 2: TAP800K390E is a thick film planar resistor designed for conduction cooling. Image source: Ohmite)
TAP800K390E also provides robust electrical isolation between the live DC bus and the grounded mounting surface. The maximum operating voltage is 5000 VDC, and the rated value of partial discharge is 4 kVRMS under the condition that the partial discharge is less than 10 Picocoulometer (pC), so long-term reliability can be achieved. These specifications ensure that the insulation can withstand repetitive high voltage stresses and switching transients typical of modern industrial drives without deterioration over time.
Heavy duty dynamic brakes for high inertia loads
Some motor drive applications place less emphasis on compact packaging than on pure energy handling. For example, industrial cranes, centrifuges, and heavy duty downhill conveyors, load reduction in these applications forces the motor to act as a generator that feeds back large amounts of kinetic energy to the drive. In these cases, the brake resistor must be able to withstand violent surges and quickly cool between cycles to avoid heat accumulation.
Ohmiti's Corrib 280 series resistors are designed for this high current, low resistance duty. The series is formed by winding corrugated resistance wires around a tubular ceramic core and fusing and fixing them with a vitreous enamel coating. This structure has several functions: corrugated resistance wires increase the surface area and accelerate heat dissipation; Ceramic core and glaze coating improve mechanical durability while promoting efficient heat transfer; The hollow core allows air to flow through the resistor body for passive cooling.
The continuous power rating of the Corrib280 series ranges from 35 watts to 1500 watts, and the resistance value of the 300 watt model is as low as 0.10 Ω. This gives the designer considerable flexibility to match the resistors to specific bus voltage, braking current and physical space constraints.
A representative example is C300KR50E (Figure 3). It has a resistance value of 0.5 Ω and a continuous free air rating of 300 W. More importantly for braking conditions, the overload rating of the Corrib280 series is 10 times the nominal power for a duration of 5 seconds (s). For C300KR50E, this corresponds to short-time pulses up to 3000 W.