https://bourns.com/ SERVO MOTOR DRIVE CIRCUITS 15 www.power-mag.com Issue 3 2025 Power Electronics Europe device. The industrial standard to prevent these random surges is to implement a surge protective device (SPD) that are specifically designed to absorb incoming energy. SPDs are commonly positioned in the circuit as the first line of defense from potential lightning strikes. These devices are proven to deliver effective protection from their advanced design that uses discrete components like metal-oxide varistors (MOVs) and Gas Discharge Tubes (GDTs). If a surge that exceeds the rated voltage of the SPD occurs, then the device will shunt excess energy away from the motor drive system. This feature helps ensure that even during extremely high voltage surges, the potentially damaging threat would be isolated to the SPD. To provide a motor drive circuit with complete power protection, the recommended protection design should include a fuse, Negative Temperature Coefficient (NTC) device, a MOV, hybrid protectors, and a GDT. Each device fulfills its niche in protecting the motor circuit. The MOV works as a clamping device, absorbing high voltages. The GDT is a low-capacitance component that, when connected in parallel to the protected circuit, diverts high currents away from the circuit, ‘crowbarring’ the power. Currently available hybrid protection devices combine MOV and GDT technology in a single space-saving package. NTCs are especially important in highinductive motor circuits, where stored energy kickback and inrush currents are commonly present. NTCs operate normally open, but when current is applied, the device warms up and acts as a short. This behavior protects the circuit from an initial current spike. Despite implementing these multiple layers of protection, redundancy is necessary to effectively protect a motor drive circuit. Fuses are considered the most reliable solution for redundant protection as they act to safeguard the circuit if every other device has failed. The Need for Rectification Full-bridge rectification converts an AC signal into a DC voltage. This is accomplished by implementing diodes into a bridge arrangement. The effect of the rectification is like the mathematical function of the absolute value of a sine wave. The resulting signal is completely positive voltage; however, it retains a strong variance and frequency from the original AC input. To achieve a stable DC voltage, designers implement a capacitor low-pass filter, which filters the 50 or 60 Hz frequency harmonics, leaving only the DC voltage. This filtering capacitor smooths out the curve, removing most of the variance in the signal. The efficacy of the filtering— reducing ripple in the voltage waveform—is determined by the capacitance value of the capacitor, which is where the cost and size constraints of the circuit must be considered when designing the filter. Power Factor Correction Some circuit designers settle for a low-pass filter, where there is a small ripple of voltage present in the rectified voltage. However, due to the shape of the waveform, the capacitor experiences large current harmonics. These harmonics are derived from the constant charging and discharging of the capacitor at a constant frequency. The resulting current waveform that flows through the capacitor contains errant harmonics, which, when reentering the electrical grid, cause disturbances and fluctuations. On a large scale, such disturbances can cause damage and power outages within buildings and to associated infrastructure. Another reason the current waveform is undesirable is the low power factor (PF), which is the ratio of real ‘useful’ power to total apparent power. The PF is affected by low real power consumption and relatively high reactive power. Real power is determined from the definition of electrical power—the multiplication of current and voltage in the component. As the current from the AC main resembles a nonsinusoidal waveform and the voltage is sinusoidal, the real, ‘useful’ power provided is low compared to the total apparent power. While real power is consumed by resistive loads, reactive power is consumed by capacitive and inductive loads (which dominate inside a servo motor system). Reactive power results from voltage and current waveforms being out of phase due to the inductive load. This power does not generate mechanical work; however, it still pulls current through the wire. The current sloshes back and forth from the electrical grid to the motor, heating up the wires, lowering the maximum voltage possible, and slowly damaging the connections. This results in the total apparent power—the Pythagorean sum of the real and reactive power—being greater than the real power. This issue shows the inefficiencies of the system that need to be corrected. A Power Factor Correction (PFC) circuit is implemented to remedy both issues of the current waveform. The most common type of PFC circuit is the high-frequency boost converter. These circuits operate by using a rapidly switching transistor to charge and discharge an inductor. Bourns’ recommendation for transistors is its Insulated Gate Bipolar Transistor (IGBT), a high-performance transistor with the characteristics of both the MOSFET and the BJT. Bourns IsoMOV hybrid protection devices operate similarly to an MOV, but leverage the low capacitance levels of the GDT, resulting in a higher performance and more reliable solution.
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