When the industrial equipment is started, the impulse current (called surge current) far higher than the normal working current will be generated. Depending on the equipment type, this start-up surge may be 10 to 30 times the steady state current. This extreme surge current is generated instantaneously, causing huge electrical and mechanical stress.
If not properly controlled, surge currents can cause circuit breaker trip, blown fuses, damage to sensitive components, and even degrade the performance of power connectors and supplies. Therefore, developing an effective surge current management strategy is essential for the reliable and safe operation of industrial systems.
One way to manage startup surges is to connect a surge current limiter (ICL) in series at the device power input. In various types of ICL, negative temperature coefficient (NTC) thermistors are widely used due to their simple design and ease of integration. The NTC thermistor is a temperature-sensitive resistor whose resistance value decreases with increasing temperature.
Figure 1: ERT-J0EG103FA NTC thermistor of Panasonic Electronic Components, nominal resistance of 10 k Ω at 25 ° C, resistance tolerance of ± 1%. Picture source: Panasonic Electronic Components)
When the industrial electrical equipment is shut down, the resistance of NTC element is relatively high. NTC element in series with load. This high cold state resistance can slow down the initial impulse current at startup, which is equivalent to a current buffer.
When flowing through a thermistor with a limited inrush current, it is heated by the resistive thermal effect. When the thermistor is heated, its resistance value will drop sharply, which will be far less than the cold resistance value. In a very short time, the thermistor transitions to a low resistance state. At this time, the input capacitor is fully charged and can flow through the normal operating current.
NTC completely exits the protection state after the surge event, which is close to the short circuit state during steady state operation. For example, an NTC with a cold resistance of 10 Ohms may drop below 0.5 Ohms after sufficient heating. This ensures that industrial equipment operates at near full voltage under steady state conditions while minimizing the thermistor's own energy losses.
Design considerations when implementing NTC restrictors
In order to ensure reliable and efficient operation, several design parameters must be considered when implementing NTC-based surge limiters.
Cold state resistance value
The cold state resistance (R25) is the rated resistance at 25 ° C and is used to determine the initial impedance when limiting the inrush current. Based on the required maximum surge current and supply voltage, the required minimum resistance can be estimated. Engineers will use Ohm's law to calculate this resistance: R=Vpeak/Imax (surge). For example, in a 230 VAC single-phase system (peak voltage approx. 325 V peak), a cold state resistance of 325/20 ≈ 16 Ω is required to limit the inrush current to 20 A peak.
Manufacturers such as TDK Electronics, VisAmetherm, and Amphenol Advanced Sensors provide NTC products with standard values such as 2 Ω, 5 Ω, 10 Ω, 22 Ω, 47 Ω at 25 ° C. The selection of suitable cold state resistance is critical because higher R25 provides better surge suppression. However, too high a value may over-limit the charging current, increase the starting time, and result in excessive initial voltage drop.