1. Why is a step voltage recommended when applying voltage to a contactor coil?
If the voltage rises or falls slowly, it can cause more bouncing of the contacts during closing or opening, leading to
unstable contact and significantly reducing the relay's service life.
2. Why does the main contact of the contactor generate heat even when no current flows after closing?
After the main contact closes, the contactor coil continues to operate, and the heat generated is caused by the coil’s normal operation.
3. Does discoloration (yellowing or blackening) of the contactor stud affect performance?
No, it does not affect performance. The stud surface uses silver plating, primarily to maintain low contact resistance. Silver naturally oxidizes over time when exposed to air, but the contact area still maintains low resistance, so performance remains unaffected.
4. Is uniform discoloration on the epoxy surface of a DC contactor with epoxy encapsulation normal?
Yes, it is normal. Epoxy resin consists of two components: epoxy resin (containing aluminum oxide powder) and a curing agent (amine-based). Amine-based curing agents tend to discolor slightly over time when exposed to air, but this occurs only on the surface and does not affect internal insulation or electrical performance.
5. What is the inrush current of a contactor?
This depends on the coil control method, which can be categorized into three types: single-coil control, dual-coil control,
and PWM control.
* Single-coil control:
Composition: Electromagnetic coil only.
Inrush current characteristic: At power-on, the inrush current is less than the rated current.
* Dual-coil control:
Composition: Two electromagnetic coils + electronic control circuit.
Inrush current characteristic: At power-on, the inrush current is greater than the rated current; refer to the datasheet or
selection manual for exact values.
* PWM control:
Composition: One electromagnetic coil + electronic control circuit.
Inrush current characteristic: At power-on, the inrush current is much larger than the rated current; refer to the datasheet or selection manual for details.
6. Does reverse electromotive force (EMF) occur when the coil control circuit is turned off, and how should it be managed?
For dual-coil and PWM control methods, the reverse EMF is very small, and no additional protection is required at the customer side. For single-coil control, the reverse EMF is large and requires additional protection. It is generally recommended to use a TVS diode (do not use a flyback diode, as it slows down the release speed and reduces electrical life).
7. How is polarity achieved in the main contacts of a contactor?
Polarity is achieved by adjusting the position of the magnets:
* Polarized type: When current flows in the forward direction, the arc deflects toward the N pole; when reversed, it deflects
toward the S pole. The magnetic field guides the arc into the arc chute for faster extinction.
* Non-polarized type: No directional magnetic field; arc movement is independent of current direction, suitable for bidirectional or AC applications.
8. Is the current-carrying capacity sufficient if the main contact’s contact area appears small?
Contact forms are classified into three types: point contact, line contact, and surface contact.
* Point contact: Small contact area, suitable for low-current, high-sensitivity applications.
* Line contact: Contact along a line, offering better stability.
* Surface contact: Large-area contact, suitable for high-current loads.
In the current market, DC contactors mainly use point contact or surface contact. Current-carrying capacity depends on the"effective contact area," not the visual size of the contact surface. Even if the apparent contact area seems small, proper pressure, material, and surface condition ensure sufficient effective contact area for required current carrying.
9. Can contact resistance be measured with a multimeter? Why or why not?
No, it cannot be accurately measured, although continuity can be checked. Contact resistance is typically in the milliohm (mΩ) range. Using a multimeter introduces lead resistance, resulting in large measurement errors. The four-terminal (Kelvin) method should be used: apply a constant current through the contact, measure the voltage drop across it with a separate voltmeter, and calculate resistance using Ohm’s Law (R = V/I).
10. What are the characteristics of capacitive loads, and how can contact damage be reduced?
When a capacitor is energized, its internal resistance is very low—almost a short circuit—so connecting a capacitive load results in a large inrush current (surge current), lasting approximately 10μs to 30ms. The magnitude varies with the circuit.
If this surge exceeds the relay's rating, the contacts may weld together (in mild cases, tapping the contactor may free
them—the weld point being a tiny molten spot, known as fusion welding). To reduce damage, use contactors with AgSnO₂ (silver tin oxide) contact material, or add a pre-charge circuit in the system.
11. Can connecting two contactors in parallel improve breaking capacity under load? Can it increase current-carrying capacity?
Connecting two contactors in parallel can increase current-carrying capacity, but it cannot improve breaking capacity under load because the contacts of the two contactors cannot switch simultaneously. One will open first and bear the full arc stress, risking failure.
12. How do pickup and release voltages change with rising ambient temperature, and why?
Due to the positive temperature coefficient of copper wire resistance (~0.374%/°C), resistance increases with temperature.
With constant voltage, the coil current decreases. However, the current required for pickup and release remains unchanged.
Therefore, higher voltage is needed to achieve the necessary magnetic force. As a result, both pickup and release voltages increase with temperature. Conversely, they decrease when temperature drops.
13. Does adding a freewheeling diode in parallel with the control coil affect actuation and release times? Why?
The actuation time remains unchanged, but the release time becomes longer. After power-off, the coil current decays slowly through the freewheeling diode loop, causing the magnetic field to persist longer and delaying contact release.
14. Can the actuation and release times vary for the same contactor?
Yes, they can vary. Microscopic changes in movable parts, as well as variations in ambient temperature, supply voltage, and usage cycles, can all cause timing fluctuations.
15. Why can dropping a contactor from a desk cause failure, and how does this compare to the shock and vibration ratings in the manual?
Most DC contactors or relays are not designed to withstand severe mechanical shocks. The impact from falling off a desk (typically 0.8–1 meter high) far exceeds the shock and vibration parameters specified in the product manual, potentially causing internal damage such as contact misalignment, coil breakage, or structural deformation, leading to failure.
16. What is the effect of operating above 2000 meters altitude on contactors?
Higher altitudes result in thinner air, leading to the following effects:
* Reduced heat dissipation: Lower air density weakens convection cooling, increasing temperature rise.
* Reduced insulation strength: Breakdown voltage decreases, requiring increased creepage distances.
* Weakened arc quenching: Arcs are harder to cool and extinguish due to lower air density, reducing breaking capacity.
Therefore, contactors used above 2000 meters should be derated (typically 10%–25%) or replaced with high-altitude-rated models.
