Q: What is the difference between a relay and a contactor?
A: In general, both terms designate an electromechanical switching device, working on the same physical principle, where a coil is used to generate a magnetic force that mechanically operates an electric contact. While the term relays is widely used in various industries for low and medium power devices, the term contactor is more common in the high power area. The magnetic “motor” is typically realized with a plunger in the center of the coil body for contactors, as opposed to a hinged armature design, which is normally used for relays.
Q: The new generation of contactors works without a gas-filling. What is the advantage of non gas-filled contactors? Can a gas filled contactor rupture?
A: In order to protect the switching contacts, and to support a quick extinguishing of switching arcs, many contactors are filled with pressurized inert gas. Nitrogen or Hydrogen is commonly used, SF6 (Sulfur-hexafluoride) is also applied, mainly in industrial applications. On the flip side, a pressurized gas filling requires a lot more effort in the design and the manufacturing process, to ensure a reliable holding of the gas throughout the lifetime of the contactor. As such contactors must be hermetically sealed by nature, it also bears the risk of rupture, when extremely strong arcing – in case of high over-currents or short-circuits – generates excess gas pressure inside the contact chamber.TE’s latest contactor designs therefore work with no inert gas, also eliminating the need for a hermetic seal. These designs are still environmentally protected against pollution and allow a gas exchange and an equalization of inside and outside pressure, which practically eliminates the risk of rupture in case of extreme arcing. Please contact TE Connectivity (TE) to learn more about the advantages of non gas-filled contactors. Link to EVC 250 Video.
Q: What are the applications for the high voltage (HV) relays and contactors?
A: In hybrid and electric vehicles, high voltage relays and contactors are typically used for the following applications:
- Main Contactor: used in both, the plus-line and the minus-line of the traction battery. The main contactors connect and disconnect the traction battery from the entire electric drivetrain in the vehicle.
- Precharge Relay: to protect the main contactors from an excess inrush current, a precharge relay is used, together with a precharge resistor, to charge the filter capacitor of the power inverter to a level of typically 90-98% of the battery voltage.
- Charger Contactor: used to establish connection between the battery charger and the traction battery, when the vehicle is connected to a charging station.
- Auxiliary Contactors: they control other electrical loads in the vehicle that are operated by the HV battery. A typical example is the electric heater for the passenger compartment of full-electric cars, where no waste heat from a combustion engine is available for this purpose.
In addition, automotive HV contactors are also sometimes used in stationary systems like DC charging stations, stationary battery storage systems, uninterrupted power supply systems and others.
Q: For which performance ranges can contactors be applied?
A: TE’s contactors portfolio includes heavy duty products for the requirements of the most powerful battery electric vehicles with a peak power up to 500kW, as well as miniaturized products for low power of serial loads and precharge applications.
Q: What continuous current can be handled by contactors?
A: In general the current carrying capability is limited by the internal heat dissipation and the thermal management. The dissipated power increases the temperature of the inner parts of the contactor and is transferred to the outside. This outside temperature rise determines the current limit for which the contactor can be used. TE recommends that the final temperature of the contact terminals should not continuously exceed 150°C. The effectivity of the cooling mechanism depends on the cross section or thermal resistance of the connected conductor outside and the environmental temperature. The heat is conducted through the electrical connections to the ambient. For a constant current the system reaches the stationary case after about 3 to 5 minutes. Example: Connected with 50mm² bus bars the EVC 250 contactor would be capable to carry 250A at 85°C ambient temperature. With a 125mm² bus bar this limit would be shifted to 375A.
Q: What overload can be handled by a contactor?
A: For peak loads of a few seconds the heat transfer process is too slow for the terminal temperature to change significantly. For longer durations the temperature rise inside the contactor could cause irreversible damage. The EVC 250 contactor for example handles 1,500A for up to 20s or 2,000A for up to 5s. Please contact TE Connectivity for more information and support.
Q: What happens to a contactor during a short circuit?
A: During a short circuit the rising current generates a repulse force which could exceed the holding force of the contacts. If the fuse is not properly adapted to the breaking and carrying capability of the contactor, the contacts can weld or excessive arcing could thermally destroy the contactor. For the EVC 175 contactor the current carrying limit is 5,000A. For the EVC 250 contactor it is 6,000A. In case of short circuits with moderate overcurrents the fuse trip time is too long to protect the system from severe damage, therefore the contactor has to break the overcurrent. A single contactor can break currents up to 2,000A at 400V within a few milliseconds. As normally two contactors are in the circuit we recommend to open both simultaneously, which increases the breaking limit up to 6,000A at 400V.
Q: What determines the coil efficiency? What are the requirements for the coil driver?
A: To separate high voltages large contact gaps are necessary. In open position the retention spring holds back the armature to provide a suitable shock resistance. To overcome these high contact gaps and forces a high magnetic flux has to be generated. To achieve the necessary forces the coils are designed with relatively low resistances. Therefore the drivers should be capable to provide currents up to 6A. Such high currents could only be applied for closing the contacts, afterwards the current has to be reduced to avoid overheating of the coil. Once the contacts are closed and the armature is in seated position, the necessary magnetic flux to hold the armature in position is smaller by one magnitude. This coil power reduction can be done with an external economizer or by an internal booster/hold coil configuration.
Q: What is important for operating a contactor with an external-economizer?
A: The operation of the external economizer should start only 100ms minimum after applying power to the coil. For the Pulse Width Modulation (PWM) TE recommends a minimum frequency of 20kHz. The minimum of the resulting oscillating coil voltage should always be above the specified hold voltage. For switch-off operation the response time of the mechanical system depends on the external termination of the coil. Therefore the PWM driver should be terminated in a way that does not slow down the contact opening. Please consider the “Circuit Recommendations” in our “Datasheets”. Link to datasheet EVC 250 Main Contactor. The coil resistance varies with the coil temperature. The magnetic force only depends on the coil current. If the PWM is set to a certain coil voltage level these thermal variations have to be taken into account. To minimize the thermal burden for the system it is preferable to control the coil current in order to neglect the influence of the temperature.
Q: What is important operating a contactor with a booster coil?
A: The booster electronic applies the full coil voltage to a separate booster coil for a limited duration. This pulse is started immediately when a voltage is applied. In case the necessary pull-in voltage was not reached within the gate time of the booster electronic, the contactor will not close. Therefore it is necessary that the minimum pull-in voltage is reached within 50ms.The termination of the hold coil is done with a 80V Zener diode. It is possible to install an extra termination diode in parallel. The termination voltage should be Vz > 33V, to keep the drop-out time short.
Q: Why is load polarity important for a contactor?
A: The breaking capability at high voltage is achieved by using magnets placed perpendicularly to the contact terminals. Both terminals are internally connected with a bridge. When the contact bridge moves away from the stationary contacts two arcs are generated. In the forward current direction, the magnets deflect the arcs to the outside resulting in a fast arc extinguishing. In reverse current direction the arcs could merge at the center causing a reduced breaking capability.
Q: What can be done if bi-directional breaking capability is required?
A: In case two contactors are used in the circuit they could be arranged with one in forward and one in reverse direction. Opening both contactors at the same time, the combined breaking capability is significantly better compared to a single contactor in forward direction.
Q: What is to be considered with silicone materials when using relays or contactors that are not hermetically sealed?
A: The use of silicone containing materials or its derivate can impact the proper function of electrical contacts. Due to the energy in the arc of a switching contact, the volatile silicone molecules are transformed to siliceous compounds which deposit on the contact surface and create insulating layers. TE Connectivity therefore strongly recommends to thoroughly test the intended silicones for their contact compatibility. Please contact TE Connectivity for more information and support.
Q: Does TE offer solutions for applications above 500VDC?
A: Depending on the insulation coordination requirements, solutions for up to 900VDC are available. Moreover, TE is currently developing solutions suitable for voltage levels up to 1000VDC with IEC 60664 compliance. Please contact TE Connectivity for details.
Q: How should a coil driver be protected against switch-off transients from the contactor coil?
A: The optimum is the use of a Zener-diode parallel to the coil driver. Please also refer to our “Datasheets” linked to each part number and to be found under “See all Documentation” in our product catalog for additional information. Learn even more in our “Automotive Application Notes” and “Relay Definitions.”
Q: Are there any hints concerning the mounting of the bus bars to the contactor?
A: Please consider the maximum allowed torques and avoid any misalignment between bus bar and the contactor terminals to ensure a uniform interface pressure. TE recommends the use of conical spring washers. When mounting a cable lug, make sure that the cable is free and not squeezed.