What Kind of Contactor Is Most Suitable for Frequency Converters

A contactor is a common electrical component used in Variable Frequency Drives (VFDs), primarily for switching buffer resistors and switching circuits. The choice of contactor directly influences the VFD’s operational quality and the electrical equipment’s running condition, thus impacting the overall system’s normal production and safety.

What type of contactor does a VFD really need?

1. Meeting Insulation Requirements for Enhanced Safety:

To meet the insulation needs of the VFD, contactors should feature reinforced flame-retardant nylon 66 casings. They must possess an isolation voltage of 1500V DC and tolerate power frequency voltages of up to 3500V. This specification enhances safety by not only eliminating potential hazards but also enabling direct installation on the chassis, thereby bolstering the VFD’s safety.

2. Single-Pole DC Design:

Contemplating that contactors function on the DC bus, they require only a single pole to switch a buffer resistor. This results in the design of single-pole DC contactors. For contactors that cater to VFDs with a power rating of 18.5kW or above, contactor current specifications should include ratings like 50A, 100A, 200A, 265A, 400A, 600A, 800A, among others.

3. High-Temperature Resistance and Dust Protection:

VFDs with low-voltage typically employ IGBT power components that generate heat during operation. In order to solve this problem, you can install a cooling fan on the casing of the frequency converter, but this will also affect the dustproof performance inside the frequency converter. Therefore, contactors installed within VFDs need to operate in high temperature environments with high dust content. To mitigate these challenges, the contactor coil is wrapped with wire that can withstand temperatures up to 180°C. Contacts and magnetic systems should be enclosed in dust covers.

4. Meeting Various Control Voltage Specifications:

VFDs operate with two primary control voltage categories: DC and AC. DC control primarily operates at DC 24V, largely due to the perceived stable performance of DC 24V for cooling fans. This is why contactors are designed for DC 24V, making them compatible with fan power supplies. On the other hand, AC control operates mainly at AC 220V and 380V. AC 220V requires the use of a control transformer to enhance contactor reliability, albeit at an increased cost, while AC 380V obviates the need for a control transformer. In essence, the choice of control voltage is a matter of preference, and all options—24V DC, 220V AC, and 380V—are viable.

5. Adhering to VFD Installation Standards:

Three-phase VFDs are often paired with three-phase AC contactors. Nevertheless, these components rarely exhibit identical shapes and installation dimensions. This necessitates considering two installation methods: using screws and clamps. Moreover, because there aren’t many manufacturers of DC contactors, the installation dimensions of contactors from different brands don’t exhibit significant differences. Therefore, when designing a VFD control cabinet, one can often directly use the contactor from the VFD manufacturer with minimal to no modifications.

Selecting the appropriate contactor is vital to ensure the effective and safe operation of a VFD and the overall system. Factors like insulation requirements, single-pole DC design, high-temperature resistance, control voltage specifications, and adherence to installation standards are key considerations when choosing the most suitable contactor for VFD applications.