Sizing a Solid-state Contactor for Hydraulic Systems

Hydraulic pumps and other inductive loads are among the more challenging applications for solid-state contactors. However, when selected correctly, solid-state contactors, unlike electromechanical contactors that need maintenance or replacement every few months, can last the lifetime of your equipment.

We will now share a case study from one of our client’s applications:

An industrial caster factory was operating a DC forklift motor. Its hydraulic pump circuit had the following operating parameters:

• 36VDC/10kW hydraulic motor, with a peak current draw of approximately 300A.
• Nominal current draw of less than 100A; due to light loads.
• The system was fused for 400A.

The operator was using a low-voltage electromechanical contactors with rated interrupting current of 100A, and uninterrupted current of 120A. However, when choosing a solid-state contactor, additional considerations are as follows:

• Solid-state contactors are not inherently bidirectional; they do not block current flowing in the reverse direction. However, for the hydraulic mains switch in this application, bidirectionality was not a requirement.
• Given that we are switching an inductive load, which produces significant Back Electromotive Force (back-EMF) at turn-off, both voltage and current margins need to be significantly higher than the nominal values. Also, a transient snub (e.g., with a freewheeling diode) is required to maximize reliability.
• Considering the motor has a peak current draw of 300A, the inrush current can be expected to reach 5-7 times this amount, or between 1.5kA and 2.1kA for approximately tens of milliseconds.
• Given that the peak sustained current draw could be 300A, the solid-state contactor and the heatsink must be able to support this current for the continuous or maximum duration without overheating.

Having considered the above, the solid-state contactor needs to be rated at least ≥100VDC, ≥300A with a peak current of ≥2.1kA.

The final recommended model is: MACH1 Solid-State Contactor, M1DS01-1000A2-P1, rated at 100VDC, 1kA (continuous@90°C), and 2.5kA (peak@90°C). The continuous current rating is much higher than 300A, as it’s the nature of how solid-state contactors scale with the peak current rating. This higher continuous current also implies a much lower ON-resistance and a much smaller thermal footprint.

If you would like to inquire how to choose a MACH-1 solid-state contactor for your hydraulics or other application, feel free to contact us or complete our technical survey here.