Braking resistor for Altivar drives, 0.7 Ohm, 330 kW
The Schneider Altivar VW3A7817 is a high-power braking resistor rated at 0.7 Ohm, capable of dissipating 330 kW at 50 °C. This is sized for large motor loads on 400 VAC three-phase systems where regenerative energy from decelerating a high-inertia load (conveyor, centrifuge, flywheel) must be dumped as heat rather than fed back to the DC bus. The 0.7 Ohm value sets the peak braking torque curve — lower resistance pulls more current and stops faster, but stresses the drive's chopper transistor. At this power level, the resistor is a standalone cabinet-grade unit, not a DIN-rail module. IP23 protection means it sheds dripping water (up to 60° from vertical) but is not washdown-rated. Install in a ventilated enclosure or a dedicated resistor cabinet with clearance for convective airflow — the resistor generates substantial heat during a braking cycle, and the IP23 rating allows natural convection cooling while keeping tools and fingers out. Connection is via M10 bar wire terminals — these are high-current lugs, not spring-cage or screw-clamp. Plan for crimp lugs with a 10 mm hole and torque the connection to the manufacturer's specification. The physical footprint is substantial: 1700 mm high, 1140 mm wide, 820 mm deep. That's a floor-standing cabinet, not a panel-mount component. Verify floor loading and access for crane or fork-truck placement before committing the BOM line.
Integration and protection
Thermal protection is handled by an external thermal overload relay (not integrated into the resistor itself). The relay must be wired in series with the resistor circuit and set to the resistor's thermal capacity curve. Without it, a stalled braking cycle (e.g., a jammed conveyor that keeps the drive in regen) can overheat the resistor element and cause a fire risk. The overload relay is a separate line item — order it alongside the resistor. Operating ambient is 0 to 50 °C; storage range is -25 to 65 °C. If the resistor sits in a non-climate-controlled area (rooftop, unheated warehouse), the storage limit is fine, but the operating limit means the resistor must not be in a 55 °C machine cabinet without derating. The 330 kW rating is already stated at 50 °C — at higher ambients, the power must be reduced per the manufacturer's derating curve.