Soft Starter Heat Dissipation and Enclosure Cooling

What drives a soft starter's heat dissipation and enclosure cooling needs? Heat comes from voltage drop across the conducting SCRs (thyristors) — typically 1-1.5 W per amp per phase while current flows through them — not from the motor's rated power. That loss falls close to zero once a bypass contactor shorts out the SCRs at full speed, so ramp duration and whether bypass is fitted matter more to cooling than horsepower. Units running continuously through the SCRs, ramping for long periods, or starting often need forced ventilation, ambient and altitude derating, and clearance from neighboring devices. This article covers SCR conduction loss, what the bypass contactor removes, duty-cycle-driven thermal sizing under AC-53a/AC-53b, enclosure ventilation options including through-panel heatsinks, and a sizing checklist.

Why Soft Starters Generate Heat During a Start

A direct-on-line contactor switches with a closed mechanical contact; contact resistance is low enough that heating is negligible. A soft starter replaces that contact with two back-to-back SCRs per phase, fired at a controlled angle so the motor terminal voltage ramps up over several seconds instead of slamming to full voltage. Each conducting SCR pair carries the full start current but drops roughly 1-1.5 V across itself while doing so. That voltage drop, multiplied by the current and the number of conducting phases, is where the heat comes from.

The ramp typically lasts 5-30 seconds, with current held at 300-400% of full-load current (FLC) by the current-limit setting. During that window, the SCR bank dissipates real power as heat, and the enclosure has to absorb it. Once the ramp ends and the motor is at speed, that loss either continues (no bypass) or drops away (bypass closes). The starter's frame size, heat sink, and any fan are chosen around the worse of those two cases — not around the motor's nameplate kW.

SCR Conduction Losses: The Core Heat Source

The dissipation figure panel builders actually use for enclosure sizing is a simple product: current through the device, times a per-amp loss factor, times the number of conducting phases.

Take a 100 A FLC motor with the current limit set to 350%: the SCRs see about 350 A during the ramp. At k = 1.25 W/A, P_diss ≈ 3 × 350 × 1.25 ≈ 1,310 W — for the duration of the ramp only, assuming no bypass yet. That is a meaningful heat pulse for a compact enclosure, even though it only lasts seconds to tens of seconds per start.

Key takeaway: Size cooling for the SCR conduction loss during the ramp, not for motor nameplate kW — a bypassed unit's steady-state heat is a fraction of its starting heat.

Bypass Contactor: Cutting Heat After the Ramp

Once the motor reaches full speed, a bypass contactor closes across the SCRs and shorts them out. Current now flows through a low-resistance mechanical contact instead of the semiconductors, and SCR conduction loss drops toward zero. What is left is contactor coil dissipation and minor leakage — a fraction of a percent of the starting loss. This is the single biggest lever a soft starter has over continuous heat load, and it is why nearly every flagship soft starter in the ABB, Schneider Electric, and Siemens ranges ships with bypass built in.

IEC 60947-4-2 separates the two cases as AC-53a (integrated bypass) and AC-53b (external bypass, added by the panel builder). Either way, the effect on enclosure heat is the same: bypassed operation means the cabinet only sees a heat pulse during starts, not a continuous load. What we see in the field: a panel with a bypassed soft starter can sit in a naturally ventilated enclosure that would cook a non-bypassed unit running the same duty cycle.

Key takeaway: A working bypass contactor is the single biggest heat-reduction feature on a soft starter; verify it actually closes at end-of-ramp during commissioning, not just on paper.

Continuous (Non-Bypassed) Duty and Derating

Some applications keep current flowing through the SCRs well past the initial ramp. Economy-tier units without a bypass option (the PSR/ATS01/3RW30 class) run this way by design. Soft-stop and pump ramp-down profiles fire the SCRs again on the way down, adding a second heat pulse per cycle. Jog duty and frequent inch-forward/inch-back operation on conveyors can keep the SCRs conducting far more of the time than a single clean start-and-run cycle.

In all of these cases, the thermal design has to assume the SCR loss is present for longer, which pushes toward forced-air cooling or a larger frame size well before the current rating alone would suggest it. Ambient temperature matters too: soft starters are typically rated at a reference ambient (commonly 40°C) and derated above it, and most datasheets also apply an altitude derating above roughly 1000 m, where thinner air cools the heat sink less effectively. Whether a given installation needs derating depends on the site — a rooftop plant room in a hot climate is a different problem than a climate-controlled panel room.

Duty Cycle, Starts Per Hour and Thermal Sizing

IEC 60947-4-2 packages start current, start time, duty percentage, and starts-per-hour into a single duty code so the thermal design has one number to check against. A rating written as 3.0-10:50 means a 3× FLC current limit, a 10-second start, and a 50% duty basis at the rated starts-per-hour figure. Two motors with identical kW but different starts-per-hour can need different starter frames purely because the SCR heat has less time to dissipate between starts.

Most feature-tier starters carry a built-in thermal model — effectively an I²t running estimate of SCR junction temperature — that blocks a restart with a too-many-starts fault if the last cycle did not leave enough time to cool. That fault is not a nuisance trip; it is the thermal model doing its job. Sizing to the actual duty cycle up front, covered in more detail in the AC-53a/AC-53b duty sizing guide, avoids hitting that fault in production.

Enclosure Cooling: Ventilation, Spacing and Forced-Air Options

Open-panel (IP20) soft starters rely on the surrounding cabinet for cooling air, so the cabinet design does as much thermal work as the starter's own heat sink. Small frames on bypassed, light-duty service — short ramp, low starts-per-hour — often get by on natural convection: enough clearance above and below the unit per the manufacturer's clearance table, and a cabinet that is not already packed with other heat-generating gear. Larger frames, non-bypassed continuous duty, or high starts-per-hour applications generally need forced air: a filtered intake fan pulling ambient air across the heat sink, sized to the worst-case P_diss figure, not the average.

Many higher-frame units in the ABB, Schneider Electric and Siemens soft starter ranges also offer a through-panel heatsink option: the heat sink projects through a cutout in the enclosure door or back panel, sealed to the outside, so the bulk of the SCR heat is rejected outside the cabinet instead of inside it. That option is worth checking before defaulting to a bigger fan or a bigger enclosure — it can keep a high-current starter in a cabinet that would otherwise need active cooling.

Key takeaway: If continuous or frequent-start duty pushes SCR conduction loss too high for natural convection, ask about the through-panel heatsink option before sizing up the fan or the enclosure.

Sizing Checklist for Heat and Cooling

Before a soft starter goes into a panel design, four questions settle the cooling approach: Is bypass built in, and does the duty cycle actually let it stay closed most of the time? What is the worst-case SCR conduction loss during the ramp (and the ramp-down, if soft stop is used)? What starts-per-hour and ambient temperature does the site actually run at, against the datasheet's reference conditions? And does the frame size in question offer a through-panel heatsink if forced air is otherwise needed? Working through soft starter selection and sizing in that order avoids picking a frame on current rating alone and then discovering a thermal problem at commissioning. It also feeds directly into the motor protection settings that the starter's own thermal model will enforce once it is running.

Conclusion

Enclosure cooling for a soft starter is really a question about the SCR conduction loss during the ramp, whether bypass removes that loss afterward, and how often the cycle repeats. Bypassed units on modest duty cycles often need nothing more than clearance and a well-ventilated cabinet. Continuous conduction, long ramps, high starts-per-hour, or hot/high-altitude sites push toward forced air or a through-panel heatsink. Getting this right at the design stage avoids nuisance too-many-starts faults and premature SCR failures later.