Understanding semiconductor control for non-motor AC loads

Hello everyone,

When you encounter the reference UL 60947-4-3, the key distinction is non-motor AC loads. UL's official transition table associates this reference with solid-state controls for non-motor loads. Within the UL 60947 series, this subject must be distinguished from motor control, particularly when evaluating resistance-heating circuits.

Separate the UL transition information from the IEC scope

UL's public table lists the transition dates to UL 60947-4-3 for non-motor solid-state controls in category NRNT as TBD, or to be determined. The table alone does not confirm the current edition of a separate UL publication or a product's approval. Purchasing decisions should check the exact model, certification record, applicable standard and conditions of use together.

The published IEC 60947-4-3:2020 covers semiconductor controllers and contactors for non-motor loads in circuits not exceeding 1,000 V AC. Its scope includes on/off devices, with or without bypass arrangements, and controllers that reduce the RMS value of AC voltage. This limit does not mean every product is rated for 1,000 V. IEC conformity must not be presented as UL approval.

SSR construction and load categories

A solid-state relay (SSR) may switch AC loads using a triac or antiparallel thyristors, depending on its design. Unlike a mechanical relay, it switches power without moving contacts; thermal limits still matter.

Read the control input and load output separately. A device operated by a DC control signal may have an AC load output. Likewise, an AC-51 resistive-load current rating cannot replace an AC-53a motor-load rating. For a motor, use the specific model's explicit motor rating.

Zero-cross switching, burst firing and phase-angle control

Zero-cross switching starts conduction near an AC voltage zero crossing. Burst firing switches complete cycles on and off; phase-angle control changes the firing point within each half-cycle. A zero-cross SSR cannot automatically provide phase-angle control. In PID temperature control, the controller's output method must suit the power stage.

For PID power control, assess the harmonics and interference associated with phase-angle operation. Zero-cross control still requires EMC consideration. Follow the manufacturer's wiring and test conditions when addressing EMI filtering and noise immunity.

A heater example: average power and conducting current differ

A single-phase, purely resistive 3 kW heater at 230 V draws approximately I = P / V = 3,000 / 230 = 13 A. Assuming constant resistance, a 50% burst-firing duty cycle produces approximately 1.5 kW average power. During the on cycles, however, the load still draws about 13 A. Halving average power does not justify selecting the SSR as though the load were 6.5 A. This calculation is an illustration, not a product rating; actual selection must also account for operating conditions.

Cooling and connection conditions

The current printed on the housing is not an unconditional carrying capacity. Read the manufacturer's derating curves for the ambient temperature, heat sink and mounting arrangement. Higher enclosure temperatures can reduce the current permitted through the same SSR. Apply the specified thermal interface material, mounting orientation and ventilation arrangements.

Obtain terminal torque, conductor size and stripping length from the relevant product instructions. Select wire ferrules, cable lugs and crimping tools to suit the connection. Fitting a higher-current SSR alone will not correct an unsuitable heat sink or a loose terminal.

Short-circuit protection and safe shutdown

Semiconductor protection requires attention to the specified fuse combination, surge withstand and I²t coordination. A stated short-circuit current rating applies with its associated protection and installation conditions. An arbitrary fuse cannot be assumed to protect the SSR during a short circuit.

An SSR off command does not provide physical isolation. Off-state leakage may remain, and a short-circuit failure can leave a heater energized. Where the risk requires it, provide independent overtemperature limiting and a suitable contactor or circuit breaker to remove power. The difference between a temperature sensor and a thermostat matters here: define the measurement, control and protection functions explicitly.

A product standard number does not establish that a device includes emergency stopping, redundant safety functions or protection against every fault. The need for these functions and their implementation follows from the machine or process assessment.

What should you look for in the documentation?

Read conformity statements and certification information separately. For example, the Carlo Gavazzi RGS data sheet lists IEC 60947-4-3 and IEC 60947-4-2 conformity separately from UL 508 component recognition. This must not be rewritten as approval to UL 60947-4-3. Base selection on the actual record for the model, rather than a general “UL compliant” statement in a quotation.

Related standards and application topics

For related subjects, see UL 60947-1 for the general framework, UL 60947-4-1 for electromechanical contactors and motor starters, UL 60947-4-2 for semiconductor motor control, UL 60947-5-1 for control-circuit devices, and IEC 60947-4-2 for the corresponding IEC motor-control scope.

To consider the application's components together, explore fuses, contactors, temperature sensors, thermocouples and SSR relays. Other product groups are available from the controllers main page.