Explosive atmospheres are one of the most sensitive areas of electric motor selection. In facilities containing gas, vapour or combustible dust, a motor's surface temperature, spark or internal arc risk can become a direct ignition source. For this reason, selecting an ATEX-certified motor is not just a power and speed calculation; it is a multi-layered engineering decision in which zone, protection type, gas/dust group and temperature class are evaluated together.

In recent years, the pressure for energy efficiency has carried the top efficiency class — IE5 synchronous reluctance (SynRM) motors — into explosive-atmosphere applications too. Because SynRM motors have no winding or cage in the rotor, they minimise rotor losses; this means both high efficiency and low surface temperature. Low surface temperature is a direct safety advantage in ATEX applications.

In this article we examine in detail how to select IE5 SynRM motors in explosive atmospheres, the special conditions imposed by drive operation, the protection types and temperature-class verification. Our aim is to clarify the field safety requirement together with its technical counterpart and to ensure fast supply of the correctly certified motor.

Zone Classification: The Basis of Selection

The ATEX directive divides explosive atmospheres into zones according to the persistence of the hazard. This classification forms the basis for which protection type of motor to choose:

  • Zone 0: A place where an explosive gas/vapour atmosphere is present continuously or for long periods. A motor is generally not used directly here.
  • Zone 1: A place where an explosive atmosphere is likely to occur occasionally in normal operation. Requires Ex d or Ex e protection.
  • Zone 2: A place where an explosive atmosphere is not expected in normal operation, and is short-lived if it occurs. Lighter protection such as Ex nA is accepted.
  • Zone 21: A place where combustible dust forms a cloud occasionally (dust). Requires Ex t protection.
  • Zone 22: A place where combustible dust is not normally present, and short-lived if it occurs (dust).

Motor selection cannot begin before the zone is determined. A wrong zone assessment leads either to an unnecessarily expensive motor or, far more dangerously, to an under-protected motor. Therefore the first step of ATEX selection is always correct zone determination.

Protection Types: Ex d, Ex e, Ex nA, Ex t

The basic logic of explosion protection is either to prevent ignition entirely or, even if ignition occurs, to contain the explosion inside the motor enclosure. The main protection types are:

  • Ex d (flameproof enclosure): A possible ignition inside the motor is contained within the enclosure; the enclosure gaps cool the flame and prevent it passing outside. Common for Zone 1.
  • Ex e (increased safety): The possibility of arcs, sparks and excessive temperature is minimised by design; no ignition source exists in internal cavities. Zone 1.
  • Ex nA (non-sparking): A construction that produces no ignition spark or arc in normal operation. An economical solution for Zone 2.
  • Ex t (dust protection): The enclosure prevents combustible dust entering the motor and prevents surface temperature igniting the dust. Zone 21/22.

In SynRM motors the protection type is chosen the same way as in their induction counterparts; the difference is that SynRM, thanks to its low rotor loss, is advantageous in temperature-class verification. The choice between an Ex d motor and an Ex e motor is often shaped by the facility's standard approach and installation practices.

Temperature Class: T1-T6 and Surface Temperature

The temperature class limits the maximum temperature the motor surface (or internal parts) can reach. This limit must stay below the ignition temperature of the gas or dust in the environment; otherwise the motor itself becomes an ignition source. For gas atmospheres the temperature classes are:

  • T1: Surface temperature up to 450 °C.
  • T2: Up to 300 °C.
  • T3: Up to 200 °C.
  • T4: Up to 135 °C.
  • T5: Up to 100 °C.
  • T6: Up to 85 °C.

As the temperature-class number rises (towards T6) the permitted surface temperature falls, meaning the protection is stricter. The lower the ignition temperature of the substance in the environment, the higher the T class required (lower surface temperature). This is where SynRM's advantage comes in: because there is almost no rotor loss, the motor's surface temperature can be kept lower than that of an induction motor of the same power, making it easier to reach stricter temperature classes.

IE5 SynRM Runs With a Drive: Why It Matters

Synchronous reluctance motors cannot start directly from the mains; they always run together with a frequency converter (drive). The drive manages the rotating field according to rotor position and drives the motor at synchronous speed. This has an important consequence in ATEX applications: the motor's temperature class and protection compliance must be verified together for the motor-drive combination.

This is because the drive applies harmonic-rich current to the motor; these harmonics create extra loss and therefore extra heating. The temperature class a motor provides on the mains may not be valid when fed by a drive. For this reason, ATEX-certified SynRM motors are tested with a specific drive type or parameter set, and the certificate is valid for that combination. Overload, insufficient cooling at low speed, or wrong parameters on the drive can violate the temperature class.

Practical rule: when selecting an IE5 SynRM ATEX motor, it is essential to evaluate the motor not alone but together with the approved drive and protection combination. If continuous operation at low speed is required, external cooling (forced ventilation) may come into play.

Gas and Dust Groups: IIB/IIC and IIIB/IIIC

Alongside the temperature class, the explosion group of the substance in the environment also determines the selection. Gas atmospheres are divided into subgroups under Group II:

  • IIA: Propane-like gases; the least critical group.
  • IIB: Ethylene-like gases; medium criticality.
  • IIC: Hydrogen and acetylene; the most critical group, requiring the strictest protection.

Dust atmospheres are divided under Group III into IIIA (combustible flyings), IIIB (non-conductive dust) and IIIC (conductive dust); IIIC is the most critical group. The motor's certificate must cover the gas/dust group of the environment in which it will be used. A motor certified for IIC can be used in a IIB environment, but not the reverse.

Nameplate and certificate compatibility is vital here. The zone, protection type, gas/dust group and temperature class must be fully readable on the motor nameplate. The certificate number and notified-body information are proof that the motor has genuinely been tested and documented.

Checklist for Correct Selection

The logical sequence to follow when selecting an IE5 SynRM motor for an explosive atmosphere is as follows. First determine the zone class of the environment (0/1/2 for gas, 20/21/22 for dust). Then identify the gas/dust group and ignition temperature of the substance; the ignition temperature sets the required T class. Next select the protection type suited to the zone (Ex d, Ex e, Ex nA, Ex t). Finally, match the drive that will run the motor and the cooling method to the combination for which the certificate is valid.

The most common mistake in this process is ignoring the drive combination. Even if the correct motor is selected, running it with a drive not stated in the certificate can invalidate ATEX compliance. For this reason the motor and drive should be supplied together, and the parameter set applied as approved by the manufacturer.

For classification and certificate details you can review our ATEX and explosion-proof motor guides and, for efficiency-class comparisons, our IE5 SynRM technical content. For the stock status of the correctly certified motor and current electric motor prices, you can request a quote.

Frequently Asked Questions

Can I run an IE5 SynRM motor directly from the mains?

No. Synchronous reluctance motors cannot start directly from the mains; they always run with a frequency converter. In ATEX applications this is even more critical, because the motor's temperature class and protection compliance are verified for the motor-drive combination. Running with a drive not stated in the certificate can invalidate ATEX compliance.

What is the advantage of SynRM motors in an ATEX environment?

Because the SynRM rotor has no winding or cage, rotor losses are minimised. This means a lower surface temperature than an induction motor of the same power. The lower surface temperature makes it easier to reach stricter temperature classes (such as T4, T5) and provides a direct safety advantage in explosive atmospheres.

How do I determine the temperature class?

The temperature class is set by the ignition temperature of the gas or dust in the environment. The motor's maximum surface temperature must stay below this ignition temperature. The lower the ignition temperature, the higher the T class required (towards T6, lower surface temperature). For correct selection it is advisable to specify the gas/dust group and ignition temperature when requesting a quote.