Can an Ex e motor be driven by a variable frequency drive?

Only if the motor is certified for operation with a converter. An increased safety (Ex e) motor that is certified only for a sinusoidal mains supply must not be put on a variable frequency drive (VFD), because the drive changes how and where the motor heats up. A converter-rated Ex e motor can be used, but the drive, its settings and the motor's thermal protection must match the conditions on the certificate.

Short answer

• IEC 60079-7 requires proof that the motor's limiting temperatures are not exceeded over its whole operating speed range on a converter, either by a type test or by calculation from representative test data.
• The type test is made with the specified converter or with a comparable one in output voltage, output current and switching frequency. The motor documentation must state the converter parameters and conditions of use.
• IEC 60079-14:2024 accepts two routes: a motor type tested with a specific converter and protective device, or a motor tested for converter operation where the converter type is not named. In the second case, the selected converter's characteristics, maximum input voltage and settings must match the motor manufacturer's specifications.
• Converter-fed Ex e motors normally rely on winding temperature sensors, at least one per phase, connected to a protective device; the converter can act as that device.
• The Ex e terminal box must cope with the voltage peaks and extra heating that a converter can cause.

Why a drive changes the safety case

An Ex e motor has no flameproof enclosure. Its safety rests on keeping every part, inside and out, below the temperature class and the limiting temperature of the insulation, in normal running and when stalled. On a mains supply this is protected by a current-dependent relay set against the time tE on the nameplate. On a converter that approach no longer works reliably:

• Extra losses. The non-sinusoidal output adds harmonic losses in the stator and rotor. IEC 60079-7 notes that the rotor may become the limiting part, particularly in cage rotors.
• Less cooling at low speed. A shaft-mounted fan moves much less air at low speed, while the drive can still deliver high torque. The motor can overheat with a current that looks normal.
• Voltage peaks. Fast switching produces voltage peaks at the motor terminals that stress insulation and can raise temperatures in the terminal box.
• Bearing currents. High-frequency common-mode voltage can drive current through the bearings, which can spark and damage them. IEC 60079-7 points to IEC TS 60034-25 for these effects and for high-frequency earthing.

The 2006 edition of IEC 60079-7 required converter-fed Ex e motors to be tested and certified as a unit with the converter specification. The 2015 edition keeps the principle but also allows testing with a comparable converter or verification by calculation.

How to apply it

Common mistakes

• Retrofitting a drive to an existing mains-only Ex e motor because the power rating fits.
• Running below the certified minimum speed with full torque.
• Leaving the winding thermistors unconnected, or wiring them to an input that is not the specified protective device.
• Changing drive settings later (switching frequency, voltage boost, speed limits) without checking the certificate.

Related on excodex

• Increased safety (Ex e): preventing sparks, arcs and hot spots
• Ex d or Ex e? Choosing between flameproof and increased safety for motors and junction boxes
• Hot surfaces in hazardous areas: luminaires, motors, heat tracing and process heat
• Which is safer, Ex d or Ex e?
• Standards space: IEC 60079-7

Standards and further reading

• IEC 60079-7:2015 (edition 5.0; consolidated 5.1 with amendment 1:2017 is current), increased safety "e": converter operation of rotating machines (5.2.8.4), winding temperature sensors (5.2.8.3), marking; publicly available as incorporated by reference: law.resource.org. The 2006 edition, as IS/IEC 60079-7: law.resource.org
• IEC 60079-14, installation: motors with converter supply; the 2007 text, published as AS/NZS 60079.14:2009 (type test as a unit with the converter, overvoltage spikes in Ex e terminal boxes): archive.org; table of significant changes in the 2024 edition (clause 6.7.3.4 b), new Annex J): cdn.standards.iteh.ai
• P. Thurnherr (convenor of IEC maintenance team MT 60079-14), "Electrical installations design, selection and installation of equipment, including initial inspection", presentation summarising IEC 60079-14:2024 (electrical machines with converter supply) at the 2024 IECEx International Hydrogen Conference, Singapore: presentation PDF
• IEC TS 60034-25, rotating electrical machines: AC electrical machines used in power drive systems (referenced in IEC 60079-7)
• M. Magyari, D. Fotău, S. Zsido, "Aspects regarding the use of variable speed drives in the case of explosion protected electric motors used in explosive environments", Annals of the University of Petroșani, Electrical Engineering, vol. 26 (2024): upet.ro

This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.