Does a variable speed drive change the temperature class of an Ex motor?

It can. The temperature class on an Ex motor is proven for a defined supply, normally sinusoidal mains at rated voltage and frequency. On a variable speed drive (VSD, also called a frequency converter or inverter) the motor can run hotter, so the marked class is only valid if the certificate explicitly covers converter supply and the installation follows the conditions it states.

Short answer

• A VSD raises motor temperature in two ways: less cooling at low speed (the shaft-mounted fan turns slower) and extra losses from the switched, harmonic-rich converter output.
• The marking does not change, but its validity does: a motor certified only for mains supply has no proven temperature class on a converter.
• Flameproof (Ex db) motors usually get a valid class on a VSD through winding temperature sensors connected to a protective device that trips the drive, or through a type test together with the converter.
• Increased safety (Ex eb) motors must be tested and certified for converter duty, with the converter type or its parameters stated in the documentation.
• The certificate or instructions define the limits: frequency or speed range, torque-speed curve, converter settings and how the thermal protection must be wired.

Why a converter makes the motor hotter

On mains supply a self-ventilated motor runs at almost constant speed, so its fan delivers constant cooling air. On a VSD the same motor may run at a fraction of rated speed while still delivering high torque. Torque needs current, current produces heat in the windings, and the fan that should remove it is now turning slowly. Manufacturers therefore give a loadability (torque-speed) curve that reduces the permitted torque at low speed.

The converter output is not a clean sine wave. Fast switching adds harmonic currents that heat the rotor cage, the stator and the iron. Steep voltage edges also stress the winding insulation and can drive currents through the bearings, which cause sparking and bearing damage. Insulated bearings, output filters or limited cable lengths are the usual countermeasures and must follow the manufacturer's instructions.

Each of these effects can push the hottest surface, inside or outside the motor, above the limit of the marked temperature class even when the current stays within its rating. The hottest point is not always the frame: the 2007 text of IEC 60079-14 notes that it can be the shaft. A current-based overload relay alone does not see it.

What the standards require

The installation standard IEC 60079-14 has long set specific conditions for motors supplied at varying frequency and voltage by a converter. In the 2007 edition (publicly readable as AS/NZS 60079.14:2009), a flameproof or type n motor needed either a type test as a unit with the specified converter and protective device, or direct temperature control by embedded sensors (or other effective measures) that disconnect the motor; a current-dependent time-lag overload device did not count as such a measure. An increased safety motor needed the type test with converter and protective device. The 2024 edition lists converter supply of electrical machines as an extended requirement and adds an informative annex on machines operated from converters. The usual routes are:

For increased safety machines, IEC 60079-7:2015 lets the temperature for converter supply be established by a type test with the specified converter or one comparable in output voltage, output current and switching frequency, or by calculation, and allows the converter, together with winding temperature sensors, to act as the safety device. The machine documentation must state the parameters and conditions for converter use. Ex e motors are more sensitive because their whole protection concept relies on limiting temperature and avoiding sparks, not on containing an explosion.

How to check an installed motor

1. Read the certificate and instructions. Look for explicit converter-supply data: frequency or speed range, torque limits, converter type or parameters, switching frequency, and the thermal protection required.
2. Check the converter settings against those data, in particular the minimum frequency, current limits and any motor parameters to be entered.
3. Check the thermal protection wiring. Sensors must be connected to the device the certificate requires, and that device must trip the drive, not only raise an alarm, where tripping is required.
4. Check the bearing and cable measures the instructions call for, such as insulated bearings or filters.
5. Record it in the verification dossier so later drive replacements or parameter changes are reviewed against the same limits.

Common mistakes

• Fitting a VSD to an existing mains-rated Ex motor to save energy, without checking the certificate. The temperature class is then not proven.
• Leaving thermistors unconnected or wired only to an alarm.
• Replacing the drive with a different model or new parameters without checking them against the motor certificate.
• Relying on current monitoring alone, which does not detect overheating from poor cooling at low speed.

Related on excodex

• Hot surfaces in hazardous areas: luminaires, motors, heat tracing and process heat
• Ex d or Ex e? Choosing between flameproof and increased safety for motors and junction boxes
• What does 'X' after the certificate number mean?
• Tools and spaces: Ex marking check, IEC 60079-14, IEC 60079-7

Standards and further reading

• IEC 60079-14:2024, Explosive atmospheres, Part 14: Electrical installations design, selection and erection (machines with converter supply; informative annex on machines operated from converters); public preview: standards.iteh.ai
• IEC 60079-14:2007 converter clauses, as published in AS/NZS 60079.14:2009: archive.org
• IEC 60079-7:2015, Explosive atmospheres, Part 7: Equipment protection by increased safety "e" (operation with a converter); public copy: law.resource.org
• IEC 60079-1, Explosive atmospheres, Part 1: Equipment protection by flameproof enclosures "d"
• 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.