IEC TC31: Harmony in Frankfurt
21 December 2016
Although it will seem some time ago when this appears in print, this is my first opportunity to report on the IEC TC31 meetings that were held along with the IEC General Meeting in Frankfurt during October. The many meetings of the various Project Teams, Working Groups and Maintenance Teams progressed well.
Working Group 27 discussed the operation of motors in hazardous areas
In my column this month, I will concentrate on some of the technical issues that we discussed in Working Group 27 in relation to the operation of motors in hazardous areas powered by variable frequency converters.
That converters can be capable of making an otherwise “safe” motor “unsafe” has been well established over many years. I first made a contribution on this subject to an IEE conference over 30 years ago.
If a motor is running at or near synchronous speed for the fundamental frequency supplied by the converter, it will be running away from synchronous speed in relation to any harmonics that may be present in the voltage wave form, resulting in the harmonic currents being magnified in relation to the fundamental frequency current.
These harmonic currents do not exist, of course, with a sinusoidal supply. Thirty years ago, it was not uncommon for the motor bearing to fail, purely as a result of the extra heating in the rotor, and for the shaft external to the motor to be the hottest external surface of a flameproof motor.
Since then, technology has moved on and, with significantly improved semiconductor devices, the most troublesome harmonics can be minimised by more nearly synthesising a sine wave through pulse-width modulation, or a similar technique.
But different converter manufacturers have their own specific designs to achieve this, and not all such converters will cause motors to behave in the identical way in respect of temperature rise from harmonic losses.
In WG 27, we have spent the last few meetings trying to get to grips with the possibility of defining important converter parameters in a way that would easily allow a motor manufacturer (or a user of a motor where the original converter has failed) to select a converter that could be described as “equivalent”, and therefore substituted for the converter that was used for the original testing programme.
The converter and motor designers within the same manufacturing organisation can often agree as to whether or not a similar but slightly different converter may subsequently be substituted. Certification of the motor may be able to take this into account, but at present it is almost impossible to predict what will happen to a motor when a converter from a different manufacturer is substituted.
The goal is to be able to define the parameters so that the potential interchangeability between converters from different manufacturers can be easily judged.
For pulse-modulated converters, the single most important feature seems to be the pulse repetition frequency, but there is not yet even a universally-accepted definition of what that means.
A separate issue which can have a similar effect on temperature is that many converters have peak output pulse amplitudes based on the input voltage to the converter, whereas other converters use a fully-stabilised voltage from which to synthesise the output frequency.
In terms of the efficiency of the converter, arguments can be made for both systems. However, in terms of the final measured temperature, the converter that does not stabilise the voltage prior to synthesis can cause unexpected heating when the converter input voltage goes high, even when the notional output voltage remains constant. Although the nominal sine wave remains unchanged, the actual peak pulse voltages seen by the stator winding will vary, and increasing the input voltage results in higher voltages at the pulse frequency.
Since most motors are designed to maximise output for minimum cost, the iron circuit is run near the saturation point. Increasing voltages above this point has the potential for increased currents without increased output from the motor, and there are additional harmonic losses.
And none of these voltage harmonic considerations cover the possibility that something failing in the converter may cause parameters such as voltage/frequency ratios to change out of specification. Some converters contain protection circuits, some do not.
Fortunately, in Zone 2, for motors with Equipment Protection Level (EPL) Gc or Dc (ATEX Category 3), we can ignore the failure of the converter and only have to worry about the continuous parameters - such as pulse repetition frequency.
However, in Zone 1 (EPL Gb or Db – ATEX Category 2) we do need to add additional protection. For a flameproof or a purged and pressurised motor, the use of direct thermal protection in the stator winding can provide the solution in many cases, but it must be remembered that safety margins need to be included as the stator winding is not necessarily the hottest part of the motor.
Increased Safety Ex eb motors remains the area where greatest concern is needed, and all certification bodies need to work with the motor manufacturer to provide a safe solution to the problems of using variable frequency converters.
About the author
SGS Baseefa Technical Manager Ron Sinclair MBE is chairman of BSI Committee EXL/31, responsible for the UK input to both European and International standards for Electrical Equipment for use in potentially explosive atmospheres. He is chairman of Cenelec TC31, represents electrical standardisation interests on the European Commission’s ATEX Standing Committee and chairs the IECEx Service Facility Certification Committee.