Centa Gears

Yaskawa is a world-leading manufacturer of inverter drives, servo drives, machine controllers and industrial robots.

ABB Drives and Motors

Rockwell Automation is a global provider of industrial automation, power, control and information solutions


Features 

Using ATEX motors with drives - the effects of harmonics

 

Using EEx d/de motors with inverter drives: keep your options open

In the past, motor users have been advised that flameproof EEx d/de motors and inverters need to be tested together when used as a system in a potentially explosive atmosphere. Dr Donald Jackson, Technical Director of Brook Crompton, argues that this is not necessary if a motor certification capable of using any inverter is established at the outset

 

The introduction of the ATEX Directive in 2003 — and the inclusion, in July 2006, of site risk assessment from DSEAR (the Dangerous Substances and Explosive Atmospheres Regulations) — established the need to review and identify areas where potentially explosive atmospheres may occur.

Typical hazards include the risk of abnormally high temperatures or the risk of sparking. The application area is categorised according to the gases or dusts present, and the likely period of exposure, as shown in in the table below.

 
Category 1
Category 2
Category 3
Safe Area
Hazard
Continuous
Short period 10-1,000 hours a year
Not usually present, only during a malfunction. Up to 10 hours a year
No
Gas
Zone 0
Zone 1
Zone 2
No
Dust
Zone 20
Zone 21
Zone 22
No
Enclosure
IP 6X

IP 6X conductive OR IP5X non-conductive
IP XX

It is only in category 2 and 3 applications in zones 2 and 3 for gases, and/or zones 21 and 22 for dust, where certified motors are needed. The risk of sparking is controlled by considering the creepage and clearances of the rotating parts including tolerances, preventing static charges, and avoiding loose parts or connections. The risk of explosion through abnormally high temperatures is controlled by designing within a temperature class (see table below).

Temperature Class
Maximum Temperature (°C)
T1
450
T2
300
T3
200
T4
135
T5
100
T6
85

Different protection concepts define the relevant surface temperatures for different applications:

• EE x d flameproof: Robust construction; restricted external surface temperatures

• EE x e increased safety: Restricted external and internal surface temperatures; limitation of stall times tE

• EE x nA non sparking: Restricted external and internal surface temperatures

• Dust non-combustible: Exclusion of dust from inside motor IP5X or IP6X; restricted external surface temperatures

The application dictates the required torque, speed range and temperature limitations.

Motor performance design tools linked to thermal models allow temperatures to be predicted to establish the motor insulation choices, as well as the maximum internal or external surface temperatures (see figure below).

These calculated results are calibrated and mirrored by measurements. Motors are tested for a wide range of:

• sizes, outputs, and pole numbers;

• inverter waveforms (PAM, PWM, PSM);

• alternative inverter settings; and

• different motor design styles.

The risks can be reduced by testing motors with different waveforms produced by a wide range of inverters from different suppliers. Having de-rating curves that are proven in practice with inverters from many manufacturers makes it is possible to cater for possible changes if the inverter is replaced later in the motor`s operating life.

Motors of differing sizes can meet the torque and speed requirements, but the correct working temperatures is chosen by using proven de-rating curves. These curves plot torque against speed to achieve a reliable thermal performance within temperature boundaries for a protection class (see graph below).

The final essential test when establishing these de-rating curves is selecting the correct thermal protection detection component that resides inside the motor.

An inverter supply can change a motor`s thermal performance by:

• increasing its running losses as a result of non-sinusoidal waveforms;

• operating at lower speeds, which reduces the fan cooling effect;

• running the motor in an under- or over-voltage configuration; or

• changing the inverter settings from those initially tested.

Thermistor protection

All of the above situations can increase motor temperatures. Brook Crompton, which pioneered the development of Ex-certified motors, recognised the benefits of fitting thermistors embedded in the windings. PTC (positive temperature coefficient) thermistors exhibit a rapidly increasing resistance at the trip temperature, which is used to operate the protective circuit. Three thermistors (shown below) are distributed around the winding, with one embedded in each phase.

Typically, for T4 certification, a 140°C trip temperature thermistor is used. Brook Crompton has developed this convenient way of protecting EEx d/de motors through testing and evaluating results using the facilities of the explosive atmospheres certification body Baseefa, based at Buxton in the UK.

The temperatures reached in a motor must have a design margin for the temperature classification. Under fault conditions, however they may arise, the thermistor protection must operate. From a safety point of view, different temperature classifications are covered in the motor sizing and the choice of trip setting. The different temperature switching components are colour coded for safety identification.

Matching

The conservative approach of using historical de-rating curves leads to a perception of substantial improvements being possible because the motor and inverter are matched with modern improved inverter waveforms.

So where might matching be possible, and what does it mean? Let me review some obvious examples.

Why not buy better steel that can accommodate the inverter waveform, thus reducing high-frequency losses? This is practical to some degree, but there is no guarantee of steel`s high-frequency performance, and steel is not usually identical from different mills, thus preventing purchasing in economic quantities. Alternatively, you could use a multi-level inverter to avoid generating the bad waveform - but can this be done at the same cost?

Changing the winding, winding style or core length to reduce motor losses is possible if orders are repeatable. However, many motor customers are not prepared to pay for this benefit and prefer off-the-shelf products. They are not willing to pay either for dedicated products or for inventory, even when supplied by Kanban.

Motor-inverter matching can be of some benefit when selecting fan motors where a trade-off of improved cooling against noise level is acceptable. Also spike-resistant wire and compatible insulation systems for an application can be an acceptable matching feature. On the inverter, choices can be made to achieve matching and to give longer motor life by reducing PWM frequencies, having the inverters installed next to the motors, and avoiding or inhibiting bipolar switching.

All of these matching techniques affect performance and operating life, but do not impinge on safety.

Benefits

Providing that a motor used with an inverter has the correct class of thermistor triplet embedded and is connected to the protection circuit, any make of inverter can be used with a high-quality EEx d/de motor, such as those supplied by Brook Crompton. The protection circuit should disconnect the motor from the supply.

The benefits to the user are:

• there are no limitations on which inverter manufacturers they use, or which models - sites that have a preference for a particular inverter supplier can say with this supplier;

• the simple approach gives users confidence that excessive surface temperatures will be detected; and

• any changes to the inverter parameters to suit particular applications, or operator changes to inverter settings, will not affect the certification conditions and prevent safety being maintained.

Brook Crompton fits thermistors as standard on motors in frames 160 and above. They can also be fitted retrospectively in winding pockets that are pre-prepared during motor manufacture, as a modification on frames 90 to 132. As always, the simplest solutions are the best.

The thermistor protection approach has been recognised as a means of conforming to the new Atex Worker Protection Directive 1999/92/EC (Atex 137).

The UK trade associations Gambica (representing drives suppliers) and Rema (representing motor suppliers) have published an excellent document, User Guide No4, Variable Speed Drives and Motors, Application of the ATEX Directives to Power Drive Systems. From this, I have extracted a few statements referring to flameproof applications.

Paragraphs 5.3.7.1 and 5.3.7.2, referring to flameproof applications, state that temperature rises should be measured, and that motors selected using manufacturers` proven loadability curves for variable-speed duty and incorporating a suitable protection device, do not need combined tests. In the absence of such curves or evidence of test, the selected motor and basic drive module (BDM) should be tested together as a unit to ensure that the motor meets the requirements of a given external temperature classification.

The Guide also states that a motor must either be type-tested for inverter duty as a unit in association with the converter (BDM/CDM) and with the protective device specified, or it should be provided with direct temperature control by embedded temperature sensors or other effective measures. The protective device must cause the motor to be disconnected. The motor and converter (BDM/CDM) combination does not need to be tested together.

Conclusion

The matching of a proven motor using the correct thermal protection component for the thermal class is a recommended route to meet the needs of the ATEX Risk Assessment which is independent of the inverter used. This procedure is mandatory no matter how the motor inverter is chosen.

All new applications of motors in hazardous areas, require correct sizing using the motor manufacturer`s de-rating curves. Brook Crompton, with its engineering expertise, can ensure an optimised result for both motor users and inverter suppliers.

Acknowledgement

The author is grateful to the AEMT for permission to include details from its Autumn 2006 Journal and to Rema and Gambica for including extracts from their User Guide No 4.

 

 


Drive Components
Linear Systems From A Single Source


 
Website Design Surrey