What actions can be taken to protect your large variable speed drives?

1) Preventive challenges :

Any breakdown initiates a crisis period. This may be more or less significant and may have three serious consequences:

• Economic impact: A production line shutdown results in a significant financial loss for your company.
• Environmental impact: A breakdown can lead to environmental risks such as river pollution, toxic vapour being discharged into the atmosphere, etc. A simple breakdown can lead to an ecological disaster.
• Human impact: Any emergency situation creates confusion and hence poses a danger to people’s safety.

It being understood that the human or ecological impacts are unacceptable, our attention will focus on the economic aspect. The criticality of the impact is adjusted according to the maintainability of the system (outdated system), funnel effects and production requirements. The focus is therefore essentially financial, taking into account production requirements, the MTBF of the product and also its MTTR, and ensuring both the technical interest and the capacity to enhance the reliability of the product.

The goal of preventive action is therefore to implement a measured action leading to economic gain through the reliability of the production system. This economic interest can be direct (reduction in maintenance costs) or indirect (linked to production costs).

The preventive budget can be optimised by means of analysis. The following elements must be taken into account:

• The system failure rate (maintenance rate and risk rate)
• MTBF produced
• MTTR of the product
• The economic impact on production

Analysis of these four criteria will enable us to come up with a proportionate response to the economic risks incurred. The choice of preventive action for power electronics thus considers the risk of failure of the Drive components in relation to the action’s likely Budget.

It should be noted that the action must be carried out in a healthy environment: the temperature of the cabinet (filters, air conditioning, ventilation duct obstruction, etc.) and any existing pollution MUST BE ADDRESSED. Dealing with environmental issues is absolutely the first act of preventive maintenance required to achieve a sustainable return on investment.

The MTBF degradation analysis will give us a meaningful indicator for triggering preventive actions.

2) Preventive actions on a variable frequency drive :

The variable frequency drive incorporates components that are sensitive to temperature and to the quality of the network.

There are several categories of components:

• Power components (Rectifier, Inverter)
• Electromechanical components (Turbines, Fans, Contactors, Relays, etc.)
• Passive components (Condensers, Snubber, Bus filters, etc.)
• Hall effect sensor
• Electrical connection (Connectors)
• Optical link (Optocoupler, Optical fibre, etc.)

Electronic components have a nominal life defined by the manufacturer. They require maintenance intervals appropriate to the conditions of use and the environment.

Power components :

Heat dissipation is an essential issue over the lifetime of power components. Reducing the sizes of the drives requires an efficient cooling system, with any lapse resulting in damage (bonding wire, leakage current, etc.) to these components and/or to the entire system.

The failure rate is 10 times higher at 100 degrees centigrade compared to 40 degrees centigrade.

It is therefore necessary to check and/or correct the following elements:

• The dissipation surface (heatsink obstruction, air flow, water flow, etc.)
• The quality of the surface resistance (frequency of grease change, gel)
• Ambient temperature (air conditioning)
• Humidity level (degradation of materials)
• Atmospheric pressure
• Quality of the turbine, ventilation
• Snubber quality (dl/dt limits and turn off)
• Contact and bus clamping quality (contact capacity: turn off and common mode circulation current)
• Signing of the avalanche effect

Electromechanical components :

Heat dissipation is an essential issue, based on the single-phase convective effect. Maintaining air flow is thus essential.

It is therefore necessary to check and/or correct the following elements:

• Air duct obstruction
• Fan change
• Turbine bearing change
• Mechanical contactor failure
• Auxiliary changes
• Relay change

Passive components :

The lifetime of the chemical capacitors depends on the operating temperature (Arrhenius law) and the ripple current.

The lifetime of a 105-degree capacitor will be four times greater on average than an 85-degree capacitor. This same law applies to the ambient temperature of the product (reduction of 10 degrees = twice the lifetime). This lifetime is reduced by heating due to the ripple current.

The capacitor is subject to little variation in lifetime due to the voltage of its connections. The rule is to use a capacitor at 80% of its nominal value.

However, it is very sensitive to HF current. It is therefore necessary to check and/or correct the following elements:

• Capacity and ESR of the bus capacities
• Bus energy return rate
• Bus balancing resistance
• No oscillations on the bus (carrier)
• Change of non-power electrochemical capacities by 105 degrees
• Check snubber (ZC value, leak, etc.)

Current measurements :

There are mainly hall effect sensors: the hall voltage is proportional to the intensity and inversely proportional to the thickness of the material. The temperature has a strong influence on the degradation of the component.

We need to check the following:

• Output voltage a I=0
• Sensor linearity

Optical components :

Optocouplers are reliable elements. Here again, however, the temperature has a significant negative impact on service lifetime. It can be of the convection type, or generated by a high current at the manifold. Furthermore, it should be noted that the CTR deteriorates over time. A safety coefficient is taken into account when designing electronic cards.

Optical fibres are sensitive to torsion and mechanical stress.

It is difficult to find a datasheet giving a lifetime of the transmitting or receiving diodes. However, feedback indicates that it is not unusual to have defective driver cards. A dynamic data transmission test allows us to qualify the operation but not the lapse of the component.

Connectors :

Pollution, oxidation and vibrations are clearly important elements in the failure of the connector. Clamping is also an important element to check.

The power connection points must be checked and generate “contact” capacities of the order of nf, which can promote common mode circulation currents.

3) Conclusions :

Efficient preventive maintenance solutions for drives exist. However, temperature has a major influence on the ageing process.

It is also worth noting a problem related to the storage of PDRs, forcing high-power drives to “reform” the intermediary bus capacitors. The forming time depends on the product’s storage time.

Reynald Boutillier

Head of DV GROUP Electronic Department