Is Your Drive System Spec Eliminating Your Best Options?

Suppose you need to make a substantial investment in one of your large critical machines. If replacing the power drive system is not part of your daily routine, you are probably not a subject matter expert. So, what options are available? If you are part of a large organisation, your engineering department or an external engineering contractor will typically manage the translation of your business needs into technical specifications. If you are in a small company without a dedicated engineering team, you might rely on technical consultants or cross-functional team members to handle this translation. Either way, when converting stakeholders’ needs into functional requirements, ensure the resulting requirements can be validated.

For simplicity’s sake, let us focus on the top priority for any mission-critical machine: reliability.

You will need to translate this high-reliability requirement into technical specifications for the drive system. However, be mindful that you are, in fact, aiming for high availability of your critical machine.
High reliability is characterised by:

• A very low failure rate (expected number of failures per 1 million (10^6) hours)

This is often expressed as Mean Time Between Failures (MTBF), the predicted elapsed time between inherent failures of a system during operation (100.000 hours is typical for a medium-voltage drive system).

• A low probability of failures leading to an unplanned shutdown of the drive system. To achieve this, you might need to build redundancy into the drive system. However, if you over-specify, for example, by demanding redundancy at the power cell level, you might exclude some topologies that could also be ideal solutions for your drive system. Therefore, avoid detailing how the implementation should be carried out where possible.

High availability indicates that the product is reliable and that the time to recover from a failure, also known as Mean Time To Repair (MTTR), is short.
If you combine these two in a formula, Availability = MTBF / (MTBF + MTTR), you get the drive system’s inherent availability.
If this availability is 99.9900, often called the 4-nines, it would allow for 52 minutes of unplanned downtime per year (24*365*0.01/100). To maintain this “four-nines” level of service with a failure occurring every 100,000 hours, your team must be able to detect, repair, and restart the system within 10 hours of any failure.

The “Mean time to repair (MTTR) of 30 minutes,’ as sometimes claimed by VFD manufacturers, sounds impressive for availability calculations. But

• What if the issue is complex?
• Can you depend on qualified support?
• How quickly can they mobilise?
• What is their track record, and are there transparent customer reviews to support it?

These questions highlight the gap between theoretical specifications and real-world operation. Therefore, it is also recommended to define operational availability that considers fault-isolation downtime, logistics-delivery downtime, and the time required to install and reconfigure replacement parts.

For the availability requirement, it makes sense to request contractually guaranteed availability (such as a percentage or a maximum annual downtime) and to include penalties or liquidated damages in the contract.

Please note that the operating environment (such as temperature and humidity) will directly affect your drive’s reliability. You need proper cooling and must keep up with regular maintenance. This is essential for keeping any drive dependable. And here is the thing: excessive heat remains one of the biggest threats to your drive’s reliability.

But honestly, the most critical factor is sound system engineering. I am talking about how the network integrates, how the mechanical and electrical components work together, and how the system is protected.

Furthermore, continuous improvements and mature, well-developed equipment ensure reliable operation over time.