Duty cycles and hidden ratings

It’s easy to assume that when you buy a “1 kW braking resistor”, you’re getting something that can safely handle 1 kW all day long. Unfortunately, that’s rarely true; the figure printed on the label – or at the top of the datasheet – doesn’t always tell the whole story. Behind that number lies something called the duty cycle, and misunderstanding it is one of the most common causes of premature resistor failure, as well as drive and application problems.

Every time a motor stops or slows down, energy from the spinning load has to go somewhere. The braking resistor’s job is to absorb that energy and convert it into heat. The problem is that no resistor can keep doing this indefinitely – at least, not without time to cool down.

That’s what the duty cycle describes: how long the resistor can safely take power before it needs a rest. For example, a resistor marked “1 kW, 10% duty” can handle 1 kW for short bursts – perhaps one second in ten – but must cool down for the remaining nine seconds. Run it continuously, and it will eventually overheat.

The confusion arises because not all manufacturers make that clear. Some relate only the peak power, while others hide the duty rating further down the page, two products that look identical online – same “kW” rating, same connector size – can behave very differently once installed.

The difference often comes down to thermal design – how well the resistor stores and releases heat. At REO, we describe this as the resistor’s thermal reservoir. A large, well-built resistor has a bigger reservoir; it can absorb and hold more heat energy before its temperature rises to a critical point. Smaller or lighter designs have a limited reservoir and heat up much faster, meaning they need longer longer cooling periods between braking events.

Even if you never see that technical detail, it’s why a heavier, better-constructed resistor usually lasts far longer than a cheaper, compact unit that claims the same rating. One has the physical capacity to manage the heat safely – the other doesn’t.

If you want something that works and keeps working, here are four simple checks:

Price – A much cheaper resistor that looks “the same” probably isn’t. manufacturers can quote impressive numbers by testing at low duty cycles. Always check whether the rating is continuous (100%) or intermittent.

Mention of duty cycle – Look for a clear statement such as “10% ED” or “40% duty.” If there’s no mention at all, ask. Never assume continuous operation.

Physical size – Heat capacity comes from mass. A resistor designed for actual continuous duty will almost always be larger and heavier than one with a low duty rating. Compact versions have their place but are only suitable for short, infrequent braking bursts and certain applications.

Cooling type – If a datasheet mentions ” forced air” “liquid cooling” or “chassis mount” those ratings rely on external fans, bonding to a heatsink or coolant systems to remove heat. If you are mounting the resistor in still air or an enclosed cabinet, use the natural air (AN) ratings, which are much lower.

By checking these four points, you will avoid most of the common pitfalls that lead to early failure. At REO, we test every braking resistor to determine how much heat its thermal reservoir can safely store before reaching its design limit. This measured value feeds into our derating curves – see figure 1, which show how long each resistor can handle a given load before the temperature becomes unsafe.

A resistor capable of handling 1 kW continuously under real conditions will always be more substantial. It utilises higher-quality materials, features a heavier construction, and incorporates a design that allows heat to escape efficiently. The result is reliability, long service life, and predictable performance.

In conclusion, if you’re ever unsure, check these points before specifying or replacing a brake resistor:

• Does the specification mention a duty cycle below 100%?
• Is the cooling method “air natural,” or does it depend on forced ventilation?
• Does the physical size and weight seem appropriate for the claimed rating?
• Are derating or duty-cycle curves available from the supplier?
• Is the price suspiciously low compared with other products?

If any of these are unclear, ask. At REO, we will review your drive parameters and recommend the correct resistor for your application. Additionally, we will also demonstrate how to interpret the duty cycle and derating information, ensuring you understand exactly what you are receiving from us and, more importantly, that it is the correct product for the given application.

After all, a braking resistor should be the most reliable and least troublesome part of your control system. It’s there to protect everything else – and if it’s specified correctly, it can do just that.

For more information, please visit https://www.reo.co.uk/brake-resistors/