Prevent 24 VDC overloads from stopping production

Siemens Ltd
By Kai Bronzel
Saturday, 30 January, 2010

Along with the laws of physics, engineers have to be aware of the law of unintended consequences. Take, for example, something as simple as the 24 VDC power supplies that feed the control circuits in automation systems as a case in point.

Years ago, 24 VDC supplies were linear units containing an input transformer, a bridge rectifier and some filtering, with a series-type voltage regulator. They were rugged and dependable, but big, heavy and not very efficient. Although they handled overloads and rarely failed, linear power supplies wasted energy and pumped heat into control cabinets.

The answer was the switchmode power supply, which is both smaller and more energy efficient than the old linear unit. Switchmode power supplies have become the most common type used today, but they have one characteristic that causes problems that linear supplies did not: the way they respond to overloads or short circuits.

The problem

When a linear power supply is overloaded, it goes out of regulation - its output voltage falls below its specified lower limit - but continues delivering current. When a switchmode power supply is overloaded - sometimes even by as little as 10% - it simply shuts down. If the supply is feeding just one load, this is not a problem, but many 24 VDC power supplies feed multiple loads, which could include HMIs, PLCs, I/O, contactors, small motors, solenoids and more.

Often a fuse or circuit breaker is installed on each load circuit to prevent a short circuit or overload on one from affecting others, but if the supply is operating near capacity, an overcurrent on one load may cause the total load on the supply to exceed its rating. Under these conditions the supply will shut down immediately (faster than a conventional circuit breaker or fuse can trip), shutting off power to all loads. This can cause disruptions in plant operations or interrupt production if enough loads are affected.

An example

For example, a 24 VDC power supply rated at 40 A feeds four circuits, each drawing 8 A and each protected by a 10 A circuit breaker. What will happen if a motor stalls or there is a short somewhere in the wiring and one of those circuits starts to pull 24 A? In theory, the breaker on that circuit should open and clear the fault, but that may take eight seconds or longer, depending on the type of breaker (see Figure 1). Meanwhile, the total load on the power supply has jumped to 48 A, a 20% overload. The power supply may shut down in a couple of seconds, or it may go into current limiting, which will increase the tripping time on the breaker still more, and then shut down. Either way, all the loads connected to it lose power.


Figure 1: A conventional circuit breaker may not clear a 240% overload for eight seconds or longer, during this time a switchmode power supply may shut down, killing power to all of its loads.

Some power supplies have a surge rating that enables them to provide current into an overload for a short time, but there is still no guarantee that the breaker will trip before the supply shuts down.

After the power supply has shut down, it may stay that way, or it may try to restart. On a restart it will sense the overload again, and shut back down - over and over again, with possibly damaging consequences for the loads.

At the same time, troubleshooting is very difficult, as there is no way to tell which load circuit caused the overload. All this has the potential to cause significant downtime, which in some plants can cost as much as $40,000 to $50,000 per minute.

A solution

It might seem that using an ‘instantaneous trip’ breaker or fuse on each load circuit would prevent the problem, but this type of breaker or fuse is likely to open on inrush surges caused by motors or capacitive loads. A breaker with the tripping curve shown in Figure 2 would trip in as little as 20 ms with an inrush of twice normal current. What is needed is something that will disconnect an overloaded circuit yet keep unaffected circuits running.

Such devices exist. Installed between a power supply and its loads, they provide adjustable overcurrent protection, selective coordination of the load circuits and current limiting. When a heavy overload occurs, they limit current to the affected circuit to prevent the power supply from shutting down, then trip if the overload continues. Some also provide status notification to both humans and control systems.

Two types are available: multichannel units (sometimes called diagnostic modules) that handle a number of loads, and single circuit units (often referred to as electronic circuit breakers). A block diagram of a multichannel unit is shown in Figure 3.


Figure 2: An ‘instantaneous trip’ breaker can trip in as little as 20 ms with an inrush of twice normal current.

Figure 3: Block diagram of the Siemens SITOP Select multichannel diagnostic module.

Inrush currents

As mentioned, some loads (motors or capacitive loads) will have high inrush currents pulling several times their running current for several milliseconds on start-up. The usual way to handle this is to put a delayed-action circuit breaker or fuse on the load in question, but if the starting inrush current is severe enough, it may require oversizing the power supply. The situation becomes worse if more than one load exhibits high inrush current, which could cause an undervoltage on start-up that might cause equipment to malfunction or the power supply to refuse to start. One way around this would also be to install an oversized power supply, but some multichannel diagnostic units offer a sequential connection delay feature to guarantee safe equipment start-up when high inrush loads are used (Figure 4).

Figure 4: Sequential connection delay guarantees safe equipment start-up.

Outputs can be programmed to start at selected intervals, which cuts down on damaging current spikes and also avoids the danger of a severe voltage drop, which could cause equipment to malfunction. And, of course, it allows the correct size power supply for the application to be specified without oversizing.

Siemens Ltd


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