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Application Notes - Machines Tripping or Resetting

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Power quality is a measure of the purity of the AC voltage waveform on your local electricity network, the purity can refer to the amplitude and frequency of the voltage as well as its distortion from the fundamental 50 or 60Hz. If should be noted that you as the customer or consumer of energy can adversely affect or change your local voltage purity. You can affect it for you but you can also affect it for your neighbours, as they can for you! In this discussion we will not look at frequency distortion, that is, flicker or harmonics.

Poor power quality or PQ can cause machines and control panels to trip or reset, this seems especially true when the machine is unattended, making it difficult to attribute the cause(s) to either external influences or to what the machine was doing at the time of the trip, indeed we may not know exactly when the machine tripped.

Machines are more commonly left unattended during night shift and shifts tend to be lighter at night and early morning, when co-incidentally the voltage levels can reach their limits, so that fact that they were unattended is not directly responsible. We therefore need to look at things like the machine loading, its process and of course, quality of the voltage supply, and indeed external factors, temperature, related machinery etcetera. In an industrial environment, there are often many large loads with 3-phase supplies, these machines can cause considerable events and fluctuations on the local electrical network; they can sometimes even affect neighbouring businesses.

Absolute Times
The first thing to establish are the (absolute) times of any trips, is there a pattern to this, is it always between 02.00 and 04.00 in the morning – high voltage a possibility; voltage and frequency are typically high at off-peak periods. If the machine trips at a peak loading times, is low voltage a possibility, voltage and frequency tend to be lower at peak loading times.

Relative Times
Perhaps there are relative times, machine A trips always after machine B is switched on or indeed switched off – is there a related loading effect or are there spikes and dips on the voltage supply related to the second machine turning on/off?

Likewise, consider when it does not trip, machine A never trips when machine C is running, is machine C absorbing the voltage transients that would otherwise affect machine A?

Temperature
Does it always happen about 2 hours after the machine has been running – ambient temperature, machine temperature related, overheating?

‘Random’
Is it purely ‘random’ (of course it will be caused by something, we need to find out what).

Investigation
Irrespective of the above, we need to start to find the cause, this if A causes B, then how is A transmitting the problem to B, what is the mechanism; the first place to look is voltage supply to the problem machine, does this supply vary outside the specified limits for the machine, are there other features of the supply that could cause resetting or shutdown. Power supply units (PSUs) can be susceptible to AC voltage supply problems, they can either temporarily fail, auto-shutdown to protect themselves or they can pass-on disturbances to the electronics they are powering, which in turn lock-up, reset or fail.

Generally, very fast positive or high voltage transients with microsecond durations don’t normally cause problems, as they contain little energy and are often filtered out by suppression components in any good power supply. When the durations of the transients have a duration in the order of milliseconds, they become more troublesome, as they contain more energy (perhaps a thousand times as much) and can pass through some suppression circuits in the PSU and cause the control panel to fault or reset. For transients to start to cause a problem they may have to be a few milliseconds in duration; these will be captured by an Electrocorder voltage or power recorder.

Voltage drops can also be a cause of problems, however generally they need to have a fairly long duration to cause the PSU to drop. If the AC voltage is generally low at the time the drop-out occurs, then the effect can be compounded and cause a problem.

To set up the logging session, first decide how long you want or need to record for, if the resetting or tripping event is daily, then recording for 24 or 48 hours is sufficient, if the events are every few days, then you will need to record for a week or so. With most Electrocorders, setting the averaging periods to 2 minutes, will give 10 days of total recording time. Program (set-up) the recorder and connect to the voltage system, as close to the problem machine or panel as possible, this ensures that the recorder will see the same voltage events.,/p>

When the machine next resets, simply download the Electrocorder and look at the average voltages, if these look acceptable, next look at the maximum voltage and minimum voltages for each of the 3 phases. These are always resolved to 1 cycle (which is 20ms for 50Hz or 16ms for 60Hz), if the voltage rose or fell for a cycle or an appreciable portion of a cycle, then the recorder will see and record it, this will be clearly seen in the Vmin or Vmax graphs.

Generally, unless the equipment is very sensitive, the AC voltage transients that cause problems are often longer than 5 milliseconds and possibly up to a few cycles in duration, this means the voltage fluctuations will be seen and captured by the Electrocorder.

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