6.8 Power limits and power reduction
The controller demands an output level – what the channel gets can be less. Four limits act on top of one another, and only the first is a setting made by the user.
Maximum output level
A permanent upper limit for the on-time of a channel. It makes sense where the heater is oversized for its zone: while heating up, the controller would work at full power, overshoot the zone and then settle in laboriously. With a limited output level the heat-up becomes calmer.
NOTE
The maximum output level limits the power, not the temperature. The channel still reaches its setpoint – only more slowly. If the limited power is no longer sufficient for that, the channel remains stuck below the warning band, see Section 6.6, “Temperature monitoring”. If you want to shape the heat-up in time and have to hold the temperature precisely, use the heat-up rate instead, see Section 6.3, “Setpoint guidance: ramp and synchronization”.Phase current budget
The channel capacity of a device exceeds its connected load several times over. This is not a planning error but the design: each channel can switch up to 16 A, while the supply is 16, 32 or 63 A depending on the device variant. A device with twelve channels could therefore supply far more current on paper than its supply line provides – and that is sufficient, because no tool loads all zones fully at the same time.
The Thermonom therefore keeps a budget per phase. If the channels on one phase together demand more than the budget allows, it switches them on in a sequence until the budget is used up; the rest follow as soon as current becomes free. This is a permanent condition of operation, not merely a phenomenon of start-up – it is simply most noticeable while heating up, because there all zones demand full power at the same time and cold heaters additionally draw more current than warm ones.
Which channel gets current first
The order is not the channel number but the demand: among the temperature-controlled channels, the one whose actual value lies furthest below its setpoint comes first. The Thermonom walks through the list in this order and switches on every channel whose current still fits into the remaining budget. If one does not fit, it is skipped – a following, smaller channel can still get in. The budget is thus filled up until no further channel fits, and redistributed in the next mains period.
NOTE
This prioritization is the reason why a zone that lies far below its setpoint makes progress even within an exhausted budget – and why a zone that sits just below its setpoint has to wait in that situation. For the user this means: an exhausted budget lengthens the heat-up, but it does not leave any zone permanently behind.Ahead of all temperature-controlled channels are two special cases that need current because they are measuring rather than heating: a running current measurement of a channel and a running controller tuning. Both would deliver a wrong result if the current were taken away from them in the middle of the measurement.
One case is exempt from this, and it is the only one: if a channel alone demands more current than the whole budget provides, it fits into no mains period – not even into one in which no other channel of the phase is heating. It is skipped every time and stays off. That is neither a defect nor an error but a limit of the setting: the channel reports WRN-10, “Phase current”, and the only remedy is to raise the maximum phase current or to reduce the heating circuit.
How much room the budget leaves is decided by the distribution of the heating circuits across the channels, and that is a matter of wiring, not of settings: the channels are permanently attached to the three phases, see Section 5.2, “Channels, phases and connected load”. The nominal currents differ considerably by type of zone – a manifold bar draws many times as much as a nozzle. If several high-current zones are on the same phase, its budget is exhausted while the other two are free.
The budget has two sources: a fixed limit of the assembly and the adjustable phase current limit in the device settings, see Section 9.2.3, “Basic settings”. This setting is useful for two reasons:
- Adaptation to the supply line. If you operate the device on a supply line with a smaller fuse, enter here the value it can carry instead of relying on the circuit breaker – the breaker shuts production down, the budget merely slows it.
- Complying with an agreed supply limit. If a maximum current draw has been agreed contractually with the energy supplier, the device can be limited to it with this value. It then keeps to the limit by itself, by staggering channels instead of exceeding it.
I²t power reduction
The power section of a channel tolerates more current briefly than permanently. Whether it is overloaded therefore does not depend on the instantaneous value but on the current over time – a high current for a short while is uncritical, a moderately raised one over a quarter of an hour is not. For this the Thermonom forms a thermal model: the square of the current, averaged over a time constant. This value is shown by the channel setting “I²t” as a percentage.
Figure 22: Up to the reduction threshold the output level remains untouched. Above it, its upper limit falls; at the limit the channel no longer outputs any power. The thresholds are device-wide values held by customer service – the diagram shows the shape, not the scale.
Two properties are important for the assessment:
- The reduction only takes effect above a reference current. A channel that draws less accumulates no load, no matter how long it heats.
- It is not a fault. The channel reports nothing, it heats more slowly. If you are looking for an inexplicably long heat-up time, you should therefore look at the I²t value: if it is high, the heater is too large for this channel and the controller is not too slow.
Heat sink
The power sections of all channels of one controller board sit on a common heat sink. Its temperature is monitored in three ways, and all three always affect the whole board – never a single channel:
| Monitoring | Effect |
|---|---|
| gradual reduction | Between two temperature thresholds, the upper limit of the output level falls as the heat sink temperature rises – from full power down to 0. This is the actual control of the dissipated heat: the device heats as much as it can dissipate. |
| switch-on inhibit | Above a threshold the board does not take any further channel into operation. The channels already running keep heating. If, while heating up a large tool, you observe that the last channels do not start for a long time, you are seeing this inhibit and not the current budget. |
| shutdown | Above the highest threshold, all channels of the board go off with an error. This is the protection of the power section and not an operating mode. |
All thresholds are board parameters and are available to customer service. The measured heat sink temperature is displayed by the device for each controller board, see Section 9.2.9, “Controller #”.
NOTE
If the power of several channels falls at the same time and for no apparent reason, check the cooling – not the channels. Always affected are the six channels of one board. For permissible ambient conditions see Section 16.2, “Permitted ambient conditions”.How to recognize a reduction
All four limits act on the output level, and all four are visible in the same place: in the channel overview the output level appears as two values as soon as they differ – first the output level the channel received, in brackets the one the controller demanded. With 62 (100) the controller wanted full power and received 62 per cent. If both values are equal, no limit is taking effect.
Which of the four limits it is, the brackets do not say – for that, a look at three things is enough:
- The maximum output level of the channel. If it is below 100 %, the cause has been found.
- The I²t load of the channel. A high value means a reduction because of overload.
- The other channels on the same phase. If several show bracketed values at the same time, the phase budget is exhausted; if all channels of one assembly show them, it is the heat sink.
NOTE
A bracketed value is not a fault. It says that the channel is heating more slowly than it could – which is normal while heating up and, during operation, an indication about the sizing. If the channel nevertheless reaches its set temperature, everything is in order.What the Thermonom does not monitor
The I²t reduction and the heat sink reduction protect the device. Monitoring of the ageing of heaters – a resistance observed over weeks, a trend, an advance warning before failure – does not exist. If you want to track the state of your heaters, there are two other means for that:
- The current limits, see Section 6.7, “Current monitoring”. A partial failure shows up as a reduced current.
- The diagnosis, see Section 10.2, “Diagnosis”. It surveys a zone and can be compared with an earlier measurement.
Related settings:REG-04, “Maximum output” · STR-05, “I²t”