Category Archives: Measurement

Open vs Closed Loop Temperature Control

We get many calls from customers that want to control the temperature of a process without a closed loop temperature controller.  In this mini article we will briefly discuss open loop temperature control vs closed loop temperature control.  First, lets simply define closed loop and open loop as it applies to common electrically heated processes:

Close loop temperature control utilizes a temperature sensor that monitors the process.   This feedback is fed to a temperature controller which controls a Power Controller that will apply power to the heater precisely and maintain the process temperature.  Some of the advantages of closed loop control are accuracy, repeatability, fast warmup times and fast recovery times.

Open loop temperature control does not utilize temperature feedback from the process.  The power delivered to the heater is set by the user, usually with a potentiometer and SCR Power Controller or a Variac (variable auto transformer).  Some of the advantages of open loop control are simplicity, low cost, reliability.

closed-vs-open-loop-temeprature-response

One of the major advantages of NuWave SCR Power Controllers and SSR Driver Boards is line voltage compensation.  Line voltage compensation corrects for changes in line voltage.  Performance improvements can be observed in closed loop processes, but especially in open loop processes.

SSRMAN-1P-line-voltage-compensation

If you have a process where the temperature accuracy is not critical, open loop control may be an option.  The only way to really tell if open loop control is suitable is to try it in your process and see if the results are acceptable.

Solid State Relay Monitor Heater Break Alarm – NEW

The new SSRMON2 mounts to a Solid State Relay and provides load break protection as well as monitoring the SSR for failure. The SSRMON2 can be wired for interrupted mode or continuous mode.  Interrupted mode forces the SSR off periodically to test the input to output logic. The SSRMON2 operates on 12 or 24VDC and can be used on line voltages from 100-600VAC .

SSRMON2
Solid State Relay Monitor Heater Break Alarm

Measuring the True RMS Current in Phase Angle Controlled Loads

Most current transformers suffer from severely inaccurate measurements of the load current in phase angle controlled loads. This is because they tend to respond mainly to the peaks of the signal which have little to do with the actual current present in the load circuit.  A true RMS analysis of the current waveform is required to get an accurate value.

Our RMS Current transformer – NWCT is a True RMS current transducer that can be ordered as 0-10V or 4-20mA output. The output is proportional to the current in the conductor passed through the sense transformer. The advantage to the NWCT is that it can sense distorted sinusoidal signals such as phase control and provide a True RMS output of the signal. The NWCT Current Transformer uses the latest in signal processing technologies to provide true RMS 4-20mA or 0-10V output of distorted AC current signals such as phase angle controlled loads.

True RMS Current Transformer NWCT

 

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