The most common and economical arrangement of SCR power control is an
SCR unit inserted between the AC power line and a constant resistance
load having a voltage rating that matches an available standard power
source. Sources with more than toaster capacity are usually in the 400
to 600 V range. Heaters that suit the process -- and are preferably
rugged -- often demand lower voltages.
current-limiting circuits are not fast enough for some sudden short
circuits. The transient overcurrent will sometimes blow your fast I2t
fuse. If the fault clears with no damage to the SCR, the process can
continue. If the process suffers cool down because you had to replace
fuses, you have recourse to a method called chop-off.
Equipment can be damaged by voltage surges and spikes. Sources vary.
Power factor capacitor switching, utility switching and lightning are
some. They are unpredictable and can come and go while you blink. It
takes special instruments that record the time, duration and peak
values to catch such transients.
by ArthurHolland | January 15, 2003 | Comments (0)
It is a given that your cables are sized properly for both steady-state
currents and predictable short-term surges. You have a few choices for
protection from short circuits and overloads.
by ArthurHolland | November 1, 2002 | Comments (0)
Control signals are put out by controllers and expect to be obeyed by
such final control devices as motorized valves, electropneumatic
valves, motor drives and SCR heater controllers. How well they are
obeyed is often pretty rough and nonlinear. Checking at this stage --
while not called calibration -- is important to control performance.
You want your processing to be accurate, consistent and reproducible
over the long term. You want to be ready when ISO compliance or your
customer's quality control calls for proof of the integrity of your
operation. So, make sure all your process-critical signals are checked
and recorded once or twice per year.
by ArthurHolland | September 1, 2002 | Comments (0)
The wider the choice is, the greater the risk. Mismatching
controllers and thermocouples during installation and maintenance has
long been a threat, not just to product yield and quality but also to
people and plant.
Of all the temperature sensors used in industry and research, the
thermocouple is the sensor of choice on grounds of temperature range,
speed of response, ruggedness and cost. Though there are other
important sensors, I will limit this article to the eight most commonly
used thermocouples known as Types J, K, T, E, N, S, R and B.
When you are buying electrical energy (kilowatt-hours), why should
your supplier care about your power factor (PF) and why should he give
you a better deal for having a high power factor? The answer: At
power factors less than one, your supplier's generators, transmission
cables and transformers hit their kVA output limit before his
generators have reached their full rated kW output.
In previous articles, I have dealt with the relatively complex subject
of feedback in temperature control loops that have troublesome lags.
Here, I will look at specific and simpler cases of feedback usage in
components of the process.
