The Case of the Mysterious Circuit Breaker
SBE certification is the emblem of professionalism in broadcast engineering. To help you get in the certification exam taking frame of mind Radio World Engineering Extra poses a typical question in every issue. Although similar in style and content to the exam questions, these are not from past exams nor will they be on future exams in this exact form.
The SBE Certification program exams test you on both scholarly knowledge (information gleaned from schooling, text or article sources — essentially secondhand information) and practical personal experience that you are expected to develop as you grow in the industry (firsthand information). The above question falls into both of these categories and is drawn from my own experience.
The answer to the question in the box is c.
First, let's eliminate the other four answer choices listed.
Regarding answer a, recent solid-state transmitters with their Class D and E amplification schemes make them notably more efficient than older traditional plate-modulated tube rigs. Tube rigs additionally have a need to power filaments and larger blowers, which when added to IR loses in a plethora of transformer cores run up the power requirements. Solid-state transmitters generally are more efficient than tube types.
Considering b, if you had reversed a phase and neutral wire in a 240-volt single phase or multiphase transmitter, the breaker would trip immediately. If any sort of connection exists between the transmitter's power phase connection points and ground, it will short the AC power. If all of the neutral connections are ground-isolated inside the transmitter, you would notice as soon as power was applied, as the control logic and probably the LED or screen displays were in trouble. It's not likely that you would be able to even move the rig into transmit mode.
The surge suppressors noted in choice d are actually voltage suppressors. The ubiquitous ordinary circuit breakers that most of us encounter do not trip on a voltage surge as they are designed to sense over-current only, so the lack of surge suppression is not the answer. Surge suppressers are invaluable to protect your station's investment in equipment but are not part of the over-current protection picture.
Selection e can be eliminated as all professional broadcast transmitters are designed to start up with a program signal input at normal modulation.
So what is happening here? A tube transmitter soft-starts by turning on necessary low-level stages, such as the blowers, filaments, supply voltage to oscillator and drivers, etc. After warm-up, another near equal surge of power is called for to power the final RF and modulation sections. Almost without exception, the high-voltage plate and screen supplies are transformer type using reactor (inductor) filter inputs that limit the current surges. The original circuit breaker could handle these incremental increases easily.
Conversely, a solid-state transmitter requires little control and/or standby current and most of the power is demanded when the transmitter begins to put out its required power output. In this particular instance, the transmitter was populated with switching power supplies that on cold start need a lot of current to "charge up" the input filter capacitors and the switching inductor. This big surge tripped the circuit breaker.
Why was it possible quickly to reset the circuit breaker and keep it running? It takes a finite amount of time for these power supply circuit elements to discharge. With quick action, we were able to reset the breaker, and with the lower "run" current demand were able to sustain operation.
You can see we have a different current demand pattern in these solid-state rigs. The tube rig circuit breaker was a standard model and its "fuse curve" (the graph characteristic of how it responds to current overloads) was flat. When the current reached the rated trip point, it tripped.
As a side note, in this particular instance, we changed the circuit breaker to an HACRS type. This acronym stands for Heating and Air Conditioning Rated Service. This type of circuit breaker is designed to ignore the first surge of current needed by HVAC systems to start motors under load and power up cold resistive heating elements. This new circuit breaker ignored the short current demand precipitated by those reactive components in the solid-state rig, after which it provided appropriate current protection for this new transmitter and the conductors that supplied power to it.
Reactive power loads (equipment that has capacitors and inductors as part of the load) are not unique to transmitters, and the nature of these components is that they have currents charging and discharging from them while they are in normal use. If you remember your AC theory, keep in mind that reactive currents are not consumed, only stored. Once discharged, they are returned to the generator.
So we have two values of power to be concerned with: the real power consumed in the functioning of the equipment, and the apparent power, which is the combination of the real power and those non-consumed charging currents. The real power is listed as VA (volt amps) and the reactive as VAR (volt amps reactive).
Power factor is the real power divided by the real power plus reactive charging currents. Most often we see this expressed as a percentage, such that a 97 kW real load divided by a 100 kW real plus reactive demand would have a power factor of 97/100 or 97 percent.
As a final thought, even though the reactive power is not consumed and is returned to the generator, this power must first be generated! So the power company wants to be paid for the extra generation needed for high power factor consumers and the larger lines to deliver that power to the customer. They collect this extra cost from the consumer by having utility meters measure the actual power consumed and the peak demand. Actual consumption is billed as one rate and demand at a penalty rate usually at some dollar figure per kW of peak demand. Part of that demand is the reactive power. American industry has made a major effort to lower demand including mitigation of reactive power to reduce the demand factor.
Don't forget, the deadline for signing up for the next cycle of SBE certification exams is April 1 for testing that will be given on April 21 at the NAB convention in Las Vegas.
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A CBRE question for April: Your standard broadcast station is authorized 25 kW non-directional day and 1 kW with a highly directional signal at night. At nighttime pattern changing time, the remote control cannot effect the change or reduce power. How long do you have to correct this problem before you must cease radiating?
a. 3 days
b. 3 hours
c. 3 minutes
d. The length of time it takes to drive to the transmitter
e. As long as it takes, provided you make notes of the corrective actions you've taken in the station log.
Buc Fitch, P.E., CPBE, AMD, is a frequent contributor to Radio World.