# Electrical questions

• posted
Hello.
I wanted ask a few questions
1. What will be the resultant freq when 2 sources with same Vrms but
frequency of 50Hz and 60Hz are used ?
2. In a R-L ckt a ac voltage is applied , such that instantaneous
power is negative for 2ms, then what will be the power factor ?
3. A sync. Motor is running at sync speed, if all of sudden D.C.
excitation is removed, then what will be the speed of the motor ?
4. A R-C ckt , supplied with DC, a bulb is connected across the
Capacitor, then what happens to the illumination, if we change the
capacitance.
5. What will happen to a DC motor supplied with AC and Induction Motor
with DC ?
Thx,
• posted
These would make quite good homework questions.
With number 4 at least, you could go and buy the bits from an electronics store and experiment, even if you couldn't work it out yourself in theory.
• posted
You don't say how they 'are used'. Insufficient information. (supplies tied in series or parallel for example)
If instantanteous power is negative for 2ms, then obviously there is some reactive power flowing (as any R-L ckt would have). But you don't specify any more about the circuit (what fraction of a cycle is the 2ms for example). Insufficient information.
Different than before excitation is removed, or perhaps the same if circumstances are 'just right'. Insufficient information.
Probably change. A simple experiment would tell you the answer. If you just think about how the voltage on the capacitor changes, that should give you a clue. Of course a light-bulb is a non-linear resistance, so if you *really* want to complicate things, you could take that into account.
For AC applied to a DC motor, you need to know what kind of DC motor. Shunt? Series? Compound? If the motor was designed to run on DC, then at best performance on AC will be lousy, at worst you will need a fire extinquisher.
Applying DC to an induction motor can be useful in certain applications. But you must be sure to apply the right amount of DC voltage, or else you may need that fire extinquisher again.
daestrom P.S. Do your own homework. You'll learn a lot more that way. And afterall, isn't that the point?
• posted
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--------------------- If the questions can't be framed fully, then who is at fault- the student who wants an easy out or the instructor who may not have thought the questions through properly?
• posted
@ Don : the questions are framed in such a way to deliberately make a dilemma in the mind of the student.
@ daestrom : I have done my homework and was thinking of writing my own answers before the replies, but thought other ways. Also please answer them considering all the possiblites ex. 1st que- consider the series as well as parallel source, and thier effect.
1. It will be indeterminate 2. This much info was available for the question. 3. For this one, it is assumed that the synchronous motor was supposed to be operating at its sync speed for a good time and then after that, the excitaion was suddenly removed. Now what will be the speed just after that and in what manner it will decrease......
and so on...
P.S : I post these questions because there is some ambiguity in my mind even after having a discussions with my peers. So please keep the " do you homework" statement to yourself. I am just asking your views, opinions. Not asking for any old-man advice.
• posted
This sure smacks of homework from someone without a clue.
Bill
• posted
Sure you were. If you can frame an intelligent question there are people here who can help you a great deal. Here is an example:
I just completed a stage play using a power "distro" (sub panel). The available power was 220 V single phase on a 50 A breaker with a 3 pin appliance outlet that I was told by the house electrician had a 30 A rating. The distro uses four 20 A breakers (2 per leg 120 vac) . the distro uses 30 ft of #6 AWG. The load was one behringer eurolight LD6230 driving 5 500W par 64', one 625 W par 64, ten 250 W par 38, two 300W par 56, 4 color bank lights (4 X 60W light chase per unit) , 2 power audio amplifiers rated at 2,500 W rms for the loads employed, and one 700 W fog machine.
• posted
First question that comes to mind is why was a 30A receptacle protected by a 50A breaker? Are there other loads on that circuit?
• posted
NO: 80 to 85 amps per leg for all loads
Even the LD6230 potentially draws too much for its branch breakers. (Assumed 26 A loads/leg)
NO: The amplifiers alone consume 25 amps per leg, 50 amps for two. Add the lights and it is worse. Given no data to suggest a reduced duty cycle for any of the lighting or audio loads, your 30 amp plug is toast.
With this much "smoke" you can drop the 700 watt fog machine off!
This looks too easy. Am I am wrong?
• posted
NO: 80 to 85 amps per leg for all loads
Even the LD6230 potentially draws too much for its branch breakers. (Assumed 26 A loads/leg)
NO: The amplifiers alone consume 25 amps per leg, 50 amps for two. Add the lights and it is worse. Given no data to suggest a reduced duty cycle for any of the lighting or audio loads, your 30 amp plug is toast.
With this much "smoke" you can drop the 700 watt fog machine off!
This looks too easy. Am I am wrong?
• posted
No other loads. This was a high school stage. I don't know if the electrician was correct or not about the rating. the plug that I bought has an extra blade which allows it to be a 30 or 50 A plug. I believe that electrically it was the same as a 50 amp outlet. it just didn't have the extra bend on the neutral pin.
• posted
It sort of points out that even when the information seems complete there are always factors that were omitted.
First, this was a stage play not a rock concert. even though large amps were employed, that rack was what happened to be already loaded up when the set up schedule was pushed up. While I can say that two 2400 watt amplifiers were plugged in, actual current used by these were minimal. In addition here was a 120 W amp used to provide sound to stage manager and backstage/dressing room. A 300W powered subwoofer was also plugged in but not emploied.
I dont know what the max current was, but the show worked fine for several days without tripping a breaker or anything even getting warm (other then the heat exchanger in the fog machine and the lamps themselves)
Note: even for rock, country, or heavy metal I find that one 20A circuit can provide plenty of sound in indoor venues regardless of the number of amplifiers that are loading it... im talking bars/clubs/theaters here, not giant concert arrays.
So lets crunch some numbers I had a 50A @ 240 vac circuit for 12,000 VA. Lets assume a totally resistive load. I measured the no load voltage at 120 vac per leg with a true RMS DVM Lets assume a "safe" load of 80% of the breaker rating or 9,600 VA. Also assume an evenly distributed load.
Now the light load: 5 @ 500 = 2500 1 @ 625 10 @ 250 = 2500 2 @ 300 = 600 4 @ 60 (only one lamp at a time on the 4 lamp chases) = 240 (only used in one scene) 1 @ 700 / intermittant duty fogger
I make it 7,165 watts, worst case plus sound.
• posted
Reminds me of a time a friend who is a professional photographer got a job shooting models on a cruise ship. He needed a 220 VAC to 120 VAC transformer to power his array of Bogen strobes from the 220 VAC power on the ship. We sized up a transformer for the nameplate power rating of the 6 or so Bogen power supplies, and he got a surplus transformer, albeit the one he got was a bit over sized. Although the current drain of the strobes was minimal until charging the capacitors, the total drain turned out to be way too much for the branch circuit on the cruise ship. Everytime the Bogen's had to recharge, they would of course all six do so in unison pulling the maximum amps.
• posted
This raises the question: did he normally run all 6 units from the same outlet?
• posted
That is a good question, maybe not. Recently I was part of a large group (approx 100)that was having our picture taken in a large hall with similar strobe set up. The photographer had the group pose for about 8 bracketed shots, on the ninth one the breaker went out and all the equipment failed.
• posted
Large arrays of amplifiers often have a sequence start feature so that a brief power interruption doesn't take out a main breaker when all the power supplies start charging at once. seems like these flash bulb guys could rig up something similar.
• posted
You'd help your credibility if you posted some of your thoughts along with each question. Then we might be able to see some of your thought process and point out variations.
But you chose not to post any significant thought, just the questions.
So here, let's start with question #3 for an example (I like machinery), "A sync. Motor is running at sync speed, if all of sudden D.C. excitation is removed, then what will be the speed of the motor ?"
Some intelligent thinking about this might be along the lines of, "On the one hand, the amount of torque that such a motor can develop is related by formula XXX. Without the DC field, then this term here would tend to zero and the developed torque would be zero. Since windage, friction, and any load are non-zero, the rotor would slow down."
But then we could add issues such as, "Residual magnetism probably exists in the rotor poles, so the torque developed wouldn't be exactly zero, but some small value YYY. Or, if the rotor is of the salient pole design, then the variations in reluctance may create some torque as well. Would these factors create enough torque to keep the rotor spinning at sync speed??"
Does the motor have Amortissier windings? This would affect just how far it 'slows down'. These windings are not typically designed for producing a torque efficiently or for a long period of time. Would they overheat? How much slip would they need to continue driving rotor and load?
See, if you had posted questions about some of the issues you supposedly have thought about, then we could see what direction you were thinking. But you didn't, you posted some very open-ended questions.
And now, above you ask, "... please answer them considering all the possibilities." Like someone should take the time to right a six page detailed review of the loss of field excitation to synchronous machinery, just because you're too lazy to frame a question in any form of specific manner. And then do another six or so pages for each of your other questions?
Sorry, I got tired of playing the game "Bring me a rock..." with people years ago. When you can figure out what color, size and composition you want in your 'rock', then at least there will be a starting point for meaningful discussions.
daestrom
• posted
|> >> Reminds me of a time a friend who is a professional photographer got a |> >> job shooting models on a cruise ship. He needed a 220 VAC to 120 VAC |> >> transformer to power his array of Bogen strobes from the 220 VAC power |> >> on the ship. We sized up a transformer for the nameplate power rating | of |> >> the 6 or so Bogen power supplies, and he got a surplus transformer, |> >> albeit the one he got was a bit over sized. Although the current drain |> >> of the strobes was minimal until charging the capacitors, the total |> >> drain turned out to be way too much for the branch circuit on the | cruise |> >> ship. Everytime the Bogen's had to recharge, they would of course all |> >> six do so in unison pulling the maximum amps. |> >> |> > |> > This raises the question: did he normally run all 6 units from the same |> > outlet? |> > |> > |> |> That is a good question, maybe not. Recently I was part of a large group |> (approx 100)that was having our picture taken in a large hall with |> similar strobe set up. The photographer had the group pose for about 8 |> bracketed shots, on the ninth one the breaker went out and all the |> equipment failed. |> | | Large arrays of amplifiers often have a sequence start feature so that a | brief power interruption doesn't take out a main breaker when all the power | supplies start charging at once. seems like these flash bulb guys could rig | up something similar.
Unfortunately, film does not behave the same when the total amount of light comes in all at once vs. one at a time. At the middle of the range it can behave reasonably consistent. But outside of this range, it does not work as well. So it's not a simple matter of just firing each flash at different times. Some exposure correction would also need to be done.