# Some question for my electrician exam , need help



## chicken steve (Mar 22, 2011)

Bddab


~cs~


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## Roger (Jul 7, 2007)

What were your answers and how did you arrive at those answers?

Roger


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## ponyboy (Nov 18, 2012)

I'm not just gonna give answers but does anyone else think #5 is a piss poor question?


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## fysnh (Feb 17, 2014)

*1*



Awg-Dawg said:


> What were your answers?


1.A2.A3.A 5.d

Just guess. ,I know nothing about VFD


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## fysnh (Feb 17, 2014)

chicken steve said:


> Bddab
> 
> 
> ~cs~


U sure?


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## FrunkSlammer (Aug 31, 2013)

on and off (stop and stop) is a digital signal, like binary 1 or 0.


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## fysnh (Feb 17, 2014)

Roger said:


> What were your answers and how did you arrive at those answers?
> 
> Roger


I did the exam. ,I failed


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## fysnh (Feb 17, 2014)

ponyboy said:


> I'm not just gonna give answers but does anyone else think #5 is a piss poor question?


If u know it, please help


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## fysnh (Feb 17, 2014)

FrunkSlammer said:


> on and off (stop and stop) is a digital signal, like binary 1 or 0.


VFD is correct?


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## ponyboy (Nov 18, 2012)

fysnh said:


> If u know it, please help


I know them all. The most helpful thing you could do for yourself is read the chapters and see what you missed


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## fysnh (Feb 17, 2014)

ponyboy said:


> I know them all. The most helpful thing you could do for yourself is read the chapters and see what you missed


My code don't have the answer , my code doesn't even mention VFD .


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## fysnh (Feb 17, 2014)

ponyboy said:


> I know them all. The most helpful thing you could do for yourself is read the chapters and see what you missed


Thanks anyway


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## Tsmil (Jul 17, 2011)

Which exam? C of Q?


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## tryme (Feb 6, 2014)

B, b, d, a, b


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## tryme (Feb 6, 2014)

Roger said:


> What were your answers and how did you arrive at those answers?
> 
> Roger


Lots like that on the C of Q. It's a way of thinking. Simplify your mind and become one with the question.


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## fysnh (Feb 17, 2014)

tryme said:


> B, b, d, a, b


Thanks


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## tryme (Feb 6, 2014)

FrunkSlammer said:


> on and off (stop and stop) is a digital signal, like binary 1 or 0.


*facepalm*


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## ablyss (Feb 8, 2014)

For first question I'd guess the occupancy sensor is analog and I'll explain why. Digital is like FrunkSlammer said, 1s and 0s. The occupancy even though runs off 120v AC, the sensor part uses analog detection, ie. Infrared. 

2. Lowering the load is the only reasonable answer.
3. No idea
4. No idea
5. Adjust Relay


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## fysnh (Feb 17, 2014)

ablyss said:


> For first question I'd guess the occupancy sensor is analog and I'll explain why. Digital is like FrunkSlammer said, 1s and 0s. The occupancy even though runs off 120v AC, the sensor part uses analog detection, ie. Infrared.
> 
> 2. Lowering the load is the only reasonable answer.
> 3. No idea
> ...


thanks


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## glen1971 (Oct 10, 2012)

fysnh said:


> 1. Which one of the following device using the analog signal?
> A. VFD. B. start/stop button. C. Occupancy sensor
> *A.. VFD's can send a 4-20 mA signal out to a PLC for speed output.*
> 
> ...


Just my two cents on what I'd put and why..


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## triden (Jun 13, 2012)

ablyss said:


> For first question I'd guess the occupancy sensor is analog and I'll explain why. Digital is like FrunkSlammer said, 1s and 0s. The occupancy even though runs off 120v AC, the sensor part uses analog detection, ie. Infrared.
> 
> 2. Lowering the load is the only reasonable answer.
> 3. No idea
> ...


Yes, but the output of an occupancy sensor is usually digital. There are people detected, or there are no people detected. Like mentioned above, 0 or 1.


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## triden (Jun 13, 2012)

fysnh said:


> 1. Which one of the following device using the analog signal?
> A. VFD. B. start/stop button. C. Occupancy sensor
> 
> Thanks


A VFD varies the frequency of the output waveform. We all know that it is an AC output, so therefore it's analog. Induction motors can not run on a digital (or DC) signal. Answer is A


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## fysnh (Feb 17, 2014)

triden said:


> A VFD varies the frequency of the output waveform. We all know that it is an AC output, so therefore it's analog. Induction motors can not run on a digital (or DC) signal. Answer is A



Same as my answer. ,thanks


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## just the cowboy (Sep 4, 2013)

fysnh said:


> 1. Which one of the following device using the analog signal?
> A. VFD. B. start/stop button. C. Occupancy sensor
> 
> 
> ...


Answers
1 a VFD 
2 d Power factor
3 a Wont start
4 d Burn up start winding
5 b Adjust if it has one

Just a side note : Is there a Split phase DC motor. I never saw one, split phase is AC.


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## fysnh (Feb 17, 2014)

just the cowboy said:


> Answers
> 1 a VFD
> 2 d Power factor
> 3 a Wont start
> ...


thanks very much ,appreciate！


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## JRaef (Mar 23, 2009)

fysnh said:


> 1. Which one of the following device using the analog signal?
> A. VFD. B. start/stop button. C. Occupancy sensor


Trick or ambiguous question, improper use of "using". If it was supposed to be "use*s* *an* analog signal", then maybe A or C, but we don't really know what they are referring to in C, and a VFD does not NEED to use an analog signal, but it MIGHT.




> 2.how to reduce VFD harmonic current?
> A.rectifiler b.lower the load
> C.raise the load
> D. Change the power factor


Trick question, are we discussing harmonic current PERCENTAGE of total demand current? Or just current itself? If it's just current itself, it can only be B. If it's total DEMAND current percentage, then none of the above.




> 3.split phase Dc motor ,when centrifugal switch cannot close ,what will happen?
> A.motor not start ,but hum
> B.motor running at low speed.
> C. Run winding will overheat
> D.the start windings will bum out


Trick question, no such thing as a "split phase DC motor". No such thing as a "phase" in DC actually.




> 4.split phase DC motor ,when centrifugal switch is stuck close by corrosion on the slides of switch, what will this probably cause?
> A.the motor may not start,but hum
> B.the motor will not reach full speed
> C.the run windings will burn out
> D. The start windings will bum out


Trick question, same issue.




> 5.a new overload relay keeps tripping ,what should be done?
> A.replace the relay
> B. adjust the relay
> C.adjust voltage
> D. Replace over load with VFD


None of the above.



> Thanks


I certainly hope this was a joke, otherwise I fear for our collective future if someone is making "electrical exams" like this...


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## JRaef (Mar 23, 2009)

triden said:


> A VFD varies the frequency of the output waveform. We all know that it is an AC output, so therefore it's analog. Induction motors can not run on a digital (or DC) signal. Answer is A


The output of a VFD is not a signal, it's power.

And just by the way, it IS in fact digital, it is a string of DC pulses that are either On or Off, in rapid succession. The motor's RESPONSE to it is _akin _to analog, because of the Inductive Time Constant in that current cannot change instantly in an inductor, it has a specific rate of rise, which the PWM output of the VFD is taking advantage of. But the output of the VFD is still technically digital (+1, 0, +1, 0, +1, 0 etc. etc. etc.) in that expanded sense of the word.


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## fysnh (Feb 17, 2014)

JRaef said:


> The output of a VFD is not a signal, it's power.
> 
> And just by the way, it IS in fact digital, it is a string of DC pulses that are either On or Off, in rapid succession. The motor's RESPONSE to it is _akin _to analog, because of the Inductive Time Constant in that current cannot change instantly in an inductor, it has a specific rate of rise, which the PWM output of the VFD is taking advantage of. But the output of the VFD is still technically digital (+1, 0, +1, 0, +1, 0 etc. etc. etc.) in that expanded sense of the word.


thanks


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## acywink (Mar 21, 2014)

just the cowboy said:


> Answers
> 1 a VFD
> 2 d Power factor
> 3 a Wont start
> ...


This...all of it. :thumbsup: I've never seen a split phase DC motor, so I would assume the question was asking about a split-phase (single-phase) AC motor. Just a few additional notes:

1 A VFD will use analog signals (4-20mA, 0-10V) so that's the only logical answer for that question
2 To change harmonic current on a VFD, I would say you can adjust the PF setting on the drive, however this doesn't specify whether you should raise or lower your PF setting - however, some drives provide THD adjustment by varying the PF setting while some do not. The most logical answer however would however be B as it relates to decreasing load which will ultimately decrease current, and therefore harmonic current; there are better ways to deal with harmonic current however. Also, A and C are definitely incorrect
3 Again assuming the question relates to a split-phase AC motor, the centrifugal switch is used to disconnect the start winding when the motor starts (usually disconnects at 70-80% of the rated speed). If this switch gets stuck open, the start winding will not get power the next time the motor is started. Since split-phase AC motors need a start winding to give the motor direction, an open centrifugal switch will cause the motor to not run
4 If the centrifugal switch gets stuck closed, the winding has the potential to burn up. The start winding is wound a bit different than the run winding and could burn up if left energized for an extended period. Once the motor has direction, the start winding is no longer needed so it can be disconnected
5 If a NEW overload is tripping, adjust the setting. There is an assumption (like many questions on the C of Q) that needs to be made; perhaps the one who installed it didn't adjust it properly. C and D are somewhat ridiculous answers

Which test are you writing, 309? Not every question will be out of the CEC, some will be trade knowledge or troubleshooting.


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