Electrical Power Engineer Test

[adnan012]

Hi,

I have to pass test for job tiltled "Electrical Power Engineer". For test preparation i wan to know which topics i need to follow?

Electric machines , 3 phase systems etc

 

[chuckey]

Learn the safety procedures. Which industry?
Frank

 

[adnan012]

Np specific industry.

This is MCQ's type test includes dsp, controls , circuit theory etc.

 

[treez]

they may well ask you to calculate the peak to peak voltage of a line to line voltage of a three phase system.
they may ask for diode currents in a three phase six pulse rectifier.
They may ask you what is the neutral current in balanced three phase system.
They may ask you “is a permanent magnet synchronous machine the same as a BLDC motor”
They may test if you know an igbt is a bjt with a mosfet gate to drive it.
They may test whather you understand that the difference between a BLDC and a stepper motor is that in a BLDC, the drive pulses should be linked in to the position of the rotor, wheras in a stepper, you don't need to sense the steppers rotor position.
They may trick you into looking at a single phase equipment which draws more than 16A...this is illegal, as its illegal to draw more than 16A from single phase mains.

 

[adnan012]

Thanks for reply.

Can you suggest any tutorials related to single phase and 3-phase systems.

 

[treez]

HERE IS SOME SIMULATION OF THEM IN THE FREE LTSPICE simulator (sorry about caps)..I Post soon

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Company you go for is GE? without knowing what is the company, or at lease there work, it is diffcult to focus for you

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but they may try to trick you, they give you a three phase machine, then they hand you a current probe, then they ask you to measure current in a three phase supply lead.....the newby will wrongly clamp the current probe round all the three leads, and he/she will measure nothing, because all the fields cancel out..but there is really current flowing there

 

[Dan Mills]

They may trick you into looking at a single phase equipment which draws more than 16A...this is illegal, as its illegal to draw more than 16A from single phase mains.

Really? Cite please, what regulation?

I work in the broadcast industry and we routinely have bigger single phase loads then that, hell that would limit you to about 4KW, and we use bigger tungsten halogen light bulbs then that upon occasion!

Given that I can go to any electrical wholesaler and order a 125A single phase plug (Not exactly a common item, but they do get used), I find this unlikely, more then that I can order single pole connectors to at least 700A, for all that smaller three phase lies are easier to deal with when you can.

To the OP: Load angles, rotor angles, subtransient reactance of a rotating machine, design of circuit discrimination, negative phase sequence and triplen harmonics... Lots of stuff that can be asked about.

Regards, Dan.

 

[schmitt trigger]

Also the basic, essential stuff:

I have this 3 phase, 405 volt load which consumes 10 KVA at a 0.7 power factor. I need to increase the power factor to 0.95, what capacitance should I use?
How much will the phase current be reduced as a result?

Perhaps calculate a reduced voltage starter for a given induction motor, and given the maximum inrush current the system may tolerate.

There are some DC motors around, so they may ask you about series, parallel, and compound configurations.

 

[treez]

Really? Cite please, what regulation?

you wont find single phase mains electric car chargers above 16A that run off single phase mains.

 

[Dan Mills]

You will however find plenty of single phase arc welders, plasma cutters and even computer server disk arrays that run to 32A or more, and there are no shortage of electric showers that rate 40A+....

Not everything is a little battery charger, and a while a 16A limit makes sense of marketing reasons in a domestic install (You can run it off a more or less standard supply) that is not the same as claiming a regulatory reason.

I suspect that the safety requirements for at least one of the safety agencies (UL?) do in fact change at that current level, which probably makes for a step change in unit cost if you exceed ~4KW, but again that is not really a legal thing.

Regards, Dan.

 

[Warpspeed]

you wont find single phase mains electric car chargers above 16A that run off single phase mains.

In Australia we have 20Amp single phase domestic power outlets, mainly used for air conditioners.
Single phase home welders and air compressors also commonly use these 20A single phase outlets.

In a factory/industrial setting very large single phase loads are quite common, and approved very heavy 3 pin plugs/wall outlets are made for this purpose.

 

[treez]

ok, but lets interpret this, we are debating at cross purposes.....if the single phase mains is hived off from a three phase supply, ie three lots of single phase mains, then yes, you can have more than 16A in each phase to neutral....but in UK and Europe domestic supplies, that isn't the case, and 16A is strictly the limit.
In Australia, you have a lot of space, there are places where only a few houses have connection to a phase to neutral, and so in Australia, more current is allowed to be drawn.
Also, in industrial environments, where the company may have payed to have their own dedicated phase to neutral wiring just for them, again it can be more than 16A.

Generally speaking , if you are using single phase mains for >16A then you are making a mistake...you should be using three phase and using the line to line say, or all three phases.

Show me a Offline electric car charger that does more than 16A..they don't exist, look at the brusa range...stops at 16A...more than that and they use three phase input

 

[Dan Mills]

Nope a standard UK electric shower is typically 8 to 12kW, I am not seeing anything here https://www.screwfix.com/c/bathrooms-kitchens/electric-showers/cat820330 that would run off a 16A line.

Electric cookers are also traditionally fed with a 32A or bigger single phase line in the UK (45A is not uncommon), so clearly no 16A limit exists except possibly the commercial one of desiring to build a charger that can be run off a spur from an existing outlet rather then needing a new circuit to be installed.

Three phase is nice, but getting it in a domestic building in the UK can be kind of expensive (Been there, done that).

 

[Warpspeed]

In Australia, you have a lot of space, there are places where only a few houses have connection to a phase to neutral, and so in Australia, more current is allowed to be drawn.

That may be true way out in the bush hundreds of miles from anywhere.

But in every suburban street, we all have three phase power available on the pole outside our homes.
I have three phase at home, but still have 20A outlets for air conditioners, and one 20A single phase outlet in the garage for an old stick welder. (My big mig welder is three phase),

The only stipulation is that if it is a 20A outlet, each one has to run back to the power board to its own individual breaker.
But these 20A outlets are very common in Oz, they look similar to 10A outlets but the pins are wider.

 

[FlapJack]

In the USA Wisconsin electric will not connect up more than one service to a home. So if you have single phase you can not have 3 phase. And 3 phase is unheard of in a home and is not readily available.

 

[Warpspeed]

Hi,

I have to pass test for job tiltled "Electrical Power Engineer". For test preparation i wan to know which topics i need to follow?

Electric machines , 3 phase systems etc

Test preparation eh ?

That sounds like you need to know about test and measurement equipment, and hooking it up correctly.
Three phase theory, and you might even be be asked about electrical safety and CPR.

Electric machines.
I suppose that means knowing about many different kinds of ac and dc motors, how they are connected, and various starting and speed control methods.

 

[sahu]

Also the basic, essential stuff:

I have this 3 phase, 405 volt load which consumes 10 KVA at a 0.7 power factor. I need to increase the power factor to 0.95, what capacitance should I use?
How much will the phase current be reduced as a result?

Perhaps calculate a reduced voltage starter for a given induction motor, and given the maximum inrush current the system may tolerate.

There are some DC motors around, so they may ask you about series, parallel, and compound configurations.

Can give here the eqeation between pf to use capacitor to increes power faced