# Questions tagged [electric-machine]

Electrical Machines are Electro-Mechanical energy converters. They maybe classified into motors (electrical -> mechanical) and generators (mechanical -> electrical).

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### How do I fix the speed of a generator?

If I need to generate power at a specified frequency, then I need to make sure that the rotor of the generator rotates at a specified speed (rpm). But when I am rotating it with steam or water how do ...
1k views

### Electrical Machines based on Electric Fields

Our teacher asked us this question during the lecture for Power Electromagnetics. We know that Electric Fields and Magnetic Fields are easily transformable into one another. And they are used greatly ...
1k views

### Isn't current flow a wrong term?

In most text books and other electronics material it's said current is flowing through the circuit. Isn't this wrong? As current is flow of charge. Is there anything such as flow of flow of charge?
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### Why is slip necessary in an Induction motor?

Why should the rotor of a 3-phase induction motor not rotate at synchronous speed?
902 views

### Electricity consumption of electrical machines - why do they consume electricity when they are not turned on?

The question is, why do electrical machines consume power when they are not turned on (they are plugged into an outlet)? Also, suppose there is a seven-port USB hub with AC power. Is the power ...
1k views

### High voltages Transmission lines?

I am having a doubt in why are the transmission lines are sent at a very high voltages? I also went through quite a few sources which said that it is to decrease the losses in the lines. BUt I am not ...
472 views

### Doubt about the volume of the air gap in an electrical device

So for the following device: I'm trying to figure out the torque. To do that, I need the area of the gap. In the solution, it says that the gap volume is: $$2gh(r_1+0.5g)\theta$$ But the part of \...
557 views

### Electric shock from laptop due to capacitive coupling?

If a person touches an exposed wire (the wire has a ground-prong) on the 20V side of a power cable while barefoot, I heard that it is possible to receive a shock of higher than 20V due to capacitive ...
6k views

### What is meant by front and back pitch of an armature winding?

I searched the internet for these definitions and found a common definition in all websites: Back Pitch- A coil advances on the back of the armature. This advancement is measured in terms of ...
3k views

### If a high rpm motor is run at lower rpm, will it produce more torque?

I did the following calculations: Max current = 1.2A, Max voltage = 12V Then power = 14.4 watts = 0.019hp Rpm = HP * 5252 ÷ torque I need a torque to 2kgf-cm = 0.144 lb-ft Gives rpm= 698. Thus this ...
3k views

### How do I modify a DC electric generator to get the same power at a lower speed?

This will be a theoretical question. Suppose that I have a DC generator, and I'm getting 300VA power from it when it rotates at 2000rpm. I want to make it run at a lower speed of about 700rpm, and I ...
297 views

### How was ceiling fan speed controlled before the invention (or availability) of Triac?

I am interested to know about the circuit diagram of the ceiling fan regulators of the old days (when Triacs were not available). I have searched on google but all of the results were related to SCR. ...
5k views

### Dry, wet and free voltage contact?

I would like to ask about three kinds of electrical contacts: dry, wet and free voltage. ...
549 views

### Will a 3 phase induction generator fight its own rotation?

So in a 3 phase induction generator with coils for the stator and magnets for on the rotor, I am wondering if the the current induced in the coils would create a magnetic field that would oppose the ...
348 views

### Programmable power supply

I have been assigned to design a kind of programmable power supply. Can't decide on a way to make it. Need help with this. This is a high level block diagram of the required system: ...
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### Why duplex lap winding has twice as many current paths as simplex lap winding?

I found the statement of question on page 346 in the book "Electric Machinery Fundamentals, fifth edition" written by Stephen J. Chapman. When I searched for how duplex lap winding looks ...
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### Why do we want higher power factor in ac motors?

Here is a simplified 3P electric motor below: This motor is simply combined of three coils facing toward to the rotator (in the center) 120 degree apart to each other. The user wants his rotator to ...
364 views

### How can the armature current of a DC shunt motor be zero at no-load?

I have seen the mathematical explanation of this explained using back EMF, where the back EMF would equal the input voltage and hence the armature current would be zero. But I'm unable to understand ...
349 views

### Why are the poles of the stator and rotor taken differently in the picture below?

The rotor and stator in the picture below have the same orientation of flux lines yet the are taken as opposite that is flux lines leaving the stator is taken as a South Pole while flux lines leaving ...
402 views

### How does this old fan regulator circuit work?

I was on the lookout for an old fan regulator. I had heard that its power consumption was constant at all speeds. I could lay my hands on this 1970 regulator. It appears to be a transformer with ...
3k views

### Could a significant amount of electricity be generated from public hand-operated generators?

So imagine you're at a bus stop at a train station. You're bored and have got nothing to do. So you see a big crank-handle sitting there doing nothing, and because you're bored, you decide to spin it ...
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### Behaviour of flux in DC motor

In given figure if we assume both sides (up and down) with equal number of coil turns (N each). And we wound(which generally we do in DC motor) coils such that they create flux and form a North-South ...
3k views

### Which parts of this rotor can be made out of plastic?

I'm asked to redesign a rotor, by making as much parts of it as possible out of plastic. The issue is its weight, we want to make it as light as possible. Especially the shaft & axis of it is the ...
128 views

### Torque production in PMSM

First and foremost I have a question regarding the MMF distribution in a Motor produced by a single coil. I found a picture here: Why does the MMF not start at 0 at 0°? When I apply the NI=\int{H \...
5k views

### Why are there no “even” harmonics in AC machines?

Why are there no "even" harmonics in AC machines? In Chapman's book, he says it's because of symmetry of the flux density wave but I don't get it...
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### Unexpected behavior of the Luenberger observer for three phase induction motor

I have been developing control software for three phase induction motor. The control software will implement the field oriented control algorithm. The considered algorithm is oriented to the rotor ...
5k views

### How to detect surges in the main AC

Electronics have a habit of breaking in my house. In the last 2 years or so, I had 3 desktops, 1 monitor, 4 cellphones going dead. They are not completely dead like they won't turn on. But in the case ...
381 views

### Voltage and current rating of electrical systems

Why voltage and current ratings of almost all electrical systems are RMS value of voltage and current not peak value of voltage and current? First I thought maybe because mostly ac input is sinusoidal ...
384 views

### Starting torque of series vs shunt DC motors

Recently, I read that starting torque of series motors are much higher than shunt motors. I try to find out what is reason behind it and I get to know that this is because in DC series motor torque is ...
905 views

### Why is DC motor speed and torque direction the same, whereas in a generator it is the opposite?

What’s the relation between speed and torque in a DC machine. I know it’s inversely proportional but how? Torque is nothing but twisting moment right? If torque increases then speed must be increased,...
383 views

### How do designers know about kilowatt power of wind turbine generators like 1.5 MW?

Generators and transformers are always designed in kVA because designers never know about the load it will provide power to. Then how do we know the wind turbine generator will deliver 1500 kW or 2 MW ...
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### Is more fuel consumed immediately when spinning reserve is brought on-line?

Spinning reserve is additional capacity (reserve) that generators which are on-line (spinning) are able to apply to the grid quickly in response to a failure elsewhere on the grid, or a spike in ...
14k views

### Will a gas generator survive an EMP?

Not really sure where to ask this question. I was watching an episode of a TV show involving a gas generator and it got me wondering whether electricity could still be generated by a gas generator ...
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### How does a large electric switch different from small?

Why do some electric appliances require to have a large electric switch to turn them on or off? If I use a small switch where a large switch is required, would there be potential possibility of ...
163 views

### Generator Frequency

How do I calculate the kW output on a generator when changing from 60Hz to 50Hz? i.e. if I have a 70kVA generator at 60Hz producing 56kW, how do I calculate what the kVA and kW's would be if I ...
480 views

### What is torque-ampere ratio at electric machines? How it is related with machine efficiency?

When I am reading a journal about SRM (switched reluctance machines) it said that outer rotor design has better efficiency due to higher torque ampere ratio. How it is related with efficiency?
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### Why starting current in star-delta starter becomes 1/√3 times that of DOL starter?

I have seen this statement in a question paper. I was asked to prove it but I can not find the logic.
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### What is actually meant by “parallel paths” in a simplex progressive lap winding of a DC Generator?

Many of my textbooks mention "parallel path" term, but I am unable to actually understand it. Also along with this it would be quite helpful to know, how the polarity of brushes is determined?
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### Which is the physical insight behind the widely approved 'space vector' notation used in the analysis of the dynamic model of electric machines?

When approaching the study of electric machines in transient condition, it is customary to discard the real physical vector quantities like magnetic field and magnetic induction and to define fake ...
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### Is it possible to connect a UPS in parallel with the mains?

Short version: When a UPS is plugged in to the mains, is it possible to plug the UPS output back to the same mains? Will that work as an emergency power backup for the mains circuit? Longer version: ...
7k views

### How does the speed of a synchronous generator changes with load?

When the generator is rotating at rated speed and let us say that the load decreases then the mechanical torque will be more than​ the electrical torque which will lead to increase in the speed of ...
134 views

### Transformer Ratio

what if we energize 50 Hz h.v side of the transformer with 40Hz, is the transformation ratio don't matter? Suppose 400/200, single phase transformer which is designed to operate at 50Hz. If you ...
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### Operation of a Growler to test armature shorts

Can someone explain to me how a Growler for testing electric motor armatures works please? I have obviously read the Wikipedia Growler page but it is still foggy. So the growler unit acts as the ...
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### What are power cube transformers?

If I'm not wrong, is the term "Power cube transformers" is less commonly used to refer "A.C/D.C Adapter"? I couldn't find the use to this term.
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### DC motor back emf and supply voltage

In this question the solution manual takes Eb as 250v and solve it to get the torque. How's that possible ? How can one takes the supply voltage as Eb? If there's something i don't or i didn't get ...
141 views

### Is it necessary to protect old devices against “new” 230V?

So, i was recently listening to old automatic music playing machines. These are devices from the year 1910 +/-. They are most likely purposed for 220V European power net. To me, most of the machines ...
710 views

### What kind of sensor is this?

I've got a m1000 automated tape dispenser and I've just recently found this sensor at the cutting blades. As far as I know the wire from the cutting blades to the main board was only for ...
302 views

### Transformer Designing

Why don't we keep low voltage winding of transformer at one loop and adjust the rest of the circuit accordingly? Like in clamp meters. Also is there any software where I can model unorthodox shapes ...