Electrical Engineering

Most motor speed controllers have all the power that goes to the motor go through a SCR (usually a TRIAC).

If you remove the SCR from such a system, no power will get to the motor and nothing will move -- the SCR is a necessary, critical part.

However, many motor speed controllers -- in particular, most variable-frequency drives (VFD), also called inverter drives, typically use some other component to transfer power to the motor -- usually MOSFETs in low-voltage systems and IGBTs in higher-voltage systems.

They don't use any SCRs, and so SCRs are not absolutely necessary for motor speed control.

The current or voltage have a frequency in an AC system. It is 50 or 60 hertz per second. It is also known as 50 or 60 cycles per second. Each cycle is set of positive and negative amplitude. Refer Sinusoidal wave for further details.

It took me a while to find out what a star connection is. In the US we call it a Y connection. Each resistor has to dissipate 1/3 of the power, or 5000 watts. then it's just a case of using P=E2/R. or R =E2/P R = 208 x 208 / 5000 = 8.66 ohms

A vacuum breaker is a mechnical device used to avoid backflow or a negative pressure in a plumbing system.

It typically consists of an air valve with a spring holding it closed. When the pressure within the system is positive, the valve remains closed. When the pressure is negative (less than atmospheric), the negative pressure overcomes the force of the spring holding the valve closed and the it opens, typically allowing normal atmospheric air to enter the system, thereby eliminating the negative pressure. Otherwise, the negative pressure might cause backflow of the plumbing system.

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because gravity control is only used in vertical position only......it has less tension

A phase out policy refers to a policy that stopped a given old policy.

yes

Static Var Compensators are the most important FACTS devices, SVc's have high accuracy, availability and fast response and therefore give better steady state and transient voltage control compared to classical shunt compensation. SVC's require large inductive and capacitive components and occupy a larger space than STATCOMs. STATCOMs give higher reactive oututs at low voltages compared to SVCs.

If it is AC then you don't need a formula, you just need a transformer.

90 degrees

The rated powers, printed on each lamp, will no longer apply. You will need to measure the load current (I) drawn by the two lamps, together with the supply voltage (U), and multiply the two values together: Power = U I

Found one DC motor having small sparking,but the commutator is light and dark bar pattern.commutator is round. We 500kw dC motor operted at 500rpm with 480Amp's .The DC motor full load current is 1050Amp's.

Why the commutator had small spaerking.

Monolithic circuit

A mini stair stepper is a workout machine that simulates climbing stairs. It contains two platforms, one for each foot, and they alternate in going up and down when a person is pushing them down.

We use the 250 ohms with the power supply because the internal resistance of a DC power supply is insufficient to develop a resistance.

You need an ohmmeter or multimeter (set in resistance measurement mode). Choose the 1kOhm range. Connect the meter's wires (red & black) each to the diode's terminals, then switch the terminals. In one case you should see a meter reading that is substantially lower than the other. If that is not the case (both read high or both read low), the diode is probably faulty.

0.00 Ohms measured between parts that normally carry current.

In electrochemistry, overvoltage is synonymous with overpotential. Look up overpotential instead of overvoltage to get details of what it is.

Basically, it is a potential that develop because of limited reaction kinetic. It works to reduce the voltage of the electrochemical cell. Think of it like this. For a given redox reaction pair that represents a chemical cell, it has a theoretical voltage determined by thermodynamics. This is typically called the open cell voltage. It's the maximum voltage that you could harness out of this reaction if you were to make this into a battery. That is all very good and all except for the little detail of reaction kinetics. In reality, you don't ever get the maximum voltage at open cell. Once current start flowing, limitations on rate of reaction at the cathode and anode develop. Reactions are not infinitely fast. Once you start the reaction, the region near the electrodes are depleted of ion species as they are oxidized or reduced into and out of the electroloyte. It takes time for more to diffuse from the bulk to the solution to the electrode/electrolyte interface. Now your reaction rate is limited. This limitation will show up as a potential, the overpotential, at the electrode. It acts to reduce the potential available to you across the entire cell. If your open cell potential of a batttery was say 2 volts, you may find a severely rate limited reaction to generate 1.9 volts of overpotential at the electrodes as you try to draw current out of the battery. When that happens, your cell actually only gives you 0.1 volts, not 2 volts. What I described above is called concentration overpotential. There are other sources of overpotential. It is much beyond the scope of Y:A to describe it. However, the phenomenon of overpotential in an electrochemical cell is a consequence of reaction kinetics. There are other concepts like equilibrium exchange current density and the use of the Tafel plot when you delve into the topic of electrochemical cells. These are things that must be considered in addition to the thermodynamic open cell potential when a cell starts to draw a current. The best batteries will be constructed out of a system in a way as to have high exchange current density and minimum overpotential. So as you draw current from the cell, you don't find yourself with vastly diminished voltage. It's ability to do work will not be self-limiting as you put it in a device that draws current.

sANta ClaRA! :))

not possible. power factor is an cosine angle between current and voltage...

It's self-started. and it has a normal speed less than that of

synchronous. but its speed decreases as the load

increases.

An induction motor has 2 sets of coils in it that are physically displaced from each other. (One is the 'start' winding, connected only during starting, and a 'run' winding, connected whenever the motor is turned on.) The sets of coils have different electrical charactreristics, and when current flows in the coils, they each create their own magnetic fields that are displaced in time by the different values of coil resistance and inductance.

The physical and timing displacements produce a combined magnetic field that rotates through the iron core and copper or aluminum bars of the rotor, inducing a current in the bars ('induction' motor....) creating a magnetic field around the bars that repels the original rotating field. This magnetic repulsion pushes the rotor around, driving the motor (and the load).

The 'start winding' has a high resistance and gets VERY hot if left connected once the motor has started. It is usually turned off (by a centrifugal switch) once the motor is running, and the 'pulsating' field from the 'run' winding keeps the rotor turning.

Some motors use a capacitor (or 'condenser') to enhance the displacement between the magnetic fields (and increase the force of repulsion) to improve starting torque, and a smaller capacitor to improve running torque and smoothness of operation.

Small, low torque motors may not have a 'start' winding, but rather a single turn, short-circuited coil ('shading ring') on part of each of its poles to create a weakly rotating magnetic field - but without a switch or capacitor.

A transfer function (also known as the system function[1] or network function and, when plotted as a graph, transfer curve) is a mathematical representation, in terms of spatial or temporal frequency, of the relation between the input and output of a linear time-invariant system with zero initial conditions and zero-point equilibrium. With optical imaging devices, for example, it is the Fourier transform of the point spread function (hence a function of spatial frequency) i.e. the intensity distribution caused by a point object in the field of view. An alternative brief definition is "a mathematical function relating the output or response of a system such as a filter circuit to the input or stimulus"[2].

Nomvula Tshabalala

It is used in variety of applications such as...

· Machine Tools such as a ball mill

· Motor generator sets

· Synchronous clocks

· Timing devices

· Synchronous condensers to condition electrical power

· Record players

· Robotics