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# faq

#### Why current flows to ground/earth without closed path?

##### Saturday, July 4, 2015

I have thought long and hard about grounding. This seems to be a topic which mystifies even many electrical engineers.

Let's pretend we have a 1,000 Volt DC source. Say the negative terminal is not connected to anything but we connect the positive terminal to a copper rod and bury it in the earth. All of a sudden, when we close the switch between the battery and the copper rod, charge will be allowed to diffuse from the positive terminal into the earth. The charge will continue to ...

#### Why the power supply frequency is 50Hz or 60Hz?

##### Sunday, June 28, 2015

Why not a 59 Hz? I prefer a prime number. If I were J.P. Morgan, I'd like to make Tesla have such a cool frequency. Or US yields to 55 Hz, and Germany get a 5 Hz bonus.
In the true world, a wide range of frequency for electrical machine could work economically, but in a real world, people make the standard, and 60 Hz simplified, e.g. the calculation. I guess Nikola Tesla had more common sense than I have. However, if I were born 100 years earlie ...

#### Why use isolated I/O modules?

##### Friday, June 26, 2015

Required precision, wire length and amount of electrical noise within the system are all considerations. There are also various levels of isolation i.e. some analog I/Os have a locally generated power supply which may or may not be isolated from general logic power or may use a separately provided supply;
Similarly, the digital signaling portion may or may not be isolated from the control bus o ...

#### What is regenerative braking in variable frequency drives?

##### Thursday, June 25, 2015

To answer your first question, regenerative braking in variable frequency drives (VFD), the premise is that whenever a motor is rotating faster than the winding are excited the motor becomes a generator. Regenerative braking means the voltage produced is "placed somewhere else". Often the excess voltage can be absorbed by the IGBTs or the addition of a braking resistor...maybe even placed back on the AC power network. I ...

#### Can I run the VFD driven pump motor down at 25 Hz?

##### Friday, June 19, 2015

If BOTH bearings are not properly insulated, operating on the variable frequency drive (VFD) will cause bearing damage. This is even more pronounced with bearings that take axial thrust loads, since there's now another contact surface in addition to the "normal" radial loading. Note that the damage MAY appear on the housing, rather than the bearing - it all depends on how the energy dissipates within the bearing region.

True, some of your observed temperature rise may be a result o ...

#### Can VFD force vector control be used to stop?

##### Monday, June 15, 2015

I have an interesting (yet frustrating) project I am trying to get to function accurately. My customer is a machine tool shop that refurbishes well casing for oil and water drilling. We installed a VFD driven system to move the pipes from area to area. The reason for the VFD is for the high torque start and stop and also the fact that not all the pipes are the same size and weight. The problem I am trying to overcome is when they are driven into the CNC lathes that clean up the threads on the ...

#### What is Disadvantages of PLC?

##### Friday, May 22, 2015

A PLC (or Programmable Logic Controller) has traditionally been a simple general purpose computer, that can be programmed using a simpler language than those used by "computer programmers". Really what it comes down to is a single (MSDOS) versus Multi tasking (Windows/Unix) computer. A traditional PLC is basically no different than a "PC" running a script interpreter. The only real distinguishing factor is in the type of I/O available and the communications. Other "variables" include the amou ...

#### Why it's difficult to measure active power under low power factor?

##### Wednesday, May 20, 2015

The short answer is that, at low displacement power factor, the active fundamental power measurement is highly sensitive to angle errors (usually introduced by current transducers). A couple of examples might make this clear. Fundamental power = fundamental volts x fundamental amps x cosine (angle between fund volts and amps). Suppose we have 230 volts and 10 amps, and we have a measurement error in the angle of +0.5 degrees.

#### Why low power factor power measurement is less accurate?

##### Monday, May 18, 2015

This is because the phase angles are almost 90 degrees apart between the voltage and the current. Since the power is the product of these two almost 90 degrees out of phase sinusoidal waveforms - any phase angle error will result in a much higher error than two sinusoidal signals that are shifted by a much smaller angle. Therefore the phase angle errors in all the instrumentation (Voltage and Current transformers) will contribute far more to errors for low power factors.

If you wan ...

#### Can I operate 60Hz (3600rpm) generator at 50Hz (3000rpm)?

##### Friday, May 15, 2015

If you operate a 3600 rpm (60 Hz) synchronous generator at 3000 rpm (50 Hz), the power output will be reduced, simply because the mass of the rotor remains constant and you reduce its rotational speed. Based on the laws of physics, kinetic energy is directly proportional to the mass and the square of the velocity. Conversely, if you operate a 3000 rpm (50 Hz) generator at 3600 rpm (60 Hz) the power output wi ...

#### Why motor produces electromagnetic interference (EMI)?

##### Thursday, May 14, 2015

Electromagnetic Interference (EMI, also called radio-frequency interference or RFI when in radio frequency) is a disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source.

All motors create magnetic fields because of the currents passing through the windings - or just because they have strong magnets (permanent magnet designs). Stronger magnets will create stronger fields - which will extend t ...

#### How to calculate motor stator winding diameter?

##### Sunday, May 10, 2015

Allowable current density is not a random choice. It is directly related to the temperature of the conductor and is dependent on the insulation (on the individual strand, on the turn, and on the completed coil), the method and efficiency of the cooling, the heat transfer properties of the surrounding magnetic materials, and the nature of the current itself (AC or DC).

As a really rough approximation, a current density of 3000 ampere / mm^2 equates to something like a temperature ri ...

#### Why single phase motor is not self starting?

##### Wednesday, May 6, 2015

In a single phase AC motor (at starting) there is only one pulsating field. For the sake of analysis only it could be represented by two rotating fields of equal magnitudes and rotating in opposite directions. If the rotor of this single phase motor is moved by any means, the stator field is still a pulsating field but the rotor induced currents will produce a rotating field and for the sake of analysis this rotating field is added to the component of the stator field rotating in the same dir ...

#### Should I oversize a motor?

##### Tuesday, April 28, 2015

Motors are sized based on the application requirements.
High starting torque (required for a high inertia load) usually translates into light loading at rated speed and voltage. The driving factor is NOT the efficiency under the loaded condition - it is the ability to start the process in the first place.

Environmental conditions also affect design choices - high ambient temperatures, high altitudes, and presence of hazardous (e.g. combustible or flammable) materials will nece ...

#### What is inductive & capacitive load?

##### Monday, April 27, 2015

Technically an Inductive Load has the current through it lagging the supply voltage across it by 90° and a Capacitive Load will have the current through it Leading at 90° in Advance of the supply voltage across it.

However, this is where the typical expression is wrong.
When most people mention Inductive or Capacitive Loads, they really imply a Resistive Load, with some element of Inductance or Capacitance. That is why others above, refer to the phase angle as being l ...

#### How to identify a motor rating without nameplate?

##### Thursday, April 23, 2015

If there is no name plate, you won't be able to know the rated voltage too. In that case, you may have to guess the approx. HP depending upon the size of the motor, select appropriate auto transformer and then run at no load by slowly increasing the voltage till it reaches steady state. I am however still not sure if this will give you the correct rated voltage. There are standard ratings of three-phase 380V, 415V & 440V. I do not know if differentiation between these is possible, because ...

#### What is Profibus?

##### Wednesday, April 22, 2015

ProfiBus is on a 2 wire RS485 platform with different protocol communication layers that actually define ProfiBus. It's widely used in Europe & Asia for industrial applications. Field communication bus, allows you to enhance the number of devices controlled by for example a PLC. It can be described as a serial network were every device connected to it has an ID number, and a range of inputs and outputs, these ranges are normally defined when creating your hardware configuration, and the d ...

#### Why we use 24V DC?

##### Thursday, February 19, 2015

When we say control the first thing comes in theory or practical is that it should have a low value of noise which is apparently achieved by DC.
- 24V is far safer limit than compared to higher values 115V AC.
- 24V DC is more resistant to control circuit drop out caused by high inrush using contactors used for starting motors or any capacitive load as well.
- Modern day electronics can now be operated at lesser voltage, Optos, relays, improving efficiency of the system and l ...

#### Why we use 4-20mA?

##### Saturday, February 14, 2015

Loops can be:
4÷20mA 2÷10V
2÷10mA 1÷5V
0÷20mA 0÷10V
0÷10mA 0÷5V

For example if we have a 4 ÷ 20mA loop, it will have a range of 20mA - 4mA = 16mA. This is domain.

By convention 4mA is the lower limit (LL = Low Limit) and 20mA upper limit (High Limit = HL), the rest of the field values are intermediate states (points).

How can we force the lower limit, quite simply, send 3 ...