FAQ

All variable frequency drives using power electronics are essentially "frequency choppers". Either the VFD is a current-sourced inverter topology, or a voltage-sourced inverter topology. To obtain the desired output (usually defined as voltage at some specific frequency), the incoming signal gets "chopped" and rectified to produce a DC pulse, which is then inverted back produce a "sample" of a fraction of a sinusoidal waveform.

The sample can ...

The question was directed to single phase motor which it is surmised to be of induction type. Depending on the load characteristics, your motor could stall or overheat (or simply not start) if the frequency is not the one it was designed for at a given load. Three phase motor is not exactly the same as a single phase motor in speed-torque characteristics. Single phase motors are meticulously designed to have start-up torque through shaded pole or capacitor, etc. They can handle certain loads ...

The source of power is an ideal voltage source having zero internal impedance with a series impedance that can have both real (resistive) and imaginary (reactive) components. This impedance is for example the transformer impedance plus the impedance of the feeder to the bus. This is called the Thevenin's equivalent circuit. This is the bus voltage. When you connect a load to the bus you are completing the circuit with another impedance. So you have a circuit with a voltage source and two impe ...

The application of a filter to a power system requires that, first, you know how much capacitive vars are necessary to accomplish the power factor correction you seek. Then, the process involves measurement of system loads at the secondary of each transformer. The measurement must provide the harmonic content of the current. This is determined by harmonic magnitude and phase angel for each harmonic at each location. Armed with this da ...

Due to the form factor=1.11, so output voltage must be multiply of form factor, so we get 11,22,33. First let us define form factor before we go into the question:

The form factor of an alternating current waveform (signal) is the ratio of the RMS (Root Mean Square) value to the average value (mathematical mean of absolute values of all points on the waveform). In case of a sinusoidal wave, the form factor is approximately 1.11.

The reason is something historical. In old ...

It depends on where the switching is taking place. The key is to keep the live system always grounded and avoid grounding neutral at more than one point.

If you are looking at an application less than 600VAC, and you have multiple sources (such as utility and generator) and you have 4-wire loads, I usually recommend switched neutral as it allows complete isolation of the sources from one another, which refers to switching neutral of a transfer switch, where neutral switching would ...

The % impedance is formally referred to as impedance voltage. It is the supply voltage, expressed as a % of rated voltage, that is required to circulate rated current through the transformer.

It is measured in the factory by a short circuit test at rated frequency. With the low-voltage winding shorted, the supply voltage to the high-voltage winding is increased until rated current flows in the transformer. ...

What the system sees with a "standard" squirrel cage induction motor connected directly to the line is the inductive loading caused by the nature of the magnetic circuits within the machine design. This can result in a reactive power demand that is anywhere from 10 to 30 percent of the real power rating of the machine (e.g. power factor lies between 0.900 and 0.700). It also gets worse (i.e. farther from unity power factor) as the machin ...

A variable frequency drive (VFD) will improve the system power factor to which it is connected, primarily because the motor reactive current is supplied by the DC bus, rather than the supply system. This will only improve the displacement power factor. Modern VFDs with AFE draw almost sinusoidal current from the supply, so power factor on the source side can be controlled up to unity and the generated harmonics are ...

Q:
I'm looking for ways to estimate the supply current to variable frequency drive (VFD) when an asynchronous motor is running under different torque and speed outputs.

Motor manufacturers usually provide the rated torque, rated current and rated speed of the motor, but when the motor is not running at the rated conditions, how can one estimate the supply current to the VFD from the actual speed and torque output of the motor?

A: ...

Alternating current flowing in a conductor sets up an alternating magnetic field which is much stronger if the conductor is surrounded by an iron-rich material, for example if it is steel wire armoured or if it is installed in a steel conduit. The currents in a twin cable, or in two single core cables feeding a single load, will be the same. They will exert opposite magnetic effects which will almost cancel, so that virtually no magnetic flux is produced if they are both enclosed in the same ...

I require urgently 75 hp motor which should be compactible for both 50Hz/60Hz, I'm very much confused as frequency changes speed will change but I'm wondering how the power rating of the motor remain same in practical power must vary but manufacturer claims power remains constant? Is it possible?

A: It is possible for a motor to deliver the same power output at 50Hz ...

I had an oil filled motor start capacitor blow once. What I think happened was when someone replaced it, rather than putting it back in its holder, they duct taped it to the side of the squirrel cage motor that it was starting. They started the motor up and walked away. The motor was a continuous duty motor so it wasn't turned off for a long time. One day, someone shut it off to do some servicing. They notice ...

Anything that changes the voltage (or current) waveform from a pure single-frequency sinusoid (in the case of AC) or a pure "flat line" (in the case of DC) is considered to be harmonic distortion.

In actual fact, there are transient conditions - events measured in the 0.001 to 0.05 second range - which cause such distortions very frequently (typically on the order of several times a second), even around the home. Such instances can be caused by switching (physically closing or open ...

In very general terms the silicon-iron magnetic cores of motors, generators, and transformers get smaller as the frequency they operate at gets higher. This of course assuming the energy delivered at the output of both cases is the same. Because of this, when you have 50 Hz core in a motor, generator or transformer, this core will generally bigger and heavier (more silicon-iron). This is because it needs to have more magnetic mass in order to be able to accommodate the extra volume of magneti ...

An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons.

What this means in reality is that a normal application for an isolation transformer is to either keep the load from generating harmful harmonics back onto the distribution bus ...

A three-phase network is, indeed, either grounded or ungrounded. What that means, to my understanding, is that, simply speaking, the neutral point on a wye connection can be either grounded or not. What I really mean when I say "ungrounded" is the situation when there's no closed zero-sequence path for current to flow. A delta-wye grounded transformer is commonly referred to as a grounding bank, because the delta side of the transformer can be left un-terminated--still connected in delta but ...

The earliest proponent of alternating current (AC), Thomas Edison pushed direct current. If I remember my history right, 60Hz was a frequency that would produce no visible lamp flicker. There were (and still remain) 25Hz, 50Hz, 133Hz, & 400Hz power systems.

The initial investment in one frequency, early power generators (hydro i.e. low frequency) would have made sudden changes to be very costly. But some of the reasons are to do with li ...

Electrical machines designed for 50Hz can usually work safely in 60Hz power supply, but not applicable to 60Hz machines to be run in 50Hz power supply. This is because impedance of 50Hz machines increases if operate in 60Hz supply which also lowers the running ampere, while if 60Hz machine run in 50Hz power system will experience increase in running ampere & overheating that will result to reduced life expectancy of the equipment. In motors coupled to water pumps or compressors increase o ...

There's not much to see physically, but electromagnetically there's a lot going on. Once the rotor containing the field winding is up to near synchronous speed, the field is energized creating a variable strength electromagnet. This electromagnet is known as the field and is rotating inside (conceptually) three coils of wire spaced 120 degrees apart. As the magnetic poles pass by the each coil voltages that are 120 degrees apart are induced, this is known as the open circuit voltage of the ge ...