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# Blog

#### Capacitors produce reactive power whereas inductors consume it

##### Friday, December 26, 2014

Assume a pure inductive load connected across an AC power supply, we will see that, the power wave has double frequency that of the voltage wave that produced it. In other words we can see that, by the time voltage wave completes one half cycle, the power wave would have completed one complete cycle. By definition, this power is called the reactive power.

We can see that, this power (reactive) consumed by the inductor is zero in any half cycle of voltage wave. Physically speaking, ...

#### Generator Static and Brushless Excitation System

##### Thursday, December 25, 2014

In hundreds MW plant we are using static excitation system, but it require more maintenance than brushless. Why not we are going for brushless system? What is the reason behind this?

As far as I know, the increased maintenance requirement is associated with the slip rings and brushes. It is not so easy to replace a static excitation system with a brushless system, as you'd have to change things in the machine rotor and shaft (remove the connections to the brushes and slip rings, ad ...

#### Induction Motor Working Principle

##### Thursday, December 25, 2014

Induction motors operate on the principle of current induction in the rotor which must rotate at a speed less than synchronous speed for induction to occur. This is referred to as slip speed and should not be confused with pole slipping.

Why induction motors have no pole slipping and why other motors have pole slipping? I will attempt to answer this for you but first let's define poles and synchronous speed in regards to electric motor operation.

Poles in an electric m ...

#### GoHz Frequency Converter Operation Guide

##### Wednesday, December 24, 2014

GoHz frequency converter is a great solution for converting 50Hz to 60Hz among different countries, also can convert 60Hz to 50Hz, in additional of frequency adjusting, the output voltage is changeable from 0-300 volts for single phase power supply, 0-520 volts for three phase power supply. This article shows the basic operation guidelines of GoHz static frequency converter, you can easy set up a converter by following this guide. ...

#### Operating 50Hz transformer on 60Hz supply

##### Wednesday, December 24, 2014

You can run the 50Hz transformer on a 60Hz supply. Now since the frequency has changed the only variable thing in the existing transformer that can change (assuming simple one input and one output) is the flux density. In its simple form we are operating the transformer (60Hz/50Hz = 1.2) at 20% lower flux density.

If the things were reversed the flux density would be 20% higher and may saturate depending on design. You can even run the transformer at 400Hz providing that the volta ...

#### Harmonics in train power system

##### Tuesday, December 23, 2014

For Metro DC traction power systems, the commonly adopted nominal voltages are 750V DC and 1500V DC. The DC substations are normally fed from a High Voltage (HV) 11 kV or 33 kV 3-phase AC network in the UK. To convert the 3-phase AC voltage to DC voltage, a modern DC substation employs a 3-winding transformer (delta-delta-star) to feed a 12-pulse rectifier (consisting of 2 x 6-pulse diode bridges, the outputs of which are connected in either parallel or series to form a 12-pulse rectifier). N ...

#### UPS Separate Earthing

##### Monday, December 22, 2014

Many people think that UPS is a magic box that solves all the electrical problems. UPS systems have capacities varying from few hundreds of VAs to massive kVAs. The only thing that is worth noting in a UPS system is that, it can be viewed as a source as well as a load. Depending upon the configuration, the grounding needs to be planned. If the UPS system is transformer based, and operating only in inverter mode (Frequency converter mode), then it is called as separately derived source and out ...

#### Why using dynamic braking resistor in motor speed controls?

##### Monday, December 22, 2014

The power developed by Induction motor during idling time if wasted in power resistor used for braking is called dynamic braking resistor. Dynamic braking resistors are used to dissipate the power generated by the motors during dynamic braking. Typically, the variable frequency drive (VFD) will rectify the power back to a DC source and the resistors are cut into the dc link via a heavy-duty contactor. The power is then converted to heat in the braking resistor grids. These resistors must be a ...

#### Harmonic Current Problem

##### Sunday, December 21, 2014

This is one of my favorite topics and have been involved for many years. The point to remember is that in Beemans, it says that while harmonic currents are theoretically possible, there is no reason to be concerned about them. That book was written sometime around 1950 or earlier. I am not sure. However, the point made is important. A harmonic problem doesn't exist until you have one. At that time we didn't have many power electronic devices.

What this means, is that harmonic curr ...

#### Distance between VFD and motor

##### Sunday, December 21, 2014

The value of critical distance depends on the propagation voltage and output rise time so as much the value of propagation voltage the critical distance will be increases. Mostly variable frequency drives (VFDs have same critical length and it already mentioned in VFD manuals so there is no need to measure the critical distance as you can get information from your distributor or manufacturer of the VFD.

Some manufacturers of large variable frequency drives integrate output circuit ...

#### Why Three Phase AC System was created?

##### Saturday, December 20, 2014

There is a widely spread error that it was introduced by Nicola Tesla. Nicola Tesla was mostly interested in 2-phase system that is usually referred as "poly-phase" system in the USA. He also created 2 phase induction motor. You can find a picture of that design from Internet. (I should mention, the first induction motor was invented and built by Ferraries from Italy, it was single-phase one and had slipping of 0,5!!! As result, efficiency was very low).

The 3-phase system was deve ...

#### Advantages of SER drives for SAG mills

##### Friday, December 19, 2014

For large mills requiring variable speed, the wound rotor motor and SER drive are economical for a total rating of approximately 2MW to 16MW. Above 16MW, the gearless drive (cyclo-converter) is typically used because gearboxes and pinion gears reach their present limit in size. Around 2MW and below, the squirrel cage/VVVF drive is simple and cost effective.

Advantages of the wound rotor/SER drive are:

1. If the SER converter drive fa ...

#### Earth fault in 220V to 110V converters

##### Friday, December 19, 2014

We have 220 DC (UPS) distributions board supplying 220v/110v converters and 220/24v converters, only in the 110 dc board we get a frequent earth fault alarms in DCS (every 30 min not a permanent ) the problem that we can not localize which feeders are making the fault due to the big number (more than 100 solenoids valve are supplied ) and when we make the voltage monitoring all of them are indicating an unbalance value between + and ...

#### Difference between Current Transformer and Voltage Transformer

##### Friday, December 19, 2014

Transformers are essentially 2 coils of different number of turns linking a common time varying flux, so that different voltages can be connected on the two sides. Three applications arise: 1. Using transformer to drive a load with voltage rating different from that available at the source. This is power transformer action. 2. Using it for instrumentation, i.e., measuring currents of hundreds/ thousands of amperes with a low range ampere meter. No 'power' is expected to be drawn through it by ...

#### Converting synchronous generator in synchronous condenser

##### Thursday, December 18, 2014

The calculations are no different than those for determining how to start a large synchronous motor, it all depends upon the type of generator and how big it is relative to the system that you are connecting it to. Take a look at the SLD (Single Line Diagram) and get the GSU (Generator Step Up) transformer rating and impedance, transmission line impedance to the grid, as well as any information that you have on the generator. Then you can determine how much the voltage will dip if the generat ...

#### Convert AC to DC power

##### Wednesday, December 17, 2014

For the Logic Power supply I want to convert AC 120/240V to 24/15/5/3.3 VDC. I am wondering what is the best topology to do this. Currently I am using a power transformer to convert down to 24vac rectify and tap off of that. Is there a way to reduce/remove the power transformer? Should I change to a flyback style of converter? I need about 5-10 Watts DC power total.

If you do not care about power factor (means no need of power factor correction), then indeed a possible solution is ...

#### Where VFD should be placed?

##### Wednesday, December 17, 2014

Most of the users installed variable frequency drive (VFD) near the MCC because it's easy to maintain the system but there are some issue's which may happen due to placing the VFD near the MCC like there is a kind voltage reflection issue if the length of the cable is more than the critical length which may damage the motor.

In this case we need to follow instructions given in the user's manual supplied with the variable frequency drive. Generally following are recommended by manuf ...

#### Can an electric motor run as a generator?

##### Tuesday, December 16, 2014

YES, but it can only be done long term with an electric motor that is designed to also run as a generator and if the generator will be paralleled with other generation the motor will need to be a synchronous motor.

I served in the US Navy on a submarine as an electrician nuclear qualified. My electrical plant includes 2 - 500 KW motor generators. When the AC end was operating as an AC motor the DC end was a DC generator that supplied power to the sub's battery, reverse current and ...

#### What is electromagnetic interference in power electronic system?

##### Tuesday, December 16, 2014

Three major issues in designing a power electronic system are Losses, EMI (Electromagnetic interference) and Harmonics. These issues affect system cost, size, efficiency and quality and it is a trade-off between these factors when we design a power converter. In power electronics, high voltages and high currents are processed by fast switching to reduce losses which are significant sources of electromagnetic noise and it cause additional costs.

Magnetic interference in power system ...

#### Effective Earthing System

##### Monday, December 15, 2014

Generally, provision of effective earthing system depends on aim and goal involved, and the environment where the earthing or grounding as the case may be, is actually required. For instance in Power Engineering, providing adequate Earthing in a substation is an important essential safety measure. The aim and goal is to provide under and around the substation, a surface of uniform potential, at near or absolute zero earth potential.

In some cases, earthing or Grounding terminology ...