Categories

# Basics

#### Induction Motor Voltage vs Current

##### Tuesday, September 29, 2015

An induction machine isn't a constant power device and the power rating is specified at a specific voltage and frequency, decrease the voltage and you decrease the power.

It's a common misconception, from looking at
P=VI

That a given power rating will try to balance this equation and the same misconception that a 100W light bulb will always draw 100 watts, when in fact power ratings are always rated at a specific voltage and frequency. At the most b ...

#### Advantages of DC hipot test compare to AC test

##### Friday, September 25, 2015

One of the advantages of using a DC test voltage is that the leakage current trip can be set to a much lower value than that of an AC test voltage. This would allow a manufacturer to filter those products that have marginal insulation, which would have been passed by an AC tester.

When using a dc hipot tester, the capacitors in the circuit could be highly charged and, therefore, a safe-discharge device or setup is needed. However, it is a good practice to always ensure that a prod ...

#### Car Power Inverter Buying Guide

##### Wednesday, September 9, 2015

In long distance driving, it's helpless when our phones or laptops run out of batteries while there is only ordinary charger on hand. Most cars don't equip with AC 110v/220v power supply. In such case, a power inverter works with the car will absolutely keep such awkwardness away from you, a car power inverter will convert 12v DC to ordinary 110v (or 120v, 220v, 230v, 240v) AC to feed regular power source to small household appliances. But, there are lots of things we need to pay attention wh ...

#### How frequency mismatch leads to grid failure?

##### Saturday, September 5, 2015

We know that the entire grid is at the exact same frequency (50Hz or 60Hz). This is because the grid is made up of synchronous machines. The difference is that of the phase angles. If you go back to the fundamentals of synchronous machines, you will see how important the frequency is to its operation. In fact, power transfer between two nodes depends largely on the angular difference between the nodes.

Load is inversely proportional to frequency. When load increases, that is curren ...

#### Using AFE VFD to avoid harmonic currents

##### Thursday, September 3, 2015

Most of the variable frequency drives (VFD) below 600 HP or so are IGBT with Diodes to supply the DC. If only six pulse the harmonics can become a concern if a number of VFDs are used on the same bus. One of the jobs I'm currently trouble shooting has 6x600 HP VFDs all connected to the same power source. This means that my harmonics are added together as there are no filters or phase shifting transformers. I expect the power factor at full load to be aro ...

#### Power Factor Correction Simple Calculation

##### Tuesday, September 1, 2015

Simple power factor correction calculation:
Motor input = P = 5 kW
Original P.F = Cosθ1 = 0.75
Final P.F = Cosθ2 = 0.90
θ1 = Cos-1 = (0.75) = 41°.41; Tan θ1 = Tan (41°.41) = 0.8819
θ2 = Cos-1 = (0.90) = 25°.84; Tan θ2 = Tan (25°.50) = 0.4843
Required Capacitor kVAR to improve P.F from 0.75 to 0.90
Required Capacitor kVAR = P (Tan θ1 &nd ...

#### Electrical Machine Resonance Problems

##### Friday, August 28, 2015

When I at Siemens (my former employer) began building gvpi stators in the 100 mw to 300 mw size, we experienced serious vibration troubles on the end turns both locally in the series connectors and globally in the winding. This was evident in both the 50 Hz and 60 Hz fleets but was more serious in the 50 Hz fleet. This problem led to several forced outages and a few total losses. The problem was that the win ...

#### Reduce the starting current of an induction motor

##### Wednesday, August 26, 2015

During the transient condition associated with acceleration of an AC machine started as an inductive load, current is drawn from the source that can be several times the full-load rating of the machine winding. To handle this current, the protective device (e.g. circuit breaker) located at the juncture of the distribution bus and the feeder to the rotating load should have an instantaneous setting and short time setting that will adequately protect the downstream equipment but will also avoid ...

#### Surge Protection

##### Wednesday, August 12, 2015

The electrical engineer doing the analysis, evaluation, and application of the surge protection of the power system surge protection must be very well experienced on all the aspects and issues involved in the system surge protection because the system is a variety of system configuration - overhead, underground, combination of overhead and underground; a variety of source connections. Near substations (small or large), near generating stations (small or large) near switching stations (small o ...

#### DC Excitation Voltage in Practical 3-phase Generator

##### Thursday, August 6, 2015

Depending on the size of the main synchronous generator (alternator), there can be a number of excitation elements.

For "separately excited" systems, the main synchronous field circuit is fed by DC which is passed to the winding through a mechanical (brush-and-collector) interface. The source feeding the interface can be an old-school DC generator or a dedicated power electronics supply.

For "self-excited" systems, there are typically one (or more) rotating elements invo ...

#### Fuse Rating vs Circuit Breaker Rating

##### Wednesday, July 29, 2015

A circuit breaker is not just a switching device - its primary function is protection, its secondary function is switching / isolation.

The fuse may be on the source side of the circuit breaker to protect the circuit breaker from the pole mount transformer PSCC (prospective short circuit current) if it is higher than the circuit breaker PSCC rating. The fuse will act as the overall system protector, should excessive short circuit current flow due to system very low impedance to the ...

#### Why motor, generator and transformer are designed for 50Hz or 60Hz?

##### Monday, July 27, 2015

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 ...

#### Core Losses

##### Thursday, July 23, 2015

There are three main losses in an un-gapped cores:

Copper loss. In low frequency, pretty much follows DC resistance, at higher frequency skin effect and proximity effect complicate resistance calculations.

Hysteresis, with goes up with gauss.

A bigger core or more turns reduce gauss, thus hysteresis loss. But, those increase copper loss. You can also use better core material, that has less loss at the gauss you are operating a ...

#### Overload and overcurrent motor protection sizing

##### Thursday, July 9, 2015

Overcurrent protects the wire, overload protects the motor.
Over load and over current protection is simple....and sometimes not. For me it is easy to think of it this way. The overload protects the motor, the overcurrent protects the wire. Because the time/current curves are very far apart from each other, don't ever think that one will adequately protect for the other.

Many electricians size the wire from the starter to the motor based on the ampacity of the overload protec ...

#### Difference between Shunt Trip and Undervoltage Trip

##### Wednesday, July 1, 2015

In this pursuit, I would like to begin with the understanding of the different word "TRIP or RELEASE", which is in fact a combined "ELECTROMECHANICAL MODULE or MECHANISM that comprises of a "mechanical latch" and a "Electrical Relay / Release" which can OPEN THE SWITCHING DEVICE e.g. a circuit breaker, by electrical "energisation" or "de-energisation", according to their characteristics.

Now coming back to o ...

#### Circuit Breaker Analyzer Tests

##### Monday, June 29, 2015

The circuit breaker analyzer will perform 3 of more specific tests.
They are the a. Timing measurement, b. Motion Measurement, c. Coil Current Measurement.

For a. The CLOSE/OPEN and multiple CLOSE-OPEN option will give you the precise trip/close timing also the reliability of your circuit breaker in effect to corrosion effects.

For b. Where you can connect analog/digital transducers to the an ...

#### Excitation System Cable Insulation

##### Saturday, June 27, 2015

Generally two factors will melt and damage the cable insulation:
1- Overheating due to excess current;
2- Insulation breakdown due to over voltage or voltage surges.

In the case of excitation system it should be noted that during field forcing the excitation current will exceed the nominal current, therefore you should review your system document to find the maximum current during field forcing or other current demand of the excitation system during the operation as for co ...

#### 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 ...

#### Balancing Generation and Load of Power Control Center

##### Sunday, June 21, 2015

It is important to recognize different time frames for the expected responses from individual generation units and from the power control center regarding balancing generation and load. The key variable here is the system frequency. If the frequency is below synchronous, you have too much demand for the available generation while frequency above synchronous indicates more generation than demand.

In the example of the outage of a 600 MW generator, frequency will initially drop at a ...

#### Minimizing Transformer Inrush Current

##### Saturday, June 13, 2015

I have used three different Methods depending on the details of a specific project plus I do not use the Method 4.

Method 1. Secondary pre-charge
This is normally the most economical Method. It uses a small pre-charge transformer to apply a voltage to the transformer's secondary and I normally use 90% of the rated transformer's secondary voltage. The required current is approximately the transformer's magnetizing current. A typical value is 0.5% of the rated w ...