Categories

# Basics

#### Why we use 115v 400Hz power supply in aircraft?

##### Saturday, January 30, 2016

Aircraft electrical components operate on many different voltages both AC and DC. However, most of the aircraft systems use 115 volts AC at 400 hertz or 28 volts DC. 26 volts AC is also used in some aircraft for lighting purposes, where the power is generated by a commutator which regulates the output voltage of 28 volts DC. AC power, normally at a phase voltage of 115V, is generated by an alternator, g ...

#### Different types of Earth Fault

##### Tuesday, January 12, 2016

Basically there are just two type earth fault and restricted earth fault.
An earth fault is a leakage of current between any conductor that normally carries current, that is the phase and neutral conductors, and the general mass of earth and anything connected to it anywhere downstream of the point within the system that the earth fault protection device is situated. The protective device for this type of fault is often referred to as an unrestricted earth fault device.

A res ...

#### Wye connection for electric-motor loss reduction

##### Friday, January 8, 2016

Q: I heard that if we have a 3 phase induction motor constantly operating under 40% of rated power, we can reduce its losses using a Wye connection. But I can't find any reference of recommendation about that. So is it a practical solution to maintain energy efficiency? And is there any practical intelligent Wye-Delta switch?

The voltage across each winding drops by sqrt(3). So the torque-speed curve is reduced (by a factor of 3). The equilibrium speed is only slightly changed, but ...

#### Relay Coordination

##### Wednesday, January 6, 2016

There are different types of relays and each type has specific applications. It looks that you are looking for overcurrent setting calculations, as can be seen from mentioning plug setting and time setting multipliers. This is a typical protection scheme in distribution systems, the story is different at transmission levels.

There are many commercial software packages available that can be used to do the calculations, but again you need to build fundamental knowledge in order to in ...

#### Difference between cogging torque and stall torque

##### Thursday, December 31, 2015

COGGING torque is defined as the attraction/interaction of the magnetic poles (typically on the rotor) to the steel teeth (typically on the stator lamination) within an un-energized motor.

STALL torque is the torque produced by a mechanical device whose output rotational speed is zero. It may also mean the torque load that causes the output rotational speed of a device to become zero, i.e., to cause stalling.

Basically - cogging means the motor cannot begin rotating, whi ...

#### Sensorless control of AC motor

##### Friday, December 25, 2015

So called "sensorless control" of AC machines means having no mechanical (speed and/or position) sensors. However, usually this term only includes vector controlled drives, i.e. it does not comprise V/f open-loop control of Induction Motors (which is the simplest technique for driving an IM motor without a mechanical sensor).

For the induction mold, sensorless vector control is a mature technology, it has been around for a few decades. Almost all reputable drive manufacturers have ...

#### How to define power source for different cables?

##### Wednesday, December 23, 2015

The rule of thumb is less than the rated voltage or the maximum capacity of the test equipment.

There are several test techniques available in literature and standards (IEEE400). All these tests have advantages and disadvantages. Sometimes the client or the utility may have specific test requirements. However selection of tests must generally be done with the following requirements in mind:

1. If it's a new installation, is it safe t ...

#### Relationship between grid frequency and load in power system

##### Thursday, December 17, 2015

Higher output power of generator (hundreds of MWs) has bigger impact on the system frequencies than low power generators, because of its massive spin momentum (torque). The active (R) load acts directly on generator speed because it creates strong magnetic field in stator that is perpendicular rotor's spinning magnetic field thus forcing an opposite momentum(torque) which slows down the turbine, this is fixed with a speed governor which controls turbine speed by manipulating its spin momentum ...

#### UPS sizing

##### Friday, December 11, 2015

Most UPS's are sized for the client load requirements. However in cases of smaller UPS systems won't comply with electrical regulations!
Why?
Let's consider a client needs a 30kW supply, to 2 100amp distribution boards. One 20 meters from the UPS and one 80 meters away. Each DB has some 32amp circuits up to 20m long. The distribution circuit is protected by 63amp BS60947 devices and the final circuits are 32amp BS60898. Specifically for a UPS system, what is going to define your UPS ...

#### The purpose of insulated bearing for Motors with VFD

##### Wednesday, December 9, 2015

There are 2 different scenarios that occur. One is a circulating current that can occur, mostly through the fact that the rotor is not perfectly formed (you can think of it as not being perfectly round) which creates a circulating current. Insulating one bearing will stop this circulation. The second scenario occurs when a motor is on a VFD (variable frequency drive) and especially a PWM VFD. This produces a charge on the rotor which wants to discharge, and the easiest path is almost always t ...

#### Brief description about maintenance in industry

##### Monday, December 7, 2015

First, there is the (NO) maintenance approach. This is essentially "run until done" style. Kind of like never changing the oil in the car engine or replacing tires and expecting infinite life. If lucky, the damage that occurs is mild and repairable (at relatively high cost) - if unlucky, complete replacement of machinery required (EXTREMELY expensive proposition, since it also has to include potentially lengthy delay in acquiring new equipment and unanticipated loss-of-production)

...

#### What will be the integration of error position in PID controller?

##### Saturday, December 5, 2015

In a position feedback PID controller the integrator component is the sum of all the position error. You could call it the hysteresis, or history of error. Regardless, it is like a running average error and tells your control system how consistently off the mark it is in general. Typically the integrator gain is small to keep a stable system, so that you don't have a strong P or D response to the I induced response itself (the gains reacting to each other's reactions is a source of instabilit ...

#### An Elementary Question About Power Factor

##### Saturday, November 28, 2015

I have an interesting dilemma at work. We have never had the ability to monitor each individual feeder emanating from the substations. I recently installed a modern recloser on one circuit which allows detailed measurement of line characteristics and discovered that we have 1) a serious imbalance among the phases, and 2) a terrible power factor on this feeder - .33 on one phase, and .65 or worse on the other two. The meter that monitors the overall bus reads a combined power factor for the tr ...

#### Difference Between Neutral and Ground

##### Thursday, November 26, 2015

Neutral is the normal return path for current coming out of the live terminal and through the load. Ground is a safety wire that (all being well) should not be carrying any current. It is connected to the chassis etc. If some goes wrong inside the box and a live wire touches the chassis, the ground connection should prevent the metal parts going to high voltage. In this pathological case, the ground wire will carry a (possibility very large) current, at least until the fuse blows or the circu ...

#### What is Impedance Matching?

##### Sunday, November 22, 2015

In substation there is Line Matching Unit (LMU) where the equipment impedance and line impedance matching is done, it is connected below coupling capacitor CC, and we have studied in network theory maximum power transfer thermo where impedance matching term is used. It is used for maximum communication signal can pass to other end with reduced loss.

This is the first part of the theory which is maximum power transfer. This is basically figuring out what should be the line impedance ...

##### Wednesday, November 18, 2015

We know the equation kva= kw/power factor. When power factor reduces, kva increases. The substation authorities have to pay a penalty if demand indicator (kva) increases over an integrating period. How the maximum demand (kva) is being calculated in substations. Are there any ways to avoid load shedding of critical loads in substations if the maximum demand indicator goes over a maximum value when power factor is reduced?

In a Tra ...

#### What is base speed of induction motor?

##### Sunday, November 8, 2015

Induction motor with squirrel cage rotor has two different speeds. The synchronous speed refers to the stator rotating magnetic field, which depends on the number of poles and frequency. The other speed is the rotor's. The rotor speed will be always slower than the stator speed, we call it slip. Without the slip, the squirrel cage induction motor doesn't have torque. Even without load the motor has slip. It increases with the load, according to the current x torque curves you can check it out ...

#### What are the causes of noise in VFD and filter?

##### Sunday, October 25, 2015

First make sure the cause of the noise is not mechanical in nature (bearings, misalignment, worn bearings, lose hardware, lubrication, etc., etc.). If the cause is identified as being electrical in nature you can check some of these.

Is the motor rated for VFD service? Although you may be able to use a regular 3 phase AC motor with some VFD's, a regular 3 phase motor is not design to work with VFD's and thus will operate with higher electrical losses than a motor which is rated for ...

#### Capacitor bank working principle for MCC panels

##### Friday, October 23, 2015

The function of a capacitor bank placed at an MCC panel is to provide reactive power to the inductive loads (improve power factor) and raise the voltage according to the system strength and capacitor bank size.

If the MCC loads also contain harmonic current distortion as is commonly produced by variable speed motor drive rectifiers, then a distortion problem can be produced should one of the principle ...

#### Precautions of VFD for hoist application

##### Tuesday, October 13, 2015

Asynchronous motors are capable of developing significantly higher starting torque than synchronous motors. It has everything to do with the slip. Two pole motors are also an issue at start, due to a significantly lower starting torque capability than 4 or more pole motors.

An encoder of some type is typically used for hoisting applications when the hoist has no mechanical load brake in the gear case. Generally, when we build flux vector hoist controls, we employ 1024 PPR, 24 VDC, ...