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Case: I am restoring a 1960s analog audio tape recorder for high quality audio use, the tape recorder is a USA model operating on 120VAC at 60Hz, the mains voltage in Singapore where the machine will be operated is 230VAC but 50Hz. Other converters that convert 230VAC/50Hz to 120VAC/60Hz have been unsuitable -- I hear high distortion and square wave noise to break through in to the audio i ...

Q: I have a situation where our UPS servicing data center rack systems are failing repetitively after replacements.

We know our mains transformer is undersized in terms of load capacity and under voltages are measured going to the input of the UPS that is providing back up power to data center in a rack mount. However these voltages measured were 110VAC and still within operating range. In another facility with the same rack systems, colleagues are saying they're seeing "brown out ...

A fault on an electrical system will continue until either a protective device operates to isolate the fault, the fault burns itself clear or the power source fails due to the load imposed by the fault, which can occur when the fault is supplied by local generators.

If the circuit has a properly adjusted protective relay and circuit breaker operating are: the relay processing time + breaker opening time + arc extinction. In the case of an unprotected circuit time will be the protec ...

Q: We have 02 motor generator sets each of same KW rating and speed etc running in parallel and having pure inductive load. Thrice it happened that 01 set got tripped on reverse power protection. 01 set operates with 0.65PF lagging while other runs with <0.5PF leading. After tripping of MG set, remaining set's current decreases from 42A to 35A. If we want to synch again two MG sets while plant is running at 100% power, is there any danger of tripping?(if remaining set got tripped, plant wi ...

As with all such questions, it depends (on the purpose of the studies, and the type of wind generator). What is modeled should be what is there. Induction generator turbines should use an induction motor model with negative real power (so that their fault current contribution, and reactive demand increase when (re) accelerating are captured). 100% inverted turbines are usually represented as a static P (possibly Q) load. Double fed turbines...may require careful thought. Obviously, dispatch/s ...

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 ...

Question: I have a 1.5 kW three phase motor running on a GoHz VFD. The VFD is connected to two phase / 220V supply, the motor is connected in Delta connection. RPM of the motor is 2800 / 3360 at 50 / 60 Hz. It is now running on 50 Hz. I need a RPM of 1400 to 1500 which means I will need to run the motor on 25 Hz which is not a problem with the GoHz VFD. But how much does it affect the motor torque? ...

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 ...

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 ...

The grounding technique depends primarily on how the system is earthed, and configuration as well as the voltage. This varies between country, and local practices, but sees below for a general idea.

MV systems like 3.3, 6.6, 11, and 33kV are generally impedance earthed so the earth fault current is limited, which means that the grounding design is relatively straightforward unless it's a small rural substation fed via overhead lines.

HV systems like 66, 132,230, 380kV te ...

We know that harmonics are generally due to the non linear loads connected at the load end. But whether harmonic distortion is increased or decreased by changing the X/R ratio of the source, feeder, transformer etc. And whether injecting power factor correction equipment into the grid will have an influence on harmonics.

Are harmonics produced only due to the non linear loads or also due to various other aspects in the tran ...

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 ...

In general, if you are using variable frequency drives (VFD) where the control mode can be programmed (as most of the modern VFDs), each VFD has its own control philosophy and some parameters to adjust how efficient they follow the specified control mode: Speed control mode (as follower to a speed command); or Torque control mode follower (as follower of a torque command). Basically the parameters to adjust in the VFDs are related to gains in their internal control loops.

I worked ...

Question: I have a motor of 100HP with FLC of 123A. It's load is Centrifugal Fan. We are currently using a Star Delta Starter. It's current in star mode various from 276A to 220A for 35secs. Is it safe? While changeover in delta it takes a peak of 376A and then comes normal 60A phase current. In this operation we are using a microprocessor based relay which is bypassed initially for 25 seconds. I want to know how safe is this operation and life of motor in long run. Or should ...

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 ...

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 ...

I have a 7KVA isolation transformer and having issues where it's tripping the circuit breakers. I have two circuit breakers, one main and one acting as a cutoff switch both installed on the primary side. Transformer manufacturer specification says 235A of inrush current with 7KVA purely resistive load. I have two questions:

1. I am fearful to load the transformer with equipment, so will the load on the secon ...

Low Voltage (400V-440V) Earthing
This discussion applies to the low voltage side of a (6.5/.400 or 11/.400 or 33/.400 kV transformers). Since your neutral is solidly grounded, the earth fault level will be sufficiently high, to trip the over current (50/51) elements. You will not need any earth fault protection. If you are running long low (really long) voltage cables you may need to provide earth fault protection.

We never design low voltage systems where the ...

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 ...

The beauty of variable frequency drive (VFD) is that the maintenance is negligible if are well maintained.
Yearly maintenance:
Tighten all nuts and bolts, all terminal blocks, cable/wire connections.
Remove dust everywhere (vacuum cleaner is good). Do not try any kind of solvent.
Check all power supply for correct voltage level.
Replace / clean filters.
GoHz maintenance plan do specify which component to change and when (for example blowers every ...