Blog

Every engineer has good points about variable frequency drive (VFD) failure reasons. Some of the phenomenons occur more frequently and some are rarer.

The sizing of VFD is important. Also the application and demands of application should be studied carefully. In some case it might be better to use soft starter than use VFD. Thus VFD is suitable for wide range of applications, some parameters should be examined before installin ...

Look at the current sense signal and MOSFET voltage and observe the wave shapes. If the converter is near light load, the transformer will be in discontinuous conduction mode. That means the transformer current starts at zero and LINEARLY ramps up to a peak value before shutting off. If you see a large initial current spike at turn on, more than can be filtered with 1/2 use of RC time constant filtering, either there is a transformer phasing problem or there is some other winding fault / load ...

The simplest means to adjust the speed of a (squirrel cage) induction motor is to vary the load applied. Since the rotor is not separately powered, adding load will reduce the shaft speed (i.e. slow the motor down). Removing load will increase the shaft speed. The upper limit is the slip speed (which is slightly below synchronous speed) for the applied frequency; the lower limit is zero speed or "stall" condition.

This does not mean the load adjustment is the best answer in terms ...

After China has built the first all DC city, during the half century that we in the U.S. are weaning ourselves from 60 Hz AC, 60Hz transformers will not be constructed by present customs. Rather, they will be switching power emulations. These emulation transformers are completely within the present state of the switching art. The technology will not use much clocked pulse width modulation, though clocked 100% duty cycle, may prevail over hysteretic l ...

There are a lot of possible ways that a MV cable can fail. It all varies depending on the constructive type, and the topology of grid it's being used in. Both terminal boxes and the underground section can fail.

Terminals can fail if
1) subjected to moisture - high humidity of the air is enough to produce failure in time;
2) PILC cables naturally lose the oil and thus insulating properties from the terminal during years of service, because of its verti ...

JOG - a small change in rotational position. A "jog" command is essentially an energy "pulse". A very quick ramp up and down of voltage and/or current is commanded with the idea that it will generate enough torque to overcome static friction and move the rotor some arbitrary fraction of a revolution. Since the duration and amplitude of the pulse are programmable, the resulting angle of the move is also variable. A drive train which contains large inertia components (either in the driven equip ...

The valve characteristic tends to linearize the response to a change in the process. For instance, if the gain if the process is linear, a 10% change can be made with a 10% valve trim movement, over the entire range of your process, then a linear trim valve will be easiest to tune, and can be made most responsive over the entire range.

If the gain of your process is non-linear, then a valve with trim cut to compensate for that non-linearity can give you a more stable process with ...

1) Continuous operation in the 75-100 percent of nameplate power rating will generally deliver optimum efficiency. However, too much "extra' HP will result in a lower power factor (since the machine is running at less than nameplate loading), which will adversely affect power consumption and utility costs in the long run. As David mentioned - having a bit more than the nameplate value can be a good thing when process changes (such as colder-than-estimated gases) occur. Of course, the best thi ...

For a given torque rating, the AC motor will almost always be physically LARGER than the equivalent torque rating in a DC design. This is particularly true for slower (< 450 rpm) shaft speeds. The reason for this is two-fold. First, the AC machine must run at a lower magnetic saturation to enable equivalent performance in terms of speed control (generally the windings are higher inductance than the DC version, since more turns and less amps). Second, the AC machine is often relying on an i ...

In an AC machine that undergoes an inductive type of start - like a squirrel cage induction or synchronous machine started "across-the-line" - the deciding factor on how often a machine can be started in succession is (almost always) the temperature reached by the bars and shorting end rings on the motor rotor. More specifically, it is the brazed joint between the two - because the braze material will "flow" at a lower temperature than the bar or ring material will deform or change properties ...

As for the good performance of a well-controlled variable frequency drive (VFD) system tank capacity is not an issue, my own criteria is to dimension them as expansion tanks, to take water hammering, and expansion due to temperature raises -if pipes are exposed to the sun for e.g., or water heating tanks are in the line.

For domestic and small civilian uses, a bigger tank will prevent start-stop cycles due to small leakages, but this will highly depend on relationship between set ...

All electrical loads (including power transmission cabling) are some combination of resistive (R), inductive (L), and capacitive (C) circuits. All AC voltage and current waveforms have some angle between them; the cosine of that angle is called "power factor" and is typically expressed as a per unit (or sometimes percentage) value.

Capacitate load in three phase is used to raise the power factor, since the circuit current in capacitive circuit leads the supply voltage. The measure ...

Energy storage will be playing an important role for the next years to come. Lots of companies are involve in the research and development of new type of battery cell designs to be used in new compensation systems technologies, like statcoms, while other relay in the use of flywheels in order to face the challenges that the new generation will be demanding.

The world is moving toward the ...

As we know that the laminations are insulated from each other to prevent the eddy current circulations between the adjacent laminations. Once lamination staking done, how we are gonna earth the core of any transformer or motor? For a large 11KV motor, I have seen a guide bar welded to the laminations at the core back side & that is grounded to body but that eventually lead to the shorting of insulations at one point.

For motors there is typically a welded bar on the back or the ...

On a centrifugal pump, the head at the discharge end should be 44% higher than if it were run at 50Hz, so the shaft torque would also be 44% higher, no? A throttling valve will restrict flow and reduce the load, but at the designed flow rate under 50Hz operation, the motor will still be operating 20% faster than it would at 50Hz. I'll take your word that this is acceptable.

What I would do would be to discuss the pump application with the manufacturer and see if it was possible to ...

Conversion of DC drive to AC drive is complex. Most important are the changes, impact or improvement in your power system (motor sizing, power factor, power quality, harmonics, efficiency, operation/maintenance, controls, etc.).

Many of the considerations for comparing AC and DC drive systems can be covered if you have a procedure for making an economic evaluation between the two technologies. Areas of comparison include: system purchase price, spare parts costs, motor cooling fac ...

Continuous level control, using a level sensor, would allow for cost-savings if applied using a properly-sized variable frequency drive (VFD). The VFD would allow more consistent level control, improve reliability in not requiring a control contactor and decrease operational costs. Employing a non-contact level sensor will supply a feedback to the VFD and would allow the pump to ramp up/down as the inflow increases/decreases. If the level goes too low, you could put the motor in an off state ...

Everybody would definitely prefer 0.0 Ohm resistance as well. However, discussing earth resistance (a.k.a. combined resistance of earthing system to the general mass of earth) involves answering the question: what is the purpose of making this resistance as low as practically possible? Or, better yet, what conditions the earthing arrangement needs to satisfy?

Does 1.0 Ohm resistance makes sense, if a $0.5MM facility is installed on solid limestone and achieving 1.0 Ohm resistance ...

Using wireless to control processes or devices is still being evaluated. Just as with earlier technologies (PLC, for instance), the trust factor simply isn't there - this is human nature when dealing with new technologies. We have done several small installations, and I recently completed a POC at a WWTP that exceeded our expectations.

To the interference issue: most wireless technologies use spread spectrum or OFDM modulation techniques, which are highly resistant to RFI. We were ...

Instrument security factor is the ratio of instrument limit primary current to the rated primary current. Instrument limit current of a metering current transformer is the maximum value of primary current beyond which current transformer core becomes saturated. Instrument security factor of current transformer is the significant factor for choosing the metering instruments which to be connected to the secondary of the current transformer. Security or Safety of the measuring unit is better, if ...