Blog

From first principles and trying to make it really simple. In an inductive circuit, the difference in potential causes a current to flow. As the current flows it induces a magnetic field. If this magnetic field in interacting with other magnetic fields (Itself or another) the other magnetic field suppresses the buildup of the magnetic field that this current is trying to establish. This inhibits the current rise as the voltage increases. In a DC circuit this happens at switch on and to a less ...

"Unearthed neutral" is the NORMAL configuration for AC electrical machines, regardless whether they are connected "delta" or "wye". In the case of delta, there is no neutral connection available. In the case of wye, the neutral point is physically located somewhere within the machine winding - but is not brought out to the world at large.

"Unearthed" neutral systems are the PREFERRED network for marine installations (particularly shipboard), since it is disadvantageous to pass curr ...

The answer depends upon the type of motor. A synchronous motor can usually be driven as an alternator by simply substituting a prime mover for the mechanical load. Things are less clear for an asynchronous/induction motor since it will depend upon whether it is attached to a three-phase supply or not.

If there is a three phase source then the prime mover has to operate at greater than synchronous speed, however its output frequency will be locked to the line frequency, assuming th ...

Depending on your local electrical code, if you need a grounded (earthed) system, you can use a 400V-Delta to 208/120V WYE transformer to feed a variable frequency drive (VFD), and then set it a 60Hz to supply 208V 60Hz to the motor. Although I do not normally recommend it, if it is appropriate for you to run the system ungrounded (unearthed), then you can feed the VFD with through a 400V-Delta to 230V-Delta transformer and then supply the motor with 230V 60Hz power from the VFD. (As with mos ...

The torque speed curves are completely different between an induction motor and slip ring motor. You need to understand the differences. If you have selected your slip ring motor to limit starting currents you can short out the slip rings and run your motor but the starting torque is reduced unless you use some features on the VFD to boost starting torque.

If you are using it as torque control on a crane then y ...

With larger motors it is normal to insulate the NDE bearing. The normal reason is that the majority of movement will come from the DE. This is done by machining out the end shield to take a sleeve into which your bearing will sit. Material selection is vital. Please be aware that this insulating requirement would include any lubrication, vibration, temperature monitoring equipment and the like. If your motor is very big the insulation material sits between the base and the pedestal but again ...

From first principle, the diesel engine can run very hard for a short period of time. This is called the STANDBY time. The PRIME POWER is when the engine runs at load factor not exceeding 80% power for any 24 hour period. Then there is the continuous rating which is BASE LOAD. This causes problems when you order a STANDBY generator at say Engine Gross Power of 1437kWm Net Engine power of 1395kWm which gives 1325kWe. Where m = mechanical and e = electrical. For the same set up, PRIME POWER Gro ...

Variable frequency drives should be protected by fuses. Why? Because the VFD itself has all the protection you require for your motor so the only item you are protecting is the Rectifier and DC bus. Then you will find it is actually really impossible to protect it as when it has a fault it is generally catastrophic and all you wish to do is to remove the faulty VFD from the network to enable the rest to continue. This must be as fast as possible to limit the damage and MCBs don’t really ...

In Europe we have a 1-phase 220v/240v power supply to home, which is ONE hot line, unlike America that have TWO hot lines (120v of each) into their homes which is therefore logically and technically a 2-phase power supply, even if they will admit the obvious.

There are two ways we can be feed our 1-phase supply from the pole mounted 3-phase 11kV line. The primary of that transformer can be feed from either one of the 3-phase lines, that would be 6.4kV to Earth, or feed from two of ...

For a surface mounted PM, the air gap flux density should be as close as possible to the value of the max BH product (energy) of the PM material. Other choices could be taken in case of special working condition has to be considered (overloads, high temperature of magnets, etc...).

If the PM has a Br of 1.2T, you will never be able to have an airgap flux density of 1.2T because in the magnetic circuit, the airgap is a "load" and the magnet is able to achieve the Br only in "no load ...

Having the motor star point earthed won't have any effect on protecting the motor windings during a fault condition.

A power system must be referenced to earth at a single point, usually at the supply transformer or generator star point. Referencing the system to earth at multiple points on a distribution system, for example, at the star point on your motor (in addition to the supply transformer star point), would be bad practice as it could lead to the undesirable condition of cre ...

The Dahlander motor refers to a specific set of optional connections to the main stator winding. These are done OUTSIDE the motor frame (or at least within the main terminal box).

This gives a single pole count for the motor, and the VFD can effectively control the machine as if it were not capable of multiple connections.

The VFD - by itself - cannot "switch" the lead connections to achieve a different pole count. However, it can modify both the applied voltage and freq ...

Harmonic waves are commonly generated by any equipment that utilizes capacitors, think VFD and a multitude of single phase equipment. You will probably have to install power factor correction to get the best from your generators and these capacitors also generate harmonics. Unlike faults, the harmonics generated are not likely to trip the equipment circuit breakers and thus affect the power supply system. It causes ripples in the voltage sinus wave, one effect this produces in practical terms ...

For an AC machine (induction OR synchronous), current required for a given load is dependent on applied voltage, overall efficiency, and power factor. With power factor at unity (1.00 per unit), current is at a minimum - but there is no appreciable amount of reactive power (either leading OR lagging).

Some machine designs have a "low" power factor, but that is typically because another performance parameter is being optimized (i.e. deemed more important to the user). For example - ...

Reactive Power is when the Current flow, caused by AC Voltage applied across a device, results in the Current flow being either ahead or behind the applied AC Voltage.

Reactive devices will store some Energy as Voltage is applied, and they will return that Energy later in the sine-wave... Think of a Spring... You put power into a Spring, then when you reduce or remove the force, such as Voltage, the Spring will spring back returning the energy put into it... No Energy is absorbed, ...

A three-winding transformer can be seen as two transformers in one and presenting a short-circuit level that will be dependent on the coupling reactance between the two secondaries. For ex., two generators feeding one common bus and with only one transformer has the least transformation cost, but the highest short-circuit level on that bus. The same generator, each one with their transformers present now the lowest Icc, but the highest transformation cost. With the 3-winding transformer, the ...

There are a couple of thoughts on this. You could connect a PLC to a VFD using a communication protocol like Ethernet IP or Modbus to name a few or using a collection of analog and digital signals. I prefer a combination of the two ideas. For command and control signals I use analog or digital signals. For all other data I use a communication protocol. The reason is that any decent technician can troubleshoot an analog or digital signal but troubleshooting a communication protocol takes a ver ...

From my view-point of a power system analyst, I see the following:
In industrial environments (LV and MV systems), the higher short-circuit level advantages are: less voltage drops due to transient loads (ex: motor starting across the line); less impact of harmonic loads (harmonic currents cannot cause too much variation on the harmonic voltage distortion where the short-circuit level is expected to be higher); less over voltages due to bolted ground faults. Disadvantages: higher incide ...

The workhorse of the electromagnetic protective relays is the induction disk type, which is used to provide a time delay function after pickup with a delay that corresponds to one of several inverse time characteristics that differ by the amount of steepness in the curve. The induction disk relay requires calibration to obtain accurate pickup and time characteristics that follow closely to that shown on the manufacturer's time current curve. Various things can happen over time to cause the re ...

Induction motors are selected on torque requirements at a specific speed not kilowatts. The mechanical power is the rated power of the motor at the shaft not the input of the electrical energy. The difference between the two is the efficiency. The torque is the mechanical output power in kW x 9950 divided by the speed in rpm. When the induction motor starts the speed is zero and the current maximum. Power of the flux interaction causes the shaft to move slightly. This small number is divided ...