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AQ: Difference between ICCB, MCCB and MCB

The aforementioned types of Circuit Breakers are used in LV System and generally based on the same operating principle.
MCB and MCCB/ICCB have a bimetal heater for overload which releases the Contact s while for short circuit the trip / electromagnet hammers itself against moving contacts. The arc created by breaking contacts is extinguished in an arc chamber. Are defined as “Thermo-magnetic “ CBs , accordingly. It is operating characteristic addressing the overload by thermal action of the bimetal strip and instantaneously dealing with short circuit occurrences by electromagnetic action.

MCB – Miniature Circuit Breaker is suitable for domestic usage. Used to protect final circuits from O/C such as Overload & Short Circuit.
i- MCB is basically made in accordance to BS 3871, is now superseded by BS EN 60898 which recognizes type B, C & D.
Type B is suitable in domestic premises.
Type C is used in commercial & industrial applications.
Type D is suitable for application where a high in-rush current is expected.
ii- MCB is of low breaking / making capacity as well as low current rating compared with MCCB/ICCB. MCBs available in different number of poles (SP, DP, TP,,).

MCCB – Molded Case Circuit Breaker & Insulated Case Circuit breaker are also current limiting devices but with high making/ breaking capacity and current ratings compared with MCB. MCCB and ICCB are almost the same and both are manufactured in accordance to NEMA AB1/AB3 to suit industrial and commercial purposes.
The advent of electronic protection increased the use of them and the scope is widened like tolerances, range of time & current adjustment. By virtue of that a good discrimination can be achieved with accuracy about ±10%.
Eventually, MCCB/ICCB has advantages in the capability of accommodating further features which can be provided as
i. RCD.
ii. Under voltage device.
iii. A shunt trip coils that enabling remote tripping.
iv. Auxiliary switches for remote monitoring and/or control.

AQ: VFD PWM and PAM definition

PWM is shorted for Pulse Width Modulation, it’s a variable frequency drive (VFD) regulate way to change the pulse width according to certain rules to adjust the output volume and waveform.

PAM is shorted for Pulse Amplitude Modulation, it’s to change the pulse amplitude according to certain rules pulse amplitude pulse train to adjust the variable frequency drive output volume and waveform.

AQ: Can a VFD reduces motor starting kick?

At zero speed the motor requires torque which is flux (voltage) and current (mostly reactive). Only a little bit of active current to compensate for the motor power losses.
Only the power losses need to be drawn from the grid at that time, which means a very small amount of current. It may produce 200% current on the motor and pull only 10% current from the grid.

Of course, as the motor is accelerating, the motor will require kW and the current pulled from the grid will increase accordingly, as the active power consumed by the motor is increasing.

Regarding the kick of torque on the motor, it is controlled by the maximum current ramp limit or through the speed reference as the ramp rate defines the current and the derivative of that rate is the current rate. For this reason, many large machines will be started using an S-Curve speed reference where the S part will adjust the torque (current) rate to avoid stressing the mechanical components, especially if there is mechanical backlash in the gears.

Actually the starting method depends on the type of motor itself SR or SQ type the voltage supply, the motor capacity and motor function, for the MV Motor a liquid or oil starter was the best solution used before.

In case the operation process required a change in the equipment speed the variable frequency drive (Air or water-cooling) based on the drive capacity is the optimum and reliable solution.

Definitely it reduces starting kick of the motor. Actually, the degree of starting kick of a motor is depending upon the starting speed of the motor. If you start your motor at low speed you will have a low starting kick but if started at high speed, you will have high starting kick. This is generally the condition for low and high kw motors. One factors of varying the speed of motors is by varying the frequency of the motors (from the formula N=120f/p) and VFD drive is use to vary the frequency, thus varying the speed of the motors. But if used for starting only, this is expensive as there is more cheaper way like using the Soft Starter or use a WRIM/Slip-ring motors with LRH/Resistor Starters or other. Normally, variable frequency drive is use on operation with speed reduction/varying requirements at required number of time or continuously.

AQ: Why transformer rating is shown in KVA?

Transformers are rated in {VA, kVA, MVA etc.} due to flows of active and reactive power through transformer. In case of transformer we have active power losses as consequence of existence inside resistance of windings (primary and secondary) and existence of active losses of ferromagnetic core and other side we have reactive power losses as consequence of existence losses of magnetic flux (primary and secondary) and existence of reactive power losses of ferromagnetic core.

[VA]=sqrt(sqr[W]+sqr[VAr])

Transformer is rated in kVA by the manufacturer to inform users about the maximum power (voltage and current) that support it, the reason for not rating it in KW is that the active power (kW) is depend on the loads (lighting, machines..)

The simple answer is: It is because the kVA (or MVA) rating is only rating that matters to express a transformer’s “capacity” to allow the “passage” of power. That capacity is the thermal capacity dictated by the current it can carry at a given ambient temperature, regardless of the power factor. So combined with its voltage ratings, kVA (or MVA) is the value that matters. kW rating does not matter as transformer can handle unity power factor or in other words, a transformer can handle kW equal to its kVA rating at any time.

Remember that a transformer, as the name suggests, is only a transformation device or a pass through device and not a power producing device like a generator or an UPS, where their capacity to produce real power (kW) is an independent limit from the thermal ( kVA) limit.

To take it a step further, if you have an ability to cool the transformer further, you can augment the kVA (or MVA) rating of a transformer. This would explain having multiple kVA/ MVA ratings on transformers with forced cooling aids installed on them.

If you think of it, this is not different from a cable or a conductor’s capacity expression. Except that a transformer can have more than one voltage levels and different ampacities on primary and secondary, but the kVA rating remains the same on either side. So that makes kVA a more convenient way to express its thermal capacity vs. the amperes alone.

AQ: Variable frequency drive key functions

Soft Starter, Auto Transformer, Electrolyte, series resistance – wound rotor- etc,). The starting factor of VFD drive is usually 1 up to 1.2 with respect to the rated load current while for Direct On line about 5-6.

Moreover and as you know the variable frequency drive can control the speed of the AC motors in accordance to the formula N=120f/P rpm
where f = the supply frequency and P = number of the Poles.
According to this formula, Motor Speed can be changed either by changing/control the frequency or by changing the number of Poles of the Motor by which step changed in the RPM will be given, while the former gives continuous variable speed as per application demand.

However, as per newly developed power Semi Conductor IGCT based on PWM VFD became the most smart, effective and efficient control device in Industries since is associated also with protective and monitoring means.

From my experience, I know that variable frequency drive plays around with the frequency which the motor operates. It starts at low speed and varies the frequency to attain maximum speed. This reduces the high starting torque usually experienced when motors are started on DOL, Star/Delta etc. When you are driving delicate materials through your conveyors or pumping liquid through pipes etc., VFD plays a useful role. It reduces hammering in pipes usually experienced when using DOL. In large hotel application, variable frequency drive could be used with pressure switches to regulate water flow and reduce hammering when guests are showing. The volume of water required will determine the speed at which the motor runs through VFD control. However, very large KW motors at high voltage level are usually started DOL due to the cost of ac drive but that is when one has enough (power) capacity otherwise it will impact on other users in the network.

AQ: Control Servo motor with a variable frequency drive

Looking at those AC drives they recommend an Induction motor. A servo motor with permanent magnets which is not quite an induction motor. So, if a servo with permanent magnets can be used instead an induction with these kinds of AC drives.

Actually, the term “Servo” makes a reference about “feedback”, it means, whether we need a control loop, we are talking in terms about Servo, in this case, we have, or we know, the “feedback” by an encoder. Typical variable frequency drive doesn’t have a input for an encoder, so, if you want to control a Servo Motor with a VFD, you can move the motor, but you can’t control it.

A servo motor can be an induction servo, a brushless servo, a reluctance servo a dc servo – each of these can be either linear or rotary and can come with a variety of feedback such as tachometer, resolver of various pole counts, incremental or absolute encoders discreet or serial interface with different bus options, laser feedback, halls etc.

Then you come to the term variable frequency drive. Brushless servo amplifiers are also vfds. Do standard inverters have proper control of induction, and brushless motors. Some allow for a software switch, some allow for a firmware download, some don’t. Will inverters accept feedback – some have it built in, most that allow it do so by option cards, many do not.

Normal input in a variable frequency drive is, digital to start or stop, and we could have an analogic input to control by potentiometer.

Using AC Drives for the servo application is quite possible, provided the application is less demanding in critical positioning purpose.
There are number of makes that showcases pinpoint positioning of motor shaft being driven by AC Drives like Hitachi SJ700 / Emerson Uni drive SP / Danfoss FC etc.

Its beneficial to opt for the AC Drives as it supports SLVC [ VFD gives almost servo-like torque at low rpms if you give it encoder feedback ], multiple motors can be accessed, torque requirement can be met if required, power dips can be sustained using VFD’s.

AQ: VFD Kinetic Buffering and Flying restart

Voltage Loss ride through with flying restart:
In this method, when the voltage sag causes the variable frequency drive to reach its undervoltage trip level, the VFD drive will shut off the inverter section and thus remove power from the motor instead of tripping. The motor will coast down during the duration of the sag and, as soon as the voltage recovers, the VFD will start into the still-spinning motor and ramp up to set speed. How much the motor speed will drop depends on the inertia of the load and the duration of the sag.
You have to configure the VFD for flying restart. During low input voltage the inverter section is cut off to maintain the DC bus voltage. If the voltage restores before the DC bus voltage goes below the tripping value, the inverter is again put on but the driven load speed has already reduced due to brief period of no voltage at the motor terminal. Flying restart feature enables the variable frequency drive to restart the Motor at the same speed at which the motor is operating thus preventing any high current. So it is basically catching a spinning motor. Without flying restart high current will be observed once the inverter section is put ON. Flying restart feature is also helpful if you want to restart a motor which is already spinning.

Kinetic backup
This option, which is also provided by some variable frequency drive manufacturers, uses the energy stored in the mechanical load to keep the DC bus voltage from dropping down to the trip level. This is accomplished by running the inverter section during a voltage sag at a frequency slightly below the motor frequency, causing the motor to act as a generator. Similar to the flying restart option, the motor speed will drop while it is acting as a generator, however the advantage is that the motor is never disconnected from the drive. This option works best for those high-inertia loads.
Kinetic buffering is a feature to prevent the variable frequency drive from tripping during voltage sags. If the VFD trips due to DC bus undervoltage there is no need for kinetic buffering.

AQ: Transformer harmonics

The harmonics are created by the loads that the transformer supplies power to. If your loads include a high percentage of electronic loads like IT equipment, electronic ballast lighting, electronic motor controls, etc., there can be a very high amount of harmonics that circulate back to the transformer. The harmonics create an increase in the neutral currents. Most standard transformers are not designed to handle the higher harmonics and corresponding high temperature. Type K rated transformers are designed withstand the higher harmonics, without derating the transformer or limiting its maximum load. There are harmonics filters on the market as well as the use of isolation transformers.

B/H curve of the magnetic material forming the transformer core is not linear, so if a sinusoidal voltage is being applied for a sinusoidal current (and hence sinusoidal flux & a sinusoidal secondary voltage), the magnetizing current is not sinusoidal. Thus the magnetizing current of a transformer having an applied sinusoidal voltage will comprise a fundamental component and various harmonics. The magnitude and composition of these harmonics will depend on the magnetizing characteristic of the core material and the value of the peak flux density.

By the way:
– The standard Transformer “Non-saturated” generate Harmonics only in transient case when the power is supplied, and after this too small time it doesn’t generate any kind of Harmonics
– The Transformer generates Harmonics if it’s saturated
– We should take care when selecting the Transformer’s Power if there are a lot of installed Non-linear Loads, so for this case, we can select the power after define the correction factor by using the special curve done by “IEC”, or calculating this factor by using a special Formula done by “UTE – France”.

All AC signals are sinusoidal and periodic. These periodic signals can be resolved into a kind of trigonometric series – fourier series which is a summation of a fundamental and multiples of fundamental frequency.

The moment there is slight distortion from sinusoidal nature , it leads to harmonics in addition to the fundamental signal.. One way is to use DC signals… no harmonics.

As long as the AC signal is perfect sinusoidal , load & source is linear, there will be no harmonics. The way to get rid of harmonics is to have perfect source and perfect loads.

The non linearity introduced due to energy storing magnetic circuits, switching circuits, energy converting & inverting circuits distort the waveform to non sinusoidal. Therefore leads to harmonics. .

The way, there are antibiotic medicines for diseases. One needs to install the filter devices, which produce counter currents to suppress the effects of harmonics. The filters contain capacitors, inductors & power electronic components which are switched in anti-phase to harmonics producing elements. Thereby absorb harmonics.

AQ: Charging Power transformer through lower rated grid auto Transformer

AQ: Frequent tripping in Unit Station Transformer

We are facing problem of frequent tripping in Unit Station Transformer -2 during heavy rain. During checking, not found any abnormalities
1. Fault UST2 OTI Trip Observation- * Direct OTI trip initiated without Alarm
* Oil temperature is normal
* Tripping contacts are not physically operated
* OTI trip contacts are operated frequently as per in Disturbance record Action taken-Spare Core of another cable used for OTI Trip contact

2. UST2 PRD Trip
* PRV contacts are not physically operated
* PRV trip contacts are operated frequently as per in Disturbance record New cable used for all contacts

Make sure for rainy conditions the marshalling box is properly closed and weather protected as moist condition also leads to tracking and may simulate tripping inadvertently. Please also make sure the tripping impulse from OTT/WTT/BT/MOG/PRD etc are driving a mechanical hand reset type VAJH type relay and the output contacts of this relays are used for trip ckt initiation.
If these are substituted by numerical relay binary inputs then also there will be a problem of spurious tripping.

Make sure the grounding of the multicore control cable if sheathed the grounding should be at TRPP panel end only, if armoured cable is used the armour grounding at the panel end only.’

I am assuming that the trip circuit is floating DC (ungrounded). If so, the moisture could be causing a “sneak” circuit, otherwise known as a “hot short” in the tripping circuit, which essentially bypasses the sensing relay contacts and actuates the tripping relay coil. I would check the cabling between the sensing relay contacts and the trip relay coil and the cabling on the hot side of the sensing relay contacts for insulation problems.