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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.

AQ: Pressure switch on three phase motor

Q: Is there a way of connecting a three phase pressure control switch on a three phase motor. Also is there a three phase float switch for a three phase submersible pump i know of a single phase switch.

A: The switch only needs to have a single contact since you use a three-phase motor controller to operate the motor. The switch is wired into the low voltage contactor coil circuit to turn the motor on and off.

You can connect a pressure switch for the purpose of motor control on its low level pressure or high level pressure. It is advisable to utilize pressure switch on the control cct, and connect the contactor coils through the auxiliary NC/NO contacts depending on whether you are interested on low pressure or high pressure control. It is not good practice to allow power cct through control ccts. You don’t need a 3 phase float switch to achieve controls of a 3phase submersible pump. You should be interested in the auxiliary terminals which will allow control flexibility for low level and upper level control. A single phase float switch will give you desired result. If you are controlling more than 1 no. 3phase motors located at different places from the same pressure signal, then your 3 phase pressure switch can be employed to control the different motors separately. In addition, some terminals could be used for indication/annunciation purposes. However, a single phase pressure switch can give you all the controls you need for a 3phase motor.

AQ: “Hissing sound” in SF 6 Gas insulated HV Switch Gear

This could be internal corona discharge. The switchgear should be de-energized and closely examined. That means pump out the SF6 and take it apart. Examine all insulating components.

Especially if the sound can be localized to portions of the switchgear which do not have bushings for connection to overhead lines. Even if the sound is in the area of air bushings, deenergizing will allow more in-depth inspection and addressing any sharp edges or cracked insulators, etc.

Take this pieces of equipment out of service immediately, perform a “hi-pot” or high potential test on the various elements of the switchgear and attempt to locate the area that is “leaking” to ground (or between phases). Inspect closely for indications of tracking on insulators from corona discharge and replace any compromised components. After component replacement, installation of new SF6 gas and other repairs, re-run the Hi-Pot test to confirm that the switchgear is able to withstand voltages at least 50% greater than the name-plate rating. Of course all of this advice is worthless if the unit has already failed.

Remember that a Hi-pot is actually a destructive test. It challenges the insulation to the point of breakdown. Check the vendor recommendations before you Hi-pot equipment that has been in-service.

AQ: Variable frequency drive Constant Torque/Variable Torque

A typical variable torque application would be a centrifugal pump. A typical constant torque application would be a conveyor, and there are positive displacement pumps that are also constant torque. Have a talk with a mechanical engineer, get them to show you curves and explain.

DBR stands for Dynamic braking resistor. Regeneration will happen when the motor rotates a speed higher than the speed which corresponds to the frequency setpoint ie.. the rotor speed is more than the speed of the rotating magnetic field.
Regeneration feeds back energy to the drive which results in DC bus overvoltage. To prevent the drive from tripping due to DC bus overvoltage the DBRs are used. The regenerative energy is discharged in the resistor as heat.

Regenerative Breaking – we used to have VFD on a vehicle rolling road. So when the car is travelling faster than the VFD, the VFD generate back into the power supply – causing a break effect. If you had a large mass- large inertia that you want to stop quickly, you need to break the load- you can do that with regenerative breaking. Otherwise, disconnecting the variable frequency drive, will mean your load just freely rotates, and that can mean it will take 30 minute to come to a stop for a large inertia.

Active Front end- I first came across this term with ABB. It is all to do with how to mitigate harmonics from VFDs. You can use phase shift transformers, but with modern electronics, you can use a opposite phase current to counter act the harmonics generated from the VFD. So the overall impact on the network is small.
In active front end technology the rectifier is basically an inverter with IGBTs.
The main advantage are:
1) Low current THD <5 %
2) It is basically a four quadrant rectifier .Referring my last post please note that you will not require a DBR with AFE. The increase in voltage of DC Bus due to regeneration can be fed back to the input AC supply in the form of energy. So you don’t require a DBR.
3) AFE drives have very good immunity to input voltage fluctuations.

Just an advice. Please go through variable frequency drive literatures (available in plenty) to have a good understanding of the different VFD technologies.
Selection of VFD requires proper understanding of the VFDs and the overall electrical system. There are lots of marketing gimmicks in the world of VFD. Always be careful before selecting a VFD specially higher KW drives.

For large drives, you need to speak with supplier to configure your machine correctly. There are many options, but yes active front ends are available. But there are other solutions; ASI Robicon use a current driven VFD, so harmonics are lessened in the first place, so an active front end is not the right terminology. It is a different solution. I used a 10MW version of that type of ac drive. I think Siemens have bought the company since.

AQ: Motor power cable – bigger or smaller?

When a choosing a power cable for a motor, we prefer using one larger diameter cable than two smaller diameter cables in parallel, although it would cost less to do so. Why?

1. Conductors/Cables/Feeders in parallel connection generally are not recommended unless there is no option, therefore it can be adopted under the following conditions:
i. Cables are of the same material and cross section area.
ii. Are of the same route and length.
iii. The sum of the current carring capacity of the parallel circuits after applying all necessary applicable correction factors should be greater than the nominal regulated current of the protective device.
iv. The current carrying capacity (before derating) shall be not less than 300A (according to the local authority/Service provider requirement/regulation).
v. Capability of addressing the Thermal & electrodynamics constraints in proper way.

2. Some designs call for parallel connection so as to:
i. Overcome the voltage drop.
ii. Avoid the difficulties of installing big size cables (bending, pulling) due corridor limitation,etc.
iii. Meet the Power demand.
iv. Mitigate the cost (Costwise).

3. For electrical Motors, two connections are normally required. One from MDB to Motor CP and other from CP to the Motor.
By virtue of the requirement of Delta/star starter, two cables are required (Mandatory) between CP & motor (one will be dead just after changing to delta connection).
While the connection from the MDB to CP will be one, sized according to the Motor rating.

However, Parallel connection of Feeders need an expert engineer(s) to meet the requirement since Short Circuit fault protection for parallel circuits require further evaluation from the Engineer that the impact of the short circuit current within the parallel section will have severe fault due to fault current path that can occur in addition subtransient contribution of the downstream system.