Different Type Of Braking Used in AC Drives:
1. Dynamic braking(DB) in the inverter comes into operation during deceleration of the motor. During deceleration, motor works like an induction generator and returns the kinetic energy of the load to DC link of the inverter. Dynamic braking control connects the DB resistor across the DC link using a power switch to dissipate the energy in the resistor. The power switch is controlled by the PWM firing pattern decided by the microprocessor, which determines the switching pattern depending on DC link voltage. This method is recommended when deceleration time is short compared to operating duration. Constant deceleration torque can be achieved by this method.
2. In regenerative braking, load kinetic energy is returned to the DC link due to induction generator operation of motor. Energy from the DC link is fed back to the source through regenerative converter or by PWM converter. This method is recommended wherein braking torque is either needed continuously or when load inertia is constantly changing making it impossible to design DB resistors which will give desired braking action under all operating conditions or when great potential of energy savings is there for loads with considerable regeneration duty.
3. During DC injection braking, DC voltage is applied between two motor terminals. Losses take place in the rotor and braking torque is generated. Braking torque generated falls with speed. This method is normally used at low speeds only for good parking ac-curacies

HARMONICS Effects Basics:
1. The main problems caused by harmonic currents are:
a. Over-loading of neutrals
b. Increase of losses in the transformers
c. Increase of skin effect
2. The main effects of the harmonics voltages are:
d. Voltage distortion
e. Disturbances in the torque of induction motors
a. Overloading of neutrals
In a three phase symmetric and balanced system with neutral, the wave-forms Between the phases are shifted by a 120° phase angle so that, when the phases Are equally loaded, the current in the neutral is zero. The presence of unbalanced Loads (phase-to-phase, phase-to-neutral etc.) allows the flowing of an Unbalanced current in the neutral.
b. Increase of losses in the transformers
The effects of harmonics inside the transformers involve mainly three aspects:
i. The iron losses are due to the hysteresis phenomenon and to the losses Caused by eddy currents; the losses due to hysteresis are proportional to The frequency, whereas the losses due to eddy currents depend on the square Of the frequency.
ii. The copper losses correspond to the power dissipated by Joule effect in the Transformer winding. As the frequency rises (starting from 350 Hz) the current tends to thicken on the surface of the conductors (skin effect); under these circumstances, the conductors offer a smaller cross section to the current flow, since the losses by Joule effect increase. These two first aspects affect the overheating which sometimes causes a de-rating of the transformer.
iii. The third aspect is relevant to the effects of the triple-N harmonics (homopolar harmonics) on the transformer winding. In case of delta winding, the harmonics flow through the winding and do not propagate upstream towards the network since they are all in phase; the delta winding therefore represent a barrier for triple-N harmonics, but it is necessary to pay particular attention to this type of harmonic components for a correct dimensioning of the transformer.
c) Increase of skin effect
When the frequency rises, the current tends to flow on the outer surface of a Conductor. This phenomenon is known as skin effect and is more pronounced At high frequencies. At 50 Hz power supply frequency, skin effect is negligible, But above 350 Hz, which corresponds to the 7th harmonic, the cross section For the current flow reduces, thus increasing the resistance and causing Additional losses and heating. In the presence of high-order harmonics, it is necessary to take skin effect into Account, because it affects the life of cables. In order to overcome this problem, It is possible to use multiple conductor cables or bus bar systems formed by More elementary isolated conductors.
d) Voltage distortion
The distorted load current drawn by the nonlinear load causes a distorted voltage Drop in the cable impedance. The resultant distorted voltage waveform is applied To all other loads connected to the same circuit, causing harmonic currents to Flow in them, even if they are linear loads. The solution consists in separating the circuits which supply harmonic generating Loads from those supplying loads sensitive to harmonics.
e) Disturbances in the torque of induction motors
Harmonic voltage distortion causes increased eddy current losses in the motors, In the same way as seen for transformers. The additional losses are due to the generation of harmonic fields in the stator, each of which is trying to rotate the motor at a different speed, both forwards (1st, 4th, 7th, ...) as well as backwards (2nd, 5th, 8th, ...). High frequency currents induced in the rotor further increase losses.

The power output of a fan is expressed as:
Q x D P
k W fan = ____________________
102 x h m x h fan
where kW fan - fan power output
Q - fan volume, in m³/s
D P - pressure developed by the fan, in mm WC
h m -Efficiency of motor
h fan -Efficiency of fan

The content is studied through online/offline resources and drafted, feedback are invited if any correction is required.
To Share Basics about NGT and NGR for generator protection: 2 Neutral grounding system:
1. generator neutral grounding resistor termed as (NGR)
2. generator neutral grounding transformer termed as (NGT)
The presence of in-plant generation adds flexibility and reliability to the consumer’s Electrical Supply Sources, but it also adds certain complexities in protection and control. The additional Protection often needed includes standard generator protective relaying utilising fault current sensing, which includes fault current travelling back to system utilising GROUNDING.
The term “grounding” describes and encompasses both systems grounding and equipment grounding. The basic difference between system and equipment grounding is that system grounding involves grounding circuit conductors that are current carrying under normal operation, where equipment grounding involves grounding of all non-current carrying metallic parts that enclose the circuit conductors. A grounding electrode or several grounding electrodes tied together as a system provides the reference ground and the means for connection to earth. Simple definitions are:
1. Grounding is arrangement through which a direct connection to the earth is established
2. Equipment grounding is the conductive path(s) installed to connect normally non-current-carrying metal parts of equipment together and to the system grounding
3. Ground fault current path is an electrically conductive path from the point of a ground fault on a wiring system through normally non-current-carrying conductors, equipment, or the earth to the electrical supply source.
Effective ground fault current path is an intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under, ground fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the over current protective device or ground fault detectors on high impedance grounded systems.
All current tries to return back to its source to complete the circuit.
A live conductor striking an un-grounded conductive metal surface will go through whatever, or whoever, connects the metal object to earth, and back to the power source.
Because having large amounts of electrical current unintentionally going through people is generally a bad thing, we want the over current protection device to trip as quickly as possible, removing the fault current before damage, injury, or death can occur. The best means of doing this for ground fault current is to provide a low enough impedance path such that ground fault current is high enough in magnitude to quickly trip the over current protective device and low enough to avoid any damages.
Generator Neutral Grounding systems are designed to allow over current devices to quickly open when a line-to-ground fault occurs. The purpose of both NGR & NGT are to limit the fault current to a level sufficient enough to operate Ground fault detection relays.
NGR also termed as Impedance Grounding (High Resistance addition)
An intentional impedance, resistance or reactance, is inserted in the ground circuit. The most common impedance ground used 600VAC and below, is high resistance grounding. A system is high resistance grounded when the connection between the neutral and ground has been made with intentional impedance (grounding resistor) in the connection. The neutral resistance is selected to limit the ground fault current that can flow to a level sufficient to pick up ground fault detection relays. One of the reasons for using a high resistance grounding system is that no trip is required on the first ground fault. The ground fault current, being limited by the resistor to a low value can be tolerated for some time, so that continuity of power to the critical load may be maintained until such time as the first ground fault can be corrected in a planned manner. High resistance grounding permits continuity of power with the first ground fault, but subsequent ground faults will result in a high magnitude phase to phase fault current that will operate the phase over current devices and power would be lost. Other reasons for high resistance system grounding are that system over voltage’s are held to acceptable levels during ground faults, and the potentially destructive effects of arcing ground faults that occur in high capacity, solidly grounded systems are reduced or eliminated.
Resistance grounding provides protection of a transformer/generator by solving the problem of transient over-voltages thereby reducing equipment damage. It accomplishes this by allowing the magnitude of fault current to be predetermined/limited by a simple Ohms law calculation I=E/R where I=fault current, E=Line to neutral voltage and R=Ohmic value of resistor. In addition, limiting fault current to predetermined maximum values permits the designer to selectively co-ordinate the operation of protective devices, which minimises system disruption and allows quick location of the fault.
NGT is neutral grounding utilised in High Voltage systems, utilising single-phase transformer
A single phase distribution transformer is utilised in conjunction with a loading resistor to provide high resistance grounding. This allows the system to operate as an un-grounded system under normal conditions, while still retaining the ability to limit fault currents during a fault condition. The primary of the grounding transformer is connected from the generator neutral to ground. The loading resistor is connected across the grounding transformer secondary. The resistor should be sized the same way as a neutral grounding resistor, except it will be reduced in value by the square of the turn’s ratio of the grounding transformer. When a ground fault occurs, downstream of the grounding transformer, ground fault current flows through the fault, back through ground to the grounding transformer. The loading resistor then limits the current flow in the secondary winding's, which in turn limits the flow of the ground fault back into the system through the primary of the grounding transformer. The resistor is normally sized to allow a primary ground fault current tin the range of 2 to 12 amps, and is rated for one minute duty. The grounding transformer should be sized accordingly. The transformer primary voltage rating should be the same as the system line-to-line voltage. The secondary voltage is normally 120 or 240 volts.
NGT is utilised for economic reasons. Let us see how:
Supposing you have an 11kV System, whose neutral you want to ground through a resistance. The desired ground fault current, let us say, is 10A. Now, if you want to connect a resistor directly in the path between the system neutral and earth, the value of the resistor would be (6350/10 = 635 Ohms) and the voltage rating of the resistor would be 6350V. A 6350V, 635 Ohms resistor would not be cheap.
Now, if you opt for a single phase 6350V/240V, neutral grounding transformer, whose 6350V winding is connected in the neutral to ground path, you can connect a simple 0.9 Ohms resistor across the 240V secondary of this neutral grounding transformer. This 0.9 Ohms resistor at 240V side will reflect multiplied by the square of the turn’s ratio; at the HV side (i.e.) 6350/240 is 26.45 whose square is 700. The 0.9 Ohms resistor connected across the 240V secondary of the neutral grounding transformer would appear as (700 x 0.9 = about 630 Ohms). And, the added advantage is that this resistor needs to be insulated only for 240V. A reduced Ohmic value resistor, with a reduced insulation rating is cheaper. And, the neutral grounding transformer can be short-time rated, to optimize on the size & cost of the neutral grounding transformer.


RFI -Radio Frequency Interference Reduction in Drive:
In the use of Variable Frequency Drives or Adjustable Speed Drives, several types of emission may be generated on the power line. These might include low-order harmonics, or EMI/RFI noise. To provide a complete power line noise solution, frequencies from the 60Hz power line and above must be considered. This includes EMI/RFI noise in the 150kHz to 30MHz band which may interfere with TV’s, radios, computers, meters and controllers.
EMI/RFI Noise Reduction
Variable Frequency Drives (VFD’s) produce noise in the 150kHz to 30MHz band, primarily contained in the switching edges of the PWM controller. To reduce noise in that band, a filter containing a combination of high frequency inductors and capacitors may be utilized. To put it simply, inductors act as open circuits and the capacitors act as short circuits at high frequencies while passing the lower power line frequencies untouched. Thus the noise must pass through a nearly open circuit and what little gets through is then shorted out. Together, these parts can decrease noise to less that one one-thousandth (-60dB) of its original value in the critical band. The following graphic demonstrates basic operation.
Design Challenges
Reducing EMI/RFI (Electromagnetic/Radio-frequency Interference) noise using filtering technologies presents significant design challenges. Since high frequency energy can "jump" around components and travel along leads, filter designers must use high permeability ferrites, specialty capacitors, short, large diameter leads and small enclosures. As VFD’s produce noise primarily on the line with respect to ground, designers should focus on what is called "Common Mode" insertion loss. A 60dB-noise reduction from 150kHz to 30MHz should provide enough EMI/RFI noise reduction in the noted frequency band to limit the disturbance of other equipment on the power line.
Compliance to Noise Standards
Considering equipment disturbances that EMI conducted emissions can cause, industrial power line noise standards have been established worldwide. Among these are the American FCC15 subpart J class A, German VDE0871A and European CISPR11A and B (CE-EMI Directive) specifications.
Although each specification limits noise in a range, an overview shows that the main focus is on frequencies from 150kHz to 30MHz with a noise maximum of around 66dBm V (2mV) on the power line. A filter would need to bring noise signals down in the order of 2V down to the critical 2mV level.
To do this, a 1000:1 (60dB) reduction ratio is needed. This can be illustrated in a highly simplified model. If a 600V source is providing 16A, it has an effective impedance of 600/16=38 ohm . If the noise on the power line is 2V, then the noise current can be represented as 2/38=0.05A. The noise current to total current ratio is 0.05/16=0.33%. An effective filter would have a minimum insertion loss of 60dB into an ideal 50ohm resistor to match the reduction ratio needed and would be designed to accommodate a noise current ratio of 2-3%. This could theoretically provide 60dB reduction for up to a 15V line noise which could result in as large as a 15A noise current for a similarly designed 600A filter. One should choose filters with generous design margins to help reduce even the most difficult noise to meet world noise standards.
Correct Filter Application
An EMI/RFI filter will be most effective when installed correctly. Although the filter reduces noise on the power lines (conducted emissions), it should be located as close as practical to the drive to reduce broadcasting of the noise (radiated emissions) from the power lines themselves. As noise is shorted to ground through the capacitors, that short must be maintained by good grounding of the filter. A short, heavy, stranded conductor from the filter chassis to the drive’s main ground bus can achieve this.
A battery braid, Litz wire, or flexible welding cable with many fine strands is recommended for best grounding performance. Radiation of noise is also a concern for power line routing as it can effectively bypass the filter. Input and output filter leads should be separated by a maximum practical distance within enclosures and should be routed separately in interconnecting conduits when used.
A combination of good grounding and input/output lead separation will assure the best possible filter operation.
Part of the Complete Solution
As transistors in drives switch at higher frequencies, drives will come down in size and price. However, EMI/RFI emissions and the filtering technologies to address them are an important concern in good drive installations. EMI/RFI filters will become a part of the complete drive system. They will reduce power line emissions and interference associated with drives by reducing noise to limits that will not disturb nearby equipment and allow for compliance with agency guidelines.

Basic Motor Formulas And Calculations
The formulas and calculations which appear below should be used for estimating purposes only. It is the responsibility of the customer to specify the required motor Hp, Torque, and accelerating time for his application. The salesman may wish to check the customers specified values with the formulas in this section, however, if there is serious doubt concerning the customers application or if the customer requires guaranteed motor/application performance, the Product Department Customer Service group should be contacted.
Rules Of Thumb (Approximation)
At 1800 rpm, a motor develops a 3 lb.ft. per hp
At 1200 rpm, a motor develops a 4.5 lb.ft. per hp
At 575 volts, a 3-phase motor draws 1 amp per hp
At 460 volts, a 3-phase motor draws 1.25 amp per hp
At 230 volts a 3-phase motor draws 2.5 amp per hp
At 230 volts, a single-phase motor draws 5 amp per hp
At 115 volts, a single-phase motor draws 10 amp per hp
Mechanical Formulas
Torque in lb.ft. = HP x 5250
rpm HP = Torque x rpm
5250 rpm = 120 x Frequency
No. of Poles
Temperature Conversion
Deg C = (Deg F - 32) x 5/9
Deg F = (Deg C x 9/5) + 32
High Inertia Loads
t = WK2 x rpm
308 x T av. WK2 = inertia in lb.ft.2
t = accelerating time in sec.
T = Av. accelerating torque lb.ft..
T = WK2 x rpm
308 x t
inertia reflected to motor = Load Inertia Load rpm
Motor rpm 2
Synchronous Speed, Frequency And Number Of Poles Of AC Motors
ns = 120 x f
P f = P x ns
120 P = 120 x f
ns
Relation Between Horsepower, Torque, And Speed
HP = T x n
5250 T = 5250 HP
n n = 5250 HP
T
Motor Slip
% Slip = ns - n
ns x 100
Symbols
I = current in amperes
E = voltage in volts
KW = power in kilowatts
KVA = apparent power in kilo-volt-amperes
HP = output power in horsepower
n = motor speed in revolutions per minute (RPM)
ns = synchronous speed in revolutions per minute (RPM)
P = number of poles
f = frequency in cycles per second (CPS)
T = torque in pound-feet
EFF = efficiency as a decimal
PF = power factor as a decimal
Basic Horsepower Calculations
Horsepower is work done per unit of time. One HP equals 33,000 ft-lb of work per minute. When work is done by a source of torque (T) to produce (M) rotations about an axis, the work done is:
radius x 2 x rpm x lb. or 2 TM
When rotation is at the rate N rpm, the HP delivered is:
HP = radius x 2 x rpm x lb.
33,000 = TN
5,250

For vertical or hoisting motion:
HP = W x S
33,000 x E
Where:
W = total weight in lbs. to be raised by motor
S = hoisting speed in feet per minute
E = overall mechanical efficiency of hoist and gearing. For purposes of estimating
E = .65 for eff. of hoist and connected gear.

For fans and blowers:
HP = Volume (cfm) x Head (inches of water)
6356 x Mechanical Efficiency of Fan

Or
HP = Volume (cfm) x Pressure (lb. Per sq. ft.)
3300 x Mechanical Efficiency of Fan

Or
HP = Volume (cfm) x Pressure (lb. Per sq. in.)
229 x Mechanical Efficiency of Fan

For purpose of estimating, the eff. of a fan or blower may be assumed to be 0.65.
Note: Air Capacity (cfm) varies directly with fan speed. Developed Pressure varies with square of fan speed. Hp varies with cube of fan speed.
For pumps:
HP = GPM x Pressure in lb. Per sq. in. x Specific Grav.
1713 x Mechanical Efficiency of Pump

Or
HP = GPM x Total Dynamic Head in Feet x S.G.
3960 x Mechanical Efficiency of Pump

where Total Dynamic Head = Static Head + Friction Head

For estimating, pump efficiency may be assumed at 0.70.
Accelerating Torque
The equivalent inertia of an adjustable speed drive indicates the energy required to keep the system running. However, starting or accelerating the system requires extra energy.
The torque required to accelerate a body is equal to the WK2 of the body, times the change in RPM, divided by 308 times the interval (in seconds) in which this acceleration takes place:
ACCELERATING TORQUE = WK2N (in lb.ft.)
308t

Where:
N = Change in RPM
W = Weight in Lbs.
K = Radius of gyration
t = Time of acceleration (secs.)
WK2 = Equivalent Inertia
308 = Constant of proportionality

Or
TAcc = WK2N
308t

The constant (308) is derived by transferring linear motion to angular motion, and considering acceleration due to gravity. If, for example, we have simply a prime mover and a load with no speed adjustment

ACRONYMS in AC DRIVES
PWM pulse width modulated) is a square wave output chopped to approximate an AC sine wave.
SCR (silicon controlled rectifier) is an electronic power device that blocks the passage of current when the voltage across is negative. When the voltage across the SCR is positive, current can pass through it if a small positive signal is applied to its control input, which is called a gate. The SCR can convert AC to DC. Using a control activated when the SCR gate is triggered, the DC amount can also be controlled.
SLD (signal level detector) is a device that compares one signal to another. The SLD outputs the result as a zero if A < B and a one if A > B. The SLD device can be created in either hardware or software. Zero speed is one type of signal generated by an SLD. Speed is compared to a constant, zero (or around zero). The output is a zero when the speed is < zero and one when speed is > zero. SLDs usually have adjustments for taking absolute values of signals, adding time delays, inverting values, etc.
TOC (timed overcurrent) can be used interchangeably with TOL. Some manufacturers use TOL to pertain to motor protection and TOC for drive protection. In this case, it protects the drive bridge from thermal overloads. If the bridge rating exceeds the motor rating, a drive may use either one.
TOL (thermal overload) extended over a long period of time can cause motor overheating, which shortens the motor’s life and could cause a fire. Electrical codes require motors to have protection against loads that exceed 100 percent of their rating after a specific time has elapsed.
UL (Underwriters Laboratories) is an independent testing organization for electrical safety.
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