More Questions about the Electrical
Power Transformers
1. Can Buck-Boost
transformers be used on 3 phase systems?
Interconnecting two or three single-phase units will readily accommodate three phase systems - refer to the corresponding three-phase section in this catalog. The number of units to be used in a 3-phase installation depends on the number of wires in the supply line. If the 3-phase supply is 4-wire Wye, then three Buck-Boost transformers are required. If the 3-phase supply is 3-wire Wye (neutral not available), two Buck-Boost transformers are needed.
2. Should Buck-Boost transformers be used to develop 3-phase 4 wire Wye circuits from 3-phase 3 wire Delta circuits?
No - a three-phase "Wye" buck-boost transformer connection should be used only on a 4-wire source of supply. A delta to Wye connection does not provide adequate current capacity to accommodate unbalanced currents fl owing in the neutral wire of the 4-wire circuit.
3. Why isn't a 'closed Delta' Buck-Boost connection recommended?
This connection requires more kVA power than a "Wye" or open delta connection, and phase shifting occurs on the output. The closed delta connection is more expensive and electrically inferior to other three-phase connections.
4. How do you know how to connect a Buck-Boost transformer?
A connection chart is provided with each unit that shows how to make the corresponding connections. These same charts are also shown in this section.
5. Can 60-Hertz Buck-Boost transformers be operated on 50-Hertz?
Due to 'saturation' of the core, 60-Hertz Buck-Boost transformers should only be operated at 60 Hertz, and not 50 Hertz. Units manufactured as 50-Hertz units will however, operate at 60 Hertz.
6. Why are Buck-Boost transformers shipped from the factory connected as isolating transformers, and not pre-connected autotransformers?
The same 4-winding Buck-Boost transformer can be connected eight different ways to provide a multitude of voltage combinations. The user when assessing the supply voltage at site can best determine the correct connection.
7. Why is the isolation transformer kVA rating shown on the nameplate instead of the autotransformer kVA rating?
Shipped as an isolating transformer, the nameplate is required to show the performance characteristics accordingly. Additionally, as an autotransformer, the eight different combinations of voltages and kava's would be impractical to list on the nameplate. A connection chart, listing the various connections, is included with each unit.
8. Do Buck-Boost transformers present a safety hazard compared to conventional autotransformers?
Buck-Boost transformers only change voltage by a small amount, such as 208 to 240 volts. This small increase does not represent a safety hazard. Conventional autotransformers, manufactured as single winding transformers, change much higher magnitudes of voltage, e.g. 480 to 240 volts. In a system where the line is grounded, it is possible to have 480 volts to ground when the expectations are that 240 volts is at the output. For this reason, qualified personnel only should maintain conventional autotransformers.
9. How does the sound level differ between Buck-Boost and isolation transformers?
Buck-Boost transformers, connected as autotransformers, will be quieter than an equivalent isolation transformer capable of handling the same load. The isolation transformer would have to be physically larger than the buck-boost transformer, and smaller transformers are quieter than larger ones. For example, a 10kVA is 35dB and a 75kVA is 50dB.
10. How do the costs compare between a Buck-Boost transformer and an Isolation transformer handling the same load?
For most Buck-Boost applications, the savings are about 75% compared to the use of an isolation transformer for the same application.
11. What is the life expectancy of a Buck-Boost transformer?
Buck-Boost transformers have exactly the same life expectancy as other dry type transformers.
12. Buck-Boost transformers are almost always installed as autotransformers. Does the National Electrical Code (NEC) permit the use of autotransformers?
Autotransformers are very common and recognized by all the safety and standard authorities. You can refer to N.E.C. Article 450-4, "Autotransformers 600 Volts, Nominal, or Less", as a reference publication. Item (a) details over-current protection for an autotransformer, and Item (b) covers an isolation transformer being field connected as an autotransformer for a Buck-Boost application.
13. When a Buck-Boost transformer is connected as an autotransformer, what is the procedure for determining the current rating of the over-current protective device, such as the fuse or circuit breaker?
The NEC Article 450-4 outlines over-current protection for autotransformers. It is reproduced as follows: "NEC 450-4 - Autotransformers 600 Volts, Nominal, or Less
(a) Over-current Protection. Each autotransformer 600 volts nominal, or less shall be protected by an individual over-current device installed in series with each ungrounded input conductor. Such over-current device shall be rated or set at not more than 125 percent of the rated full load input current of the autotransformer. An over-current device shall not be installed in series with the shunt winding.
Exception: Where the rated input current of an auto transformer is 9 amperes or more and 125 percent of this current does not correspond to a standard rating of a fuse or non-adjustable circuit breaker; the next higher standard rating described in our section shall be permitted. When the rated input current is less than 9 amperes, an over-current device rated or set at not more than 167 percent of the input current shall be permitted.
(b) Transformer Field-Connected as an Autotransformer. A transformer field-connected as autotransformers shall be identified for use at elevated voltage."
Example: A 1kVA transformer, Catalog No. BKN4EA, is rated 120 x 240 to 12 x 24 volts. It is to be connected as an autotransformer to raise 208 to 230 volts single-phase. When connected as an autotransformer in this application, the kVA rating is increased to 9.58 kVA, or 9,580 VA. This is the rating to be used for determining the full load input amps and the corresponding size of the over-current protection device, either a fuse or breaker.
Full load input amps = 9,580 Volt Amps = 46 Amp, 208 Volts.
When the full load current is greater than 9 amps, the over-current protection device - usually a fuse or nonadjustable breaker, the current rating can be up to 125 percent of the full load rating of the autotransformer input current.
Max. current rating of the over-current device = 46 amps x 125% = 57.5 amps.
The National Electrical Code, Article 450-4 (a) Exception, permits the use of the next higher standard ampere rating of the over-current device. This is shown in Article 240-6 of the N.E.C.
Max. size of the fuse or circuit breaker = 60 amps.
14. What is Nuisance Tripping and how can a Line Reactor eliminate it?
Transients due to switching on the utility line and harmonics from the drive system can cause intermittent tripping of circuit breakers. Furthermore, modern switchgear, equipped with solid-state trip sensing devices, is designed to react to peak current rather than RMS current. As switching transients can peak over 1000 volts, the resulting over-voltage will cause undesirable interruptions. A reactor added to your circuit restricts the surge current by utilizing its inductive characteristics, and therefore eliminates nuisance tripping.
15. How does a Line Reactor extend the life of switching components?
Due to the attenuation of line disturbances, the life of your solid state devices are extended when protected by the use of a Hammond line reactor.
16. Will a Hammond Line Reactor saturate?
Due to the care in the selection of the core material with its optimum flux density, Hammond line reactors will not saturate under the most adverse line conditions. Since the inductance is linear over a broader current range, equipment is protected even in extreme over-current circumstances.
17. Will a Hammond Line Reactor extend the life of your motor?
Line reactors, when selected for the output of your drive, will enhance the waveform and virtually eliminate failures due to output circuit faults. Subsequently, motor operating temperatures are reduced by 10 to 20 degrees and motor noise is reduced due to the removal of some of the high frequency harmonic currents.
18. How do Hammond Line Reactors handle Heat Dissipation?
Particular attention has been focused on the design and field-testing of this product line. The results are reactors with ideal operating features including low temperature rises and reduced losses. Hammond reactors will operate efficiently and heat dissipation in your equipment will be of minimal concern.
19. How does a Line Reactor minimize harmonic Distortion?
Nonlinear current waveforms contain harmonic distortion. By using a Hammond line reactor you can limit the inrush current to the rectifier in your drive. The peak current is reduced, the waveform is rounded and harmonic distortion is minimized. Current distortion typically is reduced to 30%. Severe Harmonic current distortion can also cause the system voltage to distort. Often, high peak harmonic current drawn by the drive, causes "fl at-topping" of the voltage waveform. Adding a reactor controls the current component, and voltage harmonic distortion is therefore reduced.
20. What level of Short Circuit Capability do Hammond Line Reactors have?
Hammond line reactors can withstand current under short circuit conditions, reducing the potential of severe damage to electronic equipment. In a short circuit, the inductance of the coil is necessary to limit over-current after the core has saturated. Hammond has extensive experience in designing and testing dry-type transformers to withstand short circuits for the most demanding applications, and this experience has been applied to line reactor design.
21. How does a Hammond Line Reactor reduce Line Notching?
Whenever a rectifier converts AC power to DC, using a nonlinear device, such as an SCR, the process of commutation occurs. The result is a notch in the voltage waveform. The number of notches is a function of both the number of pulses and the number of SCR's in the rectifier.
Line Reactors are used to provide the inductive reactance needed to reduce notching, which can adversely effect equipment operation.
22. What are DV/DT Filter Reactors?
The advent of pulse width modulated (PWM) inverters with IGBT high-speed transistors, has resulted in smaller more cost effective drives and increased switching speeds. A waveform with increased harmonics at higher frequencies is the result of these much faster switching devices, usually at frequencies of 10,000 to 20,000 Hertz.
Drives and motors often need to be separated by significant distances. For deep wells or mines, the motors are usually controlled on the surface. As a result, the distance between the drive and the motor creates long motor lead lengths. In some plant applications, the motors can withstand the harsh environment but the sensitive variable frequency drive cannot. This again results in long lead lengths to the motor.
Most manufactures of variable frequency drives will publish a recommended maximum distance between their equipment and the motor. Sometimes these recommendations create application difficulties, thus increased motor lead lengths are inevitable.
DV/DT is explained as the steep-front voltage pulses that travel down these long leads in the circuit to the motor and subsequently reverted back in a "reflective wave". When the conductors are long enough, usually 20 feet or more, the time for reflection matches the time for transmission resulting in a high amplitude 'standing wave' on the circuit. Voltage spikes of up to 2100 volts are frequently experienced for 600-volt systems, and motor winding failures are the result.
A Filter Reactor, installed in front of the motor, combines the current limiting ability of an AC line reactor plus a resistive capacitance circuit that forms a damped, low pass filter. It provides protection for the motor by slowing the rate of voltage increase and minimizing the peak voltage that occurs at the motor terminals.
The cost of a DV/DT Filter Reactor is little more than the cost of the reactor and can be mounted next to the motor, or inside the PWM enclosure.
23. What are some DV/DT applications?
The Hammond RC series DV/DT filter reactors are specifically designed for drive/motor applications with long lead lengths (usually where the motor cable length is 20 feet and greater). They are always installed between the IGBT variable frequency drive and the motor. Typical installation applications include production process lines, conveyor systems and deep wells.
24. What is the typical "RC" DV/DT Filter Reactor performance?
The RC series DV/DT filter reactors combine appropriate values of inductance, capacitance and resistance to form a filter, which reduces DV/DT and peak voltages from the PWM voltage waveform. This combined with a 3% impedance reactor that will reduce motor heating harmonics, will significantly increase the life of the motor.
Long lead length motor drive applications can experience motor terminal peak voltage spikes twice the DC bus voltage, and higher. Therefore motor terminal voltage peaks of 1200 volts for 480V drives and 1600 volts for 600V drives are not uncommon. The highest peak voltages will typically occur in lower HP applications.
25. What are Industrial Control Transformers?
A control transformer is an isolation transformer designed to provide a high degree of secondary voltage stability (regulation) during a brief period of overload condition (also referred to as "Inrush Current"). Control transformers are usually rated for 600 volts or less.
26. What is the difference between an Air Core Reactor and an Iron Core Reactor?
Air Core:
They are used primarily as current or voltage limiting devices, particularly where large currents can enter a system that uses small amounts of power. An example is the telephone system, which uses very small voltages where the current in a fault condition needs to be kept to a minimum.
Iron Core:
An iron core reactor provides the same current or voltage control on a system as its air core counterpart. Iron core units tend to be used on smaller applications where the variables need greater or more sensitive control.
27. What are General Purpose Distribution Transformers and where are they used?
General Purpose distribution transformers are rated for 600 volts and below. They are generally used for supplying appliance, lighting, motorized machine and power loads from electrical distribution systems. They are either ventilated or totally enclosed, and are available in standard ratings from 250VA up to 750kVA.
28. What are Shielded Distribution Transformers and where are they used?
Hammond shielded distribution transformers provide a copper electrostatic shield between the primary and secondary windings. The shield is grounded and thus shunts most noise and transients to the ground path rather than passing them through to the secondary. Applications for shielded transformers are similar to those above, and they are ideal for commercial or electrical installations where electronic circuitry operating at low voltage DC is present and is very sensitive to 'noise'.
29. What are K-Factor Transformers and where are they used?
K-factor transformers are used as a general-purpose transformer but are designed to withstand the variety of harmonics created in today's office and industrial environments. The expanding use of devices with switch-mode power supplies and rectifier circuits with the subsequent wave distortion requires transformers to withstand the higher harmonics in the neutral conductor in the distribution system.
30. Define K-Factor?
K-Factor is defined as a ratio between the additional losses created by the harmonics and the eddy losses at the rated 60 Hz. This factor is used to specify the size of the transformer to meet the magnitude of the harmonic load in the circuit. A standard general-purpose transformer does not have the shielding, conductor sizes, core cross-section, or the capacity in the neutral to provide the same service.
31. What is a Low Voltage General Purpose Transformer?
Hammond's low voltage general-purpose transformers provide a safe, long lasting, highly reliable power source. They are designed for general lighting and other low voltage applications. They are UL listed and CSA certified.
32. What are Buck-Boost Transformers and where are they used?
Buck-Boost transformers are control transformers with low voltage secondary windings. By field connecting the primary and secondary windings in an autotransformer configuration, they offer an economical solution to the adjustment of line voltages that are slightly above or below normal.
Buck-Boost transformers can be used to adjust stable voltages only. Fluctuating line voltages should be regulated with a Hammond Voltage Conditioner.
33. What are Autotransformers?
Autotransformers are similar to Buck-Boost transformers in that they are also an economical means of adjusting an output voltage. Autotransformers are designed to adjust the supply voltage when isolation from the line is not necessary and where local electrical codes permit. Units are designed in either a step-up or step-down application and to meet motor inrush currents.
34. What are Motor Starting Autotransformers?
Motors have a large inrush current component that requires a special design. Motor starting transformers are designed to withstand an inrush of upward of 25 times normal current. They typically are tapped on larger sizes to soft-start the motor until it is up to full RPM.
35. What are Energy Efficient (TP1) Transformers?
There is a growing movement in the electrical industry towards energy efficient products in all sectors including dry type transformers. In addition to the benefits to the environment, energy efficient transformers also can realize substantial savings in operating costs thereby having a direct impact on the initial investment evaluated over a period of time.
The specifications covering energy efficiency in transformers, is the NEMA Standards Publication, TP-1-1996, "Guide for Determining Energy Efficiency for Distribution Transformers". This specification has carefully considered the total owning cost unique for industrial or commercial installations where the load factor is an integral part of the efficiency rating.
36. What are Low Temperature Rise Transformers?
All transformers have operating losses, and heat is the product of these losses. Hammond low temperature rise transformers are designed with reduced 115°C or 80°C full load operating temperature rises. These units decrease total operating losses by 20% and 35% respectively, compared with the standard 150°C rise operating system. Hammond low temperature rise transformers provide greater efficiency under normal operating conditions, and overload capability without harm to their service life or reliability.
37. What are Encapsulated Transformers and where are they used?
These units are encapsulated and completely enclosed. The encapsulated design is especially suited for installations in harsh environments where dust, lint, moisture and corrosive contaminants are present. Typical applications include: pulp and paper plants; steel mills; food processing plants; breweries; mines; marine and shipboard installations.
38. What are Medium Voltage Transformers and where are they used?
Hammond medium voltage transformers are really 5kV class dry type distribution transformers. They are designed primarily for use in stepping down medium voltage power to a lower operating voltage for commercial, institutional or industrial applications.
39. What are Drive Isolation Transformers and where are they used?
Drive isolation transformers are designed to supply power to AC and DC variable speed drives. The harmonics created by SCR type drives requires careful designing to match the rated hp of each drive system. The duty cycle included is approximately one start every 2 hours. The windings are designed for an over-current of 150% for 60 seconds, or 200% for 30 seconds.
40. What are Clean Power Products?
Computer regulators and hard-wired line voltage conditioners protect equipment from both noise and voltage fluctuations while super isolation transformers are all designed to provide protection against frequency variation or noise related disturbances.
Interconnecting two or three single-phase units will readily accommodate three phase systems - refer to the corresponding three-phase section in this catalog. The number of units to be used in a 3-phase installation depends on the number of wires in the supply line. If the 3-phase supply is 4-wire Wye, then three Buck-Boost transformers are required. If the 3-phase supply is 3-wire Wye (neutral not available), two Buck-Boost transformers are needed.
2. Should Buck-Boost transformers be used to develop 3-phase 4 wire Wye circuits from 3-phase 3 wire Delta circuits?
No - a three-phase "Wye" buck-boost transformer connection should be used only on a 4-wire source of supply. A delta to Wye connection does not provide adequate current capacity to accommodate unbalanced currents fl owing in the neutral wire of the 4-wire circuit.
3. Why isn't a 'closed Delta' Buck-Boost connection recommended?
This connection requires more kVA power than a "Wye" or open delta connection, and phase shifting occurs on the output. The closed delta connection is more expensive and electrically inferior to other three-phase connections.
4. How do you know how to connect a Buck-Boost transformer?
A connection chart is provided with each unit that shows how to make the corresponding connections. These same charts are also shown in this section.
5. Can 60-Hertz Buck-Boost transformers be operated on 50-Hertz?
Due to 'saturation' of the core, 60-Hertz Buck-Boost transformers should only be operated at 60 Hertz, and not 50 Hertz. Units manufactured as 50-Hertz units will however, operate at 60 Hertz.
6. Why are Buck-Boost transformers shipped from the factory connected as isolating transformers, and not pre-connected autotransformers?
The same 4-winding Buck-Boost transformer can be connected eight different ways to provide a multitude of voltage combinations. The user when assessing the supply voltage at site can best determine the correct connection.
7. Why is the isolation transformer kVA rating shown on the nameplate instead of the autotransformer kVA rating?
Shipped as an isolating transformer, the nameplate is required to show the performance characteristics accordingly. Additionally, as an autotransformer, the eight different combinations of voltages and kava's would be impractical to list on the nameplate. A connection chart, listing the various connections, is included with each unit.
8. Do Buck-Boost transformers present a safety hazard compared to conventional autotransformers?
Buck-Boost transformers only change voltage by a small amount, such as 208 to 240 volts. This small increase does not represent a safety hazard. Conventional autotransformers, manufactured as single winding transformers, change much higher magnitudes of voltage, e.g. 480 to 240 volts. In a system where the line is grounded, it is possible to have 480 volts to ground when the expectations are that 240 volts is at the output. For this reason, qualified personnel only should maintain conventional autotransformers.
9. How does the sound level differ between Buck-Boost and isolation transformers?
Buck-Boost transformers, connected as autotransformers, will be quieter than an equivalent isolation transformer capable of handling the same load. The isolation transformer would have to be physically larger than the buck-boost transformer, and smaller transformers are quieter than larger ones. For example, a 10kVA is 35dB and a 75kVA is 50dB.
10. How do the costs compare between a Buck-Boost transformer and an Isolation transformer handling the same load?
For most Buck-Boost applications, the savings are about 75% compared to the use of an isolation transformer for the same application.
11. What is the life expectancy of a Buck-Boost transformer?
Buck-Boost transformers have exactly the same life expectancy as other dry type transformers.
12. Buck-Boost transformers are almost always installed as autotransformers. Does the National Electrical Code (NEC) permit the use of autotransformers?
Autotransformers are very common and recognized by all the safety and standard authorities. You can refer to N.E.C. Article 450-4, "Autotransformers 600 Volts, Nominal, or Less", as a reference publication. Item (a) details over-current protection for an autotransformer, and Item (b) covers an isolation transformer being field connected as an autotransformer for a Buck-Boost application.
13. When a Buck-Boost transformer is connected as an autotransformer, what is the procedure for determining the current rating of the over-current protective device, such as the fuse or circuit breaker?
The NEC Article 450-4 outlines over-current protection for autotransformers. It is reproduced as follows: "NEC 450-4 - Autotransformers 600 Volts, Nominal, or Less
(a) Over-current Protection. Each autotransformer 600 volts nominal, or less shall be protected by an individual over-current device installed in series with each ungrounded input conductor. Such over-current device shall be rated or set at not more than 125 percent of the rated full load input current of the autotransformer. An over-current device shall not be installed in series with the shunt winding.
Exception: Where the rated input current of an auto transformer is 9 amperes or more and 125 percent of this current does not correspond to a standard rating of a fuse or non-adjustable circuit breaker; the next higher standard rating described in our section shall be permitted. When the rated input current is less than 9 amperes, an over-current device rated or set at not more than 167 percent of the input current shall be permitted.
(b) Transformer Field-Connected as an Autotransformer. A transformer field-connected as autotransformers shall be identified for use at elevated voltage."
Example: A 1kVA transformer, Catalog No. BKN4EA, is rated 120 x 240 to 12 x 24 volts. It is to be connected as an autotransformer to raise 208 to 230 volts single-phase. When connected as an autotransformer in this application, the kVA rating is increased to 9.58 kVA, or 9,580 VA. This is the rating to be used for determining the full load input amps and the corresponding size of the over-current protection device, either a fuse or breaker.
Full load input amps = 9,580 Volt Amps = 46 Amp, 208 Volts.
When the full load current is greater than 9 amps, the over-current protection device - usually a fuse or nonadjustable breaker, the current rating can be up to 125 percent of the full load rating of the autotransformer input current.
Max. current rating of the over-current device = 46 amps x 125% = 57.5 amps.
The National Electrical Code, Article 450-4 (a) Exception, permits the use of the next higher standard ampere rating of the over-current device. This is shown in Article 240-6 of the N.E.C.
Max. size of the fuse or circuit breaker = 60 amps.
14. What is Nuisance Tripping and how can a Line Reactor eliminate it?
Transients due to switching on the utility line and harmonics from the drive system can cause intermittent tripping of circuit breakers. Furthermore, modern switchgear, equipped with solid-state trip sensing devices, is designed to react to peak current rather than RMS current. As switching transients can peak over 1000 volts, the resulting over-voltage will cause undesirable interruptions. A reactor added to your circuit restricts the surge current by utilizing its inductive characteristics, and therefore eliminates nuisance tripping.
15. How does a Line Reactor extend the life of switching components?
Due to the attenuation of line disturbances, the life of your solid state devices are extended when protected by the use of a Hammond line reactor.
16. Will a Hammond Line Reactor saturate?
Due to the care in the selection of the core material with its optimum flux density, Hammond line reactors will not saturate under the most adverse line conditions. Since the inductance is linear over a broader current range, equipment is protected even in extreme over-current circumstances.
17. Will a Hammond Line Reactor extend the life of your motor?
Line reactors, when selected for the output of your drive, will enhance the waveform and virtually eliminate failures due to output circuit faults. Subsequently, motor operating temperatures are reduced by 10 to 20 degrees and motor noise is reduced due to the removal of some of the high frequency harmonic currents.
18. How do Hammond Line Reactors handle Heat Dissipation?
Particular attention has been focused on the design and field-testing of this product line. The results are reactors with ideal operating features including low temperature rises and reduced losses. Hammond reactors will operate efficiently and heat dissipation in your equipment will be of minimal concern.
19. How does a Line Reactor minimize harmonic Distortion?
Nonlinear current waveforms contain harmonic distortion. By using a Hammond line reactor you can limit the inrush current to the rectifier in your drive. The peak current is reduced, the waveform is rounded and harmonic distortion is minimized. Current distortion typically is reduced to 30%. Severe Harmonic current distortion can also cause the system voltage to distort. Often, high peak harmonic current drawn by the drive, causes "fl at-topping" of the voltage waveform. Adding a reactor controls the current component, and voltage harmonic distortion is therefore reduced.
20. What level of Short Circuit Capability do Hammond Line Reactors have?
Hammond line reactors can withstand current under short circuit conditions, reducing the potential of severe damage to electronic equipment. In a short circuit, the inductance of the coil is necessary to limit over-current after the core has saturated. Hammond has extensive experience in designing and testing dry-type transformers to withstand short circuits for the most demanding applications, and this experience has been applied to line reactor design.
21. How does a Hammond Line Reactor reduce Line Notching?
Whenever a rectifier converts AC power to DC, using a nonlinear device, such as an SCR, the process of commutation occurs. The result is a notch in the voltage waveform. The number of notches is a function of both the number of pulses and the number of SCR's in the rectifier.
Line Reactors are used to provide the inductive reactance needed to reduce notching, which can adversely effect equipment operation.
22. What are DV/DT Filter Reactors?
The advent of pulse width modulated (PWM) inverters with IGBT high-speed transistors, has resulted in smaller more cost effective drives and increased switching speeds. A waveform with increased harmonics at higher frequencies is the result of these much faster switching devices, usually at frequencies of 10,000 to 20,000 Hertz.
Drives and motors often need to be separated by significant distances. For deep wells or mines, the motors are usually controlled on the surface. As a result, the distance between the drive and the motor creates long motor lead lengths. In some plant applications, the motors can withstand the harsh environment but the sensitive variable frequency drive cannot. This again results in long lead lengths to the motor.
Most manufactures of variable frequency drives will publish a recommended maximum distance between their equipment and the motor. Sometimes these recommendations create application difficulties, thus increased motor lead lengths are inevitable.
DV/DT is explained as the steep-front voltage pulses that travel down these long leads in the circuit to the motor and subsequently reverted back in a "reflective wave". When the conductors are long enough, usually 20 feet or more, the time for reflection matches the time for transmission resulting in a high amplitude 'standing wave' on the circuit. Voltage spikes of up to 2100 volts are frequently experienced for 600-volt systems, and motor winding failures are the result.
A Filter Reactor, installed in front of the motor, combines the current limiting ability of an AC line reactor plus a resistive capacitance circuit that forms a damped, low pass filter. It provides protection for the motor by slowing the rate of voltage increase and minimizing the peak voltage that occurs at the motor terminals.
The cost of a DV/DT Filter Reactor is little more than the cost of the reactor and can be mounted next to the motor, or inside the PWM enclosure.
23. What are some DV/DT applications?
The Hammond RC series DV/DT filter reactors are specifically designed for drive/motor applications with long lead lengths (usually where the motor cable length is 20 feet and greater). They are always installed between the IGBT variable frequency drive and the motor. Typical installation applications include production process lines, conveyor systems and deep wells.
24. What is the typical "RC" DV/DT Filter Reactor performance?
The RC series DV/DT filter reactors combine appropriate values of inductance, capacitance and resistance to form a filter, which reduces DV/DT and peak voltages from the PWM voltage waveform. This combined with a 3% impedance reactor that will reduce motor heating harmonics, will significantly increase the life of the motor.
Long lead length motor drive applications can experience motor terminal peak voltage spikes twice the DC bus voltage, and higher. Therefore motor terminal voltage peaks of 1200 volts for 480V drives and 1600 volts for 600V drives are not uncommon. The highest peak voltages will typically occur in lower HP applications.
25. What are Industrial Control Transformers?
A control transformer is an isolation transformer designed to provide a high degree of secondary voltage stability (regulation) during a brief period of overload condition (also referred to as "Inrush Current"). Control transformers are usually rated for 600 volts or less.
26. What is the difference between an Air Core Reactor and an Iron Core Reactor?
Air Core:
They are used primarily as current or voltage limiting devices, particularly where large currents can enter a system that uses small amounts of power. An example is the telephone system, which uses very small voltages where the current in a fault condition needs to be kept to a minimum.
Iron Core:
An iron core reactor provides the same current or voltage control on a system as its air core counterpart. Iron core units tend to be used on smaller applications where the variables need greater or more sensitive control.
27. What are General Purpose Distribution Transformers and where are they used?
General Purpose distribution transformers are rated for 600 volts and below. They are generally used for supplying appliance, lighting, motorized machine and power loads from electrical distribution systems. They are either ventilated or totally enclosed, and are available in standard ratings from 250VA up to 750kVA.
28. What are Shielded Distribution Transformers and where are they used?
Hammond shielded distribution transformers provide a copper electrostatic shield between the primary and secondary windings. The shield is grounded and thus shunts most noise and transients to the ground path rather than passing them through to the secondary. Applications for shielded transformers are similar to those above, and they are ideal for commercial or electrical installations where electronic circuitry operating at low voltage DC is present and is very sensitive to 'noise'.
29. What are K-Factor Transformers and where are they used?
K-factor transformers are used as a general-purpose transformer but are designed to withstand the variety of harmonics created in today's office and industrial environments. The expanding use of devices with switch-mode power supplies and rectifier circuits with the subsequent wave distortion requires transformers to withstand the higher harmonics in the neutral conductor in the distribution system.
30. Define K-Factor?
K-Factor is defined as a ratio between the additional losses created by the harmonics and the eddy losses at the rated 60 Hz. This factor is used to specify the size of the transformer to meet the magnitude of the harmonic load in the circuit. A standard general-purpose transformer does not have the shielding, conductor sizes, core cross-section, or the capacity in the neutral to provide the same service.
31. What is a Low Voltage General Purpose Transformer?
Hammond's low voltage general-purpose transformers provide a safe, long lasting, highly reliable power source. They are designed for general lighting and other low voltage applications. They are UL listed and CSA certified.
32. What are Buck-Boost Transformers and where are they used?
Buck-Boost transformers are control transformers with low voltage secondary windings. By field connecting the primary and secondary windings in an autotransformer configuration, they offer an economical solution to the adjustment of line voltages that are slightly above or below normal.
Buck-Boost transformers can be used to adjust stable voltages only. Fluctuating line voltages should be regulated with a Hammond Voltage Conditioner.
33. What are Autotransformers?
Autotransformers are similar to Buck-Boost transformers in that they are also an economical means of adjusting an output voltage. Autotransformers are designed to adjust the supply voltage when isolation from the line is not necessary and where local electrical codes permit. Units are designed in either a step-up or step-down application and to meet motor inrush currents.
34. What are Motor Starting Autotransformers?
Motors have a large inrush current component that requires a special design. Motor starting transformers are designed to withstand an inrush of upward of 25 times normal current. They typically are tapped on larger sizes to soft-start the motor until it is up to full RPM.
35. What are Energy Efficient (TP1) Transformers?
There is a growing movement in the electrical industry towards energy efficient products in all sectors including dry type transformers. In addition to the benefits to the environment, energy efficient transformers also can realize substantial savings in operating costs thereby having a direct impact on the initial investment evaluated over a period of time.
The specifications covering energy efficiency in transformers, is the NEMA Standards Publication, TP-1-1996, "Guide for Determining Energy Efficiency for Distribution Transformers". This specification has carefully considered the total owning cost unique for industrial or commercial installations where the load factor is an integral part of the efficiency rating.
36. What are Low Temperature Rise Transformers?
All transformers have operating losses, and heat is the product of these losses. Hammond low temperature rise transformers are designed with reduced 115°C or 80°C full load operating temperature rises. These units decrease total operating losses by 20% and 35% respectively, compared with the standard 150°C rise operating system. Hammond low temperature rise transformers provide greater efficiency under normal operating conditions, and overload capability without harm to their service life or reliability.
37. What are Encapsulated Transformers and where are they used?
These units are encapsulated and completely enclosed. The encapsulated design is especially suited for installations in harsh environments where dust, lint, moisture and corrosive contaminants are present. Typical applications include: pulp and paper plants; steel mills; food processing plants; breweries; mines; marine and shipboard installations.
38. What are Medium Voltage Transformers and where are they used?
Hammond medium voltage transformers are really 5kV class dry type distribution transformers. They are designed primarily for use in stepping down medium voltage power to a lower operating voltage for commercial, institutional or industrial applications.
39. What are Drive Isolation Transformers and where are they used?
Drive isolation transformers are designed to supply power to AC and DC variable speed drives. The harmonics created by SCR type drives requires careful designing to match the rated hp of each drive system. The duty cycle included is approximately one start every 2 hours. The windings are designed for an over-current of 150% for 60 seconds, or 200% for 30 seconds.
40. What are Clean Power Products?
Computer regulators and hard-wired line voltage conditioners protect equipment from both noise and voltage fluctuations while super isolation transformers are all designed to provide protection against frequency variation or noise related disturbances.
Seriously, wow.Thanks for sharing such a great blog with us.
ReplyDeleteIt is really helped to sharing blogs.partsxp group has identified alternatives to procurement.
Industrial Control Transformer
Now it is avilable to service your location Now it is avilable to service your location.