Aneka Teknik Listrik - Electrical, by ATC Automation

Gate Driver Optocouplers in Induction Cooker (4) - Finish

2009-06-22T19:53:00.001+07:00

Summary
In this article, the half-bridge series resonant and quasi resonant induction cooker topologies along with three gate driver methods were discussed. In order to reduce the design size and audible switching noise while improving power efficiency, these resonant converters are chosen.

The discrete transistor gate driver circuit is cost effective but increases design complexity while providing no safety isolation. The gate drive transformer consumes board space due its size and requires additional work and cost to achieve higher switching duty cycle above 50%. Finally, gate drive optocoupler integrated ICs saves board space through high level feature integration while providing high voltage safe isolation and noise immunity all in one package.

Gate Driver Optocouplers in Induction Cooker (3)

2009-06-22T19:50:00.003+07:00

Gate Driver Circuits for IGBT Power Switches
Three types driver circuits, the discrete transistor circuit (Figure 5), gate driver optocoupler (Figure 6) and gate driver transformer (Figure 7) can be used to drive the power switches in induction cooker application. There are several issues associated with high-frequency gate drivers; the parasitic inductances, power dissipation in the gate-drive circuit and the losses in the power switching devices in the gate driver.

Typically, the switching frequency of an induction cooker is between 25kHz to 40kHz. In order to rapidly charge turn on and off the power switch, the gate current inductance loop between the driver and power switch should be as low as possible. Hence it is advisable to design the layout of the circuit to reduce the parasitic inductances. Since the driver rapidly charge and discharge the gate capacitor of the IGBT, a higher peak gate current may be needed for proper operation. Due to this, the power dissipation within the gate drive circuit is important to manage the increase switching speed. The higher peak current is also desirable to increase the charging and discharging during turn on and off as it will help reduce the switching losses of the IGBT.
The discrete gate drivers are constructed using the bipolar transistors. NPN and PNP emitter followers can achieve reasonable drive capability. However, using several discrete components to build the driver and other functions or protection operation like Under Voltage Lockout (UVLO) is not as space efficient as using integrated ICs. Moreover discrete transistor drivers do not provide sufficient safety isolation or noise immunity.
Two types of isolation method are discussed in this article; pulse transformer and gate driver optocoupler. The pulse transformer is a traditional and simple solution which suffers from saturation limitation for a given transformer size that can reduce efficiency. Normally, a transformer can only transmit AC information and have a limited duty cycle of up to 50% due to the transformer volt-second relationship. Additional capacitor and zener diode on the secondary size can be added to allow a higher duty cycle. However, this increase the design board size and parasitic inductances which in turn increases power losses in the driver circuit.
Gate driver optocoupler ic is an integration of LED for safety isolation, transistors to provide drive current and protection functions like UVLO or Desaturation Detector. Gate driver ICs are easy to design and will save PCB board space in the application. Due to the integrated design, the drive circuitry can be located very close to the power switch which not only saves PCB space but also improves the overall noise immunity of the system. However, like any integrated ICs, power dissipation is main concern observed by designers.
For the single switch resonant converter, designer has the option of the discrete gate driver topology, gate transformer or gate driver optocoupler. As discussed in the previous section, the quasi-converter resonant voltage can be higher compared to the DC link voltage and this voltage stresses the power semiconductor switch. In most commercial low cost single switch induction cooker design, the discrete gate driver circuit is used as there is no upper power switch and both controller and the power semiconductor are able to share the same power ground. However, in cases where safety isolation and reduction of driver losses becomes an issue, the gate drive optocoupler or transformer are excellent alternatives.
For the half-bridge converter, a floating or high-side power switch needs to be driven. A high side discrete solution would increase the component count while not providing any isolation. As shown, the pulse transformer galvanic isolation solution increases in complexity for duty cycle switching above 50%. Also, the solution size is larger because of the additional discrete components on top of the transformer size. The gate driver optocoupler IC provides a good level of protection, isolation, and common-mode noise rejection. This resolves much of the problems that are associated with transformer driver or transistor discrete solution as mentioned earlier.

Gate Driver Optocouplers in Induction Cooker (2)

2009-06-22T19:48:00.001+07:00

By applying the transformer equivalent circuit, designers are able to map the load pot (secondary of transformer) to the primary side of circuit where the resonant inductor, Lr and capacitor Cr are located. From this, we can obtain the equivalent circuit for half-bridge and quasi resonant circuit, shown in Figure 3 and Figure 4. From these equivalent circuits, the operation of the induction cooker, the sizing of the resonant inductor, capacitor and control algorithm can be conceived.

In order to reduce component size, minimize switching losses and reduce audible noise during operation (above 20kHz resonant frequency), induction cooker circuit typically utilizes resonant or soft switching techniques. This circuitsoft switching technique can be subcategorized into two methods: Zero-voltage switching and Zero-current switching.
Zero-voltage switching occurs when the transistor turn-on at zero voltage. Zero-current switching refers to elimination of turn-off switching loss at zero current flow. The voltage or current administered to the switching circuit can be made zero by using the resonance created by an L-C resonant circuit. This topology is named a “resonant converter.” This allows the application to utilize resonant frequency and obtain the benefits mentioned compared to conventional hard switching techniques.
The advantages of half-bridge series resonant are stable switching, and lower cost due to streamlined design. The voltage within the circuit is limited to the level of the input voltage which reduces the voltage stress across IGBT power switch. This in-turn allows the designer to lower the cost by choosing a lower rating IGBT. The disadvantage is that the overall half-bridge control is more complicated, the size of heatsink and PCB area is bigger and insulated gate driver circuits, especially on the upper IGBT (S1 in Figure 1).
The advantage of quasi-resonant converter is that it needs only 1 IGBT power switch which reduces design size PCB and heat sink. The disadvantages are that the quasi resonant switching the high resonant voltage which can be higher than the DC input voltage stressed on to IGBT power switches. This requires a higher cost and blocking voltage power components.

Gate Driver Optocouplers in Induction Cooker (1)

2009-06-22T19:44:00.002+07:00

What is Induction Cooking?
We start by first comparing the difference between conventional gas cooking and induction cooking. In induction cooking methods, energy is transferred directly to the pot or pan while conventional cooking first generate a fire and heat energy which is then transferred to the cooking pot. Hence due to this two step energy transfer of conventional cooking, the efficiency of the induction cooking is much better.

Figure 1 & 2 shows two circuit topologies for induction cooker, the half-bridge series resonant converter Figure 1 and quasi-resonant converter Figure 2 [2]. In both topologies, there exist the resonant elements Lr and Cr. For circuit simplification, the load pot, R is assumed to be of pure resistive element. In both topologies, an ac input supply of 220V 50Hz is converted into an uncontrolled dc voltage by a full-bridge rectifier. This DC voltage is then converted into a high frequency AC voltage by the inverter IGBT (insulated gate bipolar transistors) switches, S1 and S2 in the case of the half-bridge circuit, which can be controlled using a micro-controller. Due to the high frequency switching AC, the element coil will then produce a high frequency electromagnetic field which will penetrate the ferrous material cooking pot. From Faraday’s Law and skin effect, this generates eddy current within the cooking pot which then generates heat to cook the food inside the pot.

IGBT Gate Drivers in High-Frequency Induction Cookers (3)

2009-06-21T11:52:00.003+07:00

Gate driver circuits for IGBT power switches
Three types of driver circuits, using discrete transistors (Fig. 5), gate driver optocouplers (Fig. 6) or gate driver transformers (Fig. 7) can be used to drive the power switches in the induction cooker. There are several issues associated with high-frequency gate drivers: parasitic inductances, power dissipation in the gate-drive circuit and the losses in the power switching devices in the gate driver, all of which are involved when selecting an appropriate driver circuit.

Typically, the switching frequency of an induction cooker is between 25 kHz and 40 kHz. In order to rapidly turn on and off the power switch, the gate current inductance loop between the driver and power switch should be as low as possible. Hence it is advisable to design the layout of the circuit to reduce the parasitic inductances. Since the driver rapidly charges and discharges the gate capacitor of the IGBT, a relatively high peak gate current may be needed for proper operation. A higher peak current is also desirable to increase the charging and discharging rates during turn-on and turn-off, to help reduce the switching losses of the IGBT. Due to this, managing the power dissipation within the gate drive circuit becomes increasingly important as the switching speeds are increased.
Discrete gate drivers are constructed using bipolar transistors, and NPN and PNP emitter followers can achieve reasonable drive capability. However, using several discrete components to build the driver, while simultaneously incorporating necessary operational and protective functions such as under voltage lockout (UVLO), is not as space efficient as using integrated circuits. Moreover most discrete transistor driver designs do not provide sufficient safety isolation or noise immunity.
Two methods of providing electrical isolation are pulse transformers and gate driver optocouplers. The pulse transformer is a traditional and simple solution, which, however, suffers from the potential for core saturation in a reasonably-sized transformer, resulting in reduced efficiency. A pulse transformer can only transmit AC signals, and most designs have a limited duty cycle ranging up to 50 percent due to the transformer volt-second relationship.
An additional capacitor and zener diode on the transformer secondary can be added to permit a higher duty cycle. However, this increases the circuit board size and parasitic inductances, which, in turn, increases power losses in the driver circuit.
The gate driver optocoupler IC integrates an LED light source and optical receiver for safety isolation, with transistors to provide sufficient drive current, and protection functions such as UVLO or desaturation detection.
Gate driver ICs are easy to design with, and will save PCB board space. Due to the integrated design, the drive circuitry can be located very close to the power switch, which not only saves PCB space but also improves the overall noise immunity of the system. However, as with any ICs, power dissipation is a major concern.
For the single-switch resonant converter, the designer has the option of the discrete gate driver, gate transformer or gate driver optocoupler topologies. As discussed previously, the quasiconverter resonant voltage can be higher than the DC link voltage and this voltage stresses the power semiconductor switch. In most commercial low cost single switch induction cooker designs, the discrete gate driver circuit is used as there is no upper power switch, and both the controller and power semiconductor are able to share the same power ground. However, in cases where safety isolation and reduction of driver losses becomes an issue, the gate drive optocoupler or transformer are excellent alternatives.
For the half-bridge converter, a floating or high-side power switch needs to be driven. A high-side discrete solution would increase the component count, and not provide any isolation. As shown, the pulse transformer galvanic isolation solution becomes increasingly complicated for duty cycle switching above 50 percent. Also, the solution size is larger because of the additional discrete components on top of the transformer size. The gate driver optocoupler IC provides a good level of protection, isolation, and common-mode noise rejection. This resolves many of the problems that are associated with transformer or discrete transistor drivers.

Summary
In this article, the halfbridge series resonant and quasi resonant induction cooker topologies along with three gate driver methods were discussed. In order to reduce the design size and audible switching noise while improving power efficiency, these resonant converters are chosen. The discrete transistor gate driver circuit is cost effective but increases design complexity while providing no safety isolation. The required size of the gate drive transformer consumes board space, and requires additional work, cost and board space to achieve switching duty cycles above 50 percent. Finally, the use of gate drive optocoupler ICs saves board space through high level feature integration while providing high voltage safety isolation and noise immunity all in one package.

IGBT Gate Drivers in High-Frequency Induction Cookers (2)

2009-06-21T11:45:00.003+07:00

How does an induction cooker work?
Figures 1 and 2 show two circuit topologies for induction cookers: the half-bridge series resonant converter, Fig. 1, and the quasi-resonant converter, Fig. 2. In both topologies, there exist the resonant elements Lr and Cr. For circuit simplification, the load pot, R, is assumed to be a purely resistive element.

In both topologies, an AC input supply of 220V 50 Hz is converted into an unregulated DC voltage by a full-bridge rectifier. This DC voltage is then converted into a high frequency AC voltage by the inverter IGBT (insulated gate bipolar transistor) switches—S1 and S2 in the case
of the half-bridge circuit—which can be controlled using a microcontroller. Due to the high frequency switching AC, the element coil will produce a high frequency electromagnetic field which will penetrate the ferrous material of the cooking pot. From Faraday’s Law and skin effect, this generates eddy current within the cooking pot which then generates heat to cook the food inside the pot.
By applying the transformer equivalent circuit, designers are able to map the load pot (secondary of transformer) to the primary side of the circuit where the resonant inductor, Lr, and capacitor, Cr, are located. From this, we can obtain the equivalent circuit for the half-bridge and quasi resonant circuits, shown in Figs. 3 and 4. From these equivalent circuits, the operation of the induction cooker, and the values of the resonant inductor, capacitor and control algorithm
can be derived.
In order to reduce component size, minimize switching losses and reduce audible noise during operation, induction cooker circuits typically utilize resonant or soft switching techniques. Soft switching can be subcategorized into two methods: zero-voltage switching and zero-current switching. Zero-voltage switching occurs when the transistor turns-on at zero voltage. Zero-current switching refers to the elimination of turn-off switching loss at zero current flow. The voltage or current provided to the switching circuit can be made zero by using the resonance created by an L-C circuit. This topology is named a “resonant converter.”
The advantages of a half-bridge series resonant circuit are stable switching and lower cost due to simplified design. The voltage within the circuit is limited to the level of the input voltage, which reduces the voltage stress across IGBT power switch. This, in turn, allows the designer to lower the cost by choosing an IGBT with a lower voltage rating. The disadvantage of this approach is that the control of the half-bridge circuit is relatively complicated and the required size of the heatsink and PCB area is greater, because of the high side gate driver circuit required for the upper IGBT, S1 in Fig. 1)
The advantage of a quasi-resonant converter is that it needs only one IGBT power switch, which reduces the size of the PCB and heat sink. The disadvantages are that the quasi-resonant switching develops a resonant voltage which can be higher than the DC input voltage, increasing stresses on the IGBT power switches. This requires highercost components with higher blocking
voltage capabilities.

IGBT Gate Drivers in High-Frequency Induction Cookers (1)

2009-06-21T11:41:00.001+07:00

Efficiency of induction cookers is 84 percent
Today, with the constant demand for energy saving devices, high-frequency induction cookers, already a trend in Europe, are gaining popularity in the rest of the world. These kitchen devices offer high efficiency that reduces energy usage, reduces cooking time and, simultaneously, improves user safety, particularly around children, since all heat is localized to the pan itself.

According to the U.S. Department of Energy, the typical efficiency of induction cookers is 84% compared to the 40 percent of gas cookers. In this article, two typical induction cooker designs, the halfbridge series-resonant and the quasiresonant topology, are discussed. The merits and disadvantages of these two high-frequency inverter topologies along with three gate driver circuits, discrete transistors, optocouplers integrated circuit and transformers for high frequency
operation are also discussed.

What is induction cooking?
In an induction cooktop, a magnetic field transfers electric energy directly to the object to be heated. By inducing in electric current into the ferrous cooking utensil, heat is generated in the object, and the cooking surface only gets hot from the heat reflected from the object being heated: no heat is directly produced by the induction element. Because of this direct transfer of energy, there are fewer losses, which translates to a higher level of efficiency.
This compares with conventional cooking in which a heat source, for example an electrical resistance element or a flame, transfers heat energy to the cooking pot. The two-step energy transfer is inherently less efficient than direct inductive heating.

Veritron Error Message List

2009-06-06T10:46:00.002+07:00

Error Message List (Source : Paeset2 Software)

Motor Insulation Systems (7) - Finish

2009-05-25T20:43:00.002+07:00

Significance Of Winding Configuration and Method
Figure 6 is a representation of one (1) coil of a motor winding consisting of several turns.

The coil voltage is distributed among the turns so that the turn to turn voltage is less than the full coil voltage.

If the coil is wound concentrically, each turn of the coil is wound next to the previous turn and the coil is built up in successive layers. This ensures that each turn of the coil is in contact only with immediately preceding and successive turns, and the first turns in the coil are separated from the last turns. This means that the voltage between two (2) conductors that are next to each other is always less than the full voltage that is applied to the coil. If the coils are wound randomly, the positions of the individual turns are not controlled. With random winding, it is possible that the first turn of the coil may be in contact with the last turn. If the first turn of the coil is in contact with the last turn, two (2) layers of magnet wire insulation must withstand full coil voltage. Figure 7 shows the comparison between concentric and random winding. Most motors rated for operation at 600V or less have random windings.

Motor Insulation Systems (6)

2009-05-25T20:41:00.001+07:00

Carrier Frequency
As the inverter’s carrier frequency is increased, the output current waveform supplied to the motor becomes more sinusoidal. This improved output current waveform decreases motor heating thus improving motor insulation life.
At this higher carrier frequency, however, more individual voltage pulses are output and for a given cable length, rise time and motor surge impedance, the potential for voltage overshoot increases. The power generated during this overshoot will be dissipated in the motor’s windings, and insulation life will be decreased.

Figure 5, below, shows insulation life of a generic motor, when both cable length and inverter carrier frequency (fc) are varied. Note that with a 150 ft. cable length, insulation life drops from 100,000 hours to 25,000 hours, when carrier frequency is increased from 3 to 12 Khz. The longest life occurs with short cable lengths and low carrier frequencies.

Motor Insulation Systems (5)

2009-05-25T20:39:00.000+07:00

Vibration
One mechanical stress phenomenon that is more likely to appear on inverter applications than line-started ones is resonance (when a mechanical system oscillates at it’s natural frequency). A common example of resonance is the vibrations noted on the side view mirror on an old car. As the car accelerates from standstill to freeway speeds, engine and frame vibrations are transmitted to the mirror’s mounting base. At some point during acceleration, these vibration frequencies change and the mirror stabilizes again. Motor, pump and machinery designers all take resonance into account when designing their product.

They will add mass, change support struts, or increase mounting base lengths to ensure that the item’s natural frequency is well above 60Hz. When the machine is assembled onto a base, coupled to a motor, and bolted to a concrete pad, the natural frequency decreases, but, by design, remains higher than the running speed when excited to 60Hz. However, as the machine speed is changed with an inverter, the likelihood of stumbling onto the system’s resonant frequency increases dramatically. Once resonance is reached, severe vibrations can occur in the motor, stressing stator coils, brinelling bearings, and even fatiguing bolts and castings to the breaking point. As the coils continue to move, they’ll ultimately chafe through all layers of insulation, and a failure will result. Since this new resonant point is determined not by the parts of the machine, but instead by the assembly of parts, it must be corrected at the system level. This is best done during start-up. Although additional supports can be welded onto bases and belt ratios altered, the most cost effective method to avoid these resonance frequencies is to program an offset to the critical frequency. During acceleration and deceleration the inverter will pass through the critical resonance frequency but the critical frequency offset will prevent the inverter from operating at the programmed frequency band, thus avoiding the mechanical resonance.

Voltage or Dielectric
The Dielectric properties of a material are the characteristics of the material that make the material an electrical insulator rather than a conductor. When there is a voltage difference across the thickness of an insulating material, the voltage causes a Dielectric Stress that opposes the material’s ability to prevent current from flowing through the material. The Dielectric Strength of a material is a measure of the material’s capability to withstand dielectric stress. An insulation
system’s rated operating voltage is determined by the dielectric strength of the insulating materials. If the insulation is subjected to excess voltage, it can fail suddenly and catastrophically. Gradual deterioration can be caused by voltage levels that exceed the insulation rating but do not cause catastrophic failure. The other forms of stress - thermal, environmental, mechanical and vibration - lead to a reduction in the insulation’s ability to withstand dielectric stress. The insulation ultimately fails when it can no longer withstand the applied voltage and the flow of short circuit current causes catastrophic failure.

Motor Insulation Systems (4)

2009-05-25T20:36:00.002+07:00

4. Insulation Stresses

Several different physical phenomena can cause electrical insulation to deteriorate or fail. These include thermal, contamination, mechanical, vibration, voltage, carrier frequency and the method of winding the turns of insulation.

Thermal
The service life of an insulation system is generally determined by thermal stress. All insulation systems deteriorate over a period of time due to the effects of thermal stress. If the insulation always remains at its rated temperature, it should not fail during its rated service lifetime. If the insulation continually exceeds rated temperature, its lifetime will be shortened in proportion to the level and duration of the excess temperature. The insulation may last longer than the rated lifetime if its temperature remains below the rated level for significant periods of time.
Figure 4 shows the relationship of insulation life versus temperature normalized at 25°C (100% life). Increasing the temperature to 130°C decreases insulation life to 83% from nominal. Increasing the temperature further to 155°C (Class F insulation limit) or 180°C (Class H insulation limit) will further reduce insulation life.

Environmental
Contamination of the motor windings reduces dielectric strength dramatically, especially when fast rise time, high frequency voltages from IGBT inverters, are involved. A drip-proof motor that has successfully operated in a moist, sloppy pump pit may fail in short order when transferred from line power to inverter power. This is because contaminants such as oil, salt, acid, alkalies, grease, dirt, detergents, disinfectants, carbon black, chlorines or metal dust create conductive paths along the surface of the varnished windings, especially when combined with moisture from the surrounding environment. This facilitates high frequency surface tracking, which can effectively produce short circuits between otherwise insulated portions of the windings.

Mechanical
When a motor is line-started at full voltage, the powerful magnetic fields produced push and pull the stator coils back and forth, and large inrush currents generate rapid heating of the stator conductors, causing them to expand. The surrounding iron stator core heats up less, has a lower thermal expansion rate, and doesn’t move at the same rate as the copper coils it supports. As a result, the copper coils strain against the varnish that adheres them to the core, causing fractures where the coils exit the core. Each successive across-the-line motor start repeats the cycle of flexing and expanding, worsens these breaks, and increases the chance that a conductor will abrade it’s remaining insulation and short to ground.
Once the insulation has cracks, moisture and contaminants will find their way in, further reducing insulation integrity. Similar expansion rate differences are present in the motor’s rotor circuit, where the iron core expands slower than the copper conductors (used in large motors) it holds, and faster than aluminum conductors (used in smaller motors). Generally, however, the stator windings fail before the rotor does.
During inverter starts, however, motor voltage and frequency are slowly increased rather than applied at full value. Motor coils are not subjected to the excessive heating and flexing that occurs during line starts, thus extending motor life. Only when the inverter is bypassed, does the motor experience the starting stresses described above.

Motor Insulation Systems (3)

2009-05-25T20:35:00.001+07:00

3. Design Variations

The types and amounts of insulating materials used can vary considerably from one (1) manufacturer to another. Some manufacturers may omit some or all of the paper insulating components and depend upon the varnish coated wire to serve in their places. This is, however, more typical of fractional horsepower (HP) motors, due to the cost of the added insulation in relation to the overall cost of the motor. Manufacturers often offer various motor product lines that provide a variety of application benefits at various price levels. One such example is the “inverter duty rated motor”. Some of these product lines include differences in the designs of their insulation systems. From time to time, new insulating materials are introduced with improved electrical, mechanical, thermal or chemical properties. An example of new magnet wire insulation technology is Phelps-Dodge’s Inverter Spike Resistant (ISR)® wire. This wire was originally purported to provide adequate protection against voltage overshoot caused by the fast rise times of IGBTs without the need for additional motor phase papers or sheets.
Field reports, however, have shown that this wire alone provides only a minimal extension to motor longevity.

Motor Insulation Systems (2)

2009-05-25T20:32:00.001+07:00

2. A Typical Motor Insulation System

Motor insulation systems vary considerably among the various motor manufacturers, but the following paragraphs provide a general description of the various components that comprise a typical insulation system.

Magnet wire is insulated with a thin coating of a varnish that is specifically designed as an electrical insulation material. The magnet wire insulating varnish provides the turn-to-turn insulation and a portion of the other elements of the motor insulation. In most motors, a large part of the winding-to-ground insulation is provided by a paper insulation lining in the stator slots. Paper insulation is also used to separate the windings of different phases. These components of the insulation system are called Slot Papers and Phase Papers. A rigid piece of insulation called a Top Stick Slot Wedge may be inserted in the opening of the slot to hold the windings securely in position. A diagram of a stator slot, showing the slot paper, a phase paper and the top stick are shown in Figure 2, below.

Figure 2 Stator Slot Insulation -- Slot Paper, Phase Paper and Top Stick

At each end of the stack of laminations, portions of the coils of wire, called the end-turns, pass from one slot to another. The end-turns are often separated from one another by paper insulation. Once the coils are wound into the stator laminations, the stator is dipped into a tank of insulating material, and baked, to coat the windings with another layer of insulation. This additional coating compensates for nicks and irregularities in the original coating, created during the manufacturing process and adds insulation to the magnet wire. After the additional coating cures, the stator may be dipped a second time for added protection from contaminants and moisture. This second, and subsequent dips and bakes, are typically an option offered by motor manufacturers.

Motor Insulation Systems (1)

2009-05-25T20:29:00.002+07:00

1. Motor Stator Construction

The stator of an AC motor consists of a stack of steel laminations that have coils of magnet wire set into slots.

Figure 1 is a representation of a stack of stator laminations showing the slots that receive the coils of wire. A number of coils are distributed among the slots to provide a group of coils that define each pole of the motor. For each pole, there are coils designated for connection to each phase of power.
Figure 1 Motor Stator Lamination Stack

The various electrical conductors that form the motor stator windings must be electrically insulated from each other and from the metal parts of the motor structure. Insulation is required wherever there is a difference of electrical potential between two (2) conductors. Turn-to-turn insulation prevents one (1) turn of a coil from short circuiting to an adjacent turn. Coil-to-coil insulation prevents various series or parallel connected coils from shorting to one another. Phase-to phase insulation separates the coils of one (1) phase from those of an adjacent phase. Winding-to-ground insulation prevents any part of the stator windings from shorting to the stator laminations.

Copper Wire

2009-05-03T23:31:00.003+07:00

We are able to supply copper wire with dia. 0.5mm; 0.1mm; 1mm and 2mm as per sample (it means-with out insulation = 100% copper) continuously.

Please note that:

- Offered net. prices: contact us at kecapi27@gmail.com

- Term of Payment: Cash against shipping documents.

- Loading capacity: 12MT per 20 FCL standard-container (max. 15MT)

- Quantities availability: 48 MT / month .

Commutator Surface Conditions

2009-05-02T20:34:00.002+07:00

Grade

2009-05-02T20:22:00.005+07:00

The grade of the brush is usually found stamped on the face of the carbon. The grade indicates the material composition of the brush. It represents one of the more technical challenges in carbon brush applications. Brush grades are usually classified according to the manufacturing processes and the types of materials used. The different grades in use today are derived through a variation of raw materials, molding pressures, temperature and duration of the baking process and after- treatments. These elements produce varied resistivity, hardness, and strength that in turn affect contact drop, friction and commutator filming. All grades fall within the four major categories of carbon graphite, electrographitic, graphite and metal graphite. The major characteristics of each category and a summary are listed on the opposite page.

There are only a few major carbon block makers in the world. Each of these manufacturers can make as many as 100 different grades. Each grade is designed to perform under certain operating conditions. Careful consideration is given to the actual running loads, the duty cycle, the voltage, the speed and the environment. Repco, Inc. supplies product from these major carbon manufacturers.
In addition, there are also carbon brush fabricators. A fabricator purchases carbon block, cuts it to size, adds hardware and stamps its own grade designation on the face of the brush.
Over the years, this practice has greatly added to the number of grades in the marketplace today. Our computer cross-reference system allows us to identify OEM and aftermarket brushes. Therefore, Repco, Inc. can supply carbon brushes for any motor including these major motor manufacturers:
• Baldor
• Century
• Lincoln
• Louis Allis
• Electric Machinery
• General Electric
• Gettys
• Imperial
• Indiana General
• Leeson
• Marathon
• Northwestern
• Pacific-Scientific
• Reliance
• U.S. Emerson
• Westinghouse

CARBON-GRAPHITE BRUSHES made their entrance early in the brush industry. Their properties of high hardness, material strength and pronounced cleaning action usually give long brush life under severe operating conditions, although they may not commutate as well as those with softer grades. These grades are limited to lower current densities and are used on slower machines, particularly those with flush mica commutators. The high friction generated with this type of material also makes it unattractive for some modern day applications.

ELECTROGRAPHITIC BRUSHES are baked at temperatures in excess of 2400 C. This process physically gives the material more of a graphitic structure. They generally have good commutating characteristics, but may not always be used because of high currents, or severe mechanical or atmospheric conditions. This material is fairly porous which permits treatment with organic resins. The treatments increase strength and lubricating ability which in turn helps increase brush life. These brushes are generally free from abrasive ash.

METAL GRAPHITE BRUSHES are generally made from natural graphite and finely divided metal powders. Copper is the most common metallic constituent, but tin, silver and lead are also used. This material is ideal for a variety of applications because of its low resistivity. Metal graphites are used on commutators of plating generators and wound rotor induction motors where low voltage and high brush current densities are encountered. They are also used for grounding brushes because of their low contact drop. These brushes exhibit a definite polishing action.

GRAPHITE BRUSHES are composed of natural or artificial graphite bonded with resin or pitch to form a soft brush material. Natural graphite usually contains ash, which gives the brush an abrasive, cleaning action. The fast filming properties of these brushes is beneficial in protecting the commutator or slip ring during operation in contaminated atmospheres. Their low porosity is also valuable in reducing commutator threading. They are not capable, however, of sustained operation at higher currents, like electrographic materials.

CARBON-GRAPHITE
• Used on older machines
• High strength and hardness
• Low resistance & poor commutation
• For machines that require some polishing action
• Generally, slower speed machines

GRAPHITE
• Good film characteristics
• Smooth ride
• Reduces threading
• Thermally limited
• Generally used on slip rings & FHP motors

METAL GRAPHITE
• Good for current densities
• Low contact drop
• Polishing action
• Metal content application
50% - material handling, battery charging & welding generators
60% - plating generators & rings
75% & up - rings & grounding brushes

ELECTROGRAPHITIC
• Good commutating ability
• Adequate strength with low abrasiveness
• Good friction characteristics
• Versatile
• Resistance, strength, hardness and density can be controlled through raw material
combinations
• Very widely used today in all types of motors & generators for:
steel and paper mills, excavation,
cement, transportation & aerospace

Basic Operation of AC Induction Motors (6) - FINISH

2009-05-02T20:13:00.003+07:00

Voltage and Current Waveforms

Today’s AC variable speed drive systems (up to 600 Volts and about 1500 HP) are dominated by PWM configurations. The current waveforms seen today (Figure 14) are much closer to the ideal sinusoid, thanks mostly to higher switching rates of transistors. The availability of low switching loss devices has allowed this to occur.

One of the negative aspects of the newer devices is that the low switching loss is typically accompanied by a very short transition time. This short transition between on and off states implies a high dV/dt output of the inverter. The high dV/dt results in capacitively coupled current flow according to Equation 4.

In addition to the capacitively coupled current, the high dV/dt also results in a higher peak voltage (ringup) due to cable-to-load mismatch. Finally, the high dV/dt also results in an instantaneous high voltage across the first windings within the ac motor. A companion paper, “AC Induction Motor Insulation Issues in High dV/dt Environments,” addresses this in greater detail.

Conclusions
AC induction motors are likely to continue to be increasing sources of variable speed rotating power. Their successful use in variable speed applications is a function of the collective understanding of the various parties involved in the specification, design, application, and integration of the system.

Basic Operation of AC Induction Motors (5)

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Constant Power Operation

The prior discussions regarding voltage boost and field oriented control as a means to maintain motor flux have been presented in regard to "constant torque" operation. This can also he thought of as operation "below base speed" (Figure 10).

Above the speed at which the output voltage of the controller is maximum, the controller can no longer maintain constant flux as speed is increased further (since the voltage cannot be increased to keep pace with the frequency). This is equivalent to where a DC motor begins to be "field weakened" to achieve higher speeds. Both for AC as well as DC machines, voltage (armature voltage for DC) remains constant, so for constant load current, constant output power
is available.
As the frequency supplied to an AC induction motor is increased (with voltage held constant), the resultant "field weakening" causes a reduction in the motor peak torque capability as seen in Figure 11.
This family of curves can alternatively be drawn as speed - power, rather than speed - torque curves (Figure 12). The fact that the peak power decreases as speed is increased by field weakening is the most "inherent" limitation to the "constant power speed range" of an AC induction motor drive.

A technique which is commonly employed to achieve wider speed ranges above base speed (constant power) is to utilize some of the “constant flux” speed range to augment the inherent constant power capability. By selecting a motor winding which does not require full voltage until some speed already into the desired constant power speed range, the constant power speed range can be extended as seen in Figure 13. The plots of Figure 13 show an example where the application demands a constant 100 HP from 650 RPM to 3200 RPM. By utilizing this technique, the motor size does not have to be increased in order to satisfy the wide constant power speed range.

This same technique is also used to extend the constant power speed range of DC systems as well. In the case of DC, it is to avoid commutation limits to the top speed at which constant power can be provided. For both AC and DC systems, the “price” which is paid to use this technique is an oversized source of power (higher kVA inverter or DC supply). Wind and unwind applications, along with machine tool spindles employ this technique quite commonly.

Basic Operation of AC Induction Motors (4)

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"Field Oriented" Control

In order to obtain even better yet control of AC motor torque, adjustable frequency controls often can make use of a regulation scheme known as "field-oriented" or "vector" control. This technique is intended to control the motor flux, and thereby be able to decompose the AC motor current into "flux producing" and "torque producing" components. These current components can be treated separately (in the control), then recombined to create the actual motor phase currents. This results in a solution to the boost adjustment problem, plus provides much better control of the motor torque - which allows much higher dynamic performance.

One way of looking at field oriented control is that the inverter would like to be able to have the same sort of simple, direct control of both flux and torque that is enjoyed with separately-excited dc motors. With dc motors, the flux level is controlled by simply regulating the field current, while the torque is controlled by regulating the armature current. By using field oriented control, the inverter can treat the ac induction motor as if it had the same sort of independently regulated flux and torque characteristic. When the actual induction motor phase currents are decomposed into flux and torque producing components (in the control, not in the motor), this gives the opportunity to “decouple” these two and achieve better system performance as a result.
In order to accomplish field-oriented control, the controller needs to have an accurate “model” of the motor. Over the last several years a large number of different schemes have been proposed to accomplish the “flux and torque control” desired. Many provide this control without the use of a speed feedback (tachometer) signal. These are typically referred to by the generic term of “sensorless” vector control. Many of today’s techniques also involve some sort of self-tuning at startup in order to obtain information which helps to more accurately model the motor – and thereby produce more optimal control. In addition, there are also techniques by which the models can adaptively adjust to changing conditions, such as the motor temperature going from cold to warm (which impacts the slip).

Basic Operation of AC Induction Motors (3)

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Adjustable Frequency, Variable Speed Operation

For steady-state (as opposed to starting) operation, AC induction motors offer a reasonably linear torque per amp and high power factor characteristic. This is seen in Figure 5 as the part of the speed torque curve between "breakdown RPM" and "synchronous (no load) RPM." It is this portion of the AC induction motor range of operation within which adjustable frequency drives function.

By varying both the frequency and voltage supplied to an AC motor, the controller can cause the motor to operate on a continuum of speed torque curves which allows operation in the "linear" region between breakdown and synchronous speeds (Figure 6).

This then allows the motor to operate near its optimal torque per amp or maximum efficiency point for a given load and speed.
As long as the motor flux is maintained constant while the frequency and voltage are varied, the basic "shape" of the speed torque curve will remain unchanged. The motor flux is proportional to the internal "counter-emf" divided by the frequency of that generated voltage. This can be described as:
where F is the motor flux,

Eg is the internally generated voltage due to motor rotation, f is the stator frequency, and k is a motor constant related to the winding turns, etc.
The flux paths for a four pole configuration are as seen in Figure 7 (for an instant in time). This pattern rotates at an effective speed given by Equation 1.

The motor counter-emf (Eg) can also be thought of as the voltage across the magnetizing reactance (Xm) in the equivalent circuit of Figure 3.
Maintaining constant flux while the speed (frequency) is varied can then be seen as requiring constant ratio of Eg / f (or constant Im).
Since Eg is a motor internal voltage, this needs to be related to the terminal voltage of the motor. From the AC motor equivalent circuit, it can been seen that the voltage drops across the stator resistance and leakage reactance represent the difference between Eg and the terminal voltage Vt.
If a controller were to maintain a constant ratio of TERMINAL voltage to frequency (Vt / f), rather than Eg / f, this would result in a noticeably decreasing flux level at lower speeds (frequencies).
The curves of Figure 8 demonstrate the effect of this failure to maintain the motor flux. It can be seen that the peak value of torque falls off at the reduced flux levels. In fact, the peak torque is approximately proportional to the square of the flux level, so the drop-off can be significant. The torque per amp is also (directly) proportional to the motor flux, so increased current draw for a given load (torque) will also result from reduced flux.

As a means to improve the system characteristics (beyond the curves of Figure 8), controllers often compensate for the difference between Vt and Eg in order to select the correct voltage for a given frequency. This compensation is often referred to as "voltage boost." Since the major detrimental effect of constant Vt / f is at low voltages, low frequencies (low speeds), the voltage drop across the stator leakage reactance is usually ignored (as the impedance of an inductor is proportional to frequency). This leaves the drop across the stator resistance as the major source of a discrepancy between Vt and Eg at these low speeds.
Many controllers use a value of voltage boost which compensates for the IR drop of the stator at a current equal to the motor full load amps.

Vb is the per phase (line-to-neutral) voltage boost,
Rl is the per phase stator resistance,
IFL is the motor full load current.

This would result in a voltage versus frequency characteristic as shown in Figure 9. A weakness in this technique of boosting voltage is that the value of Vb is only "correct" for a single value of load current. If the full load current is used to set the voltage boost, then the motor will be overfluxed for lighter loads, and underfluxed for overload conditions. Depending on the low speed performance required by a given application, this may or may not be a problem.
It is now common to provide a “more intelligent” voltage boost function in many controllers. This can provide a closer to optimal operating condition at low speeds, resulting in better low speed torque delivery from the system.

Basic Operation of AC Induction Motors (2)

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Speed / Torque Curves

As an AC induction motor is started, the values of resistance and reactance offered by the motor (or seen by the power source) will vary. At the instant of applying power to a stopped motor, the magnetic field is rotating much faster than the (stationary) rotor. This implies 100% slip, so R2/s is minimized. As a result, the current drawn at starting (locked rotor) conditions is quite high. Also, it is common to design rotor slots which have dramatically different impedance at high slip (say 60 Hz for starting) versus at normal running where slip is typically in the range of 0.2 - 2 Hz. This changes the values of both X2 and R2 from starting to running conditions.

As a motor accelerates to speed from a standstill, the changing impedances result in a unique characteristic developed torque and current drawn during the time of acceleration. Depending on the design of the motor, a torque / current characteristic such as one of those shown in Figure 4 would typically result. The NEMA Design B motor is considered the most "general purpose" of these characteristic shapes, with Design C and D typically used for more "difficult to start" loads. Table 2 gives some ranges of characteristics for integral HP, 1200
and 1800 RPM motors.

As can be seen from all of these speed / torque / current curves, the current drawn by an AC motor in accelerating a load up to speed can be dramatically higher than the nominal running current. At the same time, the developed torque (during acceleration) may in some cases be less than the rated full load torque. Various methods exist to control the starting current drawn by an AC motor but the torque per amp seen during (fixed frequency) starting is always much lower than at running conditions.
The nature of an AC induction motor’s acceleration to running speed is such that it can impose high stresses on both the stator and the rotor. The high current draw also stresses the upstream power system, including cabling, transformers, switchgear, etc. For this reason, there is often significant effort made to "control" AC motor starting and acceleration - both in terms of motor design as well as application.

Basic Operation of AC Induction Motors (1)

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Terminology and Equivalent Circuits

Before trying to understand the operation of AC induction motors on adjustable-frequency power (variable-speed), it will be useful to briefly review the basic fixed-frequency (constant speed) operation of AC induction motors. The fundamental electromagnetic components are the stator and rotor.
Examples of typical laminations which comprise the basic magnetic path in the stator and rotor are shown in Figure 1. In the most common configuration, the stator has three interconnected phase windings, and the rotor winding is a set of short circuited bars known as a "squirrel cage." A wound stator and an aluminum die cast (squirrel cage) rotor are seen in Figure 2.

With balanced three phase voltages applied to the windings of the stator, balanced currents flow in the three interconnected phase windings. These currents produce a magnetic field which can be thought of as "rotating" within the stator at a speed given by Equation 1.

N1 = 120 x f /P (1)
N1 = rotational speed of stator magnetic field in RPM (synchronous speed)
f = frequency of the stator current in Hz
P = number of motor magnetic poles

For various numbers of motors poles, Table 1 shows the synchronous speeds based on 60 Hz and 50 Hz frequencies.
The natural tendency is for the rotor to "follow" the rotating magnetic field, and at no-load the rotor will turn at a speed virtually equal to Nl. Any difference in the rotational speed of the magnetic field and that of the rotor will result in a voltage being induced in the rotor squirrel cage winding. The resultant rotor current interacts with the magnetic field to produce torque. The difference in rotor mechanical speed versus magnetic field rotational speed is what is known as "slip."
The equivalent circuit for an AC induction motor can help visualize some of the motor characteristics.

Figure 3 shows a typical equivalent circuit for AC induction motors. The variable resistor “R2/s”
represents the way slip causes increased current and corresponding increased torque. The greater the slip, the lower this value of resistance, and the more current is going to flow in this branch of the circuit. When the slip is virtually zero at a “no-load” condition, this resistor is seen to be a very high value. As a result, the current can be thought of as all going through the “XM” or magnetizing branch of the circuit.

Troubleshooting Drives

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