Archive for the ‘Motor’ Category

Electro Motor Starter

January 21, 2010
Untuk menggerakan elektro motor, diperlukan peralatan pendukung yaitu, motor starter atau biasa disebut starter. Saat ini dikenal ada beberapa macam jenis starter. Diantaranya seperti berikut ini.

Direct On Line (DOL) Starter
Starter model ini sangat banyak dipakai saat ini, terutama untuk motor motor kecil. Komposisi komponennya terdiri dari satu contactor dan satu proteksi arus dengan TOR atau elektronik. Kelemahan starter model ini adalah kemungkinan timbulnya arus start yang sangat tinggi. biasanya bisa mencapai 6 sampai 7 kali. Pada saat starter ini di start, torsi saat start ini juga sangat tinggi dan biasanya lebih tinggi dari kebutuhan. Ini dapat terlihat adanya lonjakan/ gerakan yang keras saat motor di start. Tingginya torsi start ini juga akan memberikan tekanan lebih pada coupling dan beban.Komponen penyusun starter ini harus mempunyai ampacity yang cukup besar. Perlu diperhitungkan juga arus saat start motor, demikian juga ukuran range overloadnya.

Star Delta Starter
Starter ini mengurangi lonjakan arus dan torsi pada saat start. Tersusun atas 3 buah contactor yaitu Main Contactor, Star Contactor dan Delta Contactor, Timer untuk pengalihan dari Star ke Delta serta sebuah overload relay. Pada saat start, starter terhubung secara Star. Gulungan stator hanya menerima tegangan sekitar 0,578 (seper akar tiga) dari tegangan line. Jadi arus dan torsi yang dihasilkan akan lebih kecil dari pada DOL Starter. Setelah mendekati speed normal starter akan berpindah menjadi terkoneksi secara Delta. Starter ini akan bekerja dengan baik jika saat start motor tidak terbebani dengan berat.

Autotransformer Starter
Starter ini pada prinsipnya hampir sama dengan Star Delta Stater yaitu dengan mengurangi arus dan torsi saat start. Pada Autotranformer terdapat beberapa tap yang dapat menurunkan tegangan line. Starter akan mengatur masuknya tegangan yang mengalir ke motor dimulai dengan tegangan yang paling rendah bertahap sampai ke tegangan normal. Jika Star Delta starter hanya dua step, dengan autotransformer bisa beberapa step. Ini berguna untuk mengurangi lonjakan arus dan torsi saat start.

Soft Starter
Softstarter sangat berbeda dengan starter lain. Alat ini mempergunakan thyristor sebagai komponen utamanya. Tegangan yang masuk ke motor akan diatur dimulai dengan sangat rendah sehingga arus dan torsi saat start juga rendah. Pada saat start ini tegangan yang masuk hanya cukup untuk menggerakkan beban dan akan menghilangkan kejutan pada beban. Secara perlahan tegangan dan torsi akan dinaikan sehingga motor akan mengalami percepatan kehingga tercapai kecepatan normal. Salah satu keuntungan mempergunakan alat ini adalah kemungkinan dilakukannya pengaturan torsi pada saat yang diperlukan, tidak terpengaruh ada atau tidaknya beban.

Frequency Drive
Frequency Drive sering disebut juga dengan VSD (Variable Speed Drive), VFD (Variable frequency Drive) atau Inverter. VSD terdiri dari 2 bagian utama yaitu penyearah tegangan AC (50 atau 60 HZ) ke DC dan bagian kedua adalah membalikan dari DC ke tegangan AC dengan frequency yang diinginkan. VSD memanfaatkan sifat motor sesuai dengan rumus sbb :

RPM = (120.f)/p

dimana

RPM : Kecepatan putar/ speed motor (RPM)

F : Frequency (Hz)

P : pole

Jadi dengan mengatur frequency tegangan yang masuk, maka kecepatan motor akan dapat diatur pula. Demikian pula pada saat start, dimulai dengan frequency rendah sampai rated frequency nya hasilnya kecepatan motor akan mengalami percepatan yang lebih halus.

Why Soft Starts?

January 20, 2010
The 3-phase induction motor is over 100 years old and obviously many design changes and variations have appeared over the years. However certain fundamental characteristics remain the same and it is the problems that these inherent features cause that electronic soft start aims to solve.

1. Direct-On-Line

Reduced Voltage Starting:
2. Star/Delta
3. Auto Transformer
4. Soft Starts

1. Direct-On-Line
The main method of starting the AC motor is direct-on-line starting. This simply means an electro-mechanical switch is opened and closed to stop and start the motor.

Disadvantages
Electrical
-High inrush current (typically 6 x full load which can cause several problems)
-Necessities over sizing of installation (particularly important on generator and UPS fed supplies)
-Limits Expansion
-Reduces service life of electrical components
Mechanical
-Excessive applied starting torque (typically 2.5 x full load)
-Increases wear on drive chain components
-Reduces service life of mechanical components

2. Star/Delta
This method requires both connections for each phase (six in all) to be taken to the starter. Three contactors are used to first connect the motor in star and then to delta after a given time. Connecting the motor in star reduces the voltage applied to each winding to about 60% of the line voltage. This reduces the starting torque and current (typically 3.5 x FLC). After a given time the motor is switched to delta connection and then runs as if direct-on-line. Its main advantages are that it is relatively simple and low cost. The major problem with this method is that the reduced voltage level is in a single stage and is fixed. sometimes this voltage is not ideal, the torque it produces (65% of full load torque) may be too small and the motor stalls or does not give complete acceleration, or if it is too great the motor still starts with a pronounced snatch. The star/delta transition will produce a second current and torque peak which is almost the equivalent of having two direct-on-line starts. On some loads the motor sometimes almost stalls during this transition time. This method of starting does however have the advantage of being a low cost and simple solution if its limitations can be tolerated.

Advantages
-Low cost and simple

Disadvantages
-Torque too high – causes snatch
-Torque too low – motor stalls
-Transition peak up to 20 x In
-Motor can stall in transition

3. Auto Transformer
This method uses transformer action to reduce the voltage applied to the motor and current seen by the supply. An improved torque/amp ratio is achieved and starting current is typically 3 x FLC, depending on the voltage rapping selected. Normally the voltage is applied to the motor in voltage steps through the transformer with the taps being selected through contactors. Typical tappings are 50%, 70%, followed by full voltage being applied to the motor. The major disadvantages are size and cost, and of course the mechanical snatch at switch on is not controllable and can still cause problems. Also once the tappings have been selected, it may be necessary to change them according to changes in load parameters.
Advantages
-Simple operation

Disadvantages
-Poor controllability
-Bulky
-Very Expensive
4. Soft Starts
The soft start is designed to apply an adjustable voltage to the motor and increase this voltage gradually over a user-selectable acceleration period. The acceleration time being dependent on the application and desired characteristics. The added advantage of this method of reduced voltage control is that the motor can also be stopped gradually by slowly reducing the output voltage to the .‘Soft Stop’ feature offers a smooth stop in many process industries such as pumps, where fast stops can result in ‘water hammer’ and mechanical damage.

Advantages
-Reduced starting current
-Reduced starting torque
-Less mechanical stress
-Improved control of acceleration and deceleration

Motor Insulation Systems (7) – Finish

May 25, 2009
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)

May 25, 2009

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)

May 25, 2009
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)

May 25, 2009
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)

May 25, 2009
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)

May 25, 2009
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)

May 25, 2009
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.

Commutator Surface Conditions

May 2, 2009