engine notes 2nd edition

ေမၚတါေတြကို အေပၚကလိုအမ်ိဳးအစါးခြဲလို႔၇ပါတယ္။ပထမဆံုး AC Polyphase ( 3 Phase ) က induction & synchronous ကို ေလ႔လါသင္႔ပါတယ္။

Induction motor မွာ wound rotor & squirrel cage rotor ဆိုၿပီးထပ္ခြဲပါတယ္။

Squirrel cage rotor က စက္ခန္းထဲအသံုးအမ်ားဆံုးၿဖစ္ၿပီး rotor မွာ ေၿကးဘါးေခ်ာင္းေတြၿမွဳပ္ၿပီး(အၿကမ္းဖ်င္းၿမင္သါေအါင္ေၿပာ၇င္) ထိပ္၂ ဖက္က ေၿကးကြင္းနဲ႔ ေ၇ွာ႔တိုက္ထါးတါပါဘဲ၊ ဒါေၿကာင္႔ ေမၚတါ၇ဲ႔ stator က transformer primary winding နဲ႔တူၿပီး rotor က transformer secondary winding ကို ေ၇ွာ႔တုိက္ထါးတါနဲ႔တူတယ္လို႔ေၿပာၿကတါၿဖစ္ပါတယ္။

AC single phase induction motors

A.C single phase induction motors ေတြဟါ သူဟါ သူ မလည္နုိင္ဘဲ ၇ိုတါကိုလွည္႔ေပးမွစလည္ပါတယ္။ ဘါလို႔လဲဆိုေတါ႔ သူ၇ဲ႔ clockwise torque = anti clockwise torque       ၿဖစ္ေနလို႔ၿဖစ္ပါတယ္။

သူကိုလည္ေစၿခင္၇င္ starting wire ( fine wire of high resistance and low reactance ) ကို series ထည္႔ေပး၇မွာၿဖစ္ပါတယ္။ အဲဒီ၀ိုင္ဒင္၇ဲ႔ angle of lag <main winding angle of lag ၿဖစ္လို႔ rotating magnetic field ၇ၿပီး စလည္နိုင္ပါတယ္။ေမၚတါ္စပိ ၇၅% ေ၇ာက္၇င္ ဗဟိုခြါအါးခလုတ္နဲ႔ ၿပဳတ္သြ့ါးပါတယ္။အခ်ိဳ႔ကလဲ STARTING WIDING မွာ ကက္ပက္စီတါ စီး၇ီး ဆက္ၿပီး နိုႈးၿကပါတယ္။

AC 3 phase induction motors

ပိုလီေဖ႔ဆိုတဲ႔သံုးသြင္ေဖ႔ induction motor ေတြက self starting ၿဖစ္ပါတယ္။ ၀န္ေပၚမူတည္ၿပီး လည္နွဳန္းက်တတ္ပါတယ္။ alternator stationary armature နဲ႔ဆင္တဲ႔ stator ၇ိွပါတယ္။

Stator - အဲဒီ stator ကို 3 phase supply ေပးတဲ႔အခါ rotating magnetic field (constant in magnitude and rotating at the same angular frequency as that of the supply voltage) ေပၚလါပါတယ္။

There are no slip rings or any DC excitation supplied to the rotor. Instead, the AC current in the stator induces a voltage across an air gap and into the rotor winding to produce rotor current and associated magnetic field.The stator and rotor magnetic fields then interact and cause the rotor to turn

credit to [Frank_D._Petruzella]_Electric_Motors_and_Control_

A three-phase motor stator winding consists of three separate groups of coils, called phases, and designated A, B, and C. The phases are displaced from each other by 120 electrical degrees and contain the same number of coils, connected for the same number of poles. Poles refer to a coil or group of coils wound to produce a unit of magnetic polarity. The number of poles a stator is wound for will always be an even number and refers to the total number of north and south poles per phase.

Typical connection of coils for a four-pole, three phase Y-connected induction motor.

credit to [Frank_D._Petruzella]_Electric_Motors_and_Control_

credit to [Frank_D._Petruzella]_Electric_Motors_and_Control_

SQUIRREL CAGE MOTOR –

Squirrel cage rotor - ၇ိူတါက ေၿမာင္းေတြထဲမွာ single stout copper conductor ထည္႔ထါးပါတယ္။ဟိုဘက္ဒီဘက္အစြန္း၂ဖက္မွာ copper ring ေတြနဲ႔ short လုပ္ထါးပါတယ္။အဲဒီ ၇ိုတါေၿကးေခ်ာင္းေတြထဲ စီးတဲ႔ လ်ပ္စီးက control လုပ္လို႔မ၇ပါဘူ။

An induction motor rotor can be either wound rotor or a squirrel cage rotor. The majority of commercial and industrial applications usually involve the use of a three-phase squirrel-cage induction motor.

A typical squirrel-cage induction motor

credit to [Frank_D._Petruzella]_Electric_Motors_and_Control

The rotor is constructed using a number of single bars short-circuited by end rings and arranged in a hamster-wheel or squirrel-cage configuration.

When voltage is applied to the stator winding, a rotating magnetic field is established. This rotating magnetic field causes a voltage to be induced in the rotor, which, because the rotor bars are essentially single-turn coils, causes currents to flow in the rotor bars. These rotor currents establish their own magnetic field, which interacts with the stator magnetic field to produce a torque. The resultant production of torque spins the rotor in the same direction as the rotation of the magnetic field produced by the stator. In modern induction

motors, the most common type of rotor has cast-aluminum conductors and short-circuiting end rings.

ေမၚတါ၇ဲ႔ စေတတါ၀ိုင္ဒင္ကို ဗို႔ေပးလိုက္ေတါ႔ ၃ သြင္ လ်ပ္စီးမို႔လို႔ လည္ေနတဲ႔ သံလုိက္စက္ကြင္းတခု စေတတါမွာေပၚလါပါတယ္။squirrel case motor ၇ဲ႔ စေတတါ ၀ုိင္ဒင္ကို ထ၇န္စေဖၚမါ တလံုး၇ဲ႔ primary winding တခုအေနနဲ႔ယူဆၿကပါတယ္။ rotor winding ကိုေတါ႔ ေ၇ွာ႔တု္ိက္ထါးတဲ႔ secondary winding တခုအေနနဲ႔ယူဆထါးၿကပါတယ္။ ထ၇န္စေဖၚမါ စကင္ဒ၇ီ၀ိုင္ဒင္ၿဖစ္တဲ႔ ၇ိုတါမွာ ညိဳ႔၇ သံလုိက္စက္ကြင္းတခုေပၚလါပါတယ္။အဲဒီ ၇ိဳတါစက္ကြင္းက စေတတါသံလုိက္စက္ကြင္းနဲ႔တြန္းဆြဲလုပ္ၿပီးေတါ႔ torque တခုေပၚလါတယ္။အဲဒါ၇ဲ႔လါး၇ာက စေတတါ၇ဲ႔ လည္ေနတဲ႔သံလုိက္စက္ကြင္းနဲ႔ လါး၇ာတူပါတယ္။

The resistance of the squirrel-cage rotor has an important effect on the operation of the motor. A high-resistance rotor develops a high starting torque at low starting current.

A low-resistance rotor develops low slip and high efficiency at full load.

 motor torque varies with rotor speed for three NEMA-type squirrel-cage induction motors:

NEMA Design B —Considered a standard type with normal starting torque, low starting current, and low slip at full load. Suitable for a broad variety of applications, such as fans and blowers, that require normal starting torque.

NEMA Design C —This type has higher than standard rotor resistance, which improves the rotor power factor at start, providing more starting torque. When loaded, however, this extra resistance causes a greater amount of slip. Used for equipment, such as a pump,

that requires a high starting torque.

NEMA Design D —The even higher rotor resistance of this type produces a maximum amount of starting torque. This type is suitable for equipment with very high inertia starts such as cranes and hoists.

Squirrel cage motor ေတြ characteristics

sychronous speed နားနီးပါး constant speed နဲ႔လည္တယ္။starting current 5 -6 ဆေလါက္၇ိွတတ္တယ္။ ပါ၀ါလုိင္း ၂ ေခ်ာင္းေၿပာင္းေပး၇င္ေၿပာင္းၿပန္လည္တယ္။လည္ေနတံုး phase တၿကိုးၿပတ္ခဲ႔၇င္ က်န္တဲ႔ လိုင္းေတြက ကါး၇င္ကို ၂ ဆေလါက္ဆြဲလိမ္႔မယ္။အိုဗါဟိၿဖစ္မယ္။ေမၚတါက synchronous speed နားေလါက္မွာ slip တခု  နဲ႔လည္မယ္။အဲဒီ slip ကေမၚတါကိုလွည္႔ေပးေနတါၿဖစ္တယ္။တကယ္လို႔ field လည္သေလါက္ rotor လည္၇င္ rotor & field ၿကား relative motion မ၇ိွေတါ႔လို႔ no voltage induced ၿဖစ္မယ္။rotor က rotating magnetic field of stator ကို slip ၿဖစ္လို႔သါ volt & current က ၇ုိတါမွာ ညိဳု႔ယူတါၿဖစ္တယ္။

Percent slip =100 x (synchronous speed – actual speed)/synchronous speed 

slip က load မ်ား၇င္လိုက္မ်ားတယ္။အဲလိုမ်ားမွ useful torque ၇မယ္။ ၆၀ ၿကိမ္နွုန္း သံုးသြင္ ေမၚတါေတြ၇ဲ႔ ပံုမွန္ slip က ၂ ကေန ၃ ၇ာခိုင္နွဳန္းေလါက္၇ိွတတ္တယ္။

ေနာက္ induction motor တမ်ိဳးက wound rotor induction motor (slip ring motor လို႔လဲေခၚပါတယ္)ၿဖစ္ပါတယ္။အ၇င္ကေၿပာခဲ႔သလို squirrel cage motor ၇ဲ႔ ၇ိဳတါလ်ပ္စီးက ထိန္းလို႔မ၇ပါဘူး။ wound rotor induction motor ၇ဲ႔ ၇ိဳတါက ကြိဳင္ပတ္ၿပီး ထိပ္မွာ slip ring နဲ႔ဆံုးထါးလို႔ အၿပင္ကေန resistance ကစါးေပးနိုင္လို႔ ၇ိဳတါလ်ပ္စီးကိုထိန္းနိုင္ပါတယ္။စနိဳးခ်ိန္ starting resistor ကိုၿဖတ္ခုိင္းလို႔ low starting current & high starting torque ၇ပါတယ္။အ၇ိွန္၇လါ၇င္ resistance ကိုေလ်ာ႔ခ်ပါတယ္။ဒါေၿကာင္႔ squirrel cage motor starting current က ပံုမွန္လ်ပ္စီးထက္ ၅ ဆေလါက္လိုေပမဲ႔ wound rotor induction motor ဆို၇င္ ၂ ဆေလါက္ဘဲလိုပါတယ္။

credit to [Frank_D._Petruzella]_Electric_Motors_and_Control

The wound-rotor induction motor (sometimes called a slip-ring motor) is a variation on the standard cage induction motors. Wound-rotor motors have a three-phase winding wound on the rotor, which is terminated to slip rings. The operation of the motor can be summarized as follows.

• The rotor slip rings connect to start-up resistors in order to provide current and speed control on start-up.

• When the motor is started, the frequency of current flowing through the rotor windings is nearly 60 Hz.

• Once up to full speed, the rotor current frequency drops down below 10 Hz to nearly a DC signal.

• The motor is normally started with full external resistance in the rotor circuit that is gradually reduced to zero, either manually or automatically.

• This results in a very high starting torque from zero speed to full speed at a relatively low starting current.

• With zero external resistance, the wound-rotor motor characteristics approach those of the squirrel cage motor.

• Interchanging any two stator voltage supply leads reverses the direction of rotation.

A wound-rotor motor is used for constant-speed applications requiring a heavier starting torque than is obtainable with the squirrel-cage type. With a high-inertia load a standard cage induction motor may suffer rotor damage on starting due to the power dissipated by the rotor. With the wound rotor motor, the secondary resistors can be selected to provide the optimum torque curves and they can be sized to withstand the load energy without failure. Starting a high-inertia load with a standard cage motor would require between 400 and 550 percent start current for up to 60 seconds.

Starting the same machine with a wound-rotor motor (slip-ring motor) would require around 200 percent current for around 20 seconds. For this reason, wound rotor types are frequently used instead of the squirrel-cage types in larger sizes.

Wound-rotor motors are also used for variable-speed service. To use a wound-rotor motor as an adjustable-speed drive, the rotor control resistors must be rated for continuous current. If the motor is used only for a slow acceleration or high starting torque but then operates at its maximum speed for the duration of the work cycle, then the resistors will be removed from the circuit when the motor is at rated speed.

In that case they will have been duty cycle–rated for starting duty only. Speed varies with this load, so that they should not be used where constant speed at each control setting is required, as for machine tools.

လ်ွပ္စစ္ ေမၚတါနဲ႔ ပန္ကါလွည္႔တဲ႔ လ်ပ္စစ္ သေဘါၤေတြမွာ သံုးၿကတါက 3 phase synchronous motor ပါဘဲ။ ေနာက္ ၿပီး 4 stroke engine သံုး cpp propulsion မွာ redundancy propulsion အေနနဲ႔ shaft generator ကို ေမၚတါေၿပာင္းၿပီးပန္ကါကိုၿပန္လွည္႔၇င္လဲ ဒီလို ေမၚတါမ်ိဳးပါဘဲ။(PTO & PTH လို႔ေၿပာၿကပါတယ္။ PTO ဆိုတါ POWER TAKE OUT – Engine ကေန shaft generator ကိုေမါင္းတါ။ PTH – power take home – shaft generator ကို ေမၚတါေၿပာင္း-မီးစက္ပါ၀ါနဲ႔ ပန္ကါလွည္႔ၿပီး အိမ္ၿပန္တါ) သူက no load to full load တင္တဲ႔ အတြင္း constant speed နဲ႔ တက္နိုင္လို႔လဲၿဖစ္ပါတယ္။သူလည္နဳန္းက ေတါ႔ ပါတဲ႔ pole အေ၇အတြက္နဲ႔ ေက်ြးတဲ႔လ်ပ္စီး၇ဲ႔ ၿကိမ္နွုန္းေပၚတည္ပါတယ္။မီးစက္တလံုးက နဂိုကတဲက rotary diode နဲ႔ ၇ိဳတါကို D.C ေပးထါးလို႔ Synchronous motor ေၿပာင္းလို႔၇ပါတယ္။

                                                                     

The three-phase synchronous motor is a unique and specialized motor. As the name suggests, this motor runs at a constant speed from no load to full load in synchronism with line frequency. As in squirrel-cage induction motors, the speed of a synchronous motor is determined by the number of pairs of poles and the line frequency.

credit to [Frank_D._Petruzella]_Electric_Motors_and_Control
၇ိုတါစလည္ဖို႔ squirrel cage motor rotor လိုdamping winding (amortisseur windings)ထဲ႔ထါးပါတယ္။

၇ိုတါလည္နွုန္း နဲ႔ စေတတါ ၇ဲ႔ လည္ေနတဲ႔ သံလုိက္စက္ကြင္းၿကားနိဳင္း၇ rpm သံုညၿဖစ္တဲ႔အခါ ၇ိုတါ သံလုိက္၀င္၇ိုးစြန္း နဲ႔ စေတတါသံလိုက္ ၀င္၇ိုးစြန္းတို႔ကပ္သါြးပါတယ္။

The operation of the motor can be summarized as follows.

• Three-phase AC voltage is applied to the stator windings and a rotating magnetic field is produced.

• DC voltage is applied to the rotor winding and a second magnetic field is produced.

• The rotor then acts like a magnet and is attracted by the rotating stator field.

• This attraction exerts a torque on the rotor and causes it to rotate at the synchronous speed of the rotating stator field.

• The rotor does not require the magnetic induction from the stator field for its excitation. As a result, the motor has zero slip compared to the induction motor, which requires slip in order to produce torque.

ေၿပာတဲ႔သေဘါက ၇ိုတါက စေတတါ၇ဲ႔လည္ေနတဲ႔ သံလိုက္စက္ကြင္းမွာ သံလုိက္ေကၚနဲ႔ကပ္သလို ကပ္ေနတယ္။ ၿပီးေတါ႔ စေတတါသံလိုက္စက္ကြင္းလည္သလို ၇ိုတါကလိုက္လည္ေနတယ္။ဒါေၿကာင္႔ zero slip ၿဖစ္ေနတယ္။

ဒါေပမဲ႔တခုလိုတါက အဲဒီ လည္ေနတဲ႔ စေတတါသံလုိက္စက္ကြင္းကို၇ိုတါကလိုက္မီေတါ႔မွ ၇ိဳတါက္ို ဒီစီေပးၿပီး သံလုိက္ေကၚနဲ႔ကပ္လို႔၇မွာ။ ဒီေတါ႔ ပထမ အမီလုိက္လည္ဘို႔ squirrel case rotor coil လို ကြိုဳင္တခုၿမွဳပ္ပတ္ထါးတတ္တယ္။ amortisseur windings လို႔ေခၚၿကတယ္။

ပထမ ၇ိုတါကို ဒီစီမေပးဘဲ ၇ိုတါနဲ႔ စေတတါသံလုိက္စက္ကြင္းၿကားလည္နွုန္းမကြါေအါင္ amortisseur winding နဲ႔၇ိုတါကိုလွည္႔ပါတယ္။

credit to [Frank_D._Petruzella]_Electric_Motors_and_Control

Synchronous motors are not self-starting and therefore require a method of bringing the rotor up to near synchronous speed before the rotor DC power is applied. Synchronous motors typically start as a normal squirrel cage induction motor through use of special rotor amortisseur windings. Also, there are two basic methods of providing excitation current to the rotor. One method is to use an external DC source with current supplied to the windings through slip rings. The other method is to have the exciter mounted on the common shaft of the motor. This arrangement does not require the use of slip rings and brushes.

အဲဒီေတါ႔အခ်ဳပ္ေၿပာ၇၇င္ induction motor ( squirrel cage or wound motor ) က ထ၇န္စေဖၚမါ primary(စေတတါ) & secondary winding(၇ိုတါ) သေဘါသံုးထါးၿပီးလည္ေနတဲ႔ စေတတါသံလုိက္စက္ကြင္းကိုတြန္းဆြဲၿပီး ၇ိုတါလိုက္လည္တါၿဖစ္ၿပီး

Synchronous motor ကေတါ႔ ၇ိုတါကို သံလိုက္ေကၚနဲ႔  လည္ေနတဲ႔ စေတတါသံလိုက္စက္ကြင္း မွာ လါ လွမ္းကပ္ၿပီးလည္တါပါဘ