Rabu, 30 Maret 2011

Electric motor

Electric motor
In the article "The classification of electrical machinery, electric motors including into the category of dynamic electric machine and it is an electromagnetic device that converts electrical energy into mechanical energy. Mechanical energy is used for, for example, rotating impeller pump, fan or blower, compressor moving, lifting materials, etc. in the industry and is also used in household electrical appliances (such as: mixers, electric drill, electric fan).

You can see an animation of this DC motor working principle here.

The electric motor is sometimes called the "work horse" of his industry, because it is estimated that motors use about 70% of the total electrical load in the industry.

Mechanism of action for all types of electric motors are generally similar (Figure 1), namely:
• An electric current in a magnetic field will exert a force.
• If the wire that carries the flow is bent into a circle / loop, then both sides of the loop, ie at right angles to the magnetic field, will gain force in the opposite direction.
• Couple style produces power play / torque to rotate the coil.
• motors have several loops on dynamo to provide a more uniform torque and the magnetic field generated by the electromagnetic structure of the so-called field coils.

In understanding an electric motor, it is important to understand what is meant by the motor load. Expense refers to the output power play / torque in accordance with the required speed. Expenses can generally be categorized into three groups:
• constant torque load, is the burden which the demand for its energy output varies with the speed of operation, but its torque does not vary. Examples of constant torque loads are conveyors, rotary kilns, and the constant displacement pump.
• Expenses with variable torque, is the load torque that varies with the speed of operation. Examples of variable torque loads are centrifugal pumps and fans (torque varies as the square of velocity).
• Expenses with constant energy, is the load torque change with demand and inversely proportional to speed. Examples for the constant power load is machine tools.
Figure 1. Working Principles of Electric Motors.


This section describes the two main types of electric motors: DC motors and AC motors. Motorcycles are classified based on the input supply, construction, and operation mechanism, and further described in the chart below.
Figure 2. Classification of Electric Motors.

1. Motor DC / Direct Current
Motor DC / DC current, as the name implies, uses direct current indirect / direct-unidirectional. DC motors are used in special applications where high torque is required ignition or acceleration is fixed for a broad range of speed.
Figure 3 shows a DC motor which has three main components:
• Pole field. In simlpe illustrated that the interaction of two magnetic poles will cause the rotation of the DC motor. DC motor has a stationary field poles and armature that moves bearing on the space between the polar terrain. Simple DC motor has two field poles: north pole and south pole. Lines of magnetic energy through the enlarged openings between the poles from north to south. For larger motors or more complex consists of one or more electromagnets. Electromagnet to receive electricity from outside resources as a provider of field structure.
• Dynamo. When the flow goes through the armature, then this flow will be an electromagnet. A cylindrical armature, connected to the countershaft to drive the load. For the case of a small DC motor, the armature rotates in a magnetic field formed by the poles, to the north and south magnetic poles switch location. If this happens, the current is turned to change the poles of north and south of the dynamo.
• commutator. This component is mainly found in DC motors. Usefulness is to reverse the direction of electric current in the armature. commutator also assist in the transmission of current between the armature and resources.
Figure 3. DC motor.

The main advantage is the speed of DC motor easily controlled and does not affect the quality of power supply. This DC motor can be controlled by adjusting:
• dynamo voltage - increasing the armature voltage will increase the speed.
• Flow field - reducing the field current will increase the speed.

DC motors are available in many sizes, but their use is generally restricted to some use of low-speed, low power usage to moderate, such as machine tools and rolling mills, because often there are problems with mechanical electric current changes direction at a larger size. Also, the motor is restricted only for use in the area is clean and not dangerous because the risk of sparks at the brush. DC motors are also relatively expensive compared to AC motors.

The relationship between speed, field flux and armature voltage is shown in the following equation:

Electromagnetic force: E = KΦN

Torque: T = KΦIa

E = electromagnetic force developed at the armature terminals (volts)
Φ = flux field which is proportional to the flow field
N = speed in RPM (revolutions per minute)
T = electromagnetic torque
He = armature current
K = constant equation

Types of DC motors / direct current

a. DC motors separate power source / Separately Excited, If the field current supplied from a separate source of DC motors it is called a separate power source / separately excited.

b. DC motor own resources / Self Excited: shunt motor. In the shunt motor, the field winding (shunt field) are connected in parallel with the armature coil (A) as shown in Figure 4. Therefore the total current in the path is the sum of field current and armature current.
Figure 4. Characteristics of DC Shunt Motor.

Following the shunt motor speed (ETE, 1997):
• The speed is practically constant independent of the load (up to a certain torque when the speed is reduced, see Figure 4) and is therefore suitable for commercial use with low initial load, such as machine tools.
• Speed ​​can be controlled by placing a detainee in the composition series with the armature (reduced speed) or by putting detainees in the flow field (velocity increases).

c. DC motor power alone: motor series. In a series motor, the field winding (shunt field) is connected in series with the armature coil (A) as shown in Figure 5. Therefore, the flow field with armature current.

Following on the series motor speed (Rodwell International Corporation, 1997; LM Photonics Ltd, 2002):
• Speed ​​limited to 5000 RPM.
• Must avoid running series motor with no load because the motor will accelerate uncontrollably.
Series motors are suitable for use requiring high starting torque, such as crane and hoist lifting equipment (see Figure 5).
Figure 5. Characteristics of DC Motor Series.

d. DC Motor Compound / Combined.
DC compound motor is a combination of series and shunt motors. In the compound motor, the field winding (shunt field) is connected in parallel and in series with the armature coil (A) as shown in Figure 6. Thus, compound motors have excellent starting torque and stable speeds. The higher the percentage of incorporation (ie the percentage of the field winding is connected in series), the higher the starting torque can be handled by this motor. For example, incorporation of 40-50% makes this bike suitable for hoist and crane lifting equipment, whereas the standard compound motor (12%) did not match (myElectrical, 2005).
Figure 6. Compound DC Motor Characteristics.

2. Motor AC / Alternating Current Behind

Motor AC / alternating current using an electric current that reverses its direction at regular intervals. AC electric motor has two basic parts power: "stator" and "rotor" as shown in Figure 7.

The stator is a component of static electricity. The rotor is rotating electrical components to rotate the motor shafts. The main advantage of AC motors, DC motors is that the speed of AC motor is more difficult to control. To overcome this disadvantage, AC motors can be equipped with variable frequency drive to increase speed while lowering power control. Induction motors are the most popular motorcycle industry because of their reliability and easier maintenance. AC induction motor is quite cheap (the price is half or less than the price of a DC motor) and also provides power to weight ratio is quite high (about twice a DC motor).

Types of Motor AC / Alternating Current Behind

a. Synchronous motor. AC synchronous motor is a motor that works on fixed speed at a certain frequency system. This motor requires direct current (DC) for power generation and has a low starting torque, and therefore synchronous motor suitable for use beginning with low load, such as air compressors, generators and motor frequency changes. Synchronous motor is able to improve system power factor, so often used in systems that use a lot of electricity.

The main component is a synchronous motor (Figure 7):
• Rotor. The main difference between synchronous motors with induction motors is that the rotor synchronous machines running at the same speed with the rotation of the magnetic field. This is possible because the rotor magnetic field is no longer induced. The rotor has permanent magnets or DC-excited, which was forced to lock in a certain position when confronted with another magnetic field.
• Stator. The stator produces a rotating magnetic field is proportional to the supply frequency.

This motor rotates at synchronous speed, which is given by the following equation (Parekh, 2003):

Ns = 120 f / P

f = frequency of the supply frequency
P = number of poles
Figure 7. Motor Sync.

b. Induction motor. Induction motors are the most common motor used in various industrial equipment. Its popularity because of its design is simple, cheap and easily available, and can be directly connected to AC power source.

Components of Induction motors have two main electrical components (Figure 8):
• Rotor. Induction motor using two types of rotor:
- Squirrel cage rotor consists of thick conductive rod that is attached in patches of parallel slots. Rods are given a short circuit at both ends by means of a short circuit ring.
- The circle rotor which has a roll of three-phase, double layer and distributed. Created by circular stator poles. Three phases heavily wire on the inside and the other end is connected to a small ring that is placed on the rod as with a brush attached to it.
• Stator. The stator is made from a number of stampings with slots to carry three-phase coils. The roll is looped for a certain number of poles. The roll was given a space geometry of 120 degrees.

Classification of induction motor

Induction motors can be classified into two main groups (Parekh, 2003):
• single-phase induction motors. This motor has only one stator winding, operates with single-phase power supply, having a squirrel cage rotor, and requires an appliance to turn on the bike. So far this bike is the most common types of motors used in household appliances such as fans, washing machines and clothes dryers, and to use up to 3 to 4 hp.
• three-phase induction motors. A rotating magnetic field generated by a balanced three-phase supply. The motor has high power capability, can have a squirrel cage rotor or coil (although 90% had a squirrel cage rotor), and ignition of its own. It is estimated that approximately 70% of motors in industrial use of this type, for example, pumps, compressors, conveyor belts, electrical networks, and grinder. Available in sizes 1 / 3 to hundreds of hp.
Figure 8. Induction Motor

Induction motor works as follows, Electricity is supplied to the stator that will generate a magnetic field. This magnetic field moving at a speed synchronous around the rotor. Rotor currents produce a second magnetic field, which is trying to fight the stator magnetic field, which causes the rotor to spin. However, in practice, the motor never worked at synchronous speed but in the "basic rate" is lower. The occurrence of differences between the two speeds are caused by "slip / slide" which increases with increasing load. Slip only occurred on induction motors. To avoid the slip rings can be mounted a sliding / slip rings, and the motor is called "motor ring shear / slip ring motor. "

The following equation can be used to calculate the percentage of slip / slide (Parekh, 2003):

% Slip = (Ns - Nb) / Ns x 100

Ns = synchronous speed in RPM
Nb = base speed in RPM

The relationship between load, speed and torque
Figure 9. Graph of Induction Motor Torque vs. Speed

Figure 9 shows the graph of torque vs. speed three-phase AC induction motor with a predefined flow. When the motor (Parekh, 2003):
• Start burning flame turns out there is an initial high current and low torque ("pull-up torque").
• Achieve 80% full speed, torque is at its highest level ("pull-out torque") and the flow began to fall.
• At full speed, or synchronous speed, torque and stator currents down to zero.

Tidak ada komentar:

Posting Komentar