Different types of Starter for Motor

Different types of Starter for Motor

1)DC
-TWO point starter

A two-point starter is used for starting dc motor which has the problem of over-speeding due to loss of load from its shaft.

-Three point starter

A three-point starter is used for starting a dc shunt or compound motor. The coil of the hold-on electromagnet is connected is series with the shunt-field coil.

- Four point starter

The 4 point starter like in the case of a 3 point starter also acts as a protective device that helps in safeguarding the armature of the shunt or compound excited DC motor against the high starting current produced in the absence of back emf at starting. The 4 point starter has a lot of constructional and functional similarity to a three point starter, but this special device has an additional point and a coil in its construction. This naturally brings about some difference in its functionality, though the basic operational characteristic remains the same. The basic difference in circuit of 4 point starter as compared to 3 point starter is that the holding coil is removed from the shunt field current and is connected directly across the line with current limiting resistance in series. 

2)AC
- Direct on line (DOL) starter

The simplest form of motor starter for the induction motor is the Direct On Line starter. The Direct On Line Motor Starter (DOL) consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection. Electromagnetic contactor which can be opened by the thermal overload relay under fault conditions.

- Star-Delta starter

Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance of voltage on the supply lines due to large starting current surges. To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed.

- Auto transformer starter

An auto-transformer starter makes it possible to start squirrel-cage induction motors with reduced starting current, as the voltage across the motor is reduced during starting.In contrast to the star-delta connection, only three motor leads and terminals are required.

- Semi-Automatic Star Delta Starter

 In semi-automatic starter Timer is not required for changing star to delta contactor. In this change-over take place as we unhold start button.

- Automatic Star Delta Starter

In automatic starter Timer is required for changing star to delta contactor. In this change-over take place after getting signal from timer. This type of starter use for higher ratting motor. Because it take more time to reach normal ratting of motor current.

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Different types of Induction Motor

Different types of Induction Motor

SINGLE PHASE INDUCTION MOTOR

1.    Split phase induction motor

There are various types of self-starting motors, known as split phase motors. Such motors have a starting winding displaced 90 electrical degrees from the main or running winding. In some types, the starting winding has a fairly high resistance, which causes the current in this winding to be out of phase with the current in the running winding. This condition produces, in effect, a rotating field and the rotor revolves. A centrifugal switch disconnects the starting winding automatically, after the rotor has attained approximately 25 percent of its rated speed.

2.    Capacitor start induction motor

With the development of high capacity electrolytic capacitors, a variation of the split phase motor, known as the capacitor start motor, has been made. Nearly all fractional horsepower motors in use today on refrigerators, oil burners, and other similar appliances are of this type. In this adaptation, the starting winding and running winding have the same size and resistance value. The phase shift between currents of the two winding is obtained by using capacitors connected in series with the starting winding.

3.    Capacitor start capacitor run induction motor

A variation of the capacitor-start motor is to start the motor with a relatively large capacitor for high starting torque, but leave a smaller value capacitor in place after starting to improve running characteristics while not drawing excessive current. The additional complexity of the capacitor-run motor is justified for larger size motors.

4.    Shaded pole induction motor

The first effort in the development of a self-starting, single phase motor was the shaded pole induction motor. This motor has salient poles, a portion of each pole being encircled by a heavy copper ring. The presence of the ring causes the magnetic field through the ringed portion of the pole face to lag appreciably behind that through the other part of the pole face. The net effect is the production of a slight component of rotation of the field, sufficient to cause the rotor to revolve. As the rotor accelerates, the torque increases until the rated speed is obtained. Such motors have low starting torque and find their greatest application in small fan motors where the initial torque required is low. 

THREE PHASE INDUCTION MOTOR

5.    Squirrel cage induction motor

Three-phase squirrel-cage motor are made of squirrel cage rotor in which copper bar are used and shorted at both end by copper ring. Three-phase squirrel-cage induction motors are widely used in industrial drives because they are rugged, reliable and economical.

6.    Slip ring induction motor

A wound-rotor motor is a type of induction motor where the rotor winding are connected through slip rings to external resistances. Adjusting the resistance allows control of the speed/torque characteristic of the motor. Wound-rotor motors can be started with low inrush current, by inserting high resistance into the rotor circuit; as the motor accelerates, the resistance can be decreased.

Compared to a squirrel-cage rotor, the rotor of the slip ring motor has more winding turns; the induced voltage is then higher, and the current lower, than for a squirrel-cage rotor. During the start-up a typical rotor has 3 poles connected to the slip ring. Each pole is wired in series with a variable power resistor. When the motor reaches full speed the rotor poles are switched to short circuit. During start-up the resistors reduce the field strength at the stator. As a result the inrush current is reduced. Another important advantage over squirrel-cage motors is higher starting torque.

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Why LT motors are delta connected and HT motors are star connected?

Why LT motors are delta connected and HT motors are star connected?

• There is both reason technical and commercial.

1.     In star, phase current is same as line current. But phase voltage is 1/1.732 times line voltage. So insulation required in case of HT motor is less.

2.     The starting current for motors is 6 to 7 times full load current. So start-up power will be large if HT motors are delta connected. It may cause instability (voltage dip) in case small Power system. In starred HT motor starting current will be less compared to delta connected motor. So starting power is reduced. Starting torque will also be reduced. (It will not be a problem as motors are of high capacity.)

3.    Also as current is less copper (Cu) required for winding will be less.

4.    LT motors are delta connected.

1.    Insulation will not be problem as voltage level is less.

2.    Starting current will not be problem as starting power in all will be less. So no problem of voltage dips.

3.    Starting torque should be large, as motors are of small capacity.

LT motors have winding delta connected.

1. In case it is having star delta starter than they are started as Star connected motor.

2. After it attains 80% of synchronous speed the changeover takes place from star to original configuration    delta.

3. In star the voltages across the winding are lesser that is 1/1.732 times that available in delta so current    is limited.

4. When it goes to delta again voltage is full line voltage so current increase even though it is lesser than     the line current it remains higher than the line current drawn in star connection at reduced voltage. So       cables for motor are sized for this current that is what it draws in delta connection.

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Brief Idea of Induction Motor

Brief Idea of Induction Motor

Induction motor: is an energy conversion device that converts electrical energy into useful rotational kinetic energy, it is an application of the Faraday's law of induction.

Sectional View

3-ph Induction Motor

 

Drawing of Induction Motor

Induction motor are the most commonly used motors in many applications. These are also called as Asynchronous Motors, because an Induction motor always runs at a speed lower than synchronous speed. Synchronous speed means the speed of the rotating magnetic field in the stator.

An Induction motor has basically two parts – Stator and Rotor

The Stator is made up of a number of stamping with slots to carry three phase winding. It is wound for a definite number of poles. The winding are geometrically spaced 120 degrees apart. Two types of rotors are used in Induction motors - Squirrel-cage rotor and Wound rotor. 

 

A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots. These bars are short-circuited at both ends by means of short-circuiting rings.

A wound rotor has three-phase, double-layer, distributed winding. It is wound for as many poles as the stator. The three phases are wyed internally and the other ends are connected to slip-rings mounted on shaft with brushes resting on them. The brushes are connected to an external resistance that does not rotate with the rotor and can be varied to change the N-T characteristics. 

In fact an Induction motor can be compared with a transformer because of the fact that just like a transformer it is a singly energized device which involves changing flux linkages with respect to a primary(stator) winding and secondary(rotor) winding.

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Different type of motors...

Different type of motors.....

Squirrel Cage Motor

 

Electric Motor

An Electric motor is a machine which converts electric energy into mechanical energy. Its action is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming’s Left-hand Rule and whose magnitude is given by F = BIl Newton.

Types of AC Motors

Classification Based On Principle of Operation:

(a) Synchronous Motors.

1. Plain

2. Super

(b) Asynchronous Motors.

1. Induction Motors:

(a) Squirrel Cage

(b) Slip-Ring (external resistance).

2. Commutator Motors:

(a) Series

(b) Compensated

(c) Shunt

(d) Repulsion

(e) Repulsion-start induction

(f) Repulsion induction

Classification Based On Type of Current:

1. Single Phase

2. Three Phase

Classification Based On Speed of Operation:

1. Constant Speed.

2. Variable Speed.

3. Adjustable Speed.

Classification Based On Structural Features:

1. Open

2. Enclosed

3. Semi-enclosed

4. Ventilated

5. Pipe-ventilated

6. Riveted frame-eye etc. 

Types of DC Motor

Most common DC motor types are-

1. Permanent-magnet motors
2. Brushed DC Motor

a.       DC shunt-wound motor

b.      DC series-wound motor

c.       DC compound motor

                                                              i.      Cumulative compound

                                                            ii.      Differentially compounded

d.      Permanent magnet DC motor

e.       Separately excited

3. Brushless DC Motor
4. Coreless or ironless DC motors
5. Printed armature or pancake DC motors
6. Universal motors

 

 

 

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Essential or need of starter with motor.

Essential or need of starter with motor.

Motors below 1 Hp is directly connect without starter because their armature resistance is very high and they have ability to afford the high current due to high resistance. So the Armature winding safe from the high starting current.

 

But a large size of motors has a very low armature resistance. if connect  this type of motor direct to Supply (3-phase Supply) then the large current will destroy the armature wading due to low resistance because motor is not running in this time. Why motor is not running in this time when we connect motor to supply? Obviously, because their is no Back E.M.F in the motor. the back E.M.F of the motor is reach at full rate when motor is running at full speed.

 

So this is the answer that why we connect a starter with motor in series.  Starter in series with motor ( I.e. Resistance) is reduce the high starting current and armature takes a low current and motor will be start. But this is not end of our story. After starting the  motor at low current, the starter resistance reduce by turning a starter handle ( not in each case, in other system or case, this can be automatically) so the armature will take high current and motor armature will be rotate at full speed ( in other words, the speed of the motor will be increase).

For more Explanation see the example.

We know that the armature current can be finding by this formula,

I_a = V-Eb/R_{a       ,}             ( I=V/R, Ohm Law)

        Where,      

I_a =Armature current

V= Supply voltage

Eb= Back _E.M.F                                           

R_a = Armature resistance

Suppose

A 5 Hp (3.73killowatt) motor with 440 volts having armature resistance 0.25 ohm resistance.

And the normal full load current is 50 amperes.

if we connect to direct to supply without starter the result will be.

So putting the values in equation

I_a= 440-0/0.25 = 1760 A

ahh! This high current will destroy armature winding because its 35.2 times high with respect to normal full load current.

1760/50 = 35.2

 

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