Starting methods of motors

NAME DAMR Fernando

INDEX NO 090130E

FIELD Mechanical

COURSE B.Sc eng semester 3

INTRODUCTION

When the motor is rotate, it has a back emf. If rotor is not rotate value of the back emf is zero. At this time very high current pass the motor. So initially motor can be burned. In a single phase induction motor, it is necessary to provide a starting circuit to start rotation of the rotor. If this is not done, rotation may be commenced by manually giving a slight turn to the rotor. The single phase induction motor may rotate in either direction and it is only the starting circuit which determines rotational direction. For small motors of a few watts, the start rotation is done by means of one or two single turn(s) of heavy copper wire around one corner of the pole. The current induced in the single turn is out of phase with the supply current and so causes an out-of-phase component in the magnetic field, which imparts to the field sufficient rotational character to start the motor. Starting torque is very low and efficiency is also reduced. Such shaded-pole motors are typically used in low-power applications with low or zero starting torque requirements, such as desk fans.

CALCULATION

(1) Capacitance vs voltage and current

(2) Capacitance (µF) vs current(mA)

Slip for the shaded pole motor

Synchronous speed of the motor

P - The number of pole pairs for the 3 phase winding

F - Frequency of the 3 phase current supplied

NS - Synchronous speed in rev/m

Rotor speed of the motor Nr = 1300 rev/m

Slip of the motor

DISCUSSIONShaded pole motor

This is a single phase induction motor. It is basically a small squirrel cage motor in which the auxiliary winding is composed of a copper ring surrounding a portion of each pole. This winding is called a shading coil. Currents in this coil delay the phase of magnetic flux in that part of the pole enough to provide a rotating magnetic field. This results the direction of rotation from the unshaded side to the shaded (ring) side of the pole. The effect produces only a low starting torque compared to other classes of single-phase motors.

D.O.L – Direct Online starter

This is the simplest type of Starter. The main component of this are a contactor and an overload relay. The main disadvantage is it drains

a very high current initially. A normal value is between 6 to 7 times the rated motor current but values of up to 9 or 10 times the rated current exist. During a direct-on-line start, the starting torque is also very high, and is usually higher than required for most applications. The motor will draw a very high inrush current for a very short time, to establish the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor.

The motor will develop Locked Rotor Torque and begin to accelerate towards full speed. As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load. The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but not the magnitude of the starting current

Disadvantages of D.O.L starter

The main disadvantage is the high initial current .Because of this high initial current ,it can create a high voltage drop in supply line .This can affect other electrical equipments which are connected to that line.

Autotransformer starting

An Auto transformer starter uses an auto transformer to reduce the voltage applied to a motor during start. The auto transformer may have a number of output taps and be set-up to provide a single stage starter, or a multistage starter. Typically, the auto transformer would have taps at 50%, 65% and 80% voltage, enabling the motor to be started at one or more of these settings.There are two ways of connecting an auto transformer starter, the most obvious way is to apply full voltage to the transformer via a contactor, and connect the motor to the tap by means of a contactor. When the motor has accelerated to full speed, or has run out of acceleration torque, the tap contactor opens, disconnecting the motor from the transformer and another contactor closes

connecting the motor to the supply. The transformer can now be disconnected from the supply.

Star-Delta starting

The Star Delta starter can only be used with a motor which is rated for connection in delta operation at the required line voltage, and has both ends each of the three windings available individually. At Start, the line voltage is applied to one end of each of the three windings, with the other end bridged together. Under this connection, the voltage across each winding is 1/v3 of line voltage and so the current flowing in each winding is also reduced by this amount.

Rotor Resistance Starter

If it is necessary to start a three phase induction motor on load then a wound rotor machine will normally be selected. Such a machine allows an external resistance to be connected to the rotor of the machine through slip rings and brushes. At start-up the rotor resistance is set at maximum but is reduced as speed inceases until eventually it is reduced to zero and the machine runs as if it is a cage rotor machine.

Reversing the motor           Some motors are expected to run on both directions. We should change the direction of the rotating magnetic field and by changing the direction of the current flow, we can achieve  this. Changing any two of the supply lines is sufficient in this process. Reversing the connections to any two of the three motor terminals can reverse the direction of rotation of 3-phase induction motor

1. Study the construction and the various parts of the 3-phase induction motor. 2. For rotor resistance starting, connect the slip-ring motor. Start the motor with full starting

resistance and then decrease the resistance in steps down to zero. 3. Take observations of the stator & rotor currents 4. For direct-on -line starting, connect the cage motor. For star-delta starting, connect the cage motor

to the terminals of the star delta switch 5. . For autotransformer starting, connect the cage motor. Take care at starting that the "Run" switch

is open and that it is not closed before the "Start" switch is opened. 6. In each case observe the starting currents by quickly reading the maximum indication of the

ammeters in the stator circuit. 7. Reverse the direction of rotation of the motor by reversing of two phases at the terminal box. The

reversal has to be made when the motor is stopped and the supply switched off.

Advantage and disadvantage

MOTOR Split-phase Capacitor shaded-pole

ADVANTAGES Different powers and speeds are available,High power factor at full load .

Can be run in different speeds ,Produce high starting torque and low starting current ,Cost effective

Cheep and reliable

DISADVANTAGESProduces a low starting torque

Produces a huge noise Has a low power factor and a low efficiency

Importance of DC motor starting method

At the starting of a DC motor, it drains a high current .Some times this can damage the winding of the motor .In order to avoid this, at initially the voltage is reduced .As we know, once a supply is connected to a three phase induction motor a rotating magnetic field will be set up in the stator, this will link and cut the rotor bars which in turn will induce rotor currents and create a rotor field which will interact with the stator field and produce rotation. Of course this means that the three phase induction motor is entirely capable of self starting. The need for a starter therefore is not, conversely enough, to provide starting but to reduce heavy starting currents and provide overload and no-voltage protection. There are a number of different types of starter including ‘The Direct On-line Starter’, ‘The Star-Delta Starter’, ‘Auto-Transformer’ and ‘Rotor resistance’ as we considered some of them in above.

References

Http\www. Single-Phase-Electric-Motors-Characteristics-Applications.html http\www.Starting-Methods-of-Induction-Motors.html http\www. faqs.cfm.htm http\www. Electric_motor.htm