Introduction:
§ Different starting methods are employed for
starting induction motors because Induction Motor draws more starting current
during starting. To prevent damage to the windings due to the high starting
current flow, we employ different types of starters.
§ The simplest form of motor starter for the
induction motor is the Direct On Line
starter. The DOL starter 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.
§ Typically, the contactor will be controlled by
separate start and stop buttons, and an auxiliary contact on the contactor is
used, across the start button, as a hold in contact. I.e. the contactor is
electrically latched closed while the motor is operating.
Principle of DOL:
§ To start, the contactor is closed, applying full line voltage to
the motor windings. The motor will draw a very high inrush current for a very
short time, 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.
§ Provided the torque developed by the motor
exceeds the load torque at all speeds during the start cycle, the motor will
reach full speed. If the torque delivered by the motor is less than the torque
of the load at any speed during the start cycle, the motor will stops
accelerating. If the starting torque with a DOL starter is insufficient for the
load, the motor must be replaced with a motor which can develop a higher
starting torque.
§ The acceleration torque is the torque developed
by the motor minus the load torque, and will change as the motor accelerates
due to the motor speed torque curve and the load speed torque curve. The start
time is dependent on the acceleration torque and the load inertia.
§ DOL starting have a maximum start current and
maximum start torque. This may cause an
electrical problem with the supply, or it may cause a mechanical problem with
the driven load. So this will be inconvenient for the users of the supply line,
always experience a voltage drop when starting a motor. But if this motor is
not a high power one it does not affect much.
Parts of DOL Starters:
(1) Contactors
& Coil.
§ Magnetic contactors are electromagnetically
operated switches that provide a safe and convenient means for connecting and
interrupting branch circuits.
§ Magnetic motor controllers use electromagnetic
energy for closing switches. The electromagnet consists of a coil of wire
placed on an iron core. When a current flow through the coil, the iron of the
magnet becomes magnetized, attracting an iron bar called the armature. An
interruption of the current flow through the coil of wire causes the armature
to drop out due to the presence of an air gap in the magnetic circuit.
§ Line-voltage magnetic motor starters are
electromechanical devices that provide a safe, convenient, and economical means
of starting and stopping motors, and have the advantage of being controlled
remotely. The great bulk of motor controllers sold are of this type.
§ Contactors are mainly used to control machinery
which uses electric motors. It consists of a coil which connects to a voltage
source. Very often for Single phase Motors, 230V coils are used and for three
phase motors, 415V coils are used. The contactor has three main NO contacts and
lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for
the control circuit. A contact is conducting metal parts which completes or
interrupt an electrical circuit.
§ NO-normally open
§ NC-normally closed
(2) Over
Load Relay (Overload protection).
§ Overload protection for an electric motor is
necessary to prevent burnout and to ensure maximum operating life.
§ Under any condition of overload, a motor draws
excessive current that causes overheating. Since motor winding insulation
deteriorates due to overheating, there are established limits on motor
operating temperatures to protect a motor from overheating. Overload relays are
employed on a motor control to limit the amount of current drawn.
§ The overload relay does not provide short circuit
protection. This is the function of over current protective equipment like
fuses and circuit breakers, generally located in the disconnecting switch
enclosure.
§ The ideal and easiest way for overload
protection for a motor is an element with current-sensing properties very
similar to the heating curve of the motor which would act to open the motor
circuit when full-load current is exceeded. The operation of the protective
device should be such that the motor is allowed to carry harmless over-loads
but is quickly removed from the line when an overload has persisted too long.
§ Normally fuses are not designed to provide
overload protection. Fuse is protecting against short circuits (over current
protection). Motors draw a high inrush current when starting and conventional
fuses have no way of distinguishing between this temporary and harmless inrush
current and a damaging overload. Selection of Fuse is depend on motor full-load
current, would “blow” every time the motor is started. On the other hand, if a
fuse were chosen large enough to pass the starting or inrush current, it would
not protect the motor against small, harmful overloads that might occur later.
§ The overload relay is the heart of motor
protection. It has inverse-trip-time characteristics, permitting it to hold in
during the accelerating period (when inrush current is drawn), yet providing
protection on small overloads above the full-load current when the motor is
running. Overload relays are renewable and can withstand repeated trip and
reset cycles without need of replacement. Overload relays cannot, however, take
the place of over current protection equipment.
§ The overload relay consists of a current-sensing
unit connected in the line to the motor, plus a mechanism, actuated by the
sensing unit, which serves, directly or indirectly, to break the circuit.
§ Overload relays can be classified as being
thermal, magnetic, or electronic.
1. Thermal Relay: As the name implies, thermal overload relays rely on the rising
temperatures caused by the overload current to trip the overload mechanism.
Thermal overload relays can be further subdivided into two types: melting alloy
and bimetallic.
2. Magnetic Relay: Magnetic overload relays react only to current excesses and are
not affected by temperature.
3. Electronic Relay: Electronic or solid-state overload relays,
provide the combination of high-speed trip, adjustability, and ease of
installation. They can be ideal in many precise applications.
Wiring of DOL Starter:
(1) Main
Contact:
§ Contactor is connecting among Supply Voltage,
Relay Coil and Thermal Overload Relay.
§ L1 of Contactor Connect (NO) to R Phase through
MCCB
§ L2 of Contactor Connect (NO) to Y Phase through
MCCB
§ L3 of Contactor Connect (NO) to B Phase through
MCCB.
§ NO Contact (-||-):
§ (13-14 or 53-54) is a normally Open NO contact
(closes when the relay energizes)
§ Contactor Point 53 is connecting to Start Button
Point (94) and 54 Point of Contactor is connected to Common wire of Start/Stop
Button.
§ NC Contact (-|/|-):
§ (95-96) is a normally closed NC contact (opens
when the thermal overloads trip if associated with the overload block)
(2) Relay
Coil Connection:
§ A1 of Relay Coil is connecting to any one Supply
Phase and A2 is connecting to Thermal over Load Relay’s NC Connection (95).
(3) Thermal
Overload Relay Connection:
§ T1,T2,T3 are connect to Thermal Overload Relay
§ Overload Relay is Connecting between Main
Contactor and Motor
§ NC Connection (95-96) of Thermal Overload Relay
is connecting to Stop Button and Common Connection of Start/Stop Button.
Wiring Diagram of DOL Starter:
Working of DOL Starter:
§ The main heart of DOL starter is Relay Coil.
Normally it gets one phase constant from incoming supply Voltage (A1).when Coil
gets second Phase relay coil energizes and Magnet of Contactor produce
electromagnetic field and due to this Plunger of Contactor will move and Main
Contactor of starter will closed and Auxiliary will change its position NO
become NC and NC become (shown Red Line in Diagram) .
§ Pushing Start Button:
§ When We Push the start Button Relay Coil will
get second phase from Supply Phase-Main contactor (5)-Auxiliary Contact
(53)-Start button-Stop button-96-95-To Relay Coil (A2).Now Coil energizes and
Magnetic field produce by Magnet and Plunger of Contactor move. Main Contactor
closes and Motor gets supply at the same time Auxiliary contact become (53-54)
from NO to NC.
§ Release Start Button:
§ Relay coil gets supply even though we release
Start button. When we release Start Push Button Relay Coil gets Supply phase
from Main contactor (5)-Auxiliary contactor (53) – Auxiliary contactor
(54)-Stop Button-96-95-Relay coil (shown Red / Blue Lines in Diagram).
§ In Overload Condition of Motor will be stopped
by intermission of Control circuit at Point 96-95.
§ Pushing Stop Button:
§ When we push Stop Button Control circuit of
Starter will be break at stop button and Supply of Relay coil is broken,
Plunger moves and close contact of Main Contactor becomes Open, Supply of Motor
is disconnected.
Motor Starting
Characteristics on DOL Starter:
§ Available starting
current: 100%.
§ Peak starting
current: 6 to
8 Full Load Current.
§ Peak starting
torque: 100%
Advantages of DOL Starter:
1. Most Economical and Cheapest Starter
2. Simple to establish, operate and maintain
3. Simple Control Circuitry
4. Easy to understand and trouble‐shoot.
5. It provides 100% torque at the time of starting.
6. Only one set of cable is required from starter
to motor.
7. Motor is connected in delta at motor terminals.
Disadvantages of DOL Starter:
1. It does not reduce the starting current of the motor.
2. High Starting Current: Very High Starting Current (Typically 6 to
8 times the FLC of the motor).
3. Mechanically Harsh: Thermal Stress on the motor, thereby
reducing its life.
4. Voltage Dip: There is a big voltage dip in the electrical installation
because of high in-rush current affecting other customers connected to the same
lines and therefore not suitable for higher size squirrel cage motors
5. High starting Torque: Unnecessary high starting torque, even
when not required by the load, thereby increased mechanical stress on the
mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain
drive, connected equipments, etc. leading to premature failure and plant
downtimes.
Features of DOL starting
§ For low- and medium-power three-phase motors
§ Three connection lines (circuit layout: star or
delta)
§ High starting torque
§ Very high mechanical load
§ High current peaks
§ Voltage dips
§ Simple switching devices
DOL is Suitable for:
§ A direct on line starter can be used if the high
inrush current of the motor does not cause excessive voltage drop in the supply
circuit. The maximum size of a motor allowed on a direct on line starter may be
limited by the supply utility for this reason. For example, a utility may
require rural customers to use reduced-voltage starters for motors larger than
10 kW.
§ DOL starting is sometimes used to start small
water pumps, compressors, fans and conveyor belts.
DOL is not suitable for:
§ The peak starting current would result in a
serious voltage drop on the supply system
§ The equipment being driven cannot tolerate the
effects of very high peak torque loadings
§ The safety or comfort of those using the
equipment may be compromised by sudden starting as, for example, with
escalators and lifts.
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