Control and Dynamics of Power Systems (1) Experiment no.
EXPERIMENT
1
Classic Control Circuits
1. Object:
The objectives of this experiment are to:
• To start and stop a three phase induction motor by Direct On Line (DOL)
Starter.
• To change the direction of a three phase induction motor rotation.
• To perform a soft start for a three phase induction motor by star/delta
starting method.
2. Theory of operation
2.1. Direct Online Motor Starter
A Direct On Line (DOL) or across the line starter applies the full line voltage to
the motor terminals. This is the simplest type of motor starter. A DOL motor
starter also has protection devices and, in some cases, condition monitoring.
Smaller sizes of direct on-line starters are manually operated; larger sizes use an
electromechanical contactor (relay) to switch the motor circuit. Solid-state direct
on line starters also exist.
A direct on line starter can be used if the high inrush current of the started 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 4KW (5HP).
DOL starting is sometimes used to start small water pumps, compressors, fans
and conveyor belts.
In the case of an asynchronous motor, such as the 3-phase squirrel-cage motor,
the motor will draw a high starting current until it has run up to full speed. This
starting current is typically 6-7 times greater than the full load current. To
reduce the inrush current, larger motors will have reduced voltage starters or
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Control and Dynamics of Power Systems (1) Experiment no.1
variable speed drives in order to minimize voltage dips to the power supply, or
series resistance and inductance can be added.
2.2. Forward/Reverse Motor Operation
To change the direction a three phase induction machine rotation, two of its
phases needs to be exchanged, thus changing the phase sequence form, say ABC
to CBA.
This can be accomplished by using two contactors, one for the forward or CW
rotation and one for the reverse or CCW rotation.
The forward and reverse contactors are mechanically interlocked i.e., if one of
them is closed the other cannot close. This is done to avoid dead short circuit in
case both the contactors closing simultaneously.
Also electrical interlocking could be provided using the contactors control
contacts.
2.3. Star/Delta Motor Starter
The Star Delta starting method is a motor starting mechanism that minimizes
the large amount of starting current that motors draw in. The Star Delta, as the
name suggests basically involves feeding the motor with 1/√3 (58%) of the full
load current until it attains speed then applying the full load current. This allows
to minimize the line current to one third of the normal (delta) value at starting
stage.
It is required three contactors i.e., the Star Contactor (KS), the Delta Contactor
(KD) and the Main Contactor (KM). However for the motor to be started in Star
Delta, its internal connection at the terminal box has to be wired in Delta-giving
it capability of receiving the full-load current at any instant.
When the power is fed into the circuit, KM allows current to flow to the motor.
Current flows into the motor and out to the KS which is the star-connected
starter. After a specified period defined by the clock delay (usually 5-10 sec) the
KD (Delta) Closes and KS opens to allow the motor to receive the full load
current and run at delta.
Traditionally, in many regions there was a requirement that all motor
connections be fitted with a reduced voltage starter for motors greater than 4KW
(5HP).This was to curb the high inrush of starting currents associated with
starting induction motors.
The star and delta contactors are electrically interlocked by using contactors
control contacts. i.e., if one of them is closed the other cannot close. This is done
to avoid dead short circuit in case both the contactors closing simultaneously
An advantage of this method could be low or reduced cost as compared to other
methods.
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Control and Dynamics of Power Systems (1) Experiment no.1
3. Instruments and Equipments
Three Phase Induction
Motor
Isolating Circuit
Breakers
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Control and Dynamics of Power Systems (1) Experiment no.1
Contactors
On Delay Timer
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Indicating Lamps
Push Buttons
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Control and Dynamics of Power Systems (1) Experiment no.1
4. Connection Digram
4.1. Direct Online Motor Starter
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4.2. Forward/Reverse Motor Operation
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Control and Dynamics of Power Systems (1) Experiment no.1
4.3. Forward/Reverse Motor Operation
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Control and Dynamics of Power Systems (1) Experiment no.1
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Control and Dynamics of Power Systems (1) Experiment no.1
5. Procedures
5.1. Direct Online Motor Starter
1. Connect the control circuit as shown in the diagram.
2. Switch on the control circuit breaker and test it first before connecting the
power circuit
3. Connect the Power circuit as shown in the diagram.
4. Switch on both circuit breakers.
5. Use the start and stop pushbuttons for motor operation.
6. Measure the currant, voltage, power and the no load speed of the motor.
5.2. Forward/Reverse Motor Operation
1. Connect the control circuit as shown in the diagram.
2. Switch on the control circuit breaker and test it first before connecting the
power circuit
3. Connect the Power circuit as shown in the diagram.
4. Switch on both circuit breakers.
5. Use the start and stop pushbuttons for motor operation.
6. Press the forward direction pushbutton.
7. Stop the motor and then press the reverse direction pushbutton to change
the Rotation of the motor.
5.3. Star/Delta Motor Starter
1. Connect the control circuit as shown in the diagram.
2. Switch on the control circuit breaker and test it first before connecting the
power circuit
3. Connect the Power circuit as shown in the diagram.
4. Switch on both circuit breakers.
5. Use the start and stop pushbuttons for motor operation.
6. Measure the current firstly in star then in delta connections.
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Control and Dynamics of Power Systems (1) Experiment no.1
6. Results
5.1. Direct Online Motor Starter
Current= ………. A
Voltage=…………V
Power =………….W
Speed =…………..RPM
5.2. Forward/Reverse Motor Operation
NO-Results
5.3. Star/Delta Motor Starter
Current in Star = …………A
Speed in Star = ………….RPM
Current in Delta= …………A
Speed in Delta =……….…RPM
7.Add Your Own Comments
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8. Use internet resources to provide:
8.1 Advantages, disadvantages and then discuss different
applications of DOL motor starter circuit
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Control and Dynamics of Power Systems (1) Experiment no.1
8.2 Advantages, disadvantages and then discuss different
applications of Forward/Reverse motor operation circuit
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8.3 Advantages, disadvantages and then discuss different
applications of Star/Delta motor starter circuit
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