Objectives1. To be able to describe the control gear components of a DOL starter2. To be able to describe the operation of a DOL starter3. To be able to explain how a three-phase motor can be reversed4. To be able to describe the operation of a reversing DOL starter5. To be able to explain the starting problems with an induction motor6. To be able to describe the operation of a star-delta starter

1. Control Gear Components of a DOL Motor StarterDOL stands for Direct-On-Line. This type of starter connects the motor straight to the electricity supply. It is made up of an isolator, 3 fuses, a contactor and an overload unit. It also has start and stop pushbut-tons.

The FusesFuses provide short circuit protection for the cables feeding the motor. They are usually HRC type (High Rupture Capacity) but can be replaced by MCBs (Miniature Circuit Breakers). Here are the IEC electrical symbols for a three phase fuse unit and a 3 pole MCB.

The isolatorThis provides a means of disconnecting the starter and the motor from the mains supply allowing for maintenance. These devices can also be padlocked in the off position. They need to be three-pole man-ually-operated switches (one pole per phase) capable of breaking the maximum motor current such as when the motor shaft is locked. If not, the switching poles inside the isolator could weld together making them useless. Here are the IEC electrical symbols for an isolator and one with integral fuses.

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Here’s a picture of a door interlocked isolator which can be locked off.

Pad-lock inserted to lock isolator in the off position

The ContactorThis is a large relay used for switching high currents. It must have at least 3 poles to switch each of the three phases and one auxiliary pole. The auxiliary pole is used as a retaining contact in the control circuit allowing the motor to stay on. The main pole contacts must be rated to switch the maximum cur-rent of the motor otherwise they weld together. Large motors also require contactors with built in arc-quenchers due to the high voltages and currents present when switching off.

Here is the IEC electrical symbol for a contactor

Notice that it contains a coil which when energised, closes the 3 main contactor poles. It also closes the two N/O auxiliary poles (13/14 and 53/54) and opens the N/C auxiliary pole (61/62).

Here’s a picture of a contactor

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The Overload UnitThis is a 3 phase device which protects the motor from damage when the mechanical load torque in-creases above the manufacturer’s rating. This can be due to mechanical wear or even a complete sei-zure of the load. Under these conditions, the motor current may be up to seven times its rating which would cause the protection fuses to blow after a long period by which time, the motor would burn out. Overload units are slow acting devices that detect currents a little above the motor rating. For example, they can be set to trip out on 1.1, 1.5 or 2 times rated current. The slow acting feature allows for large starting currents (typically 7 times rated) provided they only last for short periods. However, care must be taken not to start too often. Overload units also have auxiliary contacts to interlock with the control circuit. There are three basic types of overload unit namely thermal, magnetic and electronic. Thermal Overloads use a simple bimetallic strip that bends when hot since it carries motor current. This discon-nects the motor and switches it off. The motor will not be allowed to start again until the strip has cooled down thus protecting the motor from further damage. Magnetic Overloads work on the principle of a magnetic field. The motor current passes through a coil which operates three normally closed contacts. When the current is large enough, the contacts open disconnecting the motor. Electronic Overloads rely on a thermistor embedded in the motor. The actual temperature of the motor is sensed and disconnected if it is too hot. This is the best type of overload because it monitors the motor’s condition. Here are the IEC electrical symbols.

One of the units above has a N/C latching auxiliary contact (95/96) while the other has a N/O auxiliary contact (97/98).

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Start and Stop PushbuttonsHere are the IEC electrical symbols for pushbuttons. Notice that the start pushbutton is normally open and has contact (13/14) while the stop pushbutton is normally closed and has contact (21/22).

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2. Circuit Diagrams for a DOL StarterThere are usually two diagrams needed to show how this type of starter works. We start with the power schematic and then look at the control circuit. Finally, the operations of these components are explained in sequence.

Power SchematicThe diagram shown below makes use of the symbols covered earlier in this note. These include the fuses, isolator, contactor and overload together with auxiliary contacts used for interlocking within the control circuit.

The fuses must be rated for short circuit protection. The induction motor stator is connected in delta and the frame is earthed for safety. The contacts off C1 and OL1 are used in the control circuit covered next.

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Control CircuitThe symbols explained earlier in this lesson are used in the following circuit diagram.The circuit uses 24VAC as its supply provided by a small transformer which is not shown.

OperationThis will be explained in terms of starting, stopping and faults conditions

(a) Starting• The start button S2 must be pressed• This energises the main contactor C1, provided that the stop button S2 is not pressed and the overload OL1 has not tripped out due to over current• The motor is now connected to the supply and runs normally• The retaining contact of C1 holds it in when the start button is released.• If C1 is not working properly, the retainer will not hold in and the motor will stop.

(b) Stopping• The stop button S1 must be pressed• This disconnects the power to the contactor C1 which de-energises• The retaining contact of C1 opens disconnecting the motor from the supply making it stop• The stop button is released and the motor remains disconnected from the supply

(c) Faults conditions• If an insulation fault on the stator causes the motor frame to become live, the fuses will blow thus disconnecting the motor from the supply• If a short circuit exists between stator phase, the fuses will again blow• If the motor shaft becomes jammed or the load is excessive due to bearing wear etc, the overload unit will detect the excess current and trip. This causes the NC contact of OL1 to open and thereby de-energising contactor C1 and so disconnecting the motor from the supply.

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3. Reversing DOL StarterThis type of starter simply swaps two of the stator phases causing the rotating field to reverse direction. This causes the rotor to reverse direction also. The starter requires two contactors which are mechanically interlocked to prevent the fuses from being blown by the supply phases from being accidentally shorted-out. It also requires two start pushbuttons to select forward and reverse directions. Again, the wiring can be split into the Power Schematic and the Control Circuit.

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Circuit OperationThis can be explained in terms of starting, stopping and faults.(a) Starting - Forward• The forward start button S2 must be pressed• This energises the forward contactor C2 provided the stop button S2 is not pressed, the overload OL2 has not tripped on over current and the motor is not presently running in reverse (electrical inter-lock with N/C contact of C3)• The motor is now connected to the supply and runs in the forward direction• The retaining contact of C2 holds it in when the forward start button is released.(b) Starting - Reverse• The reverse start button S3 must be pressed• This energises the forward contactor C3 provided the stop button S2 is not pressed, the overload OL2 has not tripped on over current and the motor is not presently running forward (electrical interlock with N/C contact of C2)• The motor is now connected to the supply and runs in the reverse direction• The retaining contact of C3 holds it in when the reverse start button is released(c) Stopping• The stop button S1 must be pressed• This disconnects the power to contactor C2 (or C3) which de-energises• The retaining contact of C1 opens disconnecting the motor from the supply making it stop• The stop button is released and the motor remains disconnected from the supply(d) Faults• Fuses blow as for DOL starter• Overload OL2 trips and stops either contactor as for DOL starter

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4. Induction Motor Starting ProblemsAs we have seen previously, the torque available from an induction motor follows the curve shown below when wired up in delta.

Notice that the starting torque is a little less than the full load torque TFL and the maximum torque is about twice TFL. The motor settles down at a speed and torque that depends on the load.

We have also noted previously that an induction motor wired up in delta, draws about 7 times full load current IFL on start up. The graph below shows how current changes as the motor speeds up.

Notice that as the motor runs up to its full load speed, the current reduces as shown along the curved line until it reaches the full load current.

This occurs at a speed less than 100% NS which of course it can never reach. The actual current and speed of the motor depends on the load.

Clearly, this high starting current can cause problems in a system fed from a common distribution board. The extra large current drawn by the motor will create a voltage drop in the cables feeding the board. It is common for lights which are connected to the same supply to dim while the motor runs up to speed.

To avoid this, we can use a reduced voltage starter provided that the new starting torque is still large enough to turn the load. However, since the torque produced by the motor is related to the square of the stator voltage, halving the voltage for example, will cause the torque to drop to a quarter of its value. This may not start the load. There are two simple methods of starting an induction motor on reduced voltage namely by an auto-transformer starter or by a star-delta starter. Autotransformers can be used to set a lower starting voltage and can be adjusted to suit the load. Once the motor is up to speed, it can be shorted out to give maximum running torque. The star-delta starter is the most common method and will be explained here.

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5. Star-Delta StarterMost of the elements of this type of starter have already been covered in the DOL starter with the ad-dition of either a timer or a centrifugal switch. These devices are used to switch over from star to delta once the motor is up to speed.

The motor starts off in star. This gives it a starting current of about a third of that for delta. The motor current follows the star curve shown opposite. At around 80% of synchronous speed, the motor is switched to delta and the motor follows the delta curve instead.

This can only be done if the load torque is low enough at low speeds. A fan has a torque speed curve which is ideally suited to a star-delta starter. Notice that the change-over takes place when the load curve cuts the star curve.

As we saw previously, a starter must have a power circuit diagram and a separate control circuit dia-gram. It is sometimes cheaper to combine both circuits within a self contained, stand alone starter where the control circuit uses the same supply as the power circuit. The start and stop buttons are mounted on the front and care must be taken to isolate externally before any maintenance is carried out on it.

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Starting Operation• Press S2• C1 coil energises• Contact C1 retains• S2 can be released• Auxiliary contact C1 closes • CR timer starts timing• C2 coil energises – STAR (starting mode)• CR timer finishes timing• C2 coil de-energises • C3 coil energises – DELTA (running mode)

Electrical interlocks of C2 and C3 prevent both contacts energising together. This is in addition to the mechanical interlocks shown on the power schematic. Stopping and faults are the same as for DOL start-er.

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