AC Motor Starting Methods AC Motor Selection

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Electric Motors

Contents Basic AC Motor Control Components Starting AC Motors Examples

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AC Motor Control – Basic Components

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AC Motor Control – Basic Components

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AC Motor Control – Basic Components

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AC Motor Control – Basic Components

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AC Motor Control – Basic Components Button/Switch / Limit Switch

If a Toggle Button is pressed, itkeeps its position (does not return to previous position). A push – button makes contact when the button is pressed and breaks when the button is released.

Toggle Switch

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AC Motor Control – Basic Components Rotary/Cam Switch A rotary switch is a switch operated by rotation. These are often chosen when more than 2 positions are needed, such as a three-speed fan.

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AC Motor Control – Basic Components Relay and Contactor

A contactor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings.

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AC Motor Control – Basic Components Overcurrent Relay(Electromanetic Overcurrent Relay) Overcurrent relays are connected series to motors. So, the current that is drawn by the motor flows through the relay also. If a peak current flows over the motor in a short time interval, this case does not effect the motor. For this reason over current relay should not stop the motor for short time interval over currents (eg. Starting up the motor). For this purpose, another component which creats time delay might be added to the circuit. But, if the time interval is long, motor might be damaged. Continous over current over heats the motor and then the motor breaks down.

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AC Motor Control – Basic Components Overcurrent Relay(Thermal Overcurrent Relay) Thermal over current relays run with the heat generated by the over current that flows through motor.

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AC Motor Control – Basic Components

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AC Motor Control – Basic Components Time Relay Time relays are the components that their contacts change their states after their coils are energized. There are two time relay types.

 Time delay on high edge (direct type)

 Time delay on low edge (inverse type)

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AC Motor Control – Basic Components

Time Relay

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AC Motor Control – Basic Components

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Methods of Starting Three Phase Induction Motors The Direct On-line Starter Phase of the Grid

Terminals of coils of ACM’s can be directly connected to the voltage of the grid.

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Windings Input

Output

R

U

X

S

V

Y

T

W

Z

Electric Motors

Methods of Starting Three Phase Induction Motors The Direct On-line Starter (WYE or Star Connection) U,V,W terminals (Inputs of the coils)are connected to R S T phases and X Y Z terminals (outputs of the coils) are connected to each other. This connection is called as WYE (or star) connection and denoted as ‘λ’. Since there is 120º phase angle between the phases, line voltage is 3 times greater than phase voltage ( 380/3=220 V). Line current is equal to phase current. Phase of the Grid R S T

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Coil Input Output U V W

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X Y Z

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Methods of Starting Three Phase Induction Motors The Direct On-line Starter (Delta or Triangle Connection) If the output terminal of the first winding is connected to the input terminal of the second, output terminal of the second winding is connected to the input terminal of the third and output terminal of the third winding is connected to the input terminal of the first; then this connection is called as delta or triangle connection. This connection is denoted by  symbol. Line voltage is equal to phase voltage. Line current is 3 times greater than phase current. Phase of the Grid

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Windings Input

Output

R

U

X

S

V

Y

T

W

Z

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Methods of Starting Three Phase Induction Motors Starting currents of ACM’s are between 4 to 8 times greater than continuous currents. These peak currents last approximately 5 seconds. E.g. for a motor of three phase 1.5 [kW], the nominal voltage is 380 [V] and the nominal current is 3.7 [A]. But, its starting current is 16.5 [A]. The effects of this currents to the grid is eliminated for small power values. However, for a motor of 75 [kW], the nominal current is 140 [A] and the starting current is 980 [A]. This high currents decrease the voltage of the grid for seconds. This case is unwanted. For this reason, starting methods are applied for the motors whose power is greater than 5 [kW]. Methods are:  WYE to Delta (-) Starting  Auto Transformer Starting  Starting with Resistance

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Methods of Starting Three Phase Induction Motors

WYE to Delta (-) Starting  ACM starts with WYE connection to reduce currents. When the rotational speed of the motor achieves to nominal speed then it is switched to delta connection. Starting in star not only has the effect of reducing the motor’s start current but also the starting torque.

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Methods of Starting Three Phase Induction Motors

WYE to Delta (-) Starting  ACM starts with WYE connection to reduce currents. When the rotational speed of the motor achieves to 60 % of its nominal speed then it is switched to delta connection. Starting in star not only has the effect of reducing the motor’s start current but also the starting torque.

Switching might be achieved by time relay.

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Methods of Starting Three Phase Induction Motors

WYE to Delta (-) Starting If a motor stars with WYE connection, Uph = UL / 1.73 = 0.58 UL . However, Iph= I L . This means, the current flow through the coils decreases 33.3 %.

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Methods of Starting Three Phase Induction Motors

WYE to Delta (-) Starting In this method, in contrast with other methods, no extra components (neither resistance nor reactance) are used. Only the connection tyoe of the motor is changed. The thechnique is to decrease the start current by decreasing the start voltage. It is practical and economic. On the other hand, the voltages on the grid should be the same to implement this method.

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Methods of Starting Three Phase Induction Motors  Auto-Transformer Starting  This method of starting reduces the start current by reducing the voltage at start up. It can give lower start up currents than star-delta arrangements but with an associated loss of torque. It is not as commonly utilized as other starting methods but does have the advantage that only three connection conductors are required between starter and motor (ref 4).  Auto transformers are the transformers whose primary side and secondary side are the same (has only one coil).

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Methods of Starting Three Phase Induction Motors Starting with Resistance (series resistance to stator coils) It is connecting series resistance to the coils of the stator at start. When the start button is pressed, motor starts with resistances those are connected series to stator. After achieving the 60~65 % of the nominal speed, the resistances are disconnected for normal run. During start, the motor should be unloaded. Time relays are used for connecting and disconnecting coils.

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Methods of Starting Three Phase Induction Motors

Starting with Resistance (slip ring motor) Slip ring motor has coils also for rotor. 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. Except its starting stage, it is more like a transformer and the terminals of the rotor have a voltage value that is proportional to its number of turns.

Contactors connect and disconnect the resistances on rotor. For every stage, start current can be limited. 18.12.2012

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Methods of Starting Three Phase Induction Motors Continuous and Discontinuous Runs

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Methods of Starting Three Phase Induction Motors Changing the Direction of the Rotation of a 3PACM Sometimes it is needed to change the direction of the rotation of a three phase ACM. To achieve this, the direction of the rotating magnetic field should be change by chaning the connection of the two of the three phases (R-S-T).

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Methods of Starting Three Phase Induction Motors Changing the Direction of the Rotation of a 3PACM Sometimes it is needed to change the direction of the rotation of a three phase ACM. To achieve this, the direction of the rotating magnetic field should be change by chaning the connection of the two of the three phases (R-S-T).

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Methods of Starting Three Phase Induction Motors

Dynamic Braking in Asynchronous ACM If the voltage on motor is disconnected, its rotor does not stop immediately because of inertia. This free mechanical energy makes machine generator and if a load is connected in that time interval, the rotor stops more fast. This type of braking is called as dynamic braking. It is just related with electromagnetics and proportional to rotational speed. The mechanical energy is used as electrical energy, and so braking.

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Methods of Starting Three Phase Induction Motors

Running a 3PACM as a MPACM A three phase alternate current motor can be run as a monophase alternate current motor. There should be at least two current flow on coils which have phase angle between each other. One or two coils of the stator should be used as secondary coils in order to start a three phase motor with mono phase. A capacitor should be connected to one coil for this purpose. After that, one of the terminals of the capacitor and the terminal which has no connection with the capacitor might be connected to the mono phase grid. The power of the motor decreseases to app. 50~60 % of the nominal power of the motor in this case.

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Methods of Starting Three Phase Induction Motors Running a Mono Phase Asynchronous ACM with a Secondary Winding The rotation is the result of the rotating magnetic field, and the rotating magnetic field is produced by the stator windings in asynchronous ACM’s. The rotating magnetic field is only observed in two or three phase systems. It is not possible to generate a rotating magnetic field with only one phase that is only connected to one coil. However, if it is started then it may continue to rotate with just one coil. This coil is called as primary winding. In order to start a mono phase asynchronous motor, another coil is also used that is located with the angle of 90 degrees to primary winding. This coil is called as secondary winding. This coil makes the monophase motor runs like a two phase motor. A capacitor is connected to the secondary winding which makes the primary and secondary windings’ currents have nearly 90 degrees phase difference between each other. Secondary phase is continously connected in some types of mono phase asynchronous motors whereas in some types it is disconnected manually or automatically.

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Methods of Starting Three Phase Induction Motors Running a Mono Phase Asynchronous ACM with a Secondary Winding

Changing The Direction of a Mono Phase Asynchronous ACM with a Secondary Winding a N y L

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References: 1. 2. 3. 4.

http://320volt.com/elektrik-kumanda-teknikleri-ders-notlari/ http://megep.meb.gov.tr/mte_program_modul/modul_pdf/522EE0121.pdf http://megep.meb.gov.tr/mte_program_modul/modul_pdf/522EE0122.pdf http://www.djtelectraining.co.uk/downloads/ADL%203%20Ph%20Induction% 20Motors.pdf

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