Laboratory manual for Electronics: Motor Control: Start-Stop Push-Button Control

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Objectives

• Place wire numbers on a schematic diagram.

• Place corresponding numbers on control components.

• Draw a wiring diagram from a schematic diagram.

• Define the difference between a schematic or ladder diagram and a wiring diagram.

• Connect a start-stop push-button control circuit.

In this experiment a schematic diagram of a start-stop push-button control will be converted to a wiring diagram and then connected in the laboratory. A schematic diagram shows components in their electrical sequence without regard for the physical location of any component (Ill. 1). A wiring diagram is a pictorial representation of components with connecting wires. The pictorial representation of the components is shown in Ill. 2.

[Helpful Hint: A schematic diagram shows components in their electrical sequence without regard for the physical location of any component (Ill. 1). A wiring diagram is a pictorial representation of components with connecting wires.]

To simplify the task of converting the schematic diagram into a wiring diagram, wire numbers will be added to the schematic diagram. These numbers will then be transferred to the control components shown in Ill. 2. The rules for numbering a schematic diagram are as follows:

1. A set of numbers can be used only once.

2. Each time you go through a component the number set must change.

3. All components that are connected together will have the same number.

To begin the numbering procedure, begin at Line 1 (L1) with the number 1 and place a number 1 beside each component that's connected to L1 (Ill. 3). The number 2 is placed beside each component connected to L2 ( Ill. 4), and a 3 is placed beside each component connected to L3 (Ill. 5). The number 4 will be placed on the other side of the M load contact that already has a number 1 on one side and on one side of the overload heater ( Ill. 6). Number 5 is placed on the other side of the M load contact, which has one side numbered with a 2, and a 5 will be placed beside the second overload heater. The other side of the M load contact, which has been numbered with a 3, will be numbered with a 6, and one side of the third overload heater will be labeled with a 6. Numbers 7, 8, and 9 are placed between the other side of the overload heaters and the motor T leads.

The number 10 will begin at one side of the control transformer secondary and go to one side of the normally closed stop push button. The number 11 is placed on the other side of the stop button and on one side of the normally open start push button and normally open M auxiliary contact. A number 12 is placed on the other side of the start button and M auxiliary contact and on one side of M coil. Number 13 is placed on the other side of the coil to one side of the normally closed overload contact. Number 14 is placed on the other side of the normally closed overload contact and on the other side of the control transformer secondary winding. See Ill. 7.

Ill. 1 Schematic diagram of a basic start-stop push-button control circuit.

Ill. 2 Components of the basic start-stop control circuit.

Ill. 3 The number 1 is placed beside each component connected to L1.

Ill. 4 A number 2 is placed beside each component connected to L2.

Ill. 5 A number 3 is placed beside each component connected to L3.

Ill. 6 The number changes each time you proceed across a component.

Ill. 7 Numbers are placed beside all components.

Numbering the Components

Ill. 8 A 1 is placed beside L1, the control transformer fuse, and M load contact.

Ill. 9 The number 2 is placed beside L2 and the second load contact on M starter.

Ill. 10 Placing numbers 3, 4, 5, 6, 7, 8, and 9 beside the proper components.

Now that the components on the schematic have been numbered, the next step is to place the same numbers on the corresponding components of the wiring diagram. The schematic diagram in Ill. 7 shows that the number 1 has been placed beside L1, the fuse on the control transformer, and one side of a load contact on M starter ( Ill. 8). The number 2 is placed beside L2 and the second load contact on M starter ( Ill. 9). The number 3 is placed beside L3, the third load contact on M starter, and the other side of the primary winding on the control transformer. Numbers 4, 5, 6, 7, 8, and 9 are placed beside the components that correspond to those on the schematic diagram (Ill. 10). Note on connection points 4, 5, and 6 from the output of the load contacts to the overload heaters, that these connections are factory made on a motor starter and don't have to be made in the field. These connections are not shown in the diagram for the sake of simplicity. If a separate contactor and overload relay are being used, however, these connections will have to be made. Recall that a contactor is a relay that contains load contacts and may or may not contain auxiliary contacts. A motor starter is a contactor and overload relay combined.

The number 10 starts at the secondary winding of the control transformer and goes to one side of the normally closed stop push button. When making this connection, care must be taken to make certain that connection is made to the normally closed side of the push but ton. Since this is a double-acting push button, it contains both normally closed and normally open contacts ( Ill. 11). The number 11 starts at the other side of the normally closed stop button and goes to one side of the normally open start push button and to one side of a normally open M auxiliary contact ( Ill. 12). The starter in this example shows three auxiliary contacts: two normally open and one normally closed. It makes no difference which normally open con tact is used.

This same procedure is followed until all circuit components have been numbered with the number that corresponds to the same component on the schematic diagram ( Ill. 13).

Ill. 11 Wire number 10 connects from the transformer secondary to the stop button.

Ill. 12 Number 11 connects to the stop button, start button, and holding contact.

Connecting the Wires

Now that numbers have been placed beside the components, wiring the circuit becomes a matter of connecting numbers. Connect all components labeled with a number 1 together ( Ill. 14). All components numbered with a 2 are connected together ( Ill. 15).

All components numbered with a 3 are connected together ( Ill. 16). This procedure is followed until all the numbered components are connected together, with the exception of 4, 5, and 6, which are assumed to be factory connected ( Ill. 17).

Ill. 13 All components have been numbered.

Ill. 14 Connecting all components numbered with a 1 together.

Ill. 15 Connecting all components numbered with a 2 together.

Ill. 16 Connecting all components numbered with a 3 together.

Ill. 17 Completing the wiring diagram.

LABORATORY EXERCISES

Materials Required:

Three-phase power supply Three-phase squirrel cage induction motor or simulated load 2 double-acting push buttons (N.O./N.C. on same button) Three-phase motor starter or contactor with overload relay containing three load contacts and at least one normally open auxiliary contact

Control transformer:

Connecting a Start-Stop Push-Button Control Circuit

To connect the control circuit, follow the same procedure that was used to develop the wiring diagram. Use the schematic diagram shown in Ill. 7. It is sometimes helpful to use a highlighter to mark the diagram as connections are made.

1. Connect all components that are labeled with a number 1. Make certain to connect to a load contact on the starter or contactor.

2. Connect all components labeled with a number 2. Again make sure to connect to a load contact on the starter or contactor.

3. Connect all components labeled with a 3.

4. Wire connections 4, 5, and 6 may or may not have to be made depending on whether you are using a starter or a contactor and separate overload relay.

5. Wires 7, 8, and 9 connect from the output of the heaters on the overload relay(s) to the motor T leads. Your circuit may contain a single three-phase overload relay or three separate overload relays if you are using a contactor and separate overload relay(s).

6. Wire number 10 connects from the secondary winding of the control transformer to one side of the normally closed push button used for the stop button. If using a double-acting push button, make certain to connect to the closed side.

7. Wire number 11 connects from the other side of the normally closed push button to the normally open push button used for the start button. If a double-acting push button is being used, make certain to connect to the open side. Wire number 11 also connects to a normally open auxiliary contact on M starter. Auxiliary contacts are smaller than the load contacts and are used as part of the control circuit. Make certain to connect to one side of an open contact.

8. Wire number 12 connects from the other side of the normally open start button to the other side of the normally open auxiliary contact and to one side of the coil on M starter.

9. Wire number 13 connects from the other side of the coil on M starter to one side of the normally open contact located on the overload relay. If a three-phase motor starter is being used, or if a separate three-phase overload relay is being used, there will be only one overload contact. Note the number of contacts on the overload relay. Some overload relays contain both normally open and normally closed contacts, and some don't. Make certain that connection is made to the normally closed contact if the relay contains more than one contact. If three separate single-phase overload relays are being used, each overload relay contains an overload contact. These three contacts will have to be connected in series so that if one opens, the circuit will be broken.

10. Wire number 14 connects from the other side of the normally closed overload contact to the other side of the secondary winding on the control transformer.

11. Check with your instructor before turning on the power.

12. Test the circuit for proper operation.

13. If the circuit works properly, turn off the power and disconnect the circuit. Return the wires and components to their proper place.

QUIZ:

1. Refer to the circuit shown in Ill. 7. If wire number 11 were disconnected at the normally open auxiliary M contact, how would the circuit operate?

2. Assume that when the start button is pressed, M starter does not energize. List seven possible causes for this problem:

3. Explain the difference between a motor starter and a contactor.

4. Refer to the schematic in Ill. 7. Assume that when the start button is pressed, the control transformer fuse blows. What is the most likely cause of this trouble?

5. Explain the difference between load and auxiliary contacts.

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