Chapter 18, Problem 1SQ

Draw a control circuit with no-voltage protection. Describe how this method of wiring protects the machine operator.

To determine

Draw a control circuit with no-voltage protection and describe how this method of wiring protects the machine operator.

Explanation of Solution

The three wire control circuit uses start-stop stations, momentary contact, and holding circuit interlock connected in a parallel form with the start pushbutton in order to maintain the circuit. The no-voltage protection can be obtained using the three wire control circuit. The control circuit with no-voltage protection is drawn as Figure 1.

In Figure 1, when the start pushbutton is pressed, the control circuit can be completed using coil M and closes all the M contacts. It leads to close the motor connection to the line by contacting the power circuits to the motor. The auxiliary contact M, which is connected in parallel with the start pushbutton, is also closed. This M contact is used to maintain the circuit to coil M when the start pushbutton is released. Such a contact is called as holding or sealing or maintaining contact.

When the stop pushbutton is pressed, the circuit brakes and opens the contact M. After releasing the stop pushbutton, the circuit remains the same because the start pushbutton and holding contact is in the open state. In order to complete the circuit, the start pushbutton should be pressed again.

If the supply voltage of the circuit fails, it could lead to de-energize the circuit. When the power or supply voltage is restored, the circuit remains in the open state until the start pushbutton is pressed manually. Therefore, this arrangement is called as no-voltage protection and such an arrangement is used to protect both the operator and the equipment.

Conclusion:

Thus, the control circuit with no-voltage protection is drawn and the working process is explained.