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An industrial control circuit uses a relay (K1) to control a DC motor. The circuit is wired as follows:

- A normally closed (NC) 'Stop' pushbutton is wired in series with a momentary normally open (NO) 'Start' pushbutton and the K1 relay coil.
- A normally open (NO) contact from the K1 relay is wired in parallel across the 'Start' pushbutton.
- A second normally open (NO) contact from the K1 relay connects the motor to the power supply.

If a short circuit develops directly across the two terminals of the 'Stop' pushbutton, how will the circuit respond when an operator presses and releases the 'Start' button, and then later presses the 'Stop' button?

  1. The motor will start and continue running, but pressing the Stop button will fail to stop it.Cevap
  2. B
    The circuit's protective device will trip immediately when power is applied because of the short circuit.
  3. C
    The motor will start when the Start button is pressed, but will immediately shut off when the Start button is released.
  4. D
    The motor will fail to start entirely because the shorted Stop button creates an open circuit condition.

Cevap

The motor will start and continue running, but pressing the Stop button will fail to stop it.
The circuit described is a classic motor starter latching circuit. The parallel NO contact acts as a 'seal-in' contact, keeping the relay coil energized after the momentary Start button is released. The NC Stop button is designed to break this latched path. However, a short circuit directly across the Stop button's terminals provides a permanent alternate path for the current to flow, entirely bypassing the button's internal mechanical contacts. When the operator presses Stop, the internal contacts open, but current continues to flow freely through the short circuit. As a result, the relay coil remains energized and the motor cannot be turned off using the intended control.

Adım Adım Çözüm

1
Analyze the normal operation of the latching relay circuit.
Pressing 'Start' energizes K1, closing the parallel NO contact (latching the circuit) and the motor NO contact. The circuit stays energized through the NC 'Stop' button when 'Start' is released.
Understanding the baseline functionality is required before analyzing fault conditions.
2
Determine the electrical effect of a short circuit across the 'Stop' button terminals.
A short circuit creates a permanent zero-ohm bypass around the button's internal mechanical contacts.
To understand how the fault alters the available current paths.
3
Trace the circuit logic when 'Start' is pressed and released with the fault present.
Current flows through the bypassed 'Stop' button and the pressed 'Start' button, energizing the coil. When 'Start' is released, current continues flowing through the bypassed 'Stop' button and the latched K1 NO contact. The motor runs normally.
To determine the initial state of the motor after the start sequence.
4
Trace the circuit logic when 'Stop' is subsequently pressed.
The mechanical contacts of the 'Stop' button open, but the short circuit wire continues to provide a zero-ohm path around them. The relay coil remains energized, and the motor continues to run.
To evaluate if the intended stopping mechanism can overcome the fault.

Anahtar Kavram

Relay Latching Logic and Short Circuit Fault Analysis
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Konular

Switches and Relays

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