Industrial Electrician exam: Motor control circuits

Key points

• Stop buttons are normally closed and wired in series. Start buttons are normally open and wired in parallel.
• A holding (seal-in) contact keeps the coil energized after start is released.
• Overload contacts open the control circuit, not the motor leads directly.
• Interlocks stop forward and reverse contactors from closing together.
• Two-wire control restarts automatically after a power loss. Three-wire does not.

Study tip

Trace power from L1 through each contact to the coil. If one contact is open, the coil is off.

Try these

1. In three-wire control, extra stop buttons are wired:

   1. In parallel
   2. In series
   3. Across the coil
   4. In the motor leads
   Show answer

   B. In series Any stop must break the circuit.

2. What keeps a motor running after start is released?

   1. Overload
   2. Holding contact
   3. Timer
   4. Fuse
   Show answer

   B. Holding contact The seal-in contact bypasses the start button.

3. Which control restarts on its own after a power failure?

   1. Three-wire
   2. Two-wire
   3. Jog
   4. Reversing
   Show answer

   B. Two-wire Two-wire has no seal-in to drop out.

Keep going

PLC basics · Industrial Electrician study guide · Industrial Electrician practice questions · Mock exam