Special Control Circuits

Question 1

What are the three basic types of timing devices?

Answer 1

1. Spring-wound interval timers
2. Mechanical and electronic time switches
3. Timing relays

Question 2

Explain a Time Delay On Energization (TDOE) timing function?

Answer 2

• When the coil receives power the output contacts do not change state, the timer starts counting down to its preset value. Once the timer has reach its preset value, the output/timed contacts change state and remain changed until the coil loses power

Question 3

Explain a Time Delay On De-Energization (TDOD) function?

Answer 3

• The timed/output contacts change state as soon as the coil receives power, they remain changed as long as the coil is receiving power. Once the power to coil is cut off, the output/timed contacts remain changed and the timer starts counting down to its preset value. Once the preset value is met, the output/timed contacts change back to their normal state.

Question 4

What are some functions of electronic timing relays?

Answer 4

1. Interval timing
2. One shot timing
3. Repeat cycle timing

Question 5

What are the three methods of controlling the timing of a timing relay?

Answer 5

1. Pneumatically (Air)
2. Electronically
3. Fluid-filled dashpot

Question 6

How does Mechanical Friction Braking work?

Answer 6

• Similiar to the braking mechanism of a car
• This method of motor braking uses a friction brake mechanism and is connected to the motor shaft
• The brake mechanism is held closed by spring tension and is fitted with an electromagnetically-operated brake release
• The solenoid coil of the brake release is connected in parallel to the with the motors supply leads and is energized whenever the motor is energized
• How does this operate:
  
  • When the motor is energized, so is the brake release coil, which removes the brake friction and lets the motor run
  • When a stop button is pressed and the motor is de-energized, so is the brake release coil. When the brake release coil loses power the the spring tension applies the brake and motor stops quickly
• Downside = requires maintenance and wears quickly if used often

Question 7

How does Dynamic Braking working for DC Motors?

Answer 7

• Dynamic Braking operates a motor as a generator when the motor needs to be stopped quickly
• If the supply voltage to the armature of a DC motor is disconnected, mechanical energy continues to turn the armature, and if a supply voltage is maintained on the shunt field, the motor generates power
• To apply Dynamic Braking, a braking resistor is placed across the armature, which completes the circuit and acts as a load supplied by the EMF generated in the armature
• The current that is produced is in a direction that produces a Lorentz Force opposing the rotation of the armature, this is the Dynamic Braking Effect
• The ammount of braking can be manipulated by changing the value of the resistor, which also changes the armature current
• As the armature slows, less EMF is produced and the Dynamic Braking effect decreases. A mechanical brake is often used to complete the stopping process.

Question 8

Describe the sequence of operations for Dynamic Braking with a DC Shunt Motor

Answer 8

1. When the stop button is pressed, the motor starter contactor coil drops out, which opens the normally-open contacts to the armature circuit. The armature circuit is isolated from the supply but the shunt field stays energized
2. At the same time, the motor contactor’s normally-closed contacts that are connected in series with the braking resistor change state and close shut. This puts the braking resistor in parallel with the armature leads
3. The rotation of the armature cuts across the flux set up by the shunt field to produce a current in the armature
4. This current develops a magnetic field around the armature conductors
5. The interaction betweren the field flux and the armature field flux develops a torque that opposes the direction of rotation of the armature