Machines; Special Control Circuits Flashcards
What is the purpose of timers?
Automatically turn electrical equipment on or off and conserve energy
What are the 3 basic types of timing devices?
1) spring-wound interval timers
2) mechanical and electronic time switches
3) timing relays
True or false; spring-wound interval timers provide a timed off interval
False, timed on. Ie. When dial is released a spring drives a clock mechanism which holds contacts closed for the set time
Parking receptacle circuits are an example of what kind of timed device?
Mechanical timer
Bathtub spa motors are an example of what type of timing device?
Spring-wound interval timer
Which type of timing device uses a small electrically driven clockwork motor to control adjustable trip mechanisms?
Mechanical timers
What type of timing device do we commonly see in motor control applications? Why?
Timing relays. Because they are precise and versatile.
What are 3 methods of controlling timed operation on timing relays? Which of these methods is the most popular? Why?
Pneumatically (by air),
Electronically,
Or by a fluid-filled dashpot.
Electronically. Because electronic timing relays are smaller, more accurate, versatile, and cost-effective.
What kind of timing device can control devices over 24-hour or 7-day timing cycles.
Mechanical timers
What are the two basic types of contact actions for timing relays?
Time delay on energization (TDOE) and time delay on deenergization (TDOD)
What is a “delay on operate” contact action?
Delay on energization
What is a delay on release contact action?
Time delay on de-energization
Which type of contact action changes the normal condition of the relay contacts once the set time has elapsed?
Tdoe
What type of contact action Returns the contacts to their normal position once the set time has elapsed?
TD OD
What type of contact action would be used to allow a conveyor to empty Itself by running for a set time after the loading equipment has stopped?
TD OD
What type of contact action would be used for switching in windings during the starting of a part winding start motor?
TD OE
On a multi-function electronic timing relay what do the first four dip switches control?
The function of the relay
On a multi-function electronic timing relay what to do the last 3 dip switches control?
They determine the range of the timer
On a multi-function electronic timing relay what does the dial set?
The time delay within they selected timing range
What are three additional functions that may be included with dip switch programmable multifunction timing relays?
Interval timing
one shot timing and
repeat cycle timing
What is it called when:
- timed contacts change condition immediately -stay changed for a specified time
- return to their normal condition, even if the relay remains energized.
Interval timing
Give an example of the application of interval timing
Controlling an exhaust fan to purge a combustion chamber.
What kind of timing requires power to be applied to the relay and a start terminal on the relay
One shot timing
What type of timing function begins each time the start terminal receives a signal?
One shot timing
What kind of timing function provides a regular on and off cycle for the timed contacts?
Repeat cycle timing
What kind of Transformer connection would allow a motor to start with reduced starting current?
Wye connection
When a multi-function timing relay that is set for Interval timing has timed out what would be the condition of:
- a NO timed contact?
- a NC timed contact?
- open
- closed
When a TDOD timing relay has first energized what would be the condition of:
1) a NO timed contact
2) a NC timed contact
3) a NC, IC contact
1) closed
2) open
3) open
When a TDOE timing relay is first energized, what would be the condition of:
1) a NO timed contact
2) a NC timed contact
3) a NC, IC contact
1) open
2) closed
3) open
When a TDOE timing relay has timed out what would be the condition of:
1) a NO timed contact
2) a NC timed contact
3) a NO, IC contact
1) closed
2) open
3) closed
When a TDOD timing relay has just been deenergized, what would be the condition of:
1) a NO timed contact
2) a NC timed contact
3) a NO, IC contact
1) closed
2) open
3) open
For TDOE timing functions, when do associated contacts change state? Excluding IC contacts
After time delay
For TDOD timing functions, when do associated contacts change to their non-normal state?
Right away, when coil gets power.
For TDOD timing functions, when do associated contacts return to their normal state?
After time delay
When do IC contacts change condition?
On coil energization
When do IC contacts go back to normal condition?
When coil is deenergized
What is motor braking for?
Sometimes electric motors must come to a Full Stop quickly when the stop button is pressed and they must not be allowed to turn unless they are energized
How does a mechanical friction brake work?
Solenoid releases break when motor is energized. Spring engages break when motor is deenergized
How is the solenoid coil connected with regards to the power leads on a motor with mechanical friction breaking?
Solenoid coil of the brake release is connected in parallel with the motor power leads
When is a solenoid coil energized for a motor with mechanical friction breaking?
When the motor is energized. (Releases break when energized)
How does dynamic braking work?
- Mechanical energy continues to turn the armature after supply has been disconnected
- emf generated by the armature
- braking resistor that is placed across the armature operates on this generated emf
- current produced develops lorentz force opposing rotation of the armature.
How is the amount of braking controlled in dc dynamic braking?
Value of the braking resistor, which changes the armature current
What is another name for dynamic braking of alternating current Motors
DC injection braking
What is dc injection breaking used for?
Dynamic breaking of ac motors
How is the amount of braking force controlled in DC injection braking applications?
Amount of direct current supplied to the stator
What is the braking force proportional to for DC injection braking?
Strength of the stator and rotor magnetic fields
What is the main opposing/limiting force against current for dc injection braking?
Resistance of the stator winding
A dynamic electric brake for ac motors stops the motor by applying (ac/dc) voltage to the (solenoid/rotor/stator)
DC, stator
What is a method of braking used in induction motors?
Plugging
When is plugging used?
Applications requiring immediate stopping
How does plugging work?
Interchange supply of 2 stator-phase windings to REVERSE PHASE SUPPLY and PRODUCE TORQUE IN THE OPPOSITE DIRECTION.
Disadvantage to using plugging to stop a motor?
Generated heat from kinetic energy and coupled load. Rotor bars overheat.
What is anti-plugging?
Motor control method to prevent opposite contactor from being energized until motor has slowed or stopped.
Is an anti-plugging switch normally open or normally closed?
Normally closed
What is the function of a control transformer for motor control applications?
1) lower line voltage to safer level
2) electrically isolates control cct from power cct.
What are typical voltage ratings of control components?
240v, 208v, 120V and 24V ac
What are typical nominal line voltages for motors? (How do these compare to control voltages)
208
240
480
600
(Much higher)
According to the CEC a control circuit carries the electric signals directing the performance of a control device at the same power the device operates on.
False. Control circuit does NOT carry the power that the device controls.
What are pilot devices?
Controlling devices
- pushbuttons
- limit switches
- selector switches
- proximity switches
What kind of loads exist in a control circuit?
Relays
Solenoids
Contractor coils
Pilot lights
What do the transformer voltage ratings need to match? Primary and secondary.
- Primary needs to be rated for voltage that feeds it.
- Secondary needs to match voltage rating of control circuit components.
What else needs to be installed if control components are rated in DC? Where is it installed?
Rectifier. Secondary side
What is the capacity of a control transformer rated in?
VA
What is the range of control transformers available? With regards to capacity
25VA- 5000VA
What goes into calculating the required capacity of a control transformer?
Total steady-state (sealed) VA
And
Total inrush VA
What happens to a transformer when it is exposed to high inrush currents?
Voltage drop (V = I x R; with high inrush current and impedance comes high v-drop) that lowers output voltage.
The CEC requires control transformers to be grounded as a system except…
Except for systems that use less than 1000VA.
Can a control transformer be grounded to the metal enclosure of the power supply or bonding conductor within the enclosure?
Only if it is used in a system operating at less than 1000VA
What terminal is usually grounded when the secondary voltage of a control transformer is 120V?
X2
What class of circuit can be used for remote control circuits? Rule?
Class 1. 16-100
What size overcurent protection is required for the following wire sizes:
18awg
16awg
18 awg= 5A
16awg= 10A
What device is recommended to avoid unnecessary interruptions of current to a control transformer?
Time delay fuses
When may a control transformer NOT need secondary fusing?
If the primary fusing is not greater than 125% of the secondary current
If there IS secondary fusing and it does not exceed 125% of the secondary current, how large can the primary fuses be?
Up to 300% of the primary current of the transformer.
Name 3 styles of control transformers
Outdoor enclosed
Indoor open
Indoor enclosed
Why is dynamic braking for ac motors called DC injection braking?
Because DC is supplied to the stator winding of the ac motor
