Test 2 Flashcards
Two Types of Push Buttons
- Standard duty for coil currents of motor starters up to size 4
- Heavy duty for higher usage and higher contact ratings
Push Button P/O
- A device that provides control of a load by pressing a button which opens or closes contacts
- Two sets of momentary contacts are provided so that one is closed and the other open (Normally open / Normally closed)
Selector Switch P/O
- Are different than push buttons in that the operator nob is turned, not pushed
- Come with multiple contacts -> NO and NC
- Come in a variety of configurations
Control Station
Electrical cabinet containing a combination of push buttons, selector switches, and pilot lights
What are control relays used for?
- Control magnetic relays are used as auxiliary devices to switch control circuits and large motor starters or contactor coils
- They are also used to control small loads such as small motors, solenoids, heaters, lights, and other relays
- Primarily used in control circuits
Control Relay Notes
- Contact ratings are usually limited to 15A, and up to 600V
- They do not provide motor overload protection -> they are primarily used in 2 wire and 3 wire control systems
- Available in single or double-throw arrangements with NO/NC contacts
- Control relays differ in coil ratings, contact ratings, # of contacts, contact arrangement, size, and options
Greatest advantage of solid state relay?
- The input or control voltage is isolated from the line voltage
- This prevents voltage spikes and electrical noise from being transmitted into the control section of the circuit
Solid State Relay advantages:
- Requires minimal values of current to turn it on
- Very reliable and long life
- Not subject to arcing, wear, or deterioration
Solid State Relay Notes:
- Similar to electro-mechanical relays, but have no coil or contacts
- Uses the principle of switching transistors, SCRs, and Triacs
- A power transistor is used to connect a DC load to the line
- A solid-state relay intended to connect an AC load to the line uses a triac
Solid State Relay Reed Contact/Switch
- The coil of the relay is connected to the input voltage
- The magnetic field produced by the direct current closes a reed contact and connects the base of the power transistor to the line or the gate of the triac to the line
Solid State Relay Light Emitting Diode
- This method is generally referred to as optoisolation
- The input voltage operates an LED
- Then the LED turns on, it has the effect of connecting the base of the transistor to the line or the gate of the triac to the line.
is the most widely used method because it prevents voltage spikes and electrical noise from being transmitted from the load side of the circuit to the control side - These types of solid-state relays are commonly used in the output section of programmable controllers.
Metal Oxide Varistor
- Voltage sensitive resistor
- Has a very high resistance value
- As voltage remains below trip point, its not doing anything
- Mounted across the coil of a relay
- Drops out voltage spike
- When the voltage reaches a certain point the mov suddenly drops (can happen in 3-5 nanoseconds)
What provides surge protection to a solid state relay?
Metal Oxide Varistor
Contactors
- Electromagnetically operated switches that provide a means for connecting and interrupting branch circuits
- Difference between a contactor and a motor starter is that the contactor does not contain overloads
- Difference between contactors and relays is that contactors are mostly for 3 phase and contractors have much higher tolerance for voltage/current
- Contactors are used in combination with pilot devices to switch lighting and heating loads
- Used in cases where overload protection is provided separately
Magnetic Blowout Coils
- Large contactors have arc chutes which contain heavy copper coils called blowout coils
- The magnetic blowout coils help extinguish an electric arc which occurs when contacts open under load
- Blowout coils are large turns of wire placed in series with the contacts so that the main motor current passes through the coil and the contacts
- As contact opens -> current continues to flow through blowout coil. This current causes a magnetic field which reacts with the arc and magnetically blows arc out of the way
- Sometimes error is blown to help deflect the arc
AC Mechanically Held Contacts
- Can reduce noise over a seal
- Keeps things in the last known state if power fails so something you dont want to repeat doesnt
NEMA Stands for…
National Electrical Manufacturers Association
IEC Stands for…
International Electrotechnical Commission
What do magnetic blowout coils accomplish?
Magnetic blowout coils help to extinguish an electric arc at contacts opening under AC and DC loads
Because the coil of the magnet is usually in series with the line…
The field strength and extinguishing action are in propor-tion to the size of the arc
According to Fleming’s left-hand rule…
Motor action will tend to force the conductor in an upward direction
The function of the blowout magnet is…
To move the arc upward at the same time that the contacts are opening.
Are “ON” control stations used in multiple locations connected in series or parallel for latched-in contactors?
Parallel
