Time delays, and Breaking

On-Delay Timers

-Upon application of the voltage to the timer coil, timing starts -contacts will stay in their de-energized state until their preset time has been reached -after the preset time has been reached the contacts change states -contacts remain in their changed state until power is removed for their coil that operates them -when the power is removed the contacts return to their original state

Normally opened timed open contacts

-like the NOTC contacts, this type of contact is normally open when the coil is unpowered (de-energized), and closed by the application of power to the relay coil -however unlike the NOTC contacts, the timing action occurs upon deenegirgization of the coil rather than upon energization.

Normally closed timed close

-like the normally closed timed open contacts, this type of contact is normally clsosed when the coil is de-energized/unpowered, and is open by the application of power to the relay coil -however unlike the NCTO contact, the timing action occurs upon de-energization of the coil rather than energization.

off delay timer

-power is applied to the coil -the contacts changed state immediately as soon as coil is energized -contacts will stay in the changed state until the times function has finished timing out -timing starts when the coil is de-energized state until the timer has timed out -after timing, the contacts return to their original state

Normally closed times open contacts

-this type of contact is normally closed when the coil is unpowered (de-energized) -the contact is opened when the supplication of power to the relay coil, but only after the coil has been continuously powered for the specific amount of time

Normally open, timed closed contacts

-this type of contact is normally open when the coil is unpowered (de-energized) -the contact is closed by the application of power to the relay coil, but only after the coil has been continuously power for the specific amount of time

On Delay Timer

-upon application of voltage to the timer coil, timing starts -after the preset time, the contacts change state, -when the power is removed the contacts return to their original state.

Dynamic Braking (Rheostatic)

A method of braking in which a motor is reconnected to act as a generator immediately after it is turned off, the electrical energy can then be dissipated as heat in a resistor bank.

Regenerative braking long

A regenerative brake is a device or system which allows a vehicle to recapture and store part of the kinetic energy that would otherwise be 'lost' to heat when braking. Regenerative brakes are most commonly seen in electric or hybrid vehicles. Electric regenerative brakes have been used on electric and diesel-electric locomotives and streetcars since the mid-20th century. Braking is accomplished by switching motors to act as generators that convert motion into electricity instead of electricity into motion.

How is DC-injection braking applied to induction motors?

Direct current is injected into the stator windings after disconnecting from the AC supply. This sets up a stationary field, inducing emf. and hence currents in the rotor circuits. This method provides a high-braking torque with low losses, but the braking effect is small at high speeds for machines of normal slip and it is usually necessary to use heavy DC exciting currents to effect a quick-stop.

Solenoid Actuated Brakes

Solenoid Actuated Brakes are spring-set, electrically released, utilizing a solenoid and lever arrangement to release the brake.

How is plugging applied to AC motors?

The phase sequence of the supply is reversed by interchanging two leads. The usual arrangement for squirrel-cage motors is by a reverse contactor which closes when the stop button is operated. The supply to the motor must be disconnected as the speed nears zero. This is effected by having a reverse-rotation switch coupled to the motor shaft and connected to open the braking-contactor coil before the motor reverses. The motor takes heavy current from the supply unless resistance is added to the stator circuit.

Electromechanical Friction Braking long

This is an external brake, used for stopping the motor as well as holding the motor at a single position. It consists of a solenoid along with a drum brake arrangement. When the motor is running, the solenoid is energized; so, it keeps the brake shoes away from the rotor shaft. As the motor is turned off, the solenoid is de-energized and the brake shoe acts on the rotating shaft. There is a braking action due to the friction between the brake shoe and the rotating shaft. This kind of brake holds the load at one position, even after the motor stops. It is therefore used in applications that require the motor to be held at one position with the load, like in Cranes, Overhead Doors and hoisting equipment.

When is braking of an electric motor required?

When there is a need to bring a motor quickly to rest. To hold a motor at standstill after some operation has been completed. To check the speed rise of a motor with an overhauling load.

Electro-Mechanical braking

a drum or disc friction-type brake is used, the brake shoes being held off against spring loading by a solenoid.

Counter-current braking or plugging

achieved by reconnecting the machine so that its output torque reverses.

Regenerative braking

applied to overhauling loads; the generated voltage is either pumped back into the line or to a battery bank.

Time delay relay contacts

but be specified not only as either normally-open or normally-closed, but wether the delay operates in the direction of the closing or in the direction of opening

programable non-retentive timer

don't save accounted time upon power loss

DC Injection Braking

obtained by disconnecting the machine from the main power supply and establishing a fixed magnetic field from a DC supply which causes a stationary magnetic field to be induced in the rotor windings.

orifice

rubber gasget

programmable retentive timers

save accounted time upon loss of power

synchronous clock

timer that open and close contacts depending on the position of the hands on the clock

dashpot

timer that provides time delays by how rapidly air or liquid are allowed to pass into or out of a container through a fixed or variable diameter orifice

solid state timers

timer whose time delay is provided by a solid state electronic device enclosed by a timing delay -very accurate and good economic cost -range from fractions of a second o thousands of hours -most are designed as disposable, usually covered with epoxy resin for protection