Motor starter with overload relay.Square D

Phase Failure and Reversal Monitor Relay

If two phases of the supply to a three-phase induction motor are interchanged, themotor will reverse its direction of rotation. This action is called phase reversal. In the operation of elevators and in many industrial applications, phase reversal may result in serious damage to the equipment and injury to people using the equipment.
In other situations, if a fuse blows or a wire to a motor breaks while the motor is running, the motor will continue to operate on single phase but will experience serious overheating. To protect motors against these conditions, phase failure and reversal relays are used.

A solid-state phase monitoring relay is shown This relay provides protection in the event of a voltage unbalance or a phase reversal. The unit automatically resets after the correct voltage conditions
return. An indicating light shows when the relay is activated.

Solid-state relay. (Courtesy International Rectifier.)

Solid-state relay. (Courtesy International Rectifier.)

Open magnetic contactor with blow-out coil used for control of direct-current devices.

Open magnetic contactor with blow-out coil used  for control of direct-current devices. Contactor is of the
“clapper” type.

Different types of control relays found throughout industry

Different types of control relays found throughout industry

Eleven- and eight-pin relays

                    Eleven- and eight-pin relays

Coil 11 PIN Relay

Eight-pole control relay.

Eight-pole control relay with four normally open
and four normally closed contacts.
 
Coil 8 Pin Relay

Single-pole, double-throw, single break AC control Relay with wiring symbols.

Single-pole, double-throw, single break AC control Relay with wiring symbols.

Starter with electronic overload relay. (Courtesy Square D Company.)

Three-phase motor starter with thermal overload relay.

Three-phase solid-state overload relay.Square D

Instantaneous trip current relay. (Courtesy Allen Bradley, a Rockwell International Company.)

Dashpot type of overload relay. (Courtesy Square D Company.)

Cutaway view of a melting alloy type overload relay and overload mechanism.

Cutaway view of a bimetallic type overload relay.The mechanism of overload relay is visible.

How to combine fuses with overload relays?

How to combine fuses with overload relays?
Fuses prevent short circuits from damaging the
installation and in worst case causing a fire, and must
therefore have adequate capacities. The lower currents
are cleared by the overload relay. Here, the rated
current of the fuse does not correspond to the motor
rating but to the current, which is likely to damage the
weakest components in the installation. As mentioned
previously, the fuse provides short circuit protection
and does not provide low overcurrent protection.
The illustration on your right-hand side shows the
most important parameters that form the basis for a
successful co-ordination of fuses and overload relays.
It is essential that the fuse trips out before thermal
damage of other parts of the installation occur because
of short-circuit
 

The most important parameters that form the basis for a successful co-ordination of fuses and overload relays.
The fuse time current curve always has to be situated lower
than the limit curve (red curve) for thermal damage.
 

What overload relays do?

• Make it possible for the motor to handle harmless
temporary overloads without interrupting the
circuit, i.e. motor starting.
• Trip and open a motor circuit, if the current exceeds
its limits and and might damage the motor.
• Are reset either automatically or manually once the
overload situation has passed.
IEC and NEMA are responsible for setting the standards
as to trip classes and thus for overload relays.

Trip class designation

Generally, overload relays react to overload relay conditions according to the trip curve. Regardless of the product style (NEMA or IEC), trip classes specify the periode of time it takes the relay to open when overload occurs. The most common classes are 10, 20
and 30. The figure refers to the periode of time it takes the relay to trip. A class 10 overload relay trips within 10 seconds or less at 600% of full-load current, a class 20 overload relay trips within 20 seconds or less and a class 30 overload relay trips within 30 seconds or less.

The degree of inclination of the trip curve depends on the motor’s protection class. IEC motors are typically adapted to the application in which they are designed to operate. This implies that the overload relay is able to handle excess amounts of current, very close to its maximum capacity. The trip time is the time it takes for
a relay to trip during overload. The trip time is divided into different classes. The most common trip classes are 10, 20 and 30. Trip class 10 is the most common one for IEC motors because they are often adapted to the application. NEMA motors are applied with more built-in excess capacity, and therefore, the trip class 20
is most common.

Trip class 10 relays shut off the motor within 10 seconds at 600% of full-load current. Trip class 10 is normally used for pump motors because the run-up time of the motor is around 0.1 – 1 second. Many high inertia industrial loads require more time to start. Many of these loads require trip class 20.

 

The trip time is the time it takes for a relay to trip during overload. The trip time is divided into different classes