Motor protection

Overload relay with reclosing lockout

They should always be used where

continuous contact devices (two-wire

control) are concerned (e.g. pressure and

position switches), to prevent automatic

restarting. The reset button can be fitted as

an external feature in order to make it

accessible to all personnel. Overload

relays for example are always supplied

with manual reset. but can be converted to

automatic reset by the user.

Overload relays without reclosing

lockout

They can be used only with pulsed contact

devices (three-wire control) such as

pushbuttons etc., because on these, the

cooling of the bimetal strips cannot lead to

automatic reconnection.

Special circuitry

Special circuitry such as is found in

star-delta switches, individually

compensated motors, current

transformer-operated overload relays etc.

may require that the relay settings deviates

from the motor rated operational current.

Frequently recurring operating cycles

It makes motor protection difficult. The

relay should be set to higher than rated

motor current in view of its shorter time

constant. Motors which are rated for a high

operating frequency will stand this setting

to a certain degree. Although this will not

ensure complete protection against

overload, it will nevertheless provide

adequate protection against non-starting.

Back-up fuses and instantaneous

releases

They are needed to protect not only the

motor, but also the relay, against the

effects of short-circuits. Their maximum

rating is shown clearly on every relay and

must be adhered to without fail. Higher

ratings – chosen for instance according to

the cable cross-section – would lead to the

destruction of the motor and relay.

The following important questions and

answers give a further guide to the

behaviour of an installation with motor

protection.

To what current must the overload relay

properly be set?

To the rated motor current – no higher, no

lower. A relay set to too low a figure will

prevent the full utilization of the motor; set

too high, it will not guarantee full overload

protection. If a correctly set relay trips too

frequently, then either the load on the

motor should be reduced or the motor

should be exchanged for a larger one.

When is it right for the overload relay to

trip?

Only when the current consumption of the

motor increases due to mechanical

overloading of the motor, undervoltage or

phase failure when the motor is under full

load or thereabout, or when the motor fails

to start due to a stalled rotor.

When does the overload relay fail to trip

in good time although the motor is

endangered?

With changes in the motor which do not

cause an increase in current consumption:

Effects of humidity, reduced cooling due to

a reduction in speed or motor dirt,

temporary additional external heating of

the motor or bearing wear.

What causes destruction of the overload

relay?

Destruction will take place only in the

event of a short-circuit on the load side of

the relay when the back-up fuse is rated

too high. In most cases, this will also

endanger the contactor and motor.

Therefore, always adhere to the maximum

fuse rating specified on every relay.

3-pole overload relays should be so

connected in the case of single-phase and

DC motors so that all three poles of the

overload relay carry the current, whether

in 1-pole or 2-pole circuits.

An important characteristic feature of

overload relays conforming to IEC/EN

60947-4-1 are the tripping classes (CLASS

10 A, 10, 20, 30). They determine different

tripping characteristics for the various

starting conditions of motors (normal

starting to heavy starting duty).

EATON