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Effective:

June 1980

Supersedes I.L. 41-181.2J, dated March 1978

( | ) Denotes Change Since Previous Issue

CAUTION

Before putting relays into service, remove all

blocking which may have been inserted for the

purpose of securing the parts during shipment,

make sure that all moving parts operate freely,

inspect the contacts to see that they are clean

and close properly, and operate the relay to

check the settings and electrical connections.

1.0 APPLICATION

The type CM relay is an induction type relay designed

to protect polyphase electrical machinery against

phase unbalance or phase failure.

As shown in figure 6 the relay may be used with

either three or two-current transformers. With three

ct's the accuracy class must be at least C50; with two

ct's the accuracy class must be at least C70. Other-

wise ct errors during motor starting may cause

undesired CM tripping.

2.0 CONSTRUCTION & OPERATION

The type CM relay consists of two main current units

and their associated resistor and an indicating con-

tactor switch (ICS), or an ac indicating contactor

switch (ACS). The principal component parts of the

relay and their location are shown in figures 1 and 2.

2.1. MAIN UNIT

Each main unit has a pair of electromagnets operat-

All possible contingencies which may arise during installation, operation or maintenance, and all

details and variations of this equipment do not purport to be covered by these instructions. If further

information is desired by purchaser regarding this particular installation, operation or maintenance

of this equipment, the local ABB Power T&D Company Inc. representative should be contacted.

Printed in U.S.A .

ABB Automation Inc.

Substation Automation and Protection Division

Coral Springs, FL 33065

Type CM Phase

Balance Current Relay

ing on a single disc. The disc is damped by a perma-

nent magnet. Each disc carries its own set of

contacts with the two sets being connected in paral-

lel, in order that either disc may close the trip circuit.

The electromagnet pair are mounted face to face on

opposite sides of the disc, and so connected that the

electrical torque of one electromagnet opposes that

of the other, thus producing balanced operating

torque on the disc when the magnitudes of the cur-

rents through each of the two electromagnets are

equal. One of the electromagnets on the lower disc is

connected in series with one of the electromagnets

on the upper disc. Thus phase A current may balance

phase B current on the upper disc, and phase B cur-

rent balance C current on the lower disc. Conse-

quently

operating torque is produced on the two discs, but

with unbalanced conditions or an open phase the bal-

ance on the disc is upset and one or two sets of con-

tacts close.

Each electromagnet has a main coil located on the

center leg of an "E" type laminated structure that pro-

duces a flux which divides and returns through the

outer legs. A shading coil located on one of the outer

legs of the "E" structure causes the flux through that

leg to lag the main pole flux. The out-of-phase fluxes

thus produced in the air gap will cause disc rotation.

A resistor located to the rear of each main unit is

used as an aid in balancing the opposing torques by

controlling the current flow through the shading coils.

For some CM relays, the front electromagnet may

have adjustable plugs which are used to aid in cali-

bration.

with

balanced

system

Instruction Leaflet

41-181.2K

conditions,