K2CM

6

Output Circuits

■ Internal Circuit and Operation Description

1) Overcurrent Circuit

Overcurrent Detecting Circuit

This circuit detects when the current reaches the overcurrent operat-

ing level (115% of the set current value).

Time Setting Circuit (Inverse Type)

This circuit performs time setting using the VR (variable resistor) for

the operating time setting and obtains inverse-type characteristics

using an RC time-limiting circuit. The operating time can be set within

a range from 2 to 10 s or 8 to 40 s by operating the setting switch

using a VR. The VR covers a time range 5 times the standard range.

Start-up Detecting Circuit (Instantaneous Type)

Instantaneous-type models output a trip signal instantaneously when

the motor current exceeds the overcurrent operating value (115% of

the set current value). At the start of motor operation, a starting cur-

rent several times the rated current flows and so to prevent the motor

circuit being tripped by the starting current, instantaneous operation

is not enabled until a fixed time tc has elapsed, as shown in the fig-

ure. Instantaneous operation starts after tc has elapsed. Motor start-

ing time “to” varies, depending on motor type, within a range from

several seconds to several tens of seconds. There are even slight dif-

ferences in starting time between the same type of motors and so be

sure to set tc so that to to<tc is satisfied. If to>tc, the motor circuit will

be tripped after tc has elapsed. The fixed time limit tc at the start of

motor operation is called “lock time”. The start-up detecting circuit

detects the starting operation level (30% max. of the set current

time).

Starting Time Setting Circuit (Instantaneous Type)

This circuit performs time setting using the VR for setting the start-up

lock time and obtains fixed time-limit characteristics using an RC

time-limiting circuit.

Operation at start-up is shown in the figure below. After the motor

turns ON at point A, the motor’s starting current exceeds the start-up

operating value and so the RC time-limiting circuit starts charging. If,

for example, the motor current descends below the start-up operat-

ing value (30% max. of the set current value) at point B before the

start-up lock time, tc, has elapsed, the RC time-limiting circuit is reset

immediately and when the motor current rises above the start-up

operating value again at point C, the RC time-limiting circuit starts

charging again. After the start-up lock time has elapsed (at point D),

instantaneous operation is enabled. At the start of operation, the

motor current is at its peak immediately after operation starts. It then

lowers and settles at the rated current. The peak current is about 5 to

6 times the rated current and takes from several seconds to several

tens of seconds to settle to the rated current. This time varies largely

depending on type of motor and the nature of motor load (wt). There-

fore, it is necessary to obtain the motor’s starting time for operation

with the load and to set a start-up lock time that allows for a margin of

error. Do not set an unnecessarily long start-up lock time. If the start-

up lock time is too long and an accident due to overcurrent occurs at

the start of operation, the trip signal will not be output until the start-

up lock time has elapsed, possibly resulting in motor burnout.

With instantaneous-type

models, the circuit section

enclosed by

is

configured as shown below.

A

RS T

Test button

Reset button

To each

circuit

X

Overcurrent

indicator

Open-phase

indicator

Reverse-

phase

indicator

LED

LED

LED

X/

X/

1

To OR circuit

To each circuit

To each circuit

U

V

W

Instantaneous Type

B

C

Voltage Reverse-phase

Detection Models

2

3

M

Voltage

circuit

for test

With the voltage reverse-phase

detection models, the circuit

section enclosed by

in the

diagram on the left is configured as

shown below. The circuit section

enclosed by

is not used.

A

C

B

Inverse and Instantaneous

Types

As shown on the right, the K2CM

detects abnormalities in motor M

by checking its line current. The

motor’s current signal is detected

by the current transformer and is

processed separately for each

phase and input to the respec-

tive circuits. In each circuit, par-

allel judgement of failure such as

overcurrent, open-phase, or

reverse-phase (see note) is

made based on the input signals.

If a failure is detected in a circuit,

the circuit’s output is input to the

indication circuit to illuminate the

corresponding LED indicator and

also input to the relay drive cir-

cuit to drive relay X, resulting in a

trip signal to be externally output

from it. The three major circuits

are described below.

Note: Applies to current reverse-

phase detection models

only.

to

tc

Start

Motor current

Operating mode

Instantaneous operation enabled

Fixed time-limit

operation

Overcurrent operating value

to: Motor's starting time

tc: Lock time at start of operation

Start

Start

tc

tc

tc

Start

Trip

Output

Operation

mode

Motor

current

tc: Start-up lock time

*: Fixed time-limit operation

A

B

C

D

E

F

H

G

Overcurrent

operating value

Start-up

operating value

**: Instantaneous operation enabled

*

*

*

**

**