K8DT-AS

9

Checking Operation

Overcurrents

Gradually increase the input from 80% of the set value.

The input will equal the operating value when the input

exceeds the set value and the alarm indicator starts flashing.

Operation can be checked by the relay outputs that will start

after the operating time has passed.

Undercurrent

Gradually decrease the input from 120% of the setting and

check the operation using the same method as for

overcurrent.

How to Measure the Operating Time

Overcurrent

Change the input suddenly from 0% to 120% of the set value

and measure the time until the Unit operates.

Undercurrent

Change the input suddenly from 120% to 0% of the set value

and measure the time until the Unit operates.

Monitoring Switch-mode Power Supplies

Switch-mode Power Supplies cannot be monitored. In circuits

with a capacitor input, including switch-mode power supplies,

the input capacitor recharge current flows in pulse form as the

load current. The K8DT-AS

@ has a built-in filter as a

countermeasure against high frequencies and cannot be

used to remove pulse current.

Can a motor with a rated current of 5 A be monitored

using the K8DT?

Are there any application precautions?

The K8DT-AS1 and K8DT-AS2 cannot be used with motor

loads. Use the K8DT-AS3 in combination with the K8AC-

CT200L Current Transformer (CT). With motor loads, the

startup current and stall current will cause a current of many

times the rated current to flow. Refer to the following figure for

information on the motor startup current.

For a motor with a rating of 5 A, the startup current will be

approximately 30 A. The startup current will exceed the

overload capacity (rating: 150% for 1 s) of the K8DT-AS1 and

K8DT-AS2 and result in failure of the Relay.

To monitor the motor load, use the K8DT-AS3. (Overload

capacity: 120% of rating for continuous load, 200% of rating

for 30 s, and 600% of rating for 1 s).

The K8DT-AS3 has a large input range. Pass the conductors

multiple times through the special CT.

Concept behind Passing Conductor through the

CT When Using the K8DT-AS3

Example: Monitoring Overload of a Motor with a Rated Current of 5 A

K8DT set value:

Overcurrent detection, operating value setting: 25%, operating time: 0.1 s

Startup lock timer: 0.1 to 30 s (Set the timer according to the duration

of the startup current.)

The setting range for the K8DT-AS3 is 10% to 100% of the rated

current (i.e., 10 to 100 A). Pass the conductors through the CT five

times so that at least 10 A of current flows. The input current to the

K8DT will be 25 A (i.e., 5 A x 5 loops).

If a startup current of six times the rated current is generated, it will be

150 A (i.e., 25 A x 6). The overload capacity for the K8DT-AS3 is

200% of the rating for 30 s. The Relay will not fail even if the startup

current continues for 30 s, and it is possible to perform overload

detection.

Questions and Answers

Q

Current input

Set value

Alarm indicator

Flashing

5s

Lit

Contact output or

transistor output

A1

Output

Input terminals

Power

supply

A2

±C

100 VAC

100 VAC

100 V

A

Cycle counter

0 to 10 A

30

Ω

400 W (two in parallel)

Variable autotransformer

Example: Operating Mode = Overcurrent, Output Drive Mode =

Normally Open, and Operating Time = 5 s

Connection Diagram

Q

Q

Q

Several seconds to

several tens of seconds

Time from beginning of startup

100%

500%

K8AC-CT200L

Primary-side conductor

To the K8DT-AS3