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Questions and Answers
Q
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.
Example: Operating Mode = Overcurrent, Output Drive Mode =
Normally Open, and Operating Time = 5 s
Set value
Current input
Alarm indicator
Contact output or
transistor output
Connection Diagram
30 Ω
0 to 10 A
100 VAC
400 W (two in parallel)
Variable autotransformer
100 VAC
±
C
100 V
Cycle counter
Q
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.
Q
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.
Lit
Flashing
5s
Output
A
Input terminals
A1
Power
supply
A2
Q
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.
500%
100%
Several seconds to
several tens of seconds
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.
To the K8DT-AS3
K8AC-CT200L
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.
K8DT-AS
Time from beginning of startup
Primary-side conductor
9
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