Emerson MSM400 manual Outputs Current Output, Relay Operation

7.1OUTPUTS

7.2CURRENT OUTPUT

The operation of the current output is set up by four different parameters and is always controlled by the process variable (PV), which is normally % suspended solids. These are found in SETUP – OUTPUT – CURRENT OUTPUT.

1.Lower range value (Low range val)

This is the value of PV which corresponds to the minimum current output, either 0 or 4mA

2.Upper range value (Up range val)

This is the value of PV which corresponds to the maximum current output, 20mA.

3.Alarm action

This is the value to which the current output is driven under alarm conditions and is selectable from a list.

•3.6mA

•21mA

•Hold (i.e. hold last reading)

For alarm conditions see the section below titled ALARM.

4.0-20mA or 4-20mA setting (0/4-20mA)

This parameter sets the current output range to either 0-20mA or 4-20 and is selected from a list.

5.If required the 4-20mA can be trimmed using an external calibration meter. (For details see Section 7.8)

7.3RELAY OPERATION

The MSM400 controller offers various options for operating the relays.

There are 2 relays that are programmable to different modes, set point operation, de-sludge, alarm and fault. The mode of operation is selected through the SETUP – OUTPUT – RELAY – Relay Mode menu. The default mode for Relay 1 is set point operation and Relay 2 default mode is alarm.

a) SET POINT OPERATION

It is possible to program both relays for set point operation. These are found in SETUP – OUTPUT – RELAY. The process value (PV) controls the relays.

1.Relay on point (RL* On Point)

This is the value of PV at which each relay will turn on

2.Relay off point (RL* Off Point)

This is the value of PV at which each relay will turn off

3.Relay minimum on time (RL* Min ON)

This is the minimum time that the relay will stay on for and this takes priority over the maximum on time.

4.Relay maximum on time (RL* Max ON)

This is the maximum time that the relay will stay on for.

It is important to note that this function only operates when the minimum off time is set to a non zero value.

5.Relay minimum off time (RL* Min OFF)

Once the relay has turned off this is the minimum time before the relay will turn on again. (*=1or 2 i.e. relay one or two)

If both on and off parameters are set to zero then relay is turned off.

If the on and off points are equal (non-zero) the relay is on when PV is above the set point and off when below. If both minimum and maximum on times are set to zero (default) then they are not used.

(See also the example in “NAVIGATION IN THE MENU SYSTEM”).

b) DESLUDGE

When in desludge mode the relays operate as explained above.

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