Do we need two type of Interposing relays in Relay panel for controlling Motor & getting feedback?

Separate Relays for two application:
1. DO for Start/stop - energized by 24VDC
2. DI for getting motor status - energized by 240VAC

 

This can be tricky. Refer to the motor contactor or Motor Control Centre schematics for the appropriate voltage levels for relays. The response below may be appropriate to some situations but not to all.

1. DO for Start/stop - energized by 24VDC
Use a 24 VDC coil relay with contacts rated for 240 Vac (or whatever your motor contactor control voltage is).

2. DI for getting motor status -energized by 240VAC
Usually, an axillary contactor is used to provide a contact with no voltage on it to indicate the motor running status.

A relay with a 240Vac coil is energised using 240Vac from either the MCC or the interposing relay panel.The contacts then would be rated for 24Vdc assuming that this is what your required voltage level is.

Hope this helps.

 

Dear Summit,

Normally the DO of DCS/PLC are operated by Open collector output of Transistor or TRIAC i.e. TTL logic based on microprocessor.

Thus it is not recommended to make relay continuously energised for the long time.To achieve this, a logic is prepared such that a Relay coil (24V) is being energised for moment (2 Sec) in control panel (or in MCC) & Change over contacts of this Relay switches the control supply (usually 110VAC) of motor.

I have not seen your site, but sure that there is latching of ON/OFF command at MCC level for motor.

This also provides isolation between processor circuit & any external supply.

<b> Regarding,DI for getting motor status </b>
A DCS/PLC system sends/uses interrogating supply usually 8V/12V/24V/48V/96V DC for its digital input signals. This voltage level must be up to the field device or MCC device which has potential free change over contacts (& are being used as digital input to the PLC/DCS). NO matter how this field device or MCC device changes its <b>Potential Free</b> contact i.e it may be pressure operated Pressure Switch, it may be Temperature operated Temp Switch, It may be Change over contact of Relay** or It may be change over contact of Axillary contactor** of MCC. (<b> which is in your case</b>).

** This Relay/Axillary contactor is being operated by another or MCC control supply, what ever it may be 24VAC/DC,110VAC/DC,240VAC/DC.

Once again,This also provides isolation between processor circuit & any external supply.

Hope, the matter is clear to your kindself. pls feel free to ask if you have any query.

Regards

 

Hi Ashish,

From the information you provide, I am thinking your control circuit is 24 VDC and your drive circuit is 240VAC. I am thinking you need both relays. Else the drive circuit will "fry" the control circuit. Hope this helps!

Good luck!

 

hi

Some friends already answered. I will add some more to make it clear

1. Normally at MCC side control supply is 230 / 120 vac, so you need consider a really. But while designing circuit make sure the contact is powered by the MCC control supply. Thus you have to provide potential free contact to MCC.

2. Normally Feedback are potential free contact from MCC. i.e you have to power up integration voltage to this signals. But you say it is 240V AC signal coming from MCC then you need a relay to isolate the card form this voltage.

For control supply at MCC side refer MCC drawings.

Pandu

 

dear,

the situation is i want to control the level of a tank with 3 motors 2 are in operation and one is stand by. i have level transmitter (4-20mA) analog input. motor supply is 230vac. i want to on/off motors when level is reaches it upper and lower limits.

what strategy should be adopted?
what interposing relay size?
which controller can be use for motors one transmitter 3 thermocouples (only for indication)?

 

dear,

the situation is i want to control the level of a tank with 3 motors 2 are in operation and one is stand by. i have level transmitter (4-20mA) analog input. motor supply is 230vac. i want to on/off motors when level is reaches it upper and lower limits.

what strategy should be adopted?
what interposing relay size?
which controller can be use for motors one transmitter 3 thermocouples (only for indication)?

i dont know how to play with electrical equipment and motor s i am instrument engineer. our electrical eng went on vacations. all i have to manage. Please guide me from a b c.

 

Hello,

I couldn't agree with your comment on the DCS/PLC DO.

Please note that 24 V DC output from DCS/PLC digital output card is easily available. Normally there is power limitation, say max. 10W per channel which varies from manufacturer to manufacturer. But there is no problem of keeping them energised all the time. In fact normal design standard is keeping the shutdown DO output energised all the time and de-energise when shutdown. For your information in thousands of plants all over the world normal ESD valves are always designed like this. Their solenoids are normally energised,all the time and de-energised only when to close the valve.This is called "fail-safe" circuit designed like this to detect the cable fault.

Why for motors start/stop we use pulse signal that reason is different.

Regarding Sumit's question on using 24 V DC or 240 V AC interposing relay another gentlemen has already explained that correctly.

If you are controlling the motor from a DCS/PLC whose output card is giving 24 V DC, then definitely you need 24 V DC interposing relay with relay contact suitable for the motor control circuit voltage.
For the motor running status what kind of interposing relay to be used that completely depends on the motor control circuit. If motor control circuit is using 240 V AC, you need to use 240 V AC interposing relay/contactor with volt free contacts. If motor control circuit is using 24 V DC or 110 V AC, you need 24 V DC or 110 V AC relay respectively. The main point is to get a volt free contact. Also pl. make sure that the contacts are suitable for very low excitation current from the DCS/PLC (often in the range of 10 mA). Many times this is overlooked.

The rest of the problem is simple. You are getting 4-20 mA sensor signal. You just need to program in the DCS/PLC software which motor to start/stop depending of your setpoint and motor selection from the DCS/PLC HMI. Here you may have an on/off control varying over a set point or you may consider a gap control depending on your requirement.
Motor run/stop signal is needed for status only, this is not required for control.