Noob Question - DOL Motor Control Via PLC

[twinkletoes]

Hi,

This may sound like a stupid question, but I have been asked to create some PLC logic to control a motor that is wired up on a DOL starter.

The current setup is that the motor is wired through a MCB and contactor, which can be stopped and started via a pair of push buttons.

Currently, the only input to the PLC is the MCB and contactor for that motor (The push buttons are wired directly to the contactor)

There will be two operating modes for this motor; AUTO and MANUAL

What is required is that in MANUAL, the motor will be run via the push buttons. In AUTO the motor will run for a particular set of conditions.

With the current setup, is this possible? (I cannot see how this would be at the minute)

Regards,
twinkletoes

[corkers]

It really depends on what you need to achive..


If you trust the PLC then you could wire a selector switch in ie LOCAL / REMOTE control then wire the push buttons to the PLC and have the PLC control the operation of the DOL starter.

ie

in REMOTE mode the PLC starts the DOL starter as per the PLC program

in LOCAL mode the PLC recieves a start signal from the push button and starts the DOL starter, a signal from the stop PB (or lack thereof) would stop the DOL starter.

Alternately

A multipole change over switch could be used to isolate the plc from the DOL starter and connect a conventional stop start circuit to the DOL starter.

In some places I have worked this is the prefered method so that true local control is possible in fact one place I worked at had both systems in place for each motor starter (in excess of 1000 individual starters...) that is it was possible to start the motor in REMOTE (via the PLC), LOCAL via the PLC or MANUAL (relay logic).

One thing to be wary of is the voltages of the coils you employ, if you are using a 400V coil in the DOL starter this wouldn't really be suitable to operate directly from a PLC an interposing relay would be needed, on the other hand if you use 24VDC coils on the DOL starter you would need 24VDC for your relay logic switching. Let us know exaclel what you are looking to achive and I (we) may be able to be more specific in addressing your needs

Cheers

Andrew

[twinkletoes]

Thanks. So from what I am wanting to achieve, compared to what I have, futher modifications will be required?

In AUTO I am wanting to operate the motor for a particular set of conditions.

In MANUAL I am wanting the motor to be operated via the push buttons and cut out under a maximum limit value.

[silva.foxx]

Hi twinkletoes

You can start by listing your IO;

IN0; Overload
IN1; Manual/Auto
IN2; Start PB
IN3; Stop PB
IN4; Max limit
IN5; ...

OUT0; Contactor
OUT1; ...

State your conditions required in;

MANUAL mode
...

AUTO mode
...

[twinkletoes]

Currently the IO are as follows

I1 - MCB
I2 - Contactor
I3 - Auto mode
I4 - Manual mode
I5 - 'Auto' Condition
I6 - 'Manual' Condition

Q1 - Running Indicator
Q2 - Motor AUTO/MANUAL

In AUTO: Run motor until I5 has been made

In MANUAL: Run motor via push buttons OR until I6 has been made

That is about it.... For what I am wanting to acheive I cannot see how it would work! I am assuming that modifications will have to be made? (The push buttons are only wired into the contactor, and there is no outpur to actually activate the motor)

[tragically1969]

You have kind of answered your own questions, you will need all those signals listed back at the PLC to achieve what you want, so yes modifications will have to be made

Assuming it is a self contained DOL starter its probaly going to be best to mount the contactor set in a new box with buttons mounted on the front wired back to the PLC rather than butchering the existing DOL enclosure

[PeterW]

An alternative would be to wire the hand/auto so that you can run in hand via the pushbuttons with the PLC out of the cct, then in Auto, the PLC control the motor.

[Lancie1]

That is about it.... For what I am wanting to acheive I cannot see how it would work! I am assuming that modifications will have to be made? (The push buttons are only wired into the contactor, and there is no output to actually activate the motor)

It would work just dandy. I have wired many motors to be controlled like this, with a Auto/Manual Selector switch that selects which location has START control, the PLC or the Manual local pushbutton Start/Stop station (Stop always works from either place).

Modifications will be needed to add the new PLC control inputs and outputs to the existing motor starter. You will probably need some relays, a selector switch, and wiring back to the PLC.

I can make a schematic diagram showing how to wire it, if it would help. Where will the Auto/Manual switch be located, or will it be an HMI "software" switch?

[Lancie1]

Here is an excel spreadsheet with a motor starter schematic. Red lines are modifications to add PLC Manual/Auto mode.