Motor Schematic Drawing

[kody916]

Greetings everyone

Just happened to see this motor schematic. As mentioned in the note 1 that relay DC1 is an interposing relay which is controlled remotely by a DCS. I need to know what is CR1. What is its purpose in this schematic diagram?
does this have to do something with voltage drop?

Thanks

 

[david luchini]

Greetings everyone

Just happened to see this motor schematic. As mentioned in the note 1 that relay DC1 is an interposing relay which is controlled remotely by a DCS. I need to know what is CR1. What is its purpose in this schematic diagram?
does this have to do something with voltage drop?

Thanks

CR1 would normally be a contactor controlling the motor. The coil is energized by the control circuit, and the contacts close or open to control the motor.

The way this schematic is drawn seems off a bit. The motor power seems to be coming from the single phase control circuit, but then it wouldn't need 3 overloads.

If the motor was truly small enough to be powered from the 300va control circuit (not sure if that is even code compliant) then the CR1 is redundant.

 

[Jraef]

Greetings everyone

Just happened to see this motor schematic. As mentioned in the note 1 that relay DC1 is an interposing relay which is controlled remotely by a DCS. I need to know what is CR1. What is its purpose in this schematic diagram?
does this have to do something with voltage drop?

Thanks

More likely it's because the coil current on M1 is relatively high compared to the contact rating of DC1. So although DC1 is already an interposing relay, it's purpose was to isolate the DCS control voltage from the field. IN the starter, CR1 is an interposing relay to keep the coil current on DC1 from causing it to weld.

 

[GoldDigger]

Similar to the function of starter relay in operating starter solenoid of a car?

Tapatalk!

 

[Smart $]

More likely it's because the coil current on M1 is relatively high compared to the contact rating of DC1. So although DC1 is already an interposing relay, it's purpose was to isolate the DCS control voltage from the field. IN the starter, CR1 is an interposing relay to keep the coil current on DC1 from causing it to weld.

Probably just the manner in which you stated it, but I believe it's better stated "...CR1 is an interposing relay to keep the motor contactor coil (M) current from welding the DC1 contact."

 

[Jraef]

Probably just the manner in which you stated it, but I believe it's better stated "...CR1 is an interposing relay to keep the motor contactor coil (M) current from welding the DC1 contact."

Your saying it way is betterer.

 

[Jraef]

CR1 would normally be a contactor controlling the motor. The coil is energized by the control circuit, and the contacts close or open to control the motor.

The way this schematic is drawn seems off a bit. The motor power seems to be coming from the single phase control circuit, but then it wouldn't need 3 overloads.

If the motor was truly small enough to be powered from the 300va control circuit (not sure if that is even code compliant) then the CR1 is redundant.

This schematic is only showing the control circuit, not the power circuit for the motor. "M" is the Motor Contactor, not the motor.

 

[wsbeih]

This seems an HOA (Hand - off-- Auto) control scheme. What is shown is a 120VAC control circuit. The way i read it is:
1- When selector switch is on "Hand", "H": this trigger CR1 coil will be energized and then CR1 contact will close. As a result, the Contactor coil "M" will be energized and the "M" contacts will close. "M" contacts are not shown on control circuit because they are connected to the 480V power lines to energize the motor.
2- When selector switch is on "Auto", "A": this will be the remote control by a DCS in this case. the DCS signal is 24VDC, you cant connect it directly to the 120VAC control circuit. therefore, you need a 24VDC interposing relay to do the hand shaking between the 2 different voltages. So when DCS signal energize the "DC1" coil, the DC1 dry contact will close and the CR1 coil is energized. the rest is as mentioned in "1" above.
3- when switch in off, this is self-explanatory.

 

[hurk27]

This seems an HOA (Hand - off-- Auto) control scheme. What is shown is a 120VAC control circuit. The way i read it is:
1- When selector switch is on "Hand", "H": this trigger CR1 coil will be energized and then CR1 contact will close. As a result, the Contactor coil "M" will be energized and the "M" contacts will close. "M" contacts are not shown on control circuit because they are connected to the 480V power lines to energize the motor.
2- When selector switch is on "Auto", "A": this will be the remote control by a DCS in this case. the DCS signal is 24VDC, you cant connect it directly to the 120VAC control circuit. therefore, you need a 24VDC interposing relay to do the hand shaking between the 2 different voltages. So when DCS signal energize the "DC1" coil, the DC1 dry contact will close and the CR1 coil is energized. the rest is as mentioned in "1" above.
3- when switch in off, this is self-explanatory.

This is the way I see it also, except we would have a Jog button between terminals 4 and 5 instead of a jumper, very common circuit.

 

[kody916]

Probably just the manner in which you stated it, but I believe it's better stated "...CR1 is an interposing relay to keep the motor contactor coil (M) current from welding the DC1 contact."

could you please explain when you say " motor contactor coil (M) current from welding the DC1 contact"....i didnt get that...


Thanks

 

[Smart $]

could you please explain when you say " motor contactor coil (M) current from welding the DC1 contact"....i didnt get that...

DC1 contact is not sufficient[-ly rated] for the current draw of the motor contactor coil (circle with "M"). Otherwise, interposing relay C1 would not be necessary.

Then again, it could be just a superfluous design... like a common 120V-control motor starter converted for auxiliary 24VDC control this way because it is more economical than revising the existing 120VAC control... and the starter can operate as a standalone without a remote 24VDC signal.

 

[kody916]

DC1 contact is not sufficient[-ly rated] for the current draw of the motor contactor coil (circle with "M"). Otherwise, interposing relay C1 would not be necessary.

Then again, it could be just a superfluous design... like a common 120V-control motor starter converted for auxiliary 24VDC control this way because it is more economical than revising the existing 120VAC control... and the starter can operate as a standalone without a remote 24VDC signal.

But when CR1 is energised, you would have 120V current flowing through DC1 contact( assuming the contact is closed after DC1 coil is energised by DCS).

 

[Smart $]

But when CR1 is energised, you would have 120V current flowing through DC1 contact( assuming the contact is closed after DC1 coil is energised by DCS).

Yes... but there's no indication on the schematic as to how much current is drawn by each coil or the ratings of each contact. All we know for certain from the schematic is that main control is 120VAC and the auxiliary control's DC1 relay has a 24V coil.

 

[Jraef]

In a DCS system that is controlling an entire plant or building, the real estate in the panel is very valuable, so they now use high density interposing relays that now look like terminal blocks:



A really common spec on these small interposing "terminal block style" relays is 6A thermal, but only 3A inductive at 120VAC and the coil of a contactor would be an inductive load. A CPT rated 300VA continuous is capable of putting out well over 1000VA for coil inrush current when closing a large contactor so used ONLY for that one starter, as shown in that drawing, implies that the starter is rather large, i.e. NEMA size 3 or 4 most likely; a NEMA size 3 starter could easily draw 6A inrush. So it's not a problem for the CPT, but it is for the relay switching it meaning the amount of coil current that this is sized for implies that it will FAR exceed a relay contact rated at just 3A, which could make that relay weld shut. So they use CR1 as an interposing relay, likely an "ice cube" relay where the coil current is only something like 2-3VA. This way, that DC1 relay only sees inrush from that coil that is a fraction of an amp, but CR1 likely has contacts rated 6A or more. The little relay is in the high density marshaling panel of the DCS, and the larger CR1 interposing relay is placed in the motor control cabinet where there is more room for it.

 

[Smart $]

In a DCS system...

That's a mighty fine explanation... but the schematic note says the DC1 relay is mounted in the starter enclosure...

 

[Jraef]

That's a mighty fine explanation... but the schematic note says the DC1 relay is mounted in the starter enclosure...

So it is... I hadn't gone back and looked at it again.

So... hmmmm....

Maybe it's because the DCS company wants to control the DC1 coil power requirement because it is being driven by their system remotely and if they just said "24VDC relay" they would end up with too many unknowns? I'm fishing now though, because otherwise it makes no sense, other than as a belt and suspenders approach.

 

[hurk27]

So it is... I hadn't gone back and looked at it again.

So... hmmmm....

Maybe it's because the DCS company wants to control the DC1 coil power requirement because it is being driven by their system remotely and if they just said "24VDC relay" they would end up with too many unknowns? I'm fishing now though, because otherwise it makes no sense, other than as a belt and suspenders approach.

I think we get some engineers who get used to a way of doing things and just keep that design in all their systems, which is what I see allot of at my new job. which also has double interposing relays instead of using just one, we see this even when our DCS is a 120 volts AC out to the field control.

But putting in another point of failure makes no sense?.

 

[Jraef]

I think we get some engineers who get used to a way of doing things and just keep that design in all their systems, which is what I see allot of at my new job. which also has double interposing relays instead of using just one, we see this even when our DCS is a 120 volts AC out to the field control.

But putting in another point of failure makes no sense?.

You're unfortunately probably right, I shouldn't have wasted so much effort trying to 2nd guess what most likely was just the result of laziness.

My least favorite response to an engineering query is "Because we've done it that way for xx years and I see no good reason to change now." Yet, I probably hear that at least once per week.