Question on NO/NC contact for motor application 4

[budhiman]

In a motor protection application using relays, I have seen the trip circuit having a normally closed contact in the run circuit. What is the reasons to have normally closed contact in this application where is in normal feeder protection the relay contact is programmed as normally open? Is the reason to have normally closed in the motor to break the run circuit or is there a different reason as well? Also, when will it be good to use fail-safe contact ?

 

[itsmoked]

Not sure I'm following you but normally a Normally Closed circuit is desired so a broken wire or failed relay contact turns the thing OFF rather than a failure signalling run.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BrianPetersen]

"Fail-safe" is old terminology, "control reliable" is more recent, "functional safety" is the current one. If the stopping of the motor is a safety-related function then a risk assessment should establish the required integrity of the control circuit. ISO 13849-1 is one possibility, there are others depending on the application and on where you are in the world.

 

[waross]

Normally closed is the original standard for motor control using magnetic contactors.
There are very few exceptions.
With the assumption that the safe condition is with the motor stopped, anything that interupts the current to the contactor coil will stop the motor.
In substations and generating plants, (and other similar plants) there may be a need to do switching in the absence of grid power.
A black start is one example.
It is often desirable that power circuit breakers not trip on power dips or power loss.
These installations typically use normally open trips.
When normally open trips are used it is often required to use an independent source of tripping power.
The independent power may be a battery bank or a capacitor.
These installations are often under the care and control of a trained operator and have a rigid inspection and maintenance program.
When normally open trips are used, a failure of the tripping power source or a broken wire may make a normal or automatic trip impossible.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

A diagram might help. Low voltage motors typically are controlled via a contactor that must remain energized to run the motor. To shut down the motor, relays generally then use a normally closed contact that opens to stop the motor. If the motor is running directly from a power circuit breaker, then generally it is an energize-to-trip scheme and a normally open contact is used.

 

[waross]

Hi dpc.

If the motor is running directly from a power circuit breaker, then generally it is an energize-to-trip scheme and a normally open contact is used.

I have encountered a number of installations where a power circuit breaker was used as a motor starter.
All used a shunt trip so that the breaker would trip on loss of power.
That avoids the hassle of a battery bank.
One installation was for twelve 3000 HP motors at 13,000 Volts.
As I remember shunt trips were used.
In the Canadian code a motor must not be allowed to restart automatically after a power failure if doing so may create a hazard.
That is most easily accomplished with a NO contact scheme or with a shunt trip.
(Shunt trip: Must be energized to allow the breaker to close. Trips on loss of power. Most breakers that accept a trip unit will accept either an energize to trip or a shunt trip.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Bill,
A Shunt Trip means it is ENERGIZED to trip the breaker. What you described is an UNDERVOLTAGE trip coil, that's the kind that trips on loss of power.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

I stand corrected Jeff. Thank you.
I meant to describe an UNDERVOLTAGE trip.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Bill,

I've done a lot of installations using low voltage and medium voltage circuit breakers as motor control devices in large power plants. Never used an undervoltage release. I have seen a few. I think this is something that depends on the industry involved.

Cheers,

Dave

 

[waross]

I think that you have hit the differences.
large power plants
I think this is something that depends on the industry involved.
Further to this, an existing battery bank will make a big difference.
If the plant is using a battery bank for switching power circuit breakers, then it makes sense to use the same control plan to control large motors.
The breakers that I have seen with undervoltage release were in plants that did not already have a battery bank for switching.
Most of the plants did not have a sufficiently rigorous maintenance program to ensure proper care of the batteries.
Contrast the cost of adding a battery bank that may not be maintained with the cost of an undervoltage trip and the choice becomes easier.
Another point is that using one or more power circuit breakers it makes sense to use the same control circuits, and if a plant has no power switching but MCCs and a couple of power circuit breakers for large motors, it makes sense to keep the same control circuits as the rest of the motors use.
Despite our different exposure, I am sure that if we were reviewing a specific proposed installation together, we would quickly agree on the best way to go

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[kssschsekhar]

Hi All,

The Motor protection Relay NC (Normally Close) or NO (Normally Open) usage will be based on type of motor On/Off switching. This is core electrical engineering best practice since the beginning.

If Contactor is used for switching of Motor, then to energize contactor coil NC is best choice. Most of the other applicable contacts also NC such as Local/Remote selection etc., (As soon as coil lost control voltage, the contactor will be released without looking for any requirement)

Whereas if Air Circuit Breaker (ACB) for switching of Motor, then to energize ACB Closing coil will be used and to trip Opening Coil. As you are well aware of that ACB close/open coils should not energized continuously (best practice). Hence NO is best choice.

So in your case trip circuit observed as NC means contractor might be used for motor switching. Whereas for feeder protection relay ACB is used for switching. Hence, NO used.

In general >100 KW motors ACB will be used as switching equipment.

Note: Present available technology can perform any type of activities. However, core engineering practices always best. Because, After proper thought process those are developed.

 

[che12345]

Hi All,

1. If [Contactor] is used for switching of Motor, then to energize contactor coil NC is best
choice. Agreed. Any protections including e-stop opens the contactor will stop the motor. Contactor
opens upon voltage failure and is usually wired such that the motor shall [not re-start automatically] upon voltage resumption.
Possible risk: in case the NC contact is " welded" or "shorted" by conductive agent (H2O etc.),
the motor will not be able to switch off even under emergency situation. The fault does not show out
as it works normally/fine until disaster occurred.

2. If [ACB] is used for switching of Motor, NO contact is used to energize the [shunt release] to
open the ACB may not be ideal.
Possible risk: when power failure the ACB remained in [close position] and the motor will [re-start automatically] upon power resumption. Automatic re-start is not allowed/required in most applications.
This problem may be circumvented by using NO contact to energize the [closing coil] where the ACB is equipped with spring charging motor. The NC contact is wired in series in the [under-voltage release].
This will ensure that the ACB can be close when the voltage is available and opens automatically when voltage failed.
Note: ACB usually can be installed with [closing coil + under-voltage release] but not [shunt
release + under-voltage release]

3. In Singapore, all ACB connected to the utility are installed with NO contact which energizes the
[shunt release] to open the breaker. The ACB shall be equipped with [direct acting trip] which operates mechanically independent from the shunt release, which requires voltage to operate it. The ACB remained closed upon voltage failure.

Che Kuan Yau (Singapore)

 

[waross]

The law in Canada:
28-400 Undervoltage protection required for motors (see Appendix B)
Motors shall be disconnected from the source of supply in case of undervoltage by one of the following means,
unless it is evident that no hazard will be incurred through lack of such disconnection:
(a) when automatic restarting is liable to create a hazard, the motor control device shall provide low-voltage
protection; or
(b) when it is necessary or desirable that a motor stop on failure or reduction of voltage and automatically
restart on return of voltage, the motor control device shall provide low-voltage release.

Note:
Low-voltage protection is N/O. Does not restart automatically. Generally requires a magnetic motor starter or an ACB with an under-voltage trip. (correction)
Low-voltage release is N/C. The motor will restart automatically after a power failure. Manual motor starters and magnetic motor starters with Hand/Off/Automatic switches and similar controls such as pressure switches, level switches and thermostats to name a few..


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr waross

Please advise what do you mean " .... Generally requires a magnetic motor starter or an ACB with an under-voltage shunt trip..".
As for an ACB, there is NO such accessory as " under-voltage shunt trip". It is either "an under-voltage release" or "a shunt release". These are two different accessories, each with different functionalities. In addition, in the usual case an ACB is NOT intended to have [both] " an under-voltage release" and "a shunt release" in it.

Che Kuan Yau (Singapore)

 

[waross]

Thank you for your sharp eyes, Che Kuan Yau. I have corrected my silly mistake.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear All,
1. I will try to clarify the difference in their functionality between "an under-voltage release" and "a shunt release" installed in an ACB. After having full knowledge of their functionalities, from there on make the decision as whether to use NC or NO contact to fulfill the operation requirement and in compliance with the local regulations/law.
2. under-voltage release
a) with an open breaker, it shall be [able to close] when the voltage is = or > 85% Un,
b) with a closed breaker, it shall [open automatically] when the voltage is between 35-70% Un,
c) the values given in above a) and b) are per Standards, not field adjustable,
d) to [maintain the breaker in close position], the release [is energized continuously].
3. shunt release
a) with an open breaker, it shall be [able to close] when the voltage is < 70% Un,
b) with a closed breaker, it shall [open automatically] when the voltage is = or > 70% Un,
c) the values given in above a) and b) are per Standards, not field adjustable,
d) to [open a closed breaker], the release [is energized momentarily].

Che Kuan Yau (Singapore)

 

[waross]

Things are getting confused between different pieces of equipment.
The majority of electric motors are less than 100 HP.
These motors are typically controlled by manual motor starters or by magnetic motor starters.

There are two basic control schemes used with magnetic motor starters, two wire control and three wire control.
In both types, the overload relay contacts are normally closed and open on an overload.
Some overload relays must be reset manually, Some will reset automatically. Some may be arranged to reset either automatically or manually.
Two wire control will restart automatically after a power outage.
Three wire control must be restarted manually after a power outage.
In both schemes, the overload relay contacts are always normally closed.
Larger motors may be controlled by Air Circuit Breakers or stored energy breakers.
These are tripped by:
Removing current from a continuously energized low voltage trip coil by normally closed contacts on the protection relay.
or
Applying current to a shunt trip coil from normally open contacts on the protection relay.
In the case of the shunt trip there must be power available to trip the breaker.
This may be an additional point of failure.
In the event of a failure of control power the breaker may not be tripped remotely or automatically.
It may only be tripped off manually without control power.

where is in normal feeder protection the relay contact is programmed as normally open?

Generally you do not want your feeder breakers to trip off when the power fails.
In many instances you do want your motors to remain stopped after a power failure. Safety is not the only reason.
Your system may not withstand the surge of restarting all of the large motors simultaneously.
Some machines must have the motors restarted in sequence. eg: The oil ump must bring the oil pressure up before the large motor is started.
Loaded conveyors must be unloaded in sequence before restarting, etc.
In these cases use a scheme that will trip the breaker on failure of power. Normally closed relay contacts may be the easiest way to do this.
You may use a normally open contact scheme if you have an alternate source of power for tripping.
This may be storage batteries, a capacitive trip unit or a UPS.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Worrying about shunt or under-voltage trips in low voltage breakers is rather moot since those trips are seldom used in those breakers.

In low voltage, there is generally a circuit (relay or contactor) where the motor will run when it is powered. If the protection is simple, then it's most likely as Bill noted - an overload with a N/C contact that opens on trip. If the protection is a electronic, more complex relay, then it will likely have a N/O contact that is powered-closed when the relay is happy and opens when the relay trips or fails.

In higher voltage switchgear, it's typical to see a circuit with 2 coils. Energize one coil and the breaker closes. Energize another coil and breaker opens. These circuits require N/O contacts for the protection, so the protection relay generally uses N/O contacts.

An example of using so called "fail-safe" circuits would be when a protection relay drives an intermediate control relay. Make the protection relay contact N/C during normal operation so the control relay coil is powered. Then, if the protection relay contact or wiring to the control relay coil fail the circuit powers-off and "fails safe"

At the end of the day, the OP really needs to post more details if expecting a better answer related to a specific application.

 

[itsmoked]

Like your concise answer Hutz.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[buzzp]

Clearly, there is some confusion as to what constitutes a NO and a NC contact. NO, is one that remains open if no power is applied to it. A NC remains closed with no power applied. You pick the contact that is best for your application. Motor controls typically use a NO contact - everything good, close the contact. Protective device fails or equipment faults, the contact opens. In the power industry, things are typically done the opposite way due to regular attention and testing (no one wants to shut down a power plant because a relay fails).