Using Breakers to control motors

[mull982]

In the process of building our new plant we have many 480V starters that have solid state overloads. When we want to bump motors for the first time to verify direction, it is sometimes difficult to do without having the overloads programmed b/c the output on the overload will not close until the overload is programmed. A lot of times these motors need to be bumped before these overloads are programmed in order to verify direction.

To bypass the overloads the electricians in the field are taking the 120V control voltage and wiring it directly to the contactor coil. They are then using the 480V breaker associated with each starter to turn on the motor.

I know this is a blatent code violation but many in the field are claiming that this practice is o.k. for just temporary turning on of motors such as the bumping I described.

What are others thoughts on this temporary measure of using a breaker for controling a motor? My argument is that these breakers are not designed for such activity and should not be used as such even temporarily. What potential negatives are involved with doing this?

 

[mayanees]

Sounds perfectly okay to me.
The coil is being pulled in by a breaker that's certainly rated for the minimal coil inrush, and the motor is being bumped on and then off using the appropriately rated contactor.
John M

 

[petersonra]

I don't have an issue with it even though for the period of time the motor is being bumped there is no over current protection, which technically violates the requirement to have over current protection.

I would just program the overloads first off the motor nameplate. I can't see any reason not to.

 

[hillbilly]

The way that I understand it is you're jumpering out the coil on the starter which closes it's main (motor) contacts, and then using the breaker to bump start the motor, is that correct?

Are the motors connected to a load when you do this?
What type loads are they?
How big are the motors?

There could be serious arcing across the breaker contacts under the right circumstances..

Just my opinion

steve

 

[bth0mas20]

can you just connect a phase rotation meter in the bucket and have someone in the field spin the motor?

 

[mull982]

The way that I understand it is you're jumpering out the coil on the starter which closes it's main (motor) contacts, and then using the breaker to bump start the motor, is that correct?

Are the motors connected to a load when you do this?
What type loads are they?
How big are the motors?

There could be serious arcing across the breaker contacts under the right circumstances..

Just my opinion

steve

No the motors are not usually connected to loads

The motors range types of loads throughout the plant. Fans, Pumps, Conveyors etc..

The hp's vary. Anything from 5hp to 250hp.

can you just connect a phase rotation meter in the bucket and have someone in the field spin the motor?

Without any excitation field in the motor (cold motor) how would any voltage be produced to verify rotation back at the bucket?

 

[jjacobs]

This could be an arch flash waiting to happen. not a vaild way to do rotation check's. Your electricians should use a simple button switch in a box and jumper around the controls for a simple bump test. Motor runs only while button is pushed. much safer to use for testing.

 

[Cold Fusion]

There could be serious arcing across the breaker contacts under the right circumstances..

I'm needing a little help here. I'm neither for nor against the OP plan, I just don't understand your response.

As I understand, the OP is jogging the motor using the CB. So worst case, the CB is opening under near LRA current. I would consider that the CB would be well within it's rating doing this. If not that, what would be the circumstances that would cause serious arcing across the breaker contacts? If opening is under near LRA is your concern, why is it you don't think the CB would be well within ratings?

cf

 

[Cold Fusion]

This could be an arch flash waiting to happen. .

Again I'm not for or against the OPs plan, I just don't understand the basis for your comment.

What would be the cause of the arcflash here?

cf

 

[Jraef]

I don't see why you wouldn't come up with a simple temporary switch to directly control the contactor coil rather than switch the motor on with the breaker. That's the way most people do it. You are already going in there to mess with the coil circuit now, why not just wire in a Momentary Push Button or a Spring Return 2 position selector switch? That way they can only do harm to the motor if they stand there holding the switch closed. If they let go, it opens and the coil drops out. For temporary use like that, you could just run the 2 control wires out the door over the gasket so that you don't have to leave the door open to bump it (which is what I assume was the issue someone had with arc flash danger).

Breakers CAN do that, but it's not a good idea. As mentioned above, you might be opening the breaker under locked rotor conditions. Even a small amount of arcing in the contact chamber can compromise the breaker's future ability to safely interrupt a fault, because a carbon trace in the arc chute could possibly provide a place for the next arc under a fault condition to flash over or not extinguish fast enough. Sure, it SHOULD be OK, but every time a breaker opens under extreme load like that, you run that risk for causing just the right amount of damage to do future harm. Doing it on purpose is just tempting fate in my opinion.

 

[Cold Fusion]

Breakers CAN do that, but it's not a good idea. As mentioned above, you might be opening the breaker under locked rotor conditions. Even a small amount of arcing in the contact chamber can compromise the breaker's future ability to safely interrupt a fault, because a carbon trace in the arc chute could possibly provide a place for the next arc under a fault condition to flash over or not extinguish fast enough. .

Let's take a look at this statement, in specific, "Even a small amount of arcing in the contact chamber can compromise the breaker's future ability to safely interrupt a fault,"

So, for a 250Hp motor, FLA in the 300A range, likely an 800A frame, likely good for 25kA interupt, not once but twice. And we are going to open it under, at most 1800A. And you are saying this compromises CB?

A significant increase in risk of future failure would not fit with any known data I have ever heard of. To put it in perspective, it would seem the increased risk of future failure is far less than driving to work in the morning.

I'm not in any way against what you and others have suggested - wiring the coil to a switch. I just don't see any data that supports viable spectres of disaster that have been raised.

cf

 

[neutral]

I would say let the Electricians do their job the way they know how, (If they are Electricians and not just an installer ) I'm sure as a project engineer you might not understand what they are doing, or they might be jerking your chain. I have seen it done before when engineers get in the way of getting a job done.

 

[jim dungar]

Breakers are tested for about 50 operations at about 6x FLA. Using a breaker to "bump" a motor once should not significantly decrease the life of the breaker.

However, I do prefer to have the motor starter do the bumping.

 

[neutral]

if you just need to bump the motor to check rotation then manully close the contactor for a 1/2 second, if the motor moves then you will know the rotation if you can't see the motor then it will require 2 electricians or 1 electrician to push the contactor closed and a project Engineer to watch the motor.

 

[RHJohnson]

I'm with the earlier poster that said to use a Phase Rotation Meter. They are not very expensive, and simple to use.
Check your supply rotation: example A-B-C (you can check this at the load side of the breaker if necessary, when you close the breaker.
Open the breaker and connect to the out going "T" leads with the rotation found above.
Have someone spin motor shaft in the correct rotation required, if your "T" leads are connected to give that rotation the rotation meter will indicate proper rotation. If it is the incorrect rotation that will be indicated also.
You can use the Phase Rotation Meter to land all power cables from then on to give you the same rotation, if the motor is CW rotation for A-B-C you will then be able to connect all CW motors to A-B-C.

 

[mull982]

I'm with the earlier poster that said to use a Phase Rotation Meter. They are not very expensive, and simple to use.
Check your supply rotation: example A-B-C (you can check this at the load side of the breaker if necessary, when you close the breaker.
Open the breaker and connect to the out going "T" leads with the rotation found above.
Have someone spin motor shaft in the correct rotation required, if your "T" leads are connected to give that rotation the rotation meter will indicate proper rotation. If it is the incorrect rotation that will be indicated also.
You can use the Phase Rotation Meter to land all power cables from then on to give you the same rotation, if the motor is CW rotation for A-B-C you will then be able to connect all CW motors to A-B-C.

I am not understanding how using a phase rotation meter with just the motor will indicate anything. In other words if you have just the "T" leads connected to the motor and you spin the motor how will this indicate any phase rotation without any magentic field in the motor to generate the necessary phase voltages.

I can see how the meter can be used on the incoming supply to verify phase rotation, but I'm not seeing how it would work when just being used on a motor by itself.

 

[broadgage]

I am not understanding how using a phase rotation meter with just the motor will indicate anything. In other words if you have just the "T" leads connected to the motor and you spin the motor how will this indicate any phase rotation without any magentic field in the motor to generate the necessary phase voltages.

I can see how the meter can be used on the incoming supply to verify phase rotation, but I'm not seeing how it would work when just being used on a motor by itself.

If an induction motor is rotated by hand, then it will generally produce a three phase output voltage, which in theory could be used to verify correct rotation.
In practice though, the frequency is very low due to the low speed of rotation, not all phase rotation instruments will correctly detect this very low frequency.
Also the generated voltage is very low, perhaps too low to work the phase rotation instrument.
And this presumes that the magnetic circuit of the motor has a slight degree of permanent magnetisation, which is often but not allways the case.

 

[jim dungar]

I see some confusion.

There are two types of phase rotation meters.
One can only measure rotation on energized circuits.
The other is "self powered" and can be used to check rotation on un-energized motors. Effectively these create their own small magnetic field in the motor.

 

[don_resqcapt19]

if you just need to bump the motor to check rotation then manully close the contactor for a 1/2 second, ...

I like this idea even less than using the breaker to control the motor.

 

[buddhakii]

Sounds like a great idea, but nothing works better than visually seeing the motor turn. I don't see a problem with using the breaker, however I do like the control swich idea. Seems like most safe and efficient way to me.