Contactor/Starter Sizing

[noel0589]

Hi, I'm a recent graduate and new engineer, so my question is simple for those experience i hope. If a contactor/starter is undersized for a given elevator application, would this create excessive sparking when starting and stopping the motor at the contacts? I am trying to get a better understanding of current flow and current flow capacity through materials(specifically contacts). I know electrical energy can be dissipated by heat, but what about sparking? The reason I ask is because we seem to have less sparks emitted when using larger gauge lead wires connected to the contacts. Does this mean that the larger gauged wires are "helping" the contacts to allow more current flow? or Why else would there be excessive sparking at the contacts? Thank you in advance for any responses

Noel

 

[alehman]

An undersized contactor may have excessive arcing - and may eventually fail. Contactors should be designed (and rated for correct HP and voltage) for elevator duty because of the frequent operation. If this is a DC motor, significant arcing at the contactors may be normal. I wouldn't think the size of the wires would make any difference.

 

[Lakey]

Hi,

The arc (spark) is a phenomenon that occurs when the atmosphere between the contacts becomes ionized, usually when the contacts open or close. Arcing can be both useful and harmful.

I disagree with Alehman, with respect that if it is a DC system, and if larger gauged cables are used, then there will be a significant increase in current flow which will consequently result a larger arc across the contacts.

That is if the term “larger gauged cables” = larger cross sectional area.

Respect to all.
Lakey

 

[alehman]

Lakey - if I'm reading noel's post correctly, he reported _less_ arcing with larger gauge cable.

Regardless I have two concerns with your conclusion. First, if the cable is anywhere close to adequately sized, we're probably talking about a negligable difference in resistance as compared to the motor it is feeding.

Second, the elevator motor probably has a governor to maintain constant speed, hence constant HP output, regardless of the voltage supplied (within limits). If the voltage at the motor is lower due to high lead resistance, the current will be higher to maintain constant power. So in a theoretical sense, Noel's observation is logical. Still, unless the leads are very long or vastly different in size, I doubt the difference in resistance would make a noticable change in the arcing behavior.

 

[Lakey]

Alehman,

Apologies I did misread that.

However we can only speculate about the cables. But if the contactor is under rated, as suggested, then it may not have sufficient enough air gap to dissipate the energy in the system effectively.

Regards.

 

[digitrex]

With a smaller cable, there will be more voltage drop at the elevator motor and the starting torque will be less. The motor will take longer time to run up to full speed and hence the duration of high starting current will be longer. The contactor will carry starting currents for longer duration. The sparks on the contacts will last for longer duration.

 

[mc5w]

There are some factors that require and oversize contactor:

1. If the locked rotor code letter is over H you need an oversize contactor in order to have enough making capacity.

2. If a contactor has 3 times the minimum breaking capacity the life will be increased by a factor of 10. Telemecanique put some life versus current broken curves into the SquareD Digest a few years ago. I hung onto that edition for that reason.

3. For jogging duty the current broken will be a large fraction of the locked rotor current. For plugging duty (deliberate reversing at full speed) the plugging current in the stator can be twice locked rotor and and rotor current increases by a factor of 4.

Mike Cole, mc5w@earthlink.net