relay acring

ready961

Member
Join Date
Jan 2003
Posts
78
Hello
I am running a PLC to control 120vac 10amp relay which turns a 7.5amp motor pump on and off. The problem is that the relay is sparking/arcing when the relay is shut off. Is this do to the fact that the motor is still spinning(winding down) and creating a back current?? Could I install a diode to correct this??
Thanks
 
Any relay controlling any inductive load will arc when the contacts make or break. That's because of the energy stored in the coil of the motor or solenoid or contactor coil - it has to go someplace.

Diodes are usually used to suppress arcing on DC loads. R-C or other snubbers are used for AC. Most relay suppliers can provide a blug-in or wired in arc supressor, but you can roll your own. This has come up on hte site many times, and one of the guys with a better memory than mine will surely provide the link.
 
I just spoke to the tech dept at automation direct and they suggested using a contactor with the corrct hp rating and an overload relay. I was just using a regular relay to control the motor do to the fact that they were controled before with just a foot pedal. I figured it was the same difference.

How does the contactor differ form a regular relay?
Thanks
 
A relay is typically used for switching a low current load where contactors are usually required for higher current loads and 3 Phase. Even though one can sometimes be used in place of the other, that don't mean it's best. Consider a relay as a kind of Contactor Jr. (even though it's not really that simple).
 
A relay is designed for low-power applications (I/O signals etc.), a contactor is designed for higher power (motors, heaters etc).

The difference lies in several points. To list a few:
1. Contact point materials - with high power applications the ability to withstand arcing at the contact points without destroying the surface are important. A very low contact resistance is not.
2. Extinguishing chambers – for the real high powers, there are chambers around the contact points designed to “blow” out the arching.
3. Number of contact point per pole – regular contactors usually use two contact points per pole, a relay usually have only one.

When choosing a contactor, it is important to know what load you are connecting. I don’t know American terminology, but I will try to explain (and use European terms). The ratings are usually printed on the contactor itself:
* With regular resistances (heaters), use the contactors thermal current rating Ith.
* With motors, you have several choices, depending on how you control the motor:
- AC3: Normal start/stop, switching on 5-6xIn, breaking 1xIn (In = nominal current)
- AC4: Heavy duty, start/stop, breaking of start current, reversing etc.
* AC15: Incandescent lighting
Those are just some oft the different ratings you can find on a contactor, typically defined as a power rating (kW or hp) at various voltages. You will usually find that AC3 ratings are in the range half of the thermal.
All this applies to AC only. Switching high power DC are particularly challenging, often involving special contactors and/or combining poles in series and parallel.

Regarding the use of “snubbers”: in control circuits, use those when controlling inductive loads like relays, contactors and solenoid valves. Use free wheeling diodes with DC, R/C with AC, varistors (voltage dependent resistors) can be used with both AC and DC. Connect in parallel with the load. Using snubbers is the best way to prolong the lift of your PLC output modules, as it significantly reduces the arcing over the internal relay contacts (or reduces the reverse voltage over the output transistor).

Hope this helps to clarify....

Anders
 
Relays are designed for continuous current carrying capability, contactors are designed for making and breaking inrush currents.

Inductive loads like motors often have an inrush of 6 times full load current. Highly inductive loads like small control transformers and solenoids can have inrush currents of 10 times.

A typical 10A control relay is designed for making 6x (or 60A) and breaking .6x (or 6A). Relays are also designed for the inrush current to only last a few cycles to maybe a few seconds.

A typical 10A motor contactor is rated for making 6x (or 60A) and breaking 6x (or 60A). The contactor is designed to handle the inrush current for many (at least 30) seconds. Contactors see such large breaking currents during jogging (stopping while accelerating) and plugging (quick direction change) operations.

When an overload current relay is directly connected to a motor contactor, the result is usually called a "starter".

Contactor contacts breaking motor currents last longer due to their physical size and material composition. But, arcing and sparking can still occur, so line voltage surge suppression may still be required.
 
When switching a heavy inductive load it's best to use a contactor. This is due to the increased air gap and the use of a shorting bar provieded in a contactor. The shorting/bridging bar in a contactor is simmilat to having two contacts in series that would help to prevent, or help to snub arching when the contacts open.

I cant imagine using a 7 1/2 hp 120 volt motor controled by a relay rated at 10 amps. If your motor is truly 7.5 hp and the voltage is 120 volts your current draw would be more like 45 amps.

Mike.
 
This is an issue that has come up for me recently. While I generally use starters or contactors for motors I have occasionally used relays for motor control. Sometimes I have been limited by lack of panel space. The relays that I have used have a HP rating of 3/4 or 15 Amps at 120VAC. The motor is 1/3 HP 6 amps. We recently had to replace one when the contacts started sticking. I have used surge supressors for solenoids and relay coils that were connected to PLC relay outputs but have not used surge supression on motor loads. How do you go about sizing these for small motors?
 
If you are unable to use a contactor for your motor Rta, and you have room for a double or triple pole relay, you can jump the primary (incoming) wire to each pole. Remember to jump the secondary side as well. That way each of the poles share the current load instead of loading a single pole with the total load.
 
ready961,

This is what got you into trouble:

"I was just using a regular relay to control the motor do to the fact that they were controled before with just a foot pedal. I figured it was the same difference."

Many heavy-duty foot-pedal switches and limit switches are actually rated (by motor horsepower) to control motors directly. Also there are some relays that have a motor rating. I bet that the relay you used was rated for a 1/3 hp 120 volt motor, at 7.2 amps (if it was "motor-rated" at all). This would be the MINIMUM that would work for your 7.5 amp motor, and in the real world, it is best to go well above the minimum rating if you want the device to run for very long. Usually there is no magic black-and-white "right" size of relay or contactor to do a specific job, but there is always a minimum size for below which it will burn out!
 
Last edited:
Various Thoughts on the Relay Arcing subject.

Relay Arcing:

Here is an interesting link describing replacement of a Mechanical Contactor with an SSR :
http://www.me.psu.edu/me415/fall00/KCC2/

This rears the ugly head of when it is best to use a Electron Mechanical Contactor (EMR) vs a Solid State Relay. Here is some great information:
http://www.magnecraft.com/products/section2_01.pdf
Starting on page 6 find some great application and selection considerations for when to use an SSR and a EMR.

I have been working on an issue at work where we are forcing a “Universal Motor” (runs on AC or DC) to go either forward on reverse via relays on PLC Output Command. It is a 1/20 HP, 120VAC motor, and as described above creates a whamo arc on the 10 A contactors when the Field Coil magnetic field collapses on motor off. I am experimenting with NTE2V130 Metal Oxide Varistors (MOV) to shunt the spikes to the neutral buss.
http://www.cir.com/parts/nte/nte.htm

I’m a little under the knowledge curve with the proper use of MOV’s but I’m experimenting around with different configurations to see if I can protect the relay contactors a little better.

Research on MOV application has yielded:
http://northstanlytv.com.hosting.domaindirect.com/surge3.html
 
You may use 2 or 3 contact of the relay together in parallel.
Or to use low consumption coil contactor.
I use in most of my projects, transistor outputs with 1.7W 24 V DC contactor coil.
 
Its do to the fact the motor is still draw a load at the time the contacts are opening.

It is not uncommon to use a relay for 120 VAC motors. One a very cyclic operation the contactor should last longer. Arcing will still occur. They have more surface area to fail.

Suppressors have been used for a long time to prolong the life of contacts of relays with inductive loads.

I would think any relay manufacture would have recommendations of what to use. Givem it a shot.
 

Similar Topics

Question to anyone with ideas about my thoughts on upgrading a very vintage timer, which is being used to switch between 2 5hp domestic water...
Replies
14
Views
364
I have an application where I want to use a safety relay in combination with a flame detector as the sensing element. The flame detector has relay...
Replies
23
Views
931
Hi guys, I am looking for a good replacement for an Omron smart relay. A customer of ours has a couple of these and they are beginning to fail. As...
Replies
14
Views
1,088
Good morning guys, I'm searching for a Timer relay which accomplishes the "flasher" condition in the photo attached, however, I need the ability...
Replies
2
Views
533
Back
Top Bottom