View Full Version : Basic relay questions

03-09-2011, 10:55 AM
I've got an old Delta drill press form a closed down mill here, and it has developed an electrical issue. Someone at the mill added a foot switch that runs to a box off the motor than has hole in the top for the chuck key. When you put the chuck key in, you can use the foot switch. If the chuck key isn't in the hole, nothing works. There is a relay in the box, as well as a switch for the chuck key.

It has arced now and then since I got it, and has now stopped working unless I use the wall outlet (110) as an on/off switch. Foot switch appears to be fine.

I've pulled the relay, it's an older Cutler Hammer. I'll try to post a pic. It has two contacts on the coil, and two down below marked 'on' (a one contact relay?)

My questions: Do these things typically wear out/break? Would a cleaning do it any good? I've just done a few minutes snooping on the web.. don't know squat about electrical matters. Appears I'm dealing with a 'contact relay' as it powers a motor? There's a dizzying aray of these things.. wish Cutler Hammer had a chart of which old part #'s translate to current #'s.


03-09-2011, 11:45 AM
Relays do wear out. If the contacts are toasted you will need to either fix them or more than likely replace the relay. it sounds like the relay might have been improperly sized for the application. A motor is an inductive load and relay must be rated and sized. Given the facts that you report it sounds like the contacts are welded closed as you report it does not turn off.
You must ensure the relay is sized properly for the load. You need to get the details off the motor nameplate to source a replacement. A sketch of the components and a wiring diagram would be useful for remote diagnosis.


03-09-2011, 12:14 PM
Thanks. Will post pics and more info.

The contact does not appear to be welded to the armature, if that's what you mean. Externally all looks normal except for some dust/light scorch marks.


I took it apart a bit further, and the two lower (non coil) contacts are pretty cooked. Nothing welded, but corroded/melted for sure.

03-09-2011, 12:53 PM
I look forward to some pictures.
If you say its on all the time and the foot seems to work that seems unusual.
I think from what you describe this is what the circuit might look like.
If it is like this there are only a couple things that are wrong.
Either the Relay contacts are welded together or both switches are faulted.

03-09-2011, 01:03 PM
Here's some pics:

Nothing is welded together that I can see, but the lower two 'on' contacts are in bad shape.. maybe they were welded together and came apart when I removed the relay.


03-09-2011, 02:30 PM
It looks like the relay is properly sized. It should be easy to troubleshoot. I expect that something on the coil side is not working.
Do you follow the diagram I posted?

03-09-2011, 02:30 PM
Inductive loads can draw up to 6 times more than the rated current during startup. That relay is terribly underrated for the given load. You need a contactor type relay, which is specially designed for inductive loads and which can tolerate the unavoidable arcs better.

Here is a good document with quite detailed information:


03-09-2011, 03:03 PM
When it was operational, upon hitting the foot pedal, it would sometimes make quite a racket.. power being transmitted and cut rapidly.. now that I have this conductor in my hands the sound was partly the armature 'bouncing' I think, and the sound of arcing. I did see in my limited research that relays for starting for motors are unique... so this one is a 'regular' relay as opposed to a contactor relay?


03-09-2011, 03:42 PM
that relay has a horsepower rating,so i can be used for motor loads.

03-09-2011, 04:02 PM
that relay has a horsepower rating,so i can be used for motor loads.

This is clear as mud!

The motor is well w/in the ratings on the relay.


03-09-2011, 04:25 PM
While relays rated for HP is a good indication its suitable for inductive load, It still REALLY helps relay life to have a snubber across the contacts (Typicaly an RC snubber)

That said, that relay looks about a billion years old, And relays are well known for not lasting forever, Especialy when used anywhere near capacity, And inductive loads (especialy without a snubber) make life time even shorter. I bet the only reason it lasted this long was because it was a very well made relay. That said, they are cheap and basicly consumable these days, Like motor start caps and such.

Often relays are even socketed for ease of replacement.

03-09-2011, 04:50 PM
So what relay do I need given that motor? I can go by the motor plate, but folks here as well as printed materials posted previously say that start up, etc can far exceed those #'s.

Here's a good resource from Grainger:
http://www.grainger.com/tps/electrical_choosing_relays_motor_starters_contacto rs.pdf?cm_sp=Content-_-PDF-_-electrical_choosing_relays_motor_starters_contacto rs

1) So I know the coil voltage. 110 or 110/120.

2) Max amp and Max volt: Things start getting murky here. Motor plate says 60vlt 15 amp, but nothing at MSC matches up with that. Can I have a relay that is over-rated in either of those two categories? Would it be beneficial for the relay to be overated?

3) Total number of contacts. Mine is a 2 contact right?

Also, I *think* I can use a 'definite purpose' relay as opposed to a motor relay. The Delta motor has not been changed or modified in any way. Is that correct?



03-09-2011, 04:53 PM
Why not move the wires that were on the contact to the other (presumably unused) contact?

It'll last as long as it did originally.

03-09-2011, 05:02 PM
Both were used, and both are cooked...


03-09-2011, 05:08 PM
Just as a point of interest, I might guess that the contacts marked "ON" may actually be marked "NO" for "Normally Open." Could you possibly be viewing them upside down? Not that it matters in this case, but as I said, a point of interest.

03-09-2011, 05:10 PM
You need a 120v, 1HP@120v relay, rated at least 10A(?)
More amps or volts is totaly fine, More amps is highly benificial for relay life, More volts... Doesnt really help, but should'nt hurt either.
(Theres a small theory that higher voltage rated relays do better with inductive loads, but iv never really found much evidence of this)

I would look at a 15A relay myself, Since this is a good 'knee' in price typicaly, and should'nt be much more then a 10A relay and may last longer.
20A+ relays can start to get expensive.

(not all relays have HP ratings, And most that do have diffrent HP ratings for diffrent voltages, since higher voltage requires less amps)

Jaakko Fagerlund
03-09-2011, 05:27 PM
Just as a point of interest, I might guess that the contacts marked "ON" may actually be marked "NO" for "Normally Open." Could you possibly be viewing them upside down? Not that it matters in this case, but as I said, a point of interest.
Yup, thinking the same from the first post already :) Normally open connections would be the logical choice in a circuit like this (fail-safe).

03-09-2011, 07:28 PM
Yup, you guys are right....just testing you!


03-09-2011, 07:50 PM
Take a look here:


120 VAC coil, contacts rated for 25A, wiring diagram on the side of the relay. $9.


03-09-2011, 07:53 PM
My two cents.

Can the OP clarify what the fault is? As I read the complaint the unit runs all the time its plugged in?? The foot pedal nor the chuck key will stop its operation. Only removing the plug from the wall stops it?

I have based all my suggestion on this.

The existing relay is marked as 1HP thus is suitable for the application for the rated service life. The service life might have been exceeded. Certainly a relay with a larger rating is just fine. The relay is designed for motor usage. A snubber will be a great addition.

Instead of just saying its the relay why not troubleshoot the issue.

Other than a comment that the relay looked well used there has been no determination if it is in fact broken. If its shorted on the contact side a measure out of circuit with a Multi meter on Ohms will show a low resistance reading (less than a ohm) if the contacts are welded. If it show a high resistance out of circuit the contact side must be OK.

One way to determine if its the relay is to remove one wire from the coil If the motor still run the problem is either the relay has welded contacts or the is another path for the current to pass which is unlikely.

If the motor does not run with the coil wire off we have isolated the problem to the coil side.
If the two switches are in series as my diagram shows both have to be closed for the coil to be energized. With the power off and the unit unplugged a Multi meter on Ohms would show which one of the switches is defective.


03-09-2011, 09:45 PM
DFMiller.. correct.

I have no test equipment but will try taking a aire off the coil as suggested.



03-10-2011, 12:15 AM
The trick is dividing and conquering.
Let me know what happens when you take the wire off the coil side.
Where are you located?
Getting a cheap multimeter is something to consider. HD or most of the Big Box hardware have some cheap meters. We can always resort to cruder methods. Got a trouble light?
Got a test light for a car. We can be creative. We also don't want you get yourself electrocuted. ;-)

03-10-2011, 12:47 AM
As far as relay ratings are concerned for this application it is very simple...
Get a relay that has horsepower ratings. In this case 1Hp minimum at whatever voltage the saw is wired for (I would use 1Hp at 120 Volts to cover all bases). The coil voltage needs to be 120 (110/120 etc). It should be a 2 pole relay so it breaks (makes) both wires to the motor at the same time. That is all that is required to get a proper relay for a motor control circuit. By using a horsepower rated relay all the fudge work has already been done by the relay manufacturer.

This is how I do it and I have been building motor control panels for 20+ years and have never had a problem.... (yet :rolleyes: )

J Tiers
03-10-2011, 09:00 AM
1) You NEED at least a basic $10 meter or you have NO HOPE of doing decent troubleshooting of this stuff.

2) The problem can be caused by a bad relay, and likely ONLY a bad relay (or a wiring short) , since it is very unlikely that BOTH the footswitch AND the chuck key switch are bad at the same time.

3) rdfeil is correct, although you do not "have" to break both lines, but it is a good idea.*

4) the suggested relay appears to carry a UL rating of 1.5HP at 120V (see a page near the end of the data sheet), and is therefore perfectly suitable for this 1HP motor. There are other suitable types as well. such as this one from digikey, who are a little friendlier to order from


Also rated 1.5HP 120V when breaking both lines as recommended

5) Paralleling contacts, which seems to be what you described, is of minimal or NO benefit...... other than "carry" current. When opening, inevitably one will open before the other, which means IT carries all the current, so even in parallel, the single contact rating applies. The relay I mention is 1.5 HP when each contact breaks one power line (hot and neutral each get a contact as per rdfeil suggestion)

* One switch contact will "open all ungrounded conductors", which is what it required..... but using two will ensure that reversed wiring causes no trouble. And, as in the case with at least one of the suggested relays, the ratings may differ for using one or two. The one I suggest is rates 1.5 HP if breaking both lines, but 1HP if breaking only a single one.

03-10-2011, 02:41 PM
An important change is to put the relay contacts in the hot lead to the motor, not in the ground lead.

03-10-2011, 09:39 PM
1) You NEED at least a basic $10 meter or you have NO HOPE of doing decent troubleshooting of this stuff.

1) a meter is handy to have around for the next time something breaks too!

03-12-2011, 02:22 PM
I finally had some time to try taking one wire off the coil. Nothing. Put it back on and the whole system works as it did... foot pedal control, but the relay looks and sounds like like an arc welder half the time, the other half it works as I imagine it should.

I'm pretty sure the contacts were welded just slighty, and when I took them apart, I broke the weld. They are in really bad shape..this relay must be at the end of it's life.



03-13-2011, 12:57 PM
Sounds like you have an idea of the issue. It did seem that you can find a direct replacement relay on the Internet if you want an exact replacement.
Otherwise the Omron one looked like another option.
Good luck

03-13-2011, 05:37 PM
I think I will try one of the ones mentioned. The only Cutler Hammer units like mine are 'vintage' and generally used.. I have no interest in a used unit. Even using the same part and model #, specifications vary with the Cutler Hammer. Something a bit newer would good I'm thinking.


03-14-2011, 01:41 AM
I thought I found several that were new. If you want I will check tomorrow and send you the links.