Help w/ Contactor for DC Remote

[Eric Porter]

With the recent threads on DC remotes, I decided that I would try my hand at building my own remote. I am going to follow Tyler's design which is based off Dennis' design, but is more enclosed and more importantly allows remote operation of both a 110V vacum and 220V DCl using the same remote. My question now, is exactly what kind of contactor I need. I saw on Dennis Peacock's setup that he used a 40A 110V contactor, however Tyler only used a 15A. I know there are several other members who have also built their own remote units, so I would appreciate everyone's feedback.

I saw this 110V 25A contactor MPJA. Would this work, is the fact that it is only rated at 25A a problem?
http://www.mpja.com/directview.asp?product=15832+RL

On eBay, I also saw one, the title of the auction is below so that you can do a search and find this contactor? Would this be a better option since it is 40 amp?

Honeywell 3 pole 40amp contactor 110v coil


Any suggestions on other sources for contactors?


Thanks in advance,
Eric

[Marty Baucom]

Eric I just received this contactor Thursday evening for my remote.
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxx Tom Pritchard from this forum sent me the info. Hope this is what you are looking for.

[Tyler Howell]

Hey Eric,
The Contactor I used is rated for 15 amps at 600 VAC. I'm not an EE nor do I play one on TV but my math says that's 40.1 amps at 220V DC. enough for any hobbiest DC and your neighbors on both sides. Should be able to coral 5-7 horses with that.;)

[Don Baer]

Hey Eric,
The Contactor I used is rated for 15 amps at 600 VAC. I'm not an EE nor do I play one on TV but my math says that's 40.1 amps at 220V DC. enough for any hobbiest DC and your neighbors on both sides. Should be able to coral 5-7 horses with that.;)

um Tyler I'm afraid that's not how it works. What it means is that the contacts are rated for 15 amps and it can be used on circuits upto and including 600 Volts. peak. Thats the rating you need for a 460/480 volt circuit. Sorry about that.

[Steve Clardy]

I used a 20, as my 4hp motor was pulling 18 amps

[Larry James]

Take note, this is an overview of remote switching and the following is in no way a comprehensive explanation of how to wire and select components for remote control switching. Be aware that there is more than one way to wire this circuit.

As I understand it, the task is to control two different loads with one switch. The two loads are different voltages. You want to use a relay ( may be also called contactor) to do this.

The relay has two distinct electrical parts, the coil and the contacts. The coil and the contacts are mechanically linked together. Therefore, they each may have different electrical ratings.

The coil is an electromagnet used to open and/or close contacts. Contacts are simply switches. If you have a central heating or AC system the thermostat operates at 24 volts, yet it turns on a 120 volt fan motor or 240 volt AC compressor. The thermostat energizes (turns on) a 24 volt relay coil. The coil mechanical linkage in turn closes one or more contacts (switches) that control the higher voltage loads.

Now, substitute “remote control switch” for thermostat and you should start to see what I think you guys are trying to do. Use one switch to control one or more loads of the same of different voltages.

Because relays have two distinct electrical parts each has its own set of ratings. For example, you may use a 120 volt coil to control a 240 volt dust collection sys motor.

Each switch in a relay may be called a pole. A single pole relay has one switch, a two pole relay has two independent switches and so on. As a general rule, each hot wire to a load must be switched by a single set of contacts. A 120 volt circuit would, with one hot wire, requires one contact, a three phase motor with three hot wires would require three contacts, and so on. Never switch a ground or neutral wire!

Like all switches, contacts have maximum ratings. A full load amp (FLA) rating of 20 amps tells us that contacts may have a maximum current capacity of 20 amps, or less, continuously flowing at a given voltage. The voltage rating may be higher, but never lower than the voltage being switched.

If the “remote control switch” is switching the shop-vac directly, then it could be wired to also control a 120 volt relay coil to turn on the dust collection sys, provided the relay had the correct number of contacts and amp and voltage ratings.

The “remote control switch” only need to be able to switch the combined current of the shop-vac and relay coil.

There may also be other issues to be considered in selecting the switch and relay, and doing the wiring, best addressed by a licensed electrician.

Larry

[Tyler Howell]

um Tyler I'm afraid that's not how it works. What it means is that the contacts are rated for 15 amps and it can be used on circuits upto and including 600 Volts. peak. Thats the rating you need for a 460/480 volt circuit. Sorry about that.

Fuzzy Math. My Bad.:o
I did confirm that the Max load on the system is 8 amps.

I promise on my WWs honor to replace the contactor if I ever try to control all neighbors DC again.
I also promise never to venture into EE math on this forum againAmen

[Steve Clardy]

Fuzzy Math. My Bad.:o
I did confirm that the Max load on the system is 8 amps.

I promise on my WWs honor to replace the contactor if I ever try to control all neighbors DC again.
I also promise never to venture into EE math on this forum againAmen




:eek: :D :D :rolleyes: ;)

[Larry James]

Violations of Ohm’s Law!

The simple form of Ohm’s Law - volts = amps x resistance - is only valid for pure resistive circuits. A length of wire for example. This is direct current (DC) Ohm’s Law.

When applied to Alternating Current (AC) circuits, DC Ohm’s Law results will be incorrect.

Ask any electrician, appliance tech, HVAC tech or anyone who works on AC power circuits, when was the last time he or she used Ohm’s Law calculations for troubleshooting. Likely never. However, they use Ohm’s Law all the time - how can that be, you ask. It’s the relationship between volts, amps an ohms that is used, not the math.

It goes something like this: A circuit breaker opens. Why? The current in the circuit exceeds the max rating of the breaker. A voltage measurement confirms the correct voltage. Ohm’s Law tells us that if the voltage is constant, and the current increases, the resistance must have decreased. We look for a low resistance problem or “short circuit.”

Now, before all the double E's jump on me, that is a simplified explanation. When I’m ripping that 8’ 2x4 in my TS and it jams, and stalls the motor the breaker will pop (I hope), but not because of low resistance. It’s a magnetic motor thing that causes an increase in current.

There is an “AC version” of Ohm’s Law, but troubleshooters seldom have all the info needed to do calculations.

Remember - it is the relationship, not the math. How do you learn the relationship? You study the math!

Try as you may, you can’t violate Ohm’s Law.