Contactor for phase converter

[bob_mp]

Greetings,
I'd like to use a magnetic (or solid state) contactor to turn on/off my phase converter. The phase converter is rated for 100A at the input. It is always started and stopped without load.

I am planning on a a couple of three-way switches to control voltage to the coil. I can get a step down transformer if needed.

Some questions:

1. Does one usually break both poles to the phase converter? Can I connect one of them directly and then use the multiple poles of the contactor to break the other. I understand I won't get the full increase in load rating from switching, say, 3 poles in parallel, but say, half.

2. What's the difference between a definite purpose contactor (that's rated for motor loads) and a motor contactor? There seems a difference in price.

3. If I am buying off ebay, any recommendations on size or make? I see Allen Bradley and Siemems have a lot of offerings.

4. It seems a lot of the contactors have an additional relay block on top of the main contacts. Is this to enable latching for powering on and off the contactor?

4. Is there a benefit of using 24vDC versus AC? Does the DC coil voltage cause less hum?

Thanks!
Bob

I was looking around on ebay and there are lots of contactors. A

 

[alan camby]

A manual Disconnect switch would be much cheaper and easier.

If using a magnet contactor. I would break both poles.

Motor starters are usually more money because they include a thermal overload located under the starter. A magnetic contactor does not have a overload included.

All the big names should have quality items.
I always used Fuji at the Japanese factory I worked at.

The relay on top (on Fuji's) is for controlling one voltage with another.
Some machines have a control circuit of 24vdc and the DC power supply is not rated for the coils of all the starters. They run the 24vdc to this small relay on top and the relay switches to another voltage, say 110vac, that has less of a limit on current. In this example the coil voltage for the starter is 110vac.

I have never noticed quality starters humming. Voltage selected is up to the machine builder. Usually 48 volts or less is a touch safe voltage. Some large starters I have worked on are controlled by 480vac. It will probably be cheaper to use AC. At home I use starters with a 220vac or 120vac coil to eliminate the need for a transformer or power supply.
Just have to be carefull if the panel is open since my control switches have this voltage also. Make sure the panel mount is grounded well.

 

[alan camby]

Why not just use one of these disconnects??

 

[bob_mp]

I was looking to add a couple of switches near the lathe and mill for convenience and to keep the walls uncluttered. The phase converter will be outside, behind the shop.

Thanks,
Bob

 

[930dreamer]

How many hp is the rpc idler?

 

[bob_mp]

Hi,
It's based on a 20 HP, NEMA 256 motor. It was made by GWM and the model number is Rotogen 256.

Thanks,
Bob

 

[Norcal]

A definite purpose contactor is a throw away item & unless UL listed (or NRTL) it's is not code compliant to use, some DP contactors are "RU" a UL recognized component, not UL listed (or listed by a NRTL), so really dicey to use them, plus they are garbage.