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3 Phase starter or not?

buggsy

Plastic
Joined
Mar 8, 2013
Location
central mass
Hi, What's the reason for a starter box on any motor? Is it OK to run the motor on just an on-off switch? Inquiring minds want to know.

Thanks
 
Mr. Buggsy,

A motor starter, often known as a contactor, provides a way to start a 3 phase motor with control voltage usually less lethal than the line voltage for said motor, plus it allows for overload protection for that same motor. When properly sized for the load, the contactor will drop the load out if the motor draws more current that it's safely rated for. It also allows for the use of momentary START/STOP buttons which a bit more handy than flipping a clunky switch.

Stuart
 
Yes that is all true but yes you can run the three phase motor with a fused three phase disconnect box. One other thing three phase motor starters are not cheeps.
 
As noted above, there must be motor protection somewhere in the circuit, and if you're just going to use some type of switch, it must be horsepower rated for the size motor you intend to use it on...any old switch won't do the trick...properly.

Starter are so plentiful and cheap these days there really isn't a reason not to use one. Same goes for VFD's..all the protection and START/STOP capabilities, and they are very reasonably priced.

If you care about none of the above, you can use an old 3 pole knife switch and some frayed extension cord, just be sure you have your fire insurance paid up.:eek:

Stuart
 
Buggsy, a magnetic starter limits the effect of a high current draw on the contacts that supply electricity to the motor when you start it. After awhile ,a simple switch just wears out from being pitted each and every time the motor is turned on and off. A VFD needs to be sized to the horse power and amp rating of the motor you are using and they are only used on three phase motors. They take the place of a motor starter and have the added benefit of being able to convert single phase power as usually found in a residence to a type of three phase usable on a machine tool. Wiring a VFD is only limited by your understanding of basic electrical installation requirements and by your ability to read a manual. Safety should be your first priority. The cost of a VFD varies according to the size of the unit and who makes them. I would stay away from the very cheap chinese and korean units. Hope this helps. Jim
 
A bunch of semantics are involved here. Motors need to have a disconnect and a controller. The same common safety switch can serve as both. Overload protection can be provided by that switch If it is fusible. The fuses are rated at 125 percent of the full load current of the motor. Time delay fuses are generally used The setup I have described would be fine for a vent fan in an industrial building. Minimum essential design.
You could also operate a machine tool on something like this if you were comfortable with throwing a safety switch every time you started and stopped the machine. Not very convenient. You need a nicer controller. The drum switch is the simplest solution and will also provide reversing capability. Now you have a comfortable start stop switch a proper disconnect overload protection and reversing.
Now if you find the drum switch awkward or your machine is bigger than they make drum switches for you need to go to a starter or a contactor. The difference between the two is that a starter has overload relays and a contactor does not. Remember that the fuses in the disconnect supply overload protection.
If you're not comfortable with changing a fuse when you overload the machine You want a starter that will have a reset feature on the overload protection.
Thats about the limit of electro mechanical setups. A VFD can do all these things, and some even have their own disconnect switch. If they don't have a disconnect the plug and receptacle can also be the disconnect.
I have three lathes all of which use the cord as a disconnect and a common light switch as a controller. They are all single phase. There are common light switches that are two pole, and will stop a three phase motor. There are three phase toggle switches with resettable overload devices. With an operator standing over the machine you really don't need overload protection.
All of this semantics is from NFPA 70 known as The national electrical code.
 
With an operator standing over the machine you really don't need overload protection.

"Like wow...I was sexting this hot chick I met at Taco Bell last night and didn't notice the smoke and fire coming out of the machine, dude". "My bad Bro".:nutter:

Stuart
 
every thing above has its points but the one main reason's for a mag starter is that if the machine is running and the power goes out and when the power comes back on the machine will not as the mag starter will self disconnect and the machine will not run tell the start button / switch is pushed or turned on
 
I think you'd find that most good VFD's have the option of not starting the motor on power-up. Thus if the power goes out the motor will not re-start when the power comes back on. My feeling is that most fractional horsepower 3 phase motors don't need a magnetic starter any more that a fractional horsepower single phase motor needs one. Close the switch, the motor run. Open the switch the motor stops running. Pretty simple. The magnetic starters and starting resistors are generally used for larger 3 phase motors and can be quite expensive. That's why the VFD's are so popular. They often cost less than a magnetic stater with starting resistors to limit the inrush current upon start up. For almost all lathes that South Bend ever made, a 3 phase motor running off a VFD is a really good power system.
 
The re-start issue is the most interesting. A starter can be wired for maintained or momentary starting. On even a single lathe I would go momentary stop start so the thing will stay stopped. On a shop full of machines you definitely want momentary so the main breaker to the place doesn't open and cause a second private power failure when the utility power comes back on. There are exceptions like feedwater pumps in power houses that are usually wired to start on power up if the outtage is short. Usually done with a time relay.
 
13 and 14 are comparing apples to oranges. Square D starters are all nema standard. This is a much higher standard than the EU type stuff shown in the other post.
 
every thing above has its points but the one main reason's for a mag starter is that if the machine is running and the power goes out and when the power comes back on the machine will not as the mag starter will self disconnect and the machine will not run tell the start button / switch is pushed or turned on

Here's the best reason besides overcurrent protection.
 

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Detail, both Square D starters shown in 13&14 are two pole units. These are really not what you would choose for a three phase motor on a modern electrical system.
 
Another detail. Dont know whether you are using a phase converter or not, but someone on here that was had a problem with his starter dropping out when he started the lathe. I had no real experience with phase converters at that time, but knew they have phase imbalance often. There were many replies,and nothing worked.I sent a reply saying rotate the phases on the load side of the converter and told how to do it. He came back next day and said that cured it. A strange problem with an even stranger cure.
 
Another detail. Dont know whether you are using a phase converter or not, but someone on here that was had a problem with his starter dropping out when he started the lathe. I had no real experience with phase converters at that time, but knew they have phase imbalance often. There were many replies, and nothing worked. I sent a reply saying rotate the phases on the load side of the converter and told how to do it. He came back next day and said that cured it. A strange problem with an even stranger cure.

the issue is that the generated phase of a RPC is relatively weak.

If you imagine two identical 3 phase motors, one of them run as an rpc with no capacitor boost, when you plug the second one online, the voltage of the generated phase will be about half the nominal line volts, or less (i don't have two identical motors to try this with) . This often isn't enough volts to keep the solenoid engaged on the motor starter. moving the control transformer from the generated phase to the utility phase solves a lot of problems. or get a bigger RPC
 








 
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