Magnetic starters

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Mr Ron
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Magnetic starters

Post by Mr Ron » Fri Dec 29, 2017 12:09 pm

The only machine in my shop that runs off 220 volts is my table saw which is 3 HP. It has a magnetic starter which I know is a safety feature in case the power is interrupted. Why then are magnetic starters not used on all power tools, both 110 and 220? Just about all power tools, both wood and metalworking can be dangerous if the power is interrupted while working. My 3 HP saw has a magnetic starter, but 110 V saws don't???
Mr.Ron from South Mississippi

JackF
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Re: Magnetic starters

Post by JackF » Fri Dec 29, 2017 12:24 pm

I never wondered about that until your post. I'm guessing because of the added cost of the magnetic starter.


Jack.

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warmstrong1955
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Re: Magnetic starters

Post by warmstrong1955 » Fri Dec 29, 2017 12:45 pm

My Grizz 120V band saw has a mag starter. So did a portable Craftsman table saw my Dad had, also 120V.

I bet Jack is right though.....it's such a price driven market.....

Bill
Today's solutions are tomorrow's problems.

John Hasler
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Re: Magnetic starters

Post by John Hasler » Fri Dec 29, 2017 2:01 pm

While it's true that machines that start up when power comes back on can be dangerous and magnetic starters prevent that, the reason for using them is overload protection. Smaller motors are now made with embedded thermal cutouts (which if you think about it actually provide better protection against overheating than do the heaters on the starter). You still need starters (or equivalent protection) on machines such as mills or lathes that might otherwise start up with automatic feed enabled and crash, but I believe that the thinking is that a saw that starts up when the power comes back on will just spin harmlessly (modern "consumer" products are supposed to have guards that prevent anything (including what you are trying to cut) from touching the spinning parts).

I'd like to have starters on more of my machines, but I can't afford it.

tornitore45
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Re: Magnetic starters

Post by tornitore45 » Fri Dec 29, 2017 6:10 pm

Not arguing the value of re-start protection of any kind, but if the light go off in the shop, my hand is on the switch to turn it off. I have a flash light. Then is off to the kitchen for coffee while waiting for the power back.
Mauro Gaetano
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TomB
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Re: Magnetic starters

Post by TomB » Sat Dec 30, 2017 3:47 pm

I presume that by the words 'magnetic starter' you mean the magnetic on/off switch. There is one on my Jet Uni-Saw; its about 3 to 5 hp with a 10" blade and predates the most recent neat features. I had an interesting experience with that magnetic switch that really surprised me. Also I routinely get reprimanded on this forum for doing things that are dangerous. I'm 75 and have not yet been hurt by doing things my way so I mostly ignore those notes and figure they are probably good advice for others.

My old shop was in my cellar and it flooded too often. I keep things off the floor and hoped that the water did not get deep enough to submerge electric motors or ruin good tools. Well the last flood was too much and I have bought a new house on the top of a hill. During that flood the water got about 4 feet deep which submerged the table saw motor. It was a first for that motor but for other motors I had found that if I started them and ran them long enough to heat up the windings and bake out any moisture the motor generally survived. (Generally a bit noisier but new bearings when it was convenient would fix that.) I had also found that it was best to do the 'run and heat' asap since the presence of water in the winding degraded them over time. I had in the past disassembled and rewound a motor that had sat for several months after being submerged and when I took the old windings apart I could see the corrosion. So when faced with my table saw being submerged I took the approach of immediately (there was still water on the floor) running it to self dry. First I cleaned out the wet sawdust so air flow would not be obstructed. Its wired for 220 and I use a special extension cord to plug it into a socket that is about 3 feet off the floor and also had been submerged. Since the extension had been removed and hung high when flooding started I reconnected it and fortunately did not thread the extension under the vacuum and up to the outlet. With all that done I started the saw. It did not seem to start slow so everything was like a normal flood recovery. I let it run for an hour, put my hand on the motor enclosure and it was medium hot so I felt I was done with drying. I then turned the magnetic switch to off and the saw continued to run. I played with the switches for a moment but the saw continued to run At that point I went to where the saw plugged into the extension and unplugged it. Since I was standing in water and it was 220v I was a bit concerned. I put on a pair of dry work gloves and I already had rubber boots that were dry on the inside and top. With that I pulled the plug and socket apart. There was no shock.

After that I took the magnetic switch apart. It was full of water. I let it drain and dry, then reassembled and it worked fine.

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BigDumbDinosaur
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Re: Magnetic starters

Post by BigDumbDinosaur » Sat Dec 30, 2017 5:22 pm

TomB wrote:I presume that by the words 'magnetic starter' you mean the magnetic on/off switch.
A magnetic starter is not a switch—a switch is a manually operated device.

A magnetic starter is a type of contactor whose design has been specialized for motor control, or other loads for which automatic overload protection is desired and high inrush currents are developed at power-on. In simple terms, a contactor is a type of relay that is intended to control high current loads. The starter's overload detection feature causes the circuit to be broken if the current flow exceeds a defined maximum for a certain amount of time. The time-delay characteristic gives a motor sufficient time to accelerate its load to full RPM without the annoyance of nuisance tripping.

The starter is energized by some other control device, such as a manually operated push button that latches the starter into the actuated position, or a sensor of some sort, such as the pressure switch on an air compressor. If a push button is the control, the starter will not re-actuate following restoration of power after a power loss, a safety feature that will prevent unexpected startup of a machine.

Starters are often used to control large amounts of industrial lighting, especially HID lighting, as a single push button can be used to conveniently and safely control tens or hundreds of kilowatts of lighting.

In North America, a magnetic starter's load capacity is described by its NEMA size, with NEMA 00 being the smallest available from most distributors. Currently-available models have the option of thermal or electronic overload detection. I'm an old stick in the mud when it comes to this sort of stuff and prefer thermal overload detection. I've seen too many instances of electronic devices failing in motor control applications to trust electronic detection.
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John Hasler
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Re: Magnetic starters

Post by John Hasler » Sat Dec 30, 2017 8:25 pm

Motor starters should not be used for lighting loads. There are contactors designed for that.

As an old stick who used to design electronic motor controls, I prefer electronic overload protection to a gear stuck in a puddle of solder.

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BigDumbDinosaur
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Re: Magnetic starters

Post by BigDumbDinosaur » Sun Dec 31, 2017 12:54 am

John Hasler wrote:Motor starters should not be used for lighting loads. There are contactors designed for that.

As an old stick who used to design electronic motor controls, I prefer electronic overload protection to a gear stuck in a puddle of solder.
I never said "motor starters." :D "Magnetic starter" is a catchall term for a contactor with some sort of overload protection, not necessarily a device used to energize a motor. My Square-D catalog indicates that many of their magnetic starters are suitable for both motor and lighting loads with proper selection of (thermal) overload devices.
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John Hasler
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Re: Magnetic starters

Post by John Hasler » Sun Dec 31, 2017 8:56 am

You got me. It is common to try to use motor starters (with motor overloads) for resistive loads, though. They usually get away with it.

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Rich_Carlstedt
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Re: Magnetic starters

Post by Rich_Carlstedt » Sun Dec 31, 2017 1:07 pm

Simple answers
1. Magnetic starters cost more than switches
2. Magnetic starters offer more than just on/off ( ie interlock, thermal or overload protection)
3. Magnetic starters still require a on/off switch or control ( added $$$)
4. Magnetic starters have better contact area and materials than switches generally have. --switch designs are generally spring controlled and spring cycles affect contact force, whereas magnetic contacts are very forgiving to wear and use and have a more constant closing force
5. Magnetic starters can handle higher amp loads than normal switches as explained above and thus are seen on higher HP motors
6. Magnetic starters need only light duty interlock switches while manual swiches require interlocks to have high amp capacities ( $$ and size )

Better built/ designed machines use Magnetic starters.
Voltage has nothing to do with it , but Magnetic starters do need more room, so space needs or portability can preclude the use of Magnetic starters

Rich

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BigDumbDinosaur
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Re: Magnetic starters

Post by BigDumbDinosaur » Sun Dec 31, 2017 3:11 pm

Rich_Carlstedt wrote:Simple answers
1. Magnetic starters cost more than switches
2. Magnetic starters offer more than just on/off ( ie interlock, thermal or overload protection)
3. Magnetic starters still require a on/off switch or control ( added $$$)
4. Magnetic starters have better contact area and materials than switches generally have. --switch designs are generally spring controlled and spring cycles affect contact force, whereas magnetic contacts are very forgiving to wear and use and have a more constant closing force
5. Magnetic starters can handle higher amp loads than normal switches as explained above and thus are seen on higher HP motors
6. Magnetic starters need only light duty interlock switches while manual swiches require interlocks to have high amp capacities ( $$ and size )

Better built/ designed machines use Magnetic starters.
Voltage has nothing to do with it , but Magnetic starters do need more room, so space needs or portability can preclude the use of Magnetic starters

Rich
Good summary. One other item worth noting: many magnetic starters have replaceable contacts, hence prolonging the value of the investment in the starter.

Long ago, I designed and built a control panel for a client's dual air compressor setup in his shop. Each compressor has a 7.5 HP three-phase motor driving a two-stage pump, cutting off at 160 PSI and coming on at 145 PSI. Although the compressors are independent machines, they are cycled by a single air pressure switch, working through two magnetic starters with thermal overloads. A relay is configured to self-latch and enable the control circuits when the "start" push button is pressed—power failure would obviously result in the relay dropping out, thus preventing unexpected startup when power has returned. The circuit is also arranged so that in the event one of the thermal overloads trips the self-latching relay will drop out, preventing an unexpected startup when the overload has been reset.

Businesses receiving three-phase power are not only billed for kilowatt-hour usage, they are subject to a "demand usage" charge. Hence it is desirable to limit the maximum instantaneous load applied to the line during a one hour period, as the demand usage charge is based upon peak loading as a function of time. As integral horsepower motors may draw upwards of 10 times their full-load nameplate amperage at startup, especially if starting a high-inertia load such as an air compressor, the startup of the two compressors is staggered, using a time delay module to control one of the starters.

When the air pressure switch closes on low pressure, one compressor starts, as does the time delay period. During the time delay, a solenoid-operated valve on each compressor is cycled to drain condensate from the tank. Upon expiration of the delay, the valves are closed and the second compressor starts. Although the aggregate starting current is no different than it would be if both compressors were simultaneously started, the peak current is limited to that of one motor, helping to limit the demand usage charge for each billing period.

This setup has been in continuous use an average of six days per week for some 30 years. After three decades of nearly non-stop use, the contacts in the starters were deteriorating and needed replacement. Replacement required less that 20 minutes of downtime, with the only tools required being a screwdriver and long-nose pliers. No wiring had to be undone in the process, which made it completely painless. New contacts cost about 30 dollars per starter, which is far more economical than replacing an entire starter at 400 dollars each.
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