Questions about switch's and their ratings

Thread Starter

jonnydolt

Joined Oct 23, 2015
145
Q1.. Why is it that switch's are rated by their ith amps and not by the switch contactors amp capacity?
Q2..When choosing a switch for induction motors should we choose by AC3 or AC23 amp rating?
Q3..Why do some spec sheets only give a Kw rating for AC3 ?
Q4..Why do specs give a Kw rating when the switch's AC3 amp rating is well under what that size motor amp draw is?
Q5..What does it mean when the switch has a rating like this 250v 16(12)a ?
Q6..On spec sheets under Power P (KW) AC3 has this under 240v .. 3/2.2 and under 440v ... 5.5/3 what do these numbers represent?
Q7..if a contactor and thermal overload relay is used can a switch of a lesser ac3 amperage be used?IE:-Does the contactor and relay reduce the amp draw on a switch? and if so what is the formula?
Q7... example
  • Dual Voltage: 110V/220V; Single Phase
  • Rated Current: 2 HP / 35 Amps at 120V; 3 HP / 20 Amps at 230V .....Why can this switch only run a 3hp (2.2kw) motor if it is rated at 20 amps at 230v if a 4hp (3kw) motor has a max draw of 18.2 amps at 230v? Why can it not be used on a 4hp(3kw) motor?
Q8.. In the example below AC-21A and AC-22A for YMW-26-25 is 25 Amps and AC3 3P/1P Kw is 4/3 How can I discern what the AC-3 amp rating is at 3kw at 220v ?
upload_2019-3-24_16-59-58.png
Q9... In the example above if I wanted to run a 1 phase 240 vac 4kw 18.2 amp induction motor I would need to use YMW-26-32?
 
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shortbus

Joined Sep 30, 2009
10,045
You need to chose by what it is your switching. Motors of that size are best controlled by contactors, not just a switch.

The specs on the page you show, shows what the switch is capable of at the voltage and amperage shown. It varies because of the "flash over" point from the induction/BEMF of the motor when switched off. If you aren't sure chose one for a higher voltage/current than you actually need, as they are usually pretty close in price.

Look for something called, "Article 430 of the National Electrical Code" in Google.
 

Thread Starter

jonnydolt

Joined Oct 23, 2015
145
You need to chose by what it is your switching. Motors of that size are best controlled by contactors, not just a switch.

The specs on the page you show, shows what the switch is capable of at the voltage and amperage shown. It varies because of the "flash over" point from the induction/BEMF of the motor when switched off. If you aren't sure chose one for a higher voltage/current than you actually need, as they are usually pretty close in price.

Look for something called, "Article 430 of the National Electrical Code" in Google.
I have all the contactors and thermal overloads.I wish to learn more about switch's and also what is printed on the actual switch.I have found that spec sheets for a lot of them are unavailable.Also finding switch's that have a high enough amp rating for ac is not an easy task,Choices are very limited.Most are rated by the ith value and not the switch contacts amp value
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
I am still confused as you appear to mixing 3phase rotary switches with operating Contactors, which are animals of a different colour.
The chart you show is for the rotary switch?
Do you intend to use both and if so why?
Max.
 
The switch construction determines if it's suitable for AC or DC. AC has a zero cross events, thus it can break at zero current or voltage easily. DC contacts can weld together easily.

There is a minimum current called the wetting current for a relay contact. Like don;t expect to switch 10 mA with a contact rated for 100 A reliably.

There is a switching current rating as well. Some high voltage relays can only be switched with no signal.

That said, you will have AC switches with a HP rating, because that's the number you need to select from.

Like generators, it's not the Wattage, but the V*A rating that matters. The current in an inductor cannot change instantaneously, so for an instant in time, the generator sees the DC winding resistance. If the V*A rating of the generator isn't high enough the motor won't start.

When the motor is running, the generator sees the AC impedance.

You need to look at definite purpose contractors. You have them for tungsten (lighting loads) and motors.
With 3 phase motors you need to look at the possibility of what happens when the motor looses a phase.
When that motor is integral to a sealed compressor that's part of a $40,000 USD cryogenic refrigeration system. The costs of replacement, down time and the special refrigerants make 3 phase protection mandatory.
 

Thread Starter

jonnydolt

Joined Oct 23, 2015
145
I am still confused as you appear to mixing 3phase rotary switches with operating Contactors, which are animals of a different colour.
The chart you show is for the rotary switch?
Do you intend to use both and if so why?
Max.
All the above questions I wrote are all to help me learn more, and while I will undoubtedly use what I learn in this thread,in regards to my other thread,this thread is a thread in it's own entirety and only remotely related to the previous thread.Ya Cotton? lol :) ...I need switches to turn the motors on and off rotary or push buttons doesn't matter.From my understanding I need a contactor,a thermal relay and from there to a switch (or 2) to turn it on and off + an estop...where I think things are getting confused is that I have 5 motors all designated for there own projects IE:- a table saw (1p 2.2kw motor), a 2x 72" belt grinder (3 phase 2.2 kw motor and a XSY-AT1 vfd),a 17 cfm compressor (1p 3kw motor),an Oscillating spindle sander (1p 2.2kw motor) and the fifth motor (1P 2.2kw) not sure yet it arrived damaged so they let me keep it and replaced it,I just purchased a couple of capacitors,a few odds and ends and a project box to fix it
 
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MaxHeadRoom

Joined Jul 18, 2013
28,617
That is the point I have trying to make, if using a contactor you do not need a heavy duty rotary switch!
You have to decide how you want to end up switching it, the advantage of the contactor is low current and/or voltage on the P.B. and O.L protection on the contactor.
Also include in a E-Stop string.
WHich is it to be?
Max.
 
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Thread Starter

jonnydolt

Joined Oct 23, 2015
145
That is the point I have trying to make, if using a contactor you do not need a heavy duty rotary switch!
You have to decide how you want to end up switching it, the advantage of the contactor is low current and/or voltage on the P.B. and O.L protection on the contactor.
Also include in a E-Stop string.
WHich is it to be?
Max.
i did not know that a contactor changes amps and voltage ,nothing I have read has stated that.
 

Thread Starter

jonnydolt

Joined Oct 23, 2015
145
I am having trouble understanding what you mean by "You have to decide how you want to end up switching it" so here goes ... 240v gpo then plug and lead to the contactor and thermal relay,etc then wiring to an on switch, an off switch and an estop switch.The trouble I am having is finding AC switch's that can carry the amps needed to run the motor,The three phase motor is easy it only draws 5 amps but 1 motor draws 18.2 amps and anything that sates that it will switch 20 amps is rated by the ith amp value not the contact value which is far less on all the switches I have seen.Hence all the questions
 

Thread Starter

jonnydolt

Joined Oct 23, 2015
145
This Wikipedia article is a good starting point as to the AC-n switch applications.

https://en.wikipedia.org/wiki/Utilization_categories

A switch marked with a rating 250V 16(12)A, indicates that it is rated for a circuit voltage of up to 250V switching up to 16A resistive load and 12A inductive load.
Cool so 16A like a heater and 12A like a squirrel cage motor lol I read that previously must have missed that bit will go back and reread many thanks ...Where it gets difficult is that a 16(12)A switch is only rated for a 2hp motor at least all that I have seen are
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
I am having trouble understanding what you mean by "You have to decide how you want to end up switching it" so here goes ... 240v gpo then plug and lead to the contactor and thermal relay,etc then wiring to an on switch, an off switch and an estop switch.The trouble I am having is finding AC switch's that can carry the amps needed to run the motor,The three phase motor is easy it only draws 5 amps but 1 motor draws 18.2 amps and anything that sates that it will switch 20 amps is rated by the ith amp value not the contact value which is far less on all the switches I have seen.Hence all the questions

So it would seem you are using the rotary switch as a initial disconnect, these are not made for that function.
There are enclosure disconnects that are made for this.
So after any disconnect you would have suitable fusing, followed by a contactor that used low current control circuitry P.B. operated with a E-stop contact in the coil circuit.
The Schnieder PDF shows plenty of motor control circuits, ignore the O/L's on the R.H. side of the coil, they should be on the other side of the coil.
Max.
 

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jonnydolt

Joined Oct 23, 2015
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