Help With Math, Calculating Total Amps

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Hello Applianceville, I have a question for you math experts.

Does anyone know the formula for calculating total AMPS? If I have three different units operating in parallel on the same circuit:

#1 6.5 Amps
#2 10.1 Amps
#3 2.1 Amps

What is the total AMP draw? I though you just add 6.5+10.1+2.1 = 18.7 AMPS but apparently it doesn't work that way, the number is something lower than 18.7. I know there is a formula to calculate this, does anyone know what it is?
 
You've got it right, just add them up.

If an appliance is rated for 10.1 amps that is the maximum current. If these units have motors, that is what they draw when they start. If they all start of once, you should blow a 15A breaker.

Ken D
 
Parallel resistances...

...are the things which have a formula to evaluate them, and are less than the sum of the individual resistances.
 
Well the Unimatic is rated at 8AMPS on the model # plate, but on spin start up its amperage peaks for 15 seconds or so at 24 AMPS according to my AMP meter. I think most appliance ratings are peak operating AMPS, not the momentary start-up surge.

are the things which have a formula to evaluate them, and are less than the sum of the individual resistances.

I've heard this now from a few different sources and I'm wondering if anyone knows the formula?
 
unimatic1140 wrote:
"Well the Unimatic is rated at 8AMPS on the model # plate, but on spin start up its amperage peaks for 15 seconds or so at 24 AMPS according to my AMP meter. I think most appliance ratings are peak operating AMPS, not the momentary start-up surge."

**********************

Your mention of peak operating amps makes sense, but 15 seconds is too long to be called a start-up surge, and 24 amps can't be right, it would take out a 15 amp breaker for sure.

Test your amp meter, a 60W light bulb should read about half an amp.

Ken D.
 
Excuse My 'P'

Thanks Jon, I need to do some calculations for the Super Unimatic.

Ken, its 24 AMPS for sure; my 1955 Unimatic is 25 AMPS. I have multiple meters all are within five years old and they all read the same, even my old Frigidaire Wattmeter reads 24 AMPS when it’s converted to AMPS. The early Frigidaire Tech-Talk manuals have a discussion about this 24 AMP draw and tell the installers to make absolutely sure the washer is on a 15AMP time-delay fuse. It won’t take out a 15AMP circuit unless it’s left in the start winding for too long, but 15 seconds is well within its tolerance.
 
I did not know a time delay fuse would hold that much current for 15 seconds. Interesting. I bet the lights dim when it goes into spin!

I see this is for the Super Unimatic. Make sure the relay you use for the motor can handle the that current.

Ken D.
 
Yes the lights do dim a bit when it first goes into spin. I have each Unimatic on its own circuit so they can all go into spin at once. I have 200amp service in the house so its not an issue.

We have some great electronic surplus stores here that sell all kinds of electric and electronic parts real cheap. I bought several 16 amp (operating rating) relays to use for the motor, heater and pumps. The relays on the Computer Interface kit are only rated to 3 amps but I figure any component that draws over 1 amp I will use a 16 amp operating relay in between the board and the washer component to keep any electrical pressure off of the computer interface.
 
most circuit breakers are what's known as "time delay" Induction motors frequently draw several times the amount of power during starting as they do when running. For instance, the refrigerator I have pulls 2200 watts instantaiously when starting, but only 280 during running (using a "watts up" meter) Watts can be converted to amperes by taking wattage X voltage = amperes. The time delay fuse keeps the circuit from tripping during these surges. Start-up Surges however are rarely noted on the spec plate however. What you may be seeing as the max amperege draw is how much the machine would take with all it's features on. For instance, when the heater AND the motor in a dishwasher are both running at the same time.

Many times, the amerage draw is done from "lock up" power. In other words, if you were to get something in a blender that keeps the motor from turning, it will pull much more power than when the motor is freewheeling. This will be the maximum rating.

Maximum current ratings also take into account the motor type. There are two motors used typically in appliances. Induction motors are big, quiet, torquey and slow. These are usually found in most large appliances like washers, fans and other appliances that have continuous duty. Brushed motors are the exact opposite they are small, fast, noisey, and not very torquey. They are commonly found in power tools, blenders, vacuums, and other small locations. Frequently, they are geared down to the applicaion they are being used in. These motors do not require starting devices like induction motors do either. They will however pull much more power, and sometimes even burn out if locked up!
 
With speakers for example, two sets fo 8 ohm speaker will be a total resistance of only 4 ohms.

8Ohms over two paths = 4 Ohms. (8/4 =2 ) meaning the receiver will be passing more currnet than it wants to and the electronics can blow.

On a regular line-voltage circuit, more amps will flow with parallel loads becasue certainly the resistance to curren flow drops with each additional load. However the voltage in parallel cicruits stays the same (110v) to wach load; and the aperage IS the sum of the amps.

In series circuits the VOLTAGE is divided by the number of reistances/loads. For example to use a 600V DC subway 3rd rail for task lighting on the tracks they simply put five 120 volt light bulbs in series; each one gets 120 volts.

OHM's Law (resistance) is
E= I*R
Where E is electomotive force (volts)
I is Ampreage (why I dont remeber) and
R is resistance, defined in ohms.

Once you know two the other can be figured out algebraically.
A 5,200 watt heating elemment on 220v is 23.63 amps

220v/23.63a =9.31 ohms, therefore

E/R = 120/9.31 the samee element on 120 volts is 12.88 amps *120 volts= 1,547 watts!

(remember the GE dryers in the 60's that had an element that could be used on either voltage?).

ALso think of a regualar house circuit. The more items plugged into ONE circuit the greater the amps. They are after all in parallel!
 
Subway lighting

I've seen those 5-lamp arrays inside the older lower Manhattain tunnels in NYC. I never thought about about them powering them off the third rail, but that makes perfect sense! The New York subway is pretty fascinating. One of the most fascinating things was how they used rotary converters to change the 25hz AC current into DC to power the trains in the days before solid-state rectifiers.
 
Electrifying subject matter....

ok, do tell...

What 25hz AC?

I guess that is why old incandescent subway bulbs/lamps would "strobe" noticably.

So what is a rotary converter?

Thought to ponder:
Can't I have ONE post/thread that doesn't turn to AC/DC?
LOL

Come to think of it I beleive the AMPS are the AMPS just add em up. But the OHMS (resistance) would be by formula for parallel loads. Does this make ANY sense?

In the speaker example, I meant to say:
2 sets of 8 Ohm speakers connected in parallel.
 
Not to derail this thread, but wasn't there a topic a while back about a Watts-Up meter? I tried a search last night but came up empty handed... I've been thinking it'd be handy to perform some real-world tests. Anyone out there have one??

Cory
 
T

tolivac

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25Hz power was used by railways to "combine" the advantages of DC and AC power.the 25Hz power could be transmitted and transformed by transformers like regular AC power.The frequency was low enough so electric locomotive DC motors could operate off it WITHOUT rectification.Modern AC loco motors weren/t available then-most electric locomotive designed involved DC motors.To control the speed on the electric 25Hz locomotive transformer tap changers were used-IN GG-1 locomotive the transformer had 25 taps-25 speeds.The top 9 speeds were locked out to prevent blowing the 25Hz feeders and overhead lines if two or more GG-1 were coupled together.They were rated at about 4700 HP each.
On Universal and Induction motors-universal(brush type) could produce high torque-but due to their small frame and armature windings-for only a short period of time.For the applications favoring universal motors-they pack a lot of power in a small and light size.Thats a good reason why "contractors table" saws have geared down universal motors-essentially a "skilsaw" gear motor mounted to the saw trunnions.If you put a 1.5Hp induction motor on the stand type contractors saw-it would be too heavy to lift-and too difficult to run from a generator or temporary power feed.That geared down universal motor in the saw could generate up to 3Hp.You just wouldn't want to rip Oak planks on it all day-that motor would overheat-the induction one would not.To put it another way-a 1.5Hp universal motor is lighter and smaller than the induction one and draws about half the amps.And since the universal motor has no start windings or caps,less start power.
A "rotary converter" could be considered a mechanical rectifier-a device to convert AC to DC-yes was most commonly used before the invention of mercury,seleenium,and silicone rectifiers.It could be considered an induction motor turning a DC generator.Pipe organs used them until present for the 12V relay and switching systems in the organs-the DC generator was often turned by the organ blower motor.Now most pipe organs have solid state rectifier systems.These are more compatible with the solid state logic and switching (digital) installed in organs today.Some very early broadcast transmitters had MANY Motor-gen converters in them-for tube filaments,Bias,Low voltage DC plate,and high voltage DC plate.Haven't worked on any of those-but read manuals on them just in case.Those transmitters had interlock systems in them so you couldn't activate the LV and HV plate supplies unless the filament and bias motor gens were on and generating voltage.Oh-Oh-got to sign off a transmitter--
 
Rotary converters

"A "rotary converter" could be considered a mechanical rectifier-a device to convert AC to DC-yes was most commonly used before the invention of mercury,seleenium,and silicone rectifiers.It could be considered an induction motor turning a DC generator."

Actually, that was a later development, whereas the AC motor and DC generator were combined electrically into one unit. Older machines were actually "M-G" (motor generator) sets and consisted of an AC motor turning a DC generator via a shaft and were totaly separate electrically.

And yes, simply totalling up the amps is the way to get total amp draw. The plate rating doesn't take into account the starting surge.
 

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