Start Capacitors & Run Capacitors for Electric Motors - Differences Explained by TEMCo

so this video I'm gonna dress the

difference between a motor start

capacitor and a motor run capacitor now

there's a couple of fundamental

differences between these two and it has

to do mostly with their construction and

their ability to handle current for

extended periods of time now a motor run

capacitor will be something like this

here and it's engaged in the circuit all

the time these are often used for phase

delay or power factor correction you'll

see these a lot in HVAC units and larger

single-phase motors they're an oil

cooled they're generally in a metal case

now they're filled with oil they tend to

be fairly low capacitance rating so an

example here I've got 15 micro farad's

this one here is 5 micro farad's fairly

small capacitance or values and that's

all that's really necessary often for

the for the run operation on a motor now

a start capacitor on the other hand

these tend to be a bit larger both

physically although always but they're

also larger in capacitance values so for

example this guy here I've got 432 516

micro farad this is something to note on

these capacitors start capacitor to have

a range they're not dead on a lot of the

times that's because the phase shift

produced by running AC current through a

start capacitor isn't as precise or the

precision isn't necessary to get a motor

turning it just needs to be able to

generate a fairly large shift in order

for the motor to start turning

internally on these you have pretty much

the same construction that's two

conductive plates with an insulating

layer in between the two and it's

wrapped too generally in a coil around

and around and around so the two plates

aren't touching each other electrically

anyway and you'll generate a charge on

one and it it transfers to the other in

the case of AC or in DC it'll charge and

hold like a battery now these

Pasteur's motor start and run capacitors

are AC rated capacitors and when you

look on the label of these you'll see

the voltage rating here so for example

this one says here 250 volts ac a

littler one here 220 to 250 volts and on

the run caps here I've got 370 to 440

when you're sizing up capacitors if

you're not replacing one already if

you're just sizing one to start with you

want to make sure that the voltage

rating of your capacitor is about one

and a half times your line rated voltage

this is because the voltage ratings on

these is not RMS but rather peak to peak

voltage on a 240 volt system run

capacitors you'll see 370 to 440 volts

480 volt systems you'll often see 600

volt rated capacitors used on those and

it's just to account for the extra peak

to peak current or I mean voltage levels

that that are present in such a thing

now the other big difference between

these two is duty cycle run motor run

capacitors are a hundred percent duty

cycle they can remain in the circuit

indefinitely charged up so in this case

if I have my I have my box here 240

volts AC and I can connect this right

across this capacitor here like that and

I can switch this on if we measure it

here we can put our voltmeter on AC and

see that there is indeed voltage present

on the terminals right there get 244

point 7 volts and this one's fine too

just leave here like I said one hundred

percent duty cycle you can leave it on

and just in there if you had a motor

running with this or your air

conditioner pump or something along

those lines

this kind of capacitor it just is left

in the circuit

you can use motor run capacitors as

spark capacitors however you cannot use

start capacitors as motor run capacitors

now I'm going to show you why here in

just a second

the disadvantage if you try to use motor

run capacitors that's still live so

don't want to touch it

the disadvantage to using motor run

capacitors as they start up in the

circuit is generally for the startup of

the motor to be able to develop torque

you need a very high capacitance value

to get 150 or 400 micro farad's out of

15 micro farad capacitors you'd need an

entire array of these if you've got the

space for that then that would work out

fine for you now and this is still

energized like I said as you can see

there's there's no effect of having this

in the circuit continuously so let me go

ahead and turn that off and I'm going to

bleed down the capacitor with a resistor

real quick just to make sure that

there's no voltage left on it I don't

want to get shocked there we go

set the meter to DC because when you

turn these capacitors off if you watch

my other video on start caps about

bleeder is that really done resistors

you'll see that when you turn these off

if you turn them off during a period in

the AC waveform other than the zero

crossing these can actually store a DC

voltage up to lethal levels I'm gonna

show you the start capacitor and when we

put a start capacitor into a

current-carrying circuit here it's good

for a couple of seconds now a lot of

times the failure of these will be two -

an overloaded motor or a motor that

doesn't start up all the way or if you

have the centrifugal cutout switch that

doesn't cut out in time and it holds the

capacitor in the circuit too long well

let's show you what happens here safety



when you leave these things engaged for

more than a couple of seconds here

they don't fare too well now if you do

open up your HVAC unit or your motor and

you find you find something like this

inside go ahead and give a Temko a call

we carry a full line of replacement

capacitors it's area code five one zero

four zero three four zero six one and

we'd be happy to hook you up with some

brand new start or run capacitors for

your application to get you back going


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