High startup amps normal

J

jaddy

Plastic

Joined

Sep 8, 2013

Location

misssissippi

• Dec 23, 2017

• #1

I have a 25 hp rpc. Been using it for a while. Been using a 100 amp breaker to start it up. Today i put an amp clamp on one of the starting legs it peaked as high as 475amps only for about 5 sec once up and running no load runs 18 amps Is this expected? It has 2 starting capacitors? Do i need more or larger? Wish i could put a pony motor on but there is. No shaft sticking out to mount a pulley.

 

rons

Diamond

Joined

Mar 5, 2009

Location

California, USA

• Dec 23, 2017

• #2

You probably wouldn't be able type your post if there was a 475 amp current for 5 sec.

Your meter is waiting for some stability before figuring out what to display on the lcd panel.

 

S

Shawn Ghormley

Stainless

Joined

Jun 29, 2007

Location

Amarillo, TX

• Dec 23, 2017

• #3

Did you build the RPC, post some pics of it. Yes the amps will be high on start up.

 

J

jaddy

Plastic

Joined

Sep 8, 2013

Location

misssissippi

• Dec 23, 2017

• #4

rons said:

You probably wouldn't be able type your post if there was a 475 amp current for 5 sec.

Your meter is waiting for some stability before figuring out what to display on the lcd panel.

Click to expand...

5 sec maybe exaggerated that was the last number i seen (it was happening pretty fast) before it reached full rpm

 

J

jaddy

Plastic

Joined

Sep 8, 2013

Location

misssissippi

• Dec 23, 2017

• #5

its a used one don’t remember the brand

 

G

Guest

Guest

• Dec 23, 2017

• #6

High inrush current on a 3600 rpm electric motor is common. Just a measly 3/4 HP bench grinder will draw 15+ amps on start up and then run on 1 amp if unloaded once up to speed.

 

JST

Diamond

Joined

Jun 16, 2001

Location

St Louis

• Dec 23, 2017

• #7

25 HP draws about 100A. It would not be considered odd to draw 500 to 600A as a starting surge.

 

Jraef

Titanium

Joined

Aug 10, 2004

Location

San Francisco Bay Area

• Dec 23, 2017

• #8

JST said:

25 HP draws about 100A. It would not be considered odd to draw 500 to 600A as a starting surge.

Click to expand...

I'm thinking it's higher than that!
Single phase motor charts stop at 10HP, but 10HP 230V is 50A, so extrapolating to 25HP puts it at 125A, so Starting Current could be as high as 750A. 475A is looking pretty good now, isn't it?

And I agree on the meter issue, it's likely too slow of a sample rate to react to what's really taking place, so it's averaging and showing you that.

Don't worry about it though. The magnetic trips on a 100A residential circuit breaker are likely set to around 800-1000A, and the thermal trips would likely take up to about 15 seconds or so to trip at 475A.

 

D

DMF_TomB

Diamond

Joined

Dec 13, 2008

Location

Rochester, NY, USA

• Dec 24, 2017

• #9

jaddy said:

I have a 25 hp rpc. Been using it for a while. Been using a 100 amp breaker to start it up. Today i put an amp clamp on one of the starting legs it peaked as high as 475amps only for about 5 sec once up and running no load runs 18 amps Is this expected? It has 2 starting capacitors? Do i need more or larger? Wish i could put a pony motor on but there is. No shaft sticking out to mount a pulley.

Click to expand...

.
.
most motors pull about 400% of normal amps on starting up to normal rpm

 

9100

Diamond

Joined

Nov 1, 2004

Location

Webster Groves, MO

• Dec 24, 2017

• #10

Unless he has a series of switched capacitors to optimize the startup ramp, most of the time the third leg will be way off peak and you can expect the motor to struggle. Five seconds is a long time for a motor without additional flywheel to start up. I think this is a case for being happy that it starts at all.

Bill

 

rons

Diamond

Joined

Mar 5, 2009

Location

California, USA

• Dec 24, 2017

• #11

The initial jolt on your meter locked it out to prevent you from seeing a current reading during those first 5 seconds. A decaying exponential is not what cheap clamp-on meters are for.

 

JST

Diamond

Joined

Jun 16, 2001

Location

St Louis

• Dec 24, 2017

• #12

If you want to know what it really is, a meter with 'peak hold' is what you want.

But several hundred amps is to be expected unless you have a soft-start circuit. And even WITH one it will be high, just not AS high.

 

T

TonyStark

Aluminum

Joined

Dec 23, 2017

• Jan 3, 2018

• #13

I am having an issue similar to the OP with a rotary phase converter. I know their are high startup amps - but much do you you usually need to oversize your breaker / wiring for starting an RPC?

 

J

jaddy

Plastic

Joined

Sep 8, 2013

Location

misssissippi

• Jan 4, 2018

• #14

TonyStark said:

I am having an issue similar to the OP with a rotary phase converter. I know their are high startup amps - but much do you you usually need to oversize your breaker / wiring for starting an RPC?

Click to expand...

Mine has a 100amp breaker and works fine.

 

JST

Diamond

Joined

Jun 16, 2001

Location

St Louis

• Jan 4, 2018

• #15

Usually a time delay trip breaker set at 125% of nominal full load RPC amps. IF the loads are not powered through the same breaker