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Reasonable inrush current 1/8hp coolant motor

M

madmachinst

Stainless
Joined
Jan 15, 2007
Location
Central FL
welding contacts and blowing fuses after having upgraded fuses and 1/2 ed teh amperage by rewiring motor to 240 from 120. Engineer from Finder told me expect 10-15 inrush current on start up. I am thinking like "NO WAY!" is that reasonable? Do I need to break my head and figure out how to shove a 20 amp DPDT relay base in there or is there something else wrong? Motor says 1.2 amps @ 240 and I have an 8 amp relay on there. 20 amp relay base made for 4 poles so imagine how much wider that thing is.
 
I can't imagine having starting issues (inrush) on a 1/8HP motor.

I doubt the inrush is 10-15 X. Probably more like 7-9X.

But most fuses for motors will be slow blow, which means they can 'ignore' inrush. Do you have fast blow fuses in there? That might be why.
 
If it is a centrifugal pump that is always wet I can see 10A to start. What I don't get is welding of 8A contacts. I've always been fine using a contactor rated 1.5 - 2X the FLA of the motor. Never gone so far as to rate the thing at inrush. But... I'm not an electrician or an EE.
 
well the contacts didn't weld I shaked that little module around and knocked it on my desk and now it is moving nicely. coolant pump is dry right now, tank is empty. Now I do have 6 amp 10X38 500V a.c. 100ka fuses ( beats me if that is fast or slow blow), those are the same fuses on my machine servo though.
 
I'll just get some long wires with clips on them, disconnect the old wires, hence bypassing the fuse holders. put am amprobe on that and run it momentarily to get a reading.
 
I was too scared to jump and bypass the fuses. went to ace and picked up 15 amp slow blow fuses. Boom big spark visible with cabinet closed. Then when called buddy and inspecting cabinet I come across Varistors v150la20a. Every time after I cleaned that motor and got it rotating nicely, when trying to start I blew a fuse. I only blew one fuse???? Out of the 2 in there one on each leg. Now the varistor is toasted. This might have been my problem? Varistor gets over voltaged with new voltage ( rewired pump to take advantage of higher voltage and lower amperage)and just send a surge to that one fuse that blows? Here are pics
IMG_20210701_171649229[1].jpg
Mind you that is the inside of the receptacle where the coolant pump was plugged in. Originally 120VAC. Now I got it to 240VAC. So Now the varistor, should I look for a correct one and how should I wire them? Get the same two original varistors and one to each live leg and the other legs of the varistors together going to ground lug? Bottom line, was that my whole problem?
 
These varistors should have been wired to ground. Get two news ones, of the same value, and wire them to ground not differentially across the 240 power. If they are for surge arresting you want the surge to be dumped to ground. So make sure the ground connection is good all the way to the power source.
 
yep that is what I said. I went to try it out without varistor no problem. Yep split the amps dbl the volts and reconfig the varistors. Now where do I get those?
 
ignator said:
These varistors should have been wired to ground. Get two news ones, of the same value, and wire them to ground not differentially across the 240 power. If they are for surge arresting you want the surge to be dumped to ground. So make sure the ground connection is good all the way to the power source.
Click to expand...

Not correct. You wire them as the design requires.

Not sure I followed everything correctly. But what I think I read is that you converted the motor from 120 to 240. You had 150 volt varistors so unless you changed them to 300 volt varistors you would expect them to blow when you energized the circuit, which they did - never did catch how they are wired in the circuit. I would expect the varistor to be across the motor leads - this way when the contact opens and the motor winding magnetic field collapses, the varistor shunts the energy caused by the collapsing magnetic field which keeps the voltage down and prevents damaging contact arcing and excessive voltage across the motor windings. The varistors start conducting around their rated voltage but otherwise are like an open circuit.

Varistors are rated in voltage and energy (joules). Looks like those are in parallel???? I have never done that, but maybe they did that to increase the energy capability.

I would just put a 300 volt varistor with a larger rating or put 2 in with the same rating. Not sure off the top of my head how the motor voltage affects the energy. To be on the safe side you could just go bigger on the energy.

In big 3 phase motors you put lighting arresters (varistors) across the leads and capacitors from each leg to ground. The lightning arresters clamp the peak voltages and the capacitors knocks the surge front down (increases the dv/dt of the surge wavefront).
 
so, before I put new varistors in there to dump energy to ground. I will check my ground point on the receptacle pictured in post #7 all the way to the main ground in the machine panel. I want to see very little resistance in there. Then I take 2 varistors and parallel their output leg ( Is one leg output and one input?), hook that up to ground, and the other legs hook them up to the live legs in the receptacle and call that a day. Thanks guys for those links.
 
Yup. Do not put the varistor to ground! Go across the 240 and make sure its a 300 volt varistor or varistors if you use 2 in parallel. They need to be on the motor side of the contacts.
 
Um, recheck the wiring from when you rewired 240->120?
Check ohms from all wires to the case?
The rule of thumb I've always used is 3X FLA for start/inrush current.
When something is blowing fuses it usually isn't a problem with the fuse.
 
Almost certainly the 150 V varistors were line to neutral, unless the original installer was a moron. So likely no issue of drastically over-volting them.

You can argue about whether to use one higher voltage across the 240V, or two lower voltage ones each across 120V. Both systems work, one keeps the voltage to ground under control, the other does not.

However, the real question is whether they were rated for the energy. If not, that would explain them blowing. And if somehow they got wired across the 240 after all, that would explain it as well.

Inrush current: Most smaller motors are fairly high resistance, which will limit the inrush. But not necessarily to very low levels.

However, if you use a contactor or switch that is HP rated for the motor, it will handle the inrush current, there is no need whatsoever to over-spec the contactor to handle inrush. That is the good thing about the HP rating system, it "just works", and no need for complex engineering calculations to find the smallest usable contactor.
 
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