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Old 03-09-2016, 01:15 PM   #1
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Default 3 phase to single phase

I am working with an air compressor motor. Auto mechanic shop. The owner installed a new Baldor L1501T motor 2 years ago. Single phase, 230v, 7.5hp. I was called because it tripped the 50 amp breaker. One of the capacitors (run, black electrolytic) blew the end out. I ordered all new capacitors. Same spec as original. 1 start and 2 run. Installed them and everything seemed fine. A little over 2 weeks later, I was called again. The capacitors did not blow out this time, specs on one were way out. These capacitors are made in China. New out of the box, the readings taken with a capacitor checker did not quite match up to the specs written on the capacitor cases. I installed them anyway with owner approval. I have got new ones coming. There are 3 white leads from the motor case to capacitor bank, all white, no labels. I don't know how to check those leads.
This is a pretty old Quincy compressor. It originally had a 3 phase 7.5 hp motor. 3 phase power not available at this shop. My question is; can a 3 phase 7.5 hp motor be replaced with a single phase 7.5 hp motor? I don't know proper terminology here. Does a single phase motor have to be upsized to provide enough oomph? The mechanics say the compressor parts are working fine. Thanks
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Old 03-09-2016, 01:30 PM   #2
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The horsepower rating is a measure of how much power it puts out. Generally, a 7.5HP is a 7.5HP, regardless of how many phases. You shouldn't need to upsize it.

Someone wiser than me may know more about torque curves, etc, and might be able to say whether one is better than the other, but it's been done many, many times on compressors and I've never heard talk of any problems like that.

Can't help you with the white leads....
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Old 03-09-2016, 01:56 PM   #3
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As far as I know the main reason you go with 3 phase over single phase is efficiency, 3 phase is more efficient. For an air compressor I'd expect the single phase motor to consume more power and run hotter than its three phase equivalent.

I wish I could be more help but I don't work with capacitor start/run single phase motors.
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Old 03-09-2016, 02:02 PM   #4
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I'm a little confused in that it seems you are working on a 3-phase motor in a shop where 3-phase is not available.

What did I misunderstand?

Edit: OK, looks like the owner changed the motor and your just trying to confirm what he did is OK. I agree with the others that a 7.5HP motor does the same work whether it's 3-phase or single-phase.
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Old 03-09-2016, 02:03 PM   #5
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Yes. A 3 phase motor can be replaced with the equivelant single phase motor as long as Hp ratings are the same. HP is rated at the shaft. Your efficiency and PF will be way worse with a single phase motor though.
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Old 03-09-2016, 02:04 PM   #6
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Quote:
Originally Posted by hardworkingstiff View Post
I'm a little confused in that it seems you are working on a 3-phase motor in a shop where 3-phase is not available.

What did I misunderstand?
It was a three phase motor and they replaced it with some sort of capacitor run/start single phase motor. Now it's tripping a 50 amp breaker, which is odd to me, and killing capacitors.

I derped and looked at 3 phase motors haha.
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Last edited by NC Plc; 03-09-2016 at 02:10 PM.
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Old 03-09-2016, 02:05 PM   #7
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You might want to look into the ambient temperature.

What do the amps look like as the pressure is building in relation to the nameplate rating?
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Old 03-09-2016, 02:07 PM   #8
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A 7.5 HP 240v motor is rated for an FLA of 40A. The breaker protecting it should be 100A by CEC
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Old 03-09-2016, 02:08 PM   #9
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A 7.5 HP 240v motor is rated for an FLA of 40A. The breaker protecting it should be 100A by CEC
I looked at the wrong table.
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Old 03-09-2016, 02:10 PM   #10
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I think I know what the problem with the capacitors is, "made in China". Probably full of PCB's too.
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Old 03-09-2016, 02:11 PM   #11
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Isn't a 7.5 HP motor at that voltage gonna pull 22 amps? Why would it trip the breaker while running?
Table 430.248 says a single-phase 7.5HP motor at 230V should have a 40A current rating.

I wonder what the voltage is when the motor is running?

What's the amperage?

Is it possible the motor is not cutting out the start capacitor when it should? What effect will that have on the motor?
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Old 03-09-2016, 02:11 PM   #12
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You might want to look into the ambient temperature.

What do the amps look like as the pressure is building in relation to the nameplate rating?
Ambient temp caused one of my air compressors to keep tripping its own thermal breakers, had to install a thermostat / exhaust fan / etc just for it.
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Old 03-09-2016, 02:12 PM   #13
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Table 430.248 says a single-phase 7.5HP motor at 230V should have a 40A current rating.
Yea I'm a tard, I went to engineering toolbox instead of pulling out my NEC and looked at a 3 phase table without thinking.
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Old 03-09-2016, 02:15 PM   #14
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Ambient temp caused one of my air compressors to keep tripping its own thermal breakers, had to install a thermostat / exhaust fan / etc just for it.
Also, how often is this compressor cycling on/off? Is the compressor large enough for the shop usage of compressed air?

One other thing you might try, lower the cutout pressure if the shop can deal with it, that might reduce some of the strain on the motor.
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Old 03-09-2016, 02:16 PM   #15
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Yea I'm a tard, I went to engineering toolbox instead of pulling out my NEC and looked at a 3 phase table without thinking.
Not a tard, just moving fast. It's not like we all haven't done a similar thing.
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Old 03-09-2016, 02:17 PM   #16
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Also, how often is this compressor cycling on/off? Is the compressor large enough for the shop usage of compressed air?

One other thing you might try, lower the cutout pressure if the shop can deal with it, that might reduce some of the strain on the motor.
The air compressor sits at 170ish pressure, I put a thing-a-mah-jigger on it that reduced the air line pressure to 80. That's about 30 psi higher than we need for the equipment to operate.

The problem was while the air compressor was rated for continuous duty when it was running 24/7 it was overheating in the storage building.

The causes of it running 24/7, when it is already larger than the project calls for cfm wise, was a faulty pressure switch and a leak in the air line I have to fix next week.

ALSO, for some ****ing reason it didn't come with a hobs meter on it, and I hate when air compressors do not come with one of those included, so I have to install one tomorrow. They let me know if the compressor is running more than it should so I can sniff out problems.
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Old 03-09-2016, 02:26 PM   #17
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I tried to find the motor online and Baldor doesn't seem to recognize L1501T
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Old 03-09-2016, 02:32 PM   #18
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I tried to find the motor online and Baldor doesn't seem to recognize L1501T
Same, nothing came up for me.
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Old 03-09-2016, 03:41 PM   #19
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What frame size and is that motor run 1800 or 3600 ?? Sometime can end up a spl ( specail )rating on them..

I kniw someone mention 50 amp breaker.. That is too small for thst motor it should be at least 80 amps that will give little more current to start up..

Did that motor have thermal protection ??
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Old 03-09-2016, 06:09 PM   #20
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Baldor 1509T = 7.5HP, 3450 RPM, 213T frame, cap start/cap run, no O/L, 30.0 amps.

Baldor 1510T = 7.5HP, 1725 RPM, 251T frame, cap start/cap run, no O/L, 33.0 amps.

When a capacitor blows, it's usually the start one, and very likely because of too many starts per hour or even more likely too long of a starting time. If the voltage is low or the load is high, the motor will cycle in and out of start. This is really hard on start caps, and sometimes happens when a 3Ø motor is replaced with a 1Ø. 1Ø motors do not have as much pull-up torque as 3Ø ones do.

Check the unloader, if it starts under pressure, a 1Ø model will have a hard time where a 3Ø will be ok.

How long does it take to come up to speed? Does it come part way up, then sort of cycle a bit then the rest of the way up?

Generally speaking, caps are designed for not more than 20 starts per hour and not more than 3 seconds per start.

If known, how long did the capacitor sit before it was installed. If more than a couple of years, it'll need to be reformed before full voltage is applied or it'll blow up after a while. Maybe right away.......
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