Eddie702
Licensed Electrician
- Location
- Western Massachusetts
- Occupation
- Electrician
a 3hp single phase motor will not start the same compressor as easily as a three phase motor, especially a 480 volt one.
motor circuit
- Thread starter sportzcoach
- Start date
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TwoBlocked
Senior Member
- Location
- Bradford County, PA
- Occupation
- Industrial Electrician
Feeder and branch wire gauge should be checked, too. If the breaker wasn't upsized, the wiring might not have been either.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Common misconception. Torque is torque is torque. HP is torque at speed. It’s no different.
ptonsparky
Tom
- Occupation
- EC - retired
The breaker up size does not require an increase in BC ampacity.
Provided the circuit breaker is not used for OL protection also.
ptonsparky
Tom
- Occupation
- EC - retired
Most single phase motors of that size have OL protection, at least for the ones I've seen.
You do bring a point that has been touched on. Most of these motors will start and run on a breaker sized at the 125% IF there is not a mechanical problem preventing it.
Three seconds at LRA means there is a problem.
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While that’s true, different types of motors of the same HP can have drastically different starting torques.
kwired
Electron manager
- Location
- NE Nebraska
did they change the supply voltage to 240?
kwired
Electron manager
- Location
- NE Nebraska
250% still gives you max SCGFP of 15.
kwired
Electron manager
- Location
- NE Nebraska
most general purpose induction motors particularly in ~1800 RPM version will have similar enough characteristics they are usually interchangeable though. Main thing is if it has enough torque to start accelerating the load or not. Once it starts accelerating the single phase version possibly takes longer to reach full speed but it usually can do it.
SSDriver
Senior Member
- Location
- California
- Occupation
- Electrician
If almost everything else is the same the starting Torque should be the same. The main difference you will notice between a single phase motor and a three phase is the efficiency and power factor are Usually much better on a three phase motor.
kwired
Electron manager
- Location
- NE Nebraska
I don't know about that. I often see single phase motors with high power factor marked on nameplate compared to a three phase motor of similar design.
With the specific example in the OP, I agree. But there are a lot of motor types with either very high or very low starting torques.
DC series-wound vs shaded pole, are extreme examples.
The post I was replying to didn’t limit the motor type.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
230224-1047 EST
The University of Michigan electrical engineering professor, Benjamin F. Bailey, that was coauthor of the book "Alternating-Current Machinery", was contacted by Detroit Edison in the 1920s, and asked the question of whether it was possible to design a single phase motor to serve as a refrigeration compressor power source. He thought about this for a moment, and said yes.
The motor design that resulted from this was the capacitor run single phase motor. The major problem in the development of this motor was the creation of a suitable capacitor. In its basic form the capacitor run motor has no switching components, and thus allows the design of a motor that could be hermetically sealed for long life with a compressor. I have a freezer with this design that has run for over 50 years with no maintenance. It was the early 1930s, and possibly slightly earlier, that the totally sealed motor and compressor came into existence.
I never met Bailey or Gault, but I did use their book. J. G. Tarboux was my teacher for both DC and AC machinery. Tarboux was an outstanding teacher. I did know Gault's wife when she became a secretary at the Electronic Defense Group where I worked part time as I went to school.
A straight single phase induction motor is basically an inductor and resistance in series.
A capacitor run motor is the plain single phase motor with a parallel network of a series, capacitor, inductor, and resistance. This parallel path shifts the net input current more toward neutral. Thus, the net effect of a capacitor run induction motor is a better power factor than a straight inductive induction motor.
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The University of Michigan electrical engineering professor, Benjamin F. Bailey, that was coauthor of the book "Alternating-Current Machinery", was contacted by Detroit Edison in the 1920s, and asked the question of whether it was possible to design a single phase motor to serve as a refrigeration compressor power source. He thought about this for a moment, and said yes.
The motor design that resulted from this was the capacitor run single phase motor. The major problem in the development of this motor was the creation of a suitable capacitor. In its basic form the capacitor run motor has no switching components, and thus allows the design of a motor that could be hermetically sealed for long life with a compressor. I have a freezer with this design that has run for over 50 years with no maintenance. It was the early 1930s, and possibly slightly earlier, that the totally sealed motor and compressor came into existence.
I never met Bailey or Gault, but I did use their book. J. G. Tarboux was my teacher for both DC and AC machinery. Tarboux was an outstanding teacher. I did know Gault's wife when she became a secretary at the Electronic Defense Group where I worked part time as I went to school.
A straight single phase induction motor is basically an inductor and resistance in series.
A capacitor run motor is the plain single phase motor with a parallel network of a series, capacitor, inductor, and resistance. This parallel path shifts the net input current more toward neutral. Thus, the net effect of a capacitor run induction motor is a better power factor than a straight inductive induction motor.
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Note the OP said at start. Not much torque at zero speed.
SSDriver
Senior Member
- Location
- California
- Occupation
- Electrician
Just an observation on my part if you look at most single phase motors. They're in the low 80% efficiency and PF. Most all newer 3 PH motors are around 90-92%
kwired
Electron manager
- Location
- NE Nebraska
Well there was mention of the item originally having a three phase motor on it, which majority of those are induction motors, particularly in the under 100 HP ranges.
kwired
Electron manager
- Location
- NE Nebraska
Well don't you need torque to break free from standstill? Single phase motor with open aux/start winding or three phase motor with "lost phase" would have no torque if rotor is not turning at all, if rotor is turning it may continue to turn unless the connected load demands more torque than is being produced.
Zero speed is locked-rotor torque. Usually in the neighborhood of 125% of full load torque for an induction motor.
JDRhodes
Electronic Technician
- Location
- Tuscaloosa,Alabama
- Occupation
- Electronic Technician
There are a few approaches to troubleshooting. The first one is a process of elimination. The simplest advice I can give without any further knowledge other than what you have discussed. Is to drop the load (the motor) then allow the motor contactor via HOA switch to energize the coil of the motor starter. If it starts under normal operation then there's a possiblity you could have an issue with the motor itself. You would need to meg the motor with a digital mulitmeter or DMM. Fluke and a bunch of other manufacturers make specific meters for megging motors, either digital or analog meters. I always use a Fluke 1587, which has never failed me. When ever you start a high inductive load, you will see an inrush current. It settles down shortly after and runs at the FLA/FLC of the motor nameplate.
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