Electrician Talk banner
1 - 20 of 25 Posts

·
Registered
Joined
·
307 Posts
Discussion Starter · #1 ·
Is it possible to connect a 3 phase motor to 240V single phase without a convertor? I'm not comfortable with motors as I used to be a construction electrician and just switched to commercial.

 

·
Electrical Simpleton
Joined
·
3,350 Posts
You can also use another 3 phase motor to generate a third phase. In doing so you feed the generating motor single phase.

Pete

p.s. pretty sure it isn't code compliant but in a pinch it will work.
 

·
Electrical Simpleton
Joined
·
3,350 Posts
Pete, what you described is a rotary phase converter (as opposed to a static converter that uses capacitors). Article 455 allows it.
True.. but what I proposed is not a "listed" application... just a barnyard way of making it work.:laughing:

Pete
 

·
Registered
Joined
·
307 Posts
Discussion Starter · #7 ·
Thanks guys. Now I understand why the previous motor lasted only 2 years. It was installed by someone else and now I'm suppose to replace it. There was no OL or anything else, just a switch.
 

·
Bilge Rat
motors and controls.........
Joined
·
9,497 Posts
Is it possible to connect a 3 phase motor to 240V single phase without a convertor? I'm not comfortable with motors as I used to be a construction electrician and just switched to commercial.

View attachment 34615
If a 3Ø motor is connected to single phase power, it won't know what direction to turn, so it'll need its shaft turned a bit in order to get started. It'll also produce roughly 1/4 of its rated HP.

For a small motor like the on in the pic, the best way to run it on single phase is to use a VFD that's designed for single phase input. All VFDs output 3Ø, and will operate the motor very close to its rated HP.

Once you get over about 3HP, the VFD route gets expensive, but for smaller motors it's a reasonably economical way to go.

Just for info, a static phase converter (the capacitor kind) will allow a 3Ø motor to operate on single phase power, but it'll produce about half of its rated HP. A rotary phase converter will allow about 3/4 of its rated HP. Both will start the motor on its own, but with seriously reduced starting torque.
 

·
Bilge Rat
motors and controls.........
Joined
·
9,497 Posts
Please elaborate. This is a new one.
In this case, you're custom building a rotary phase converter.

By running a basic 3Ø motor on single phase, it'll manufacture the 3rd phase which can be used to run another 3Ø motor.

A ∆ connected motor works better than a wye, and if you add some capacitors you don't have to spin the shaft by hand to get it to start.
 

·
Registered
Scada Supervisor
Joined
·
4,559 Posts
A 1hp vfd that has 230v 1 phase in and 230v 3 phase out runs about $177 from automation direct, I just bought one it work great. BUT your motor is realy a 1.25 HP due to the service factor rating. A 2HP drive is $271 or so.
 

·
Fond of three phase
Joined
·
1,649 Posts
Yeah we call that a rotary phase converter.



I really don't like them. There a pain in the arse to troubleshoot.
Personally, I think it's a poor excuse for a three phase source. IIRC, the motors sound sick, like they're single phasing. If you check the current draw on all three phases, you'll find a gross current imbalance.
Back in the good old days, there was no choice, but today!
VFD's are so reasonably priced and work so well. :thumbsup:
 

·
Registered
Joined
·
5,648 Posts
... BUT your motor is realy a 1.25 HP due to the service factor rating. A 2HP drive is $271 or so.
Just for general info:

Most motor mfrs will tell you that you cannot use the Service Factor of a motor AND run them from a VFD. One or the other, not both. Or put another way, all motors should be considered 1.0SF if run from a VFD.
 

·
Donuts > Fried Eggs
Joined
·
17,042 Posts
Y'know, it's sad, I work in a company that has a freaking motor rewind shop and I still can't get a clear explanation of service factor.

"A service factor of 1.25 allows you to run the motor at 125% of of the nameplate horsepower rating... but it damages the life expectancy by overheating the windings." Then what the hell is the point? I can run a 1.0SF motor at 125% and damage it just fine, too. :rolleyes:

I've never seen an official explanation of how these companies are actually calculating SF.
 

·
Registered
Scada Supervisor
Joined
·
4,559 Posts
Y'know, it's sad, I work in a company that has a freaking motor rewind shop and I still can't get a clear explanation of service factor.

"A service factor of 1.25 allows you to run the motor at 125% of of the nameplate horsepower rating... but it damages the life expectancy by overheating the windings." Then what the hell is the point? I can run a 1.0SF motor at 125% and damage it just fine, too. :rolleyes:

I've never seen an official explanation of how these companies are actually calculating SF.
I was told it was to compensate for the lack of different sizes. We had a air compressor that was 50 HP with a sf of 1.25 and the manufacture said it was because they did not make a 60 HP and that was what they needed. They said it would not hurt the motor and it ran for at least 20 yr that I know of.
 

·
Registered
Joined
·
5,648 Posts
Y'know, it's sad, I work in a company that has a freaking motor rewind shop and I still can't get a clear explanation of service factor.

"A service factor of 1.25 allows you to run the motor at 125% of of the nameplate horsepower rating... but it damages the life expectancy by overheating the windings." Then what the hell is the point? I can run a 1.0SF motor at 125% and damage it just fine, too. :rolleyes:

I've never seen an official explanation of how these companies are actually calculating SF.
The "official" wording in NEMA MG-1 is less vehement, it says

"A motor operating at a service factor greater than 1.0 will have a reduced life expectancy compared to a motor running at its rated nameplate horsepower".

The PURPOSE of using SF is really for OEMs who don't want to have to buy a larger motor for just a few more percent power requirement. For example a torque-speed curve on a pump comes out to be 11BHP, technically they should spec a 15HP motor. But if the OEM wants to be cheap, he uses a 10HP with a 1.15SF. It will last out the warranty, that's all he cares about. It's the USER who pays for that decision later. But if the machine is not going to get used much, they may never know anyway.
 

·
Donuts > Fried Eggs
Joined
·
17,042 Posts
...But if the OEM wants to be cheap, he uses a 10HP with a 1.15SF. It will last out the warranty, that's all he cares about....
Alright, that's a good way of looking at it: It's a warrantied way of overloading the motor.

So how are these guys determining the SF? I assume it doesn't follow the old Arrhenious rule-of-thumb where every 10° increase reduces service life by 50%. Like cowboy said, seems like some motors can run in SF indefinitely.
 

·
Registered
Joined
·
5,648 Posts
Alright, that's a good way of looking at it: It's a warrantied way of overloading the motor.

So how are these guys determining the SF? I assume it doesn't follow the old Arrhenious rule-of-thumb where every 10° increase reduces service life by 50%. Like cowboy said, seems like some motors can run in SF indefinitely.
An SF rating is a requirement in NEMA MG-1 design specs if the motor is to be labeled as 'General Purpose'. But even though it is required, they go on to say "you shouldn't use it". It's weird, that's why the IEC people can't understand why we do it. But it has a lot to do with history. Long ago, the NEMA MG-1 description of SF used to mention "short time" overloads. But that was so nebulous that they either had to define it more specifically, or water it down to being generic by getting rid of all mention of time. They chose to water it down, probably because the specifics became way too complicated.
 

·
Fond of three phase
Joined
·
1,649 Posts
Thanks guys. Now I understand why the previous motor lasted only 2 years. It was installed by someone else and now I'm suppose to replace it. There was no OL or anything else, just a switch.
After I posted my reply, earlier today, I thought to myself, Why not just replace it with a dual voltage 1.5 HP 1760 single phase motor. It doesn't seem to be anything special, like a motor that is designed to be an integral part of a machine. They even make explosion proof, single phase motors. :rolleyes:
 

·
Bilge Rat
motors and controls.........
Joined
·
9,497 Posts
Just for general info:

Most motor mfrs will tell you that you cannot use the Service Factor of a motor AND run them from a VFD. One or the other, not both. Or put another way, all motors should be considered 1.0SF if run from a VFD.
A lot of nameplates these days will state something like '1.0 SF using VFD power' or something to that effect when the normal SF is 1.15 or even higher.
 
1 - 20 of 25 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top