Discussion:
Replacing a 3ph motor with single phase
(too old to reply)
DaveC
2014-07-14 16:00:10 UTC
Permalink
I need to provide jog function on a machine (run the motor for a few seconds
under load) without use of VFD**. It was done on this small printing press
with a 3 HP, 3-phase motor but the customer has no multiphase power so I¹m
changing to a 220 ³single phase² 5 HP motor.

Is there any inherent issue with starting an asynchronous motor under load? A
3ph motor does this with ease, but can the same be said of an async motor?
Should I be looking at a particular design of async motor?

Also, reversing an asynch motor‹is this a straightforward thing to do? I'm
thinking about bringing the directional wiring inside the motor's terminal
box out to separate contactors. I'm also thinking about cross-connecting aux
contacts in the contactors to eliminate possibility of actuating both
simultaneously and some kind of timer relay to provide delay between forward
and reverse actuation.

Any observations or experience you¹d like to share would be greatly
appreciated.

Thanks!


** The customer has said ³absolutely no VFD². Apparently his business
neighbor has had nothing but troubles with VFDs (blowing the supply fuses;
smoking at least one VFD) and he is terrified of them. It¹s an emotional
issue, not a logical one so no amount of discussion can change his mind.
We¹ve tried. 
u***@downunder.com
2014-07-14 19:07:49 UTC
Permalink
Post by DaveC
I need to provide jog function on a machine (run the motor for a few seconds
under load) without use of VFD**. It was done on this small printing press
with a 3 HP, 3-phase motor but the customer has no multiphase power so I¹m
changing to a 220 ³single phase² 5 HP motor.
Most likely the original three phase motor was a typical squirrel cage
AC induction motor, with three sets of coils 120 degrees apart running
at asynchronous speed (slightly below the synchronous speed 3600 rpm
or some submultiple). The new single phase motor is also an
asynchronous motor, the only difference is that there are only two
sets of windings 90 degrees apart. For unidirectional motors, the
other coil might be weaker "starter coil" and to get the required 90
phase shift a starter capacitor is used between the hot ends of the
coils. Sometimes the starter winding is disconnected.

As far as understand, in the US many single phase motors are
bidirectional with a fixed capacitor between equal power windings. To
reverse the direction, you just select which end of the capacitor you
couple the line voltage.
Post by DaveC
Is there any inherent issue with starting an asynchronous motor under load?
Please remember that an induction motor can take about 6 times the
nominal current at startup. Traditionally big three phase motors have
been started with wye/delta starters, but these days soft starters
(triacs) are used.
Post by DaveC
A
3ph motor does this with ease, but can the same be said of an async motor?
Should I be looking at a particular design of async motor?
Both are asynchronous induction motors, of course you have to check
that it can handle full power in both directions.
Post by DaveC
Also, reversing an asynch motor‹is this a straightforward thing to do?
With symmetrical windings, just select correct winding (or end of
starter capacitor).
Post by DaveC
I'm
thinking about bringing the directional wiring inside the motor's terminal
box out to separate contactors. I'm also thinking about cross-connecting aux
contacts in the contactors to eliminate possibility of actuating both
simultaneously and some kind of timer relay to provide delay between forward
and reverse actuation.
If you need to reverse the direction quickly, you need some load
(resistor) to dump the rotational energy), otherwise the current
demand would be much more than 6x relative to nominal current.
Mike Perkins
2014-07-14 21:56:26 UTC
Permalink
Post by u***@downunder.com
Post by DaveC
I need to provide jog function on a machine (run the motor for a few seconds
under load) without use of VFD**. It was done on this small printing press
with a 3 HP, 3-phase motor but the customer has no multiphase power so I¹m
changing to a 220 ³single phase² 5 HP motor.
Most likely the original three phase motor was a typical squirrel cage
AC induction motor, with three sets of coils 120 degrees apart running
at asynchronous speed (slightly below the synchronous speed 3600 rpm
or some submultiple). The new single phase motor is also an
asynchronous motor, the only difference is that there are only two
sets of windings 90 degrees apart. For unidirectional motors, the
other coil might be weaker "starter coil" and to get the required 90
phase shift a starter capacitor is used between the hot ends of the
coils. Sometimes the starter winding is disconnected.
As far as understand, in the US many single phase motors are
bidirectional with a fixed capacitor between equal power windings. To
reverse the direction, you just select which end of the capacitor you
couple the line voltage.
Post by DaveC
Is there any inherent issue with starting an asynchronous motor under load?
Please remember that an induction motor can take about 6 times the
nominal current at startup. Traditionally big three phase motors have
been started with wye/delta starters, but these days soft starters
(triacs) are used.
Post by DaveC
A
3ph motor does this with ease, but can the same be said of an async motor?
Should I be looking at a particular design of async motor?
Both are asynchronous induction motors, of course you have to check
that it can handle full power in both directions.
Post by DaveC
Also, reversing an asynch motor‹is this a straightforward thing to do?
With symmetrical windings, just select correct winding (or end of
starter capacitor).
Post by DaveC
I'm
thinking about bringing the directional wiring inside the motor's terminal
box out to separate contactors. I'm also thinking about cross-connecting aux
contacts in the contactors to eliminate possibility of actuating both
simultaneously and some kind of timer relay to provide delay between forward
and reverse actuation.
If you need to reverse the direction quickly, you need some load
(resistor) to dump the rotational energy), otherwise the current
demand would be much more than 6x relative to nominal current.
It might not be much more than the starting current:
https://www.selinc.com/WorkArea/DownloadAsset.aspx?id=2427

A concern is that single phase induction motors have a worse starting
torque than a 3 phase motor.

Furthermore a single phase motor is significantly larger than a 3-phase
motor, so the OP's choice of a 5hp motor is going to be *much* bigger
than a 3hp 3-phase motor.

I would have thought a single to 3-phase inverter/converter would have
been the way forward.
--
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk
DaveC
2014-07-14 22:03:51 UTC
Permalink
Post by Mike Perkins
I would have thought a single to 3-phase inverter/converter would have
been the way forward.
So does everyone except the customer.
Mike Perkins
2014-07-14 22:33:53 UTC
Permalink
Post by DaveC
Post by Mike Perkins
I would have thought a single to 3-phase inverter/converter would have
been the way forward.
So does everyone except the customer.
There are some passive varieties around though they need to be matched
to the motor.

They are, in effect, nothing different to that already done in a single
phase motor!
--
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk
j***@gmail.com
2014-07-15 03:37:53 UTC
Permalink
I would have thought a single to 3-phase inverter/converter would have > been the way forward. So does everyone except the customer.
How about one of them oldfangled rotary phase converters ? they can be kludged to gether for cheap per HP. What's more, as long as the current astays within limits it can be used for other three phase machines.

In fact if that machine has any ancillary motors such as a coolant or cutting oil pump or some such, that will likely be three phase as well. the rotary convertor (as a VFD would) fixes it all. It is like the old fashioned way of doing it.

This customer may be a siliconophobic, and I do not blame him really. Some of these guys grew up when TVs had those damn unreliable tubes in them, yet lasted seven years before needing repair. So we canot call him an idiot or whatever.

the rotary phase convertor, unless you buy one, is made with a large AC motor. I am not sure of all the details as I have not done it persoanally. however, this sounds like it is right up youer customer's alley. no silicon. No triacs, IGFETs, none of that. Just a couple of big caps and a couple of motors.

BTW, the more machines (within the current limit) that youi hook up, the more efficient it runs. So I'm told.
Dan Coby
2014-07-14 22:40:27 UTC
Permalink
Post by DaveC
I need to provide jog function on a machine (run the motor for a few seconds
under load) without use of VFD**. It was done on this small printing press
with a 3 HP, 3-phase motor but the customer has no multiphase power so I¹m
changing to a 220 ³single phase² 5 HP motor.
Is there any inherent issue with starting an asynchronous motor under load? A
3ph motor does this with ease, but can the same be said of an async motor?
Should I be looking at a particular design of async motor?
Also, reversing an asynch motor‹is this a straightforward thing to do? I'm
thinking about bringing the directional wiring inside the motor's terminal
box out to separate contactors. I'm also thinking about cross-connecting aux
contacts in the contactors to eliminate possibility of actuating both
simultaneously and some kind of timer relay to provide delay between forward
and reverse actuation.
Any observations or experience you¹d like to share would be greatly
appreciated.
Thanks!
** The customer has said ³absolutely no VFD². Apparently his business
neighbor has had nothing but troubles with VFDs (blowing the supply fuses;
smoking at least one VFD) and he is terrified of them. It¹s an emotional
issue, not a logical one so no amount of discussion can change his mind.
We¹ve tried.
Does your customer have any objection to a non-electronic converter? A
common means of running 3 phase motors from a single phase supply is via
the use of rotary phase converters.

http://en.wikipedia.org/wiki/Rotary_phase_converter


Dan
Don Kelly
2014-07-15 01:32:51 UTC
Permalink
Post by Dan Coby
Post by DaveC
I need to provide jog function on a machine (run the motor for a few seconds
under load) without use of VFD**. It was done on this small printing press
with a 3 HP, 3-phase motor but the customer has no multiphase power so I¹m
changing to a 220 ³single phase² 5 HP motor.
Is there any inherent issue with starting an asynchronous motor under load? A
3ph motor does this with ease, but can the same be said of an async motor?
Should I be looking at a particular design of async motor?
Also, reversing an asynch motor‹is this a straightforward thing to do? I'm
thinking about bringing the directional wiring inside the motor's terminal
box out to separate contactors. I'm also thinking about
cross-connecting aux
contacts in the contactors to eliminate possibility of actuating both
simultaneously and some kind of timer relay to provide delay between forward
and reverse actuation.
Any observations or experience you¹d like to share would be greatly
appreciated.
Thanks!
** The customer has said ³absolutely no VFD². Apparently his business
neighbor has had nothing but troubles with VFDs (blowing the supply fuses;
smoking at least one VFD) and he is terrified of them. It¹s an emotional
issue, not a logical one so no amount of discussion can change his mind.
We¹ve tried.
Does your customer have any objection to a non-electronic converter? A
common means of running 3 phase motors from a single phase supply is via
the use of rotary phase converters.
http://en.wikipedia.org/wiki/Rotary_phase_converter
Dan
Such rotary phase converters have been commonly used in the past and can
be cobbled together from surplus equipment such as a 5-7.5HP 3 phase
motor driven by a single phase motor <1HP at an estimate.
I should think that this is the cheapest alternative.
It is bulky but it means that the original 3 phase 3HP potor can be
kept. Capacitors can help.

However, the sizing of the original motor is probably based on the load
current at start -and a HP rating to fit this and also fit an rms HP
limit equivalent. It may be that a 3HP single phase motor -capacitor
start designed for max torque near standstill would do the job as the
duty cycle is the same as before.
Check the specific info for the original motor and a replacement. The
single phase replacement will be expensive and there would likely be
mounting problems.
It's a bit of a toss up- mainly economics
--
Don Kelly
remove the cross to reply
j***@gmail.com
2014-07-15 03:41:28 UTC
Permalink
"Dan Coby 5:40 PM (5 hours ago)
Other recipients:
Does your customer have any objection to a non-electronic converter? A common means of running 3 phase motors from a single phase supply is via the use of rotary phase converters. http://en.wikipedia.org/wiki/Rotary_phase_converter Dan


?i missed thaty somehow. You beat me to it. they used to build those things on furniture dllie and welding tank carts and shit. You start the thing up and it does the job. The only thing is if you try to start too many motors at once you can stall it, which will blow a breaker of course.

So you start the convertor first, then the biggest machine you got and then all the smaller ones in order of HP.
d***@krl.org
2014-07-15 20:20:39 UTC
Permalink
Post by j***@gmail.com
So you start the convertor first, then the biggest machine you got and then all the smaller ones in order of HP.
Do you have any rational for this starting order? I have made several rotary converters, and never bothered to have any certain starting order for driven motors.

Dan
j***@gmail.com
2014-07-15 20:23:31 UTC
Permalink
"Do you have any rational for this starting order? I have made several rotary converters, and never bothered to have any certain starting order for driven motors."
It is just what I was told, and it seemed to make sense. If it isn't necessary then it isn't. It might make it a little easier on the equipment or something. That kinda makes sense but no, I just took their word for it.
Clifford Heath
2014-07-16 00:07:37 UTC
Permalink
Post by d***@krl.org
Post by j***@gmail.com
So you start the convertor first, then the biggest machine you got and then all the smaller ones in order of HP.
Do you have any rational for this starting order? I have made several rotary converters, and never bothered to have any certain starting order for driven motors.
Startup power transients. If you start your largest machine last, it'll
overload the converter during startup.
j***@gmail.com
2014-07-16 07:43:20 UTC
Permalink
"Startup power transients. If you start your largest machine last, it'll
overload the converter during startup. "

Maybe I had it wrong ? It was like they werre saying thsat all the motors on the line were aiding in the conversion process. the more that better, ;to a certain point. Damifino, it was a long long time ago.
d***@krl.org
2014-07-16 13:45:37 UTC
Permalink
Post by Clifford Heath
Startup power transients. If you start your largest machine last, it'll
overload the converter during startup.
Have you actually found this to be true in real life. I am under the impression that starting all the smaller motors first will help prevent the converter from being overloaded during start up of the largest machine.

Consider that if you have a 3 phase motor running on a single phase, you can then use this to start another 3 phase motor running on single phase power.

So each smaller motor running will help start the motor in the largest machine.

Dan
u***@downunder.com
2014-07-16 20:38:30 UTC
Permalink
Post by d***@krl.org
Post by Clifford Heath
Startup power transients. If you start your largest machine last, it'll
overload the converter during startup.
Have you actually found this to be true in real life. I am under the impression that starting all the smaller motors first will help prevent the converter from being overloaded during start up of the largest machine.
Consider that if you have a 3 phase motor running on a single phase, you can then use this to start another 3 phase motor running on single phase power.
So each smaller motor running will help start the motor in the largest machine.
Dan
Starting the smallest would make sense only if the small motors are
synchronous motors (preferable with big flywheels). A synchronous
motor turns into a synchronous generator when it rotates even slightly
above synchronous speed.

Thus, the small motors are started and achieve synchronous frequency.
Then the big motor is started, heavily loading the original generator,
dropping the feed frequency below the initial small motor synchronous
frequency, thus the small motors will also act as generator for a
while, feeding some of the flywheel energy into the big motor in
addition to the primary power.

However, for asynchronous motors, which will act as generators at
10-20 % overspeed, the primary power source frequency would have to
drop by 20-40 %, so that the small asynchronous motors could help
starting the big motor.

In most practical cases, I do not see how the small motors could help
start the last big motor.
d***@krl.org
2014-07-17 20:37:56 UTC
Permalink
Post by u***@downunder.com
Then the big motor is started, heavily loading the original generator,
dropping the feed frequency below the initial small motor synchronous
frequency,
The feed frequency comes from the power grid. So when the big motor is started, the feed frequency does not change.

Dan
j***@gmail.com
2014-07-18 03:00:17 UTC
Permalink
"In most practical cases, I do not see how the small motors could help
start the last big motor. "

I kinda agree. there is a such thing as power factor, and mayber them supplying some current during the startup of big Bertha. Drawing on say five of them, if they help to keep the phase right, maybr it does do something.

However, we are not generally taling one 25 foot horizontal boring mill versus the other three phase devices like the IBM Selectrics, the copyting machine and, well, the lighting.

If you got three phase lighting and no three phase power there is somethning wrong. Usually, there is a eire to run one machine, like a Bridgeport in your basement. According to what I've heard, it is really rare that you use all the current capacity. Nice to have it of course, but you only need it for a split second. The only time running the current goes up is when you bury an endmill into some tough material. Maybe the OP's associate is the type to do that.

Maybe. On manual machines which is what people use to make homeade guns and shit like that, you never need all that umch power. When you run productionn is when you pull the juice. Maybe that guy is running production in his basement.
j***@gmail.com
2014-07-18 03:05:36 UTC
Permalink
"eire"
Believe it or not that means "desire".
josephkk
2014-07-18 02:01:19 UTC
Permalink
Post by d***@krl.org
Post by Clifford Heath
Startup power transients. If you start your largest machine last, it'll
overload the converter during startup.
Have you actually found this to be true in real life. I am under the impression that starting all the smaller motors first will help prevent the converter from being overloaded during start up of the largest machine.
Consider that if you have a 3 phase motor running on a single phase, you can then use this to start another 3 phase motor running on single phase power.
So each smaller motor running will help start the motor in the largest machine.
Dan
Other motors do not contribute much to starting capability. What they do
contribute to is fault current, because the faulted voltages are trying to
dynamically brake the motor.

?-)
j***@gmail.com
2014-07-18 02:41:42 UTC
Permalink
"Have you actually found this to be true in real life. I am under the impression that starting all the smaller motors first will help prevent the converter from being overloaded during start up of the largest machine. "
Maybe I had that bass akwards. If all the motors on the line help the process, it SHOULD better to start all the small ones fisrt.

Somehow, at the time it seemed to make sense the other way. Also, I seem to remember somme designs with somewhat of a flywheel. Only a few.

Thing about starting all the small stuff first though is the whole thing is under a normal load, and then has to take the surge. Also, alot of people who have these things are home shop achinists who might be working alone and have no reason to have more thatn one machine running at the same time. Many probably only have one three phas machine in the first place.

I just relayed what I was told. If it is wrong I appreciate the correction.
Mike Perkins
2014-07-18 10:21:24 UTC
Permalink
Post by d***@krl.org
Post by j***@gmail.com
So you start the convertor first, then the biggest machine you got
and then all the smaller ones in order of HP.
Do you have any rational for this starting order? I have made
several rotary converters, and never bothered to have any certain
starting order for driven motors.
Rotary converters are more forgiving of start-up currents, electronic
ones less so.

If the converter/inverter was unable to supply the full start current,
as indeed may well be the case in many instances, it would be wise to
start the largest motor first to minimise peak current draw and get any
loss of regulation done with quickly, then allow the others to come
onstream.

I have had the condition where an electronic inverter wasn't man enough
to bring motor up to speed with a light load, a load that I couldn't
remove, where it just ended up spinning slowly and getting very hot.

Many electronic inverters have a frequency ramp to help with running up
motors!
--
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk
j***@gmail.com
2014-07-22 06:50:02 UTC
Permalink
"Many electronic inverters have a frequency ramp to help with running up
motors! "

this is what the OP's customer fears. He, well if he took a VFD it would be a DC motore with a three phase genny on it. that's th way I gt it.
u***@downunder.com
2014-07-15 06:11:55 UTC
Permalink
Post by DaveC
I need to provide jog function on a machine (run the motor for a few seconds
under load) without use of VFD**. It was done on this small printing press
with a 3 HP, 3-phase motor but the customer has no multiphase power so I¹m
changing to a 220 ³single phase² 5 HP motor.
The classical trick running an existing three phase motor from a
single phase supply is to connect capacitors between the three motor
terminals and then connect the single phase source between two of the
terminals.

Of course, the voltage is now lower, so less power is available.
Perhaps a autotransformer could be used to beef up the voltage. Having
intermediate taps in the autotransformer could be used to alter
starter or tongue performance.
d***@krl.org
2014-07-15 20:51:09 UTC
Permalink
Post by u***@downunder.com
The classical trick running an existing three phase motor from a
single phase supply is to connect capacitors between the three motor
terminals and then connect the single phase source between two of the
terminals.
Of course, the voltage is now lower, so less power is available.
Perhaps a autotransformer could be used to beef up the voltage. Having
intermediate taps in the autotransformer could be used to alter
starter or tongue performance.
If you look over in R.C.Metalworking you will find a lot of information on rotary converters. Some of the people posted results of actual tests.
Paul Hovnanian P.E. was one of the earliest contributors.

Dan
Don Kelly
2014-07-16 02:46:18 UTC
Permalink
Post by u***@downunder.com
Post by DaveC
I need to provide jog function on a machine (run the motor for a few seconds
under load) without use of VFD**. It was done on this small printing press
with a 3 HP, 3-phase motor but the customer has no multiphase power so I¹m
changing to a 220 ³single phase² 5 HP motor.
The classical trick running an existing three phase motor from a
single phase supply is to connect capacitors between the three motor
terminals and then connect the single phase source between two of the
terminals.
Yes, you could do this-I'm not sure, without further analysis, that
there will not be a considerable hit on the effective motor rating.
The phase converter,even without capacitors will work. The limiting
current is dependent on the larger machine-and is not a hit on the
capacity of the smaller load machine.

However, in this case of a jog function- If a single phase capacitor
start single phase motor has the same starting torque as the 3 phase
motor (and with the right capacitor- this may well be done) a single
phase motor may well fit the needs-electrically-but likely not
mechanically with respect to mounting.
Post by u***@downunder.com
Of course, the voltage is now lower, so less power is available.
Perhaps a autotransformer could be used to beef up the voltage. Having
intermediate taps in the autotransformer could be used to alter
starter or tongue performance.
--
Don Kelly
remove the cross to reply
DaveC
2014-07-16 04:48:50 UTC
Permalink
Post by Don Kelly
However, in this case of a jog function- If a single phase capacitor
start single phase motor has the same starting torque as the 3 phase
motor (and with the right capacitor- this may well be done) a single
phase motor may well fit the needs-electrically-but likely not
mechanically with respect to mounting.
I (OP) am concerned about jogging. The single-phase motor will be using its
start winding, and the associated high current will heat the winding. Jogging
means usually repeated, sometimes back-and-forward direction changes, short
motor actuations.

With all this jogging the centrifugal start switch will be abused over time
which probably means early failure.
josephkk
2014-07-18 02:07:44 UTC
Permalink
Post by DaveC
Post by Don Kelly
However, in this case of a jog function- If a single phase capacitor
start single phase motor has the same starting torque as the 3 phase
motor (and with the right capacitor- this may well be done) a single
phase motor may well fit the needs-electrically-but likely not
mechanically with respect to mounting.
I (OP) am concerned about jogging. The single-phase motor will be using its
start winding, and the associated high current will heat the winding. Jogging
means usually repeated, sometimes back-and-forward direction changes, short
motor actuations.
With all this jogging the centrifugal start switch will be abused over time
which probably means early failure.
Two easy answers:
a. use a cap run motor
b. use a current sensing start switch

Also this will almost always be within the start time of the jogger motor.

?-)
Don Kelly
2014-07-22 04:59:16 UTC
Permalink
Post by DaveC
Post by Don Kelly
However, in this case of a jog function- If a single phase capacitor
start single phase motor has the same starting torque as the 3 phase
motor (and with the right capacitor- this may well be done) a single
phase motor may well fit the needs-electrically-but likely not
mechanically with respect to mounting.
I (OP) am concerned about jogging. The single-phase motor will be using its
start winding, and the associated high current will heat the winding. Jogging
means usually repeated, sometimes back-and-forward direction changes, short
motor actuations.
With all this jogging the centrifugal start switch will be abused over time
which probably means early failure.
Your point is valid- I'd be less concerned about the switch than with
the start winding itself -second thoughts about "rms HP"
The motor would have to be cap start (not necessarily cap run)-yes,
there may be a high current at the time that the switch operates
Cap run optimises operation near synchronous speed -not at start.
How long will the motor run, when jogging? Does it get up tYour point
about the

There is another problem- mounting the motor- it will be larger
physically than the polyphase motor.

It also comes down the starting torque needed -any idea?
Don't rule out DC motors fed from an ordinary diode bridgr-
--
Don Kelly
remove the cross to reply
Paul Hovnanian P.E.
2014-08-15 21:53:50 UTC
Permalink
How about a hydraulic motor or actuator, accumulator and small pump driven
by single phase?
--
Paul Hovnanian mailto:***@Hovnanian.com
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