Replacing a 3ph motor with single phase

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

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?
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.
appreciated.
Thanks!
neighbor has had nothing but troubles with VFDs (blowing the supply fuses;
issue, not a logical one so no amount of discussion can change his mind.
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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.

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.

Both are asynchronous induction motors, of course you have to check that it can handle full power in both directions.
With symmetrical windings, just select correct winding (or end of starter capacitor).

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.
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On 14/07/2014 20:07, snipped-for-privacy@downunder.com wrote:


It might not be much more than the starting current: https://www.selinc.com/WorkArea/DownloadAsset.aspx?id $27
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.
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So does everyone except the customer.
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On 14/07/2014 23:03, DaveC wrote:

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!
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On 7/14/2014 9:00 AM, DaveC wrote:







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
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On 14/07/2014 3:40 PM, Dan Coby wrote:







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
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Don Kelly
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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.
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On 14/07/2014 11:11 PM, snipped-for-privacy@downunder.com wrote:


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.

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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.
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its

Jogging

short

time

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.
?-)
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On 15/07/2014 9:48 PM, DaveC wrote:

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-
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Have you looked at single-phase AC gearmotors? It looks like Siemens has s ome. ...How important is jogging speed? If the only reason they had a 3HP motor was for the torque, a gearmotor may match up well with the torque re quirements and provide greater adjustment control by slowing the speed?
For the record, I have no experience with AC gearmotors - I'm speculating m ore than anything...
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How about a hydraulic motor or actuator, accumulator and small pump driven by single phase?
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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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Are 3 HP single phase motors available at reasonable cost? Usually it's three phase at such power levels, and a better solution may be a three-phase motor plus a Variable Frequency Drive. See the Automation Direct website.
Joe Gwinn
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