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.

Reply to
DaveC
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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.

Reply to
upsidedown

It might not be much more than the starting current:

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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 
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Reply to
Mike Perkins

So does everyone except the customer.

Reply to
DaveC

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 
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Reply to
Mike Perkins

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.

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Dan

Reply to
Dan Coby

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

Reply to
Don Kelly

How about one of them oldfangled rotary phase converters ? they can be klud ged 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 cutti ng oil pump or some such, that will likely be three phase as well. the rota ry convertor (as a VFD would) fixes it all. It is like the old fashioned wa y 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 mot or. I am not sure of all the details as I have not done it persoanally. how ever, this sounds like it is right up youer customer's alley. no silicon. N o triacs, IGFETs, none of that. Just a couple of big caps and a couple of m otors.

BTW, the more machines (within the current limit) that youi hook up, the mo re efficient it runs. So I'm told.

Reply to
jurb6006

Other recipients: Does your customer have any objection to a non-electronic converter? A com mon means of running 3 phase motors from a single phase supply is via the u se of rotary phase converters.

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nverter 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.

Reply to
jurb6006

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.

Reply to
upsidedown

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

Reply to
dcaster

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.

Reply to
jurb6006

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

Reply to
dcaster

Startup power transients. If you start your largest machine last, it'll overload the converter during startup.

Reply to
Clifford Heath

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.

--
Don Kelly 
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Reply to
Don Kelly

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.

Reply to
DaveC

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.

Reply to
jurb6006

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

Reply to
dcaster

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.

Reply to
upsidedown

The feed frequency comes from the power grid. So when the big motor is started, the feed frequency does not change.

Dan

Reply to
dcaster

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