Full Load Soft Start With Hybrid Phase Converter 6

[briggs22]

I need full load starting torque from a 3-phase 120-208/230 motor, and want to use a soft starter (Gavazzi RSE4812-B) on a hybrid phase converter (GenTec/Baldor) to limit current on the 1-phase 240V line. Is a heftier 3-phase supply a better solution?
The phase converter is a hybrid rotary type which uses 2 phases (as single phase 240V) also connected to the load, and generates the third phase using a modified motor. It passes motor start loads directly back to the line.
The soft starter controls 2 phases using SCRs (third pole directly connected) to ramp Vmax with a Vstart setpoint to control starting torque.
At full load starting torque, won't the motor start current be the same as the DOL start current, and passed through directly to the line?

 

[jraef]

Yes, at the full starting torque setting (see * below) on most soft starting devices, the starting current will be exactly the same as DOL. There is no free lunch. No point in adding it to the circuit if you need locked rotor torque at start up.

Describe why you feel you need the full starting torque at start up. Are you fully aware of what that means and how the torque-speed characteristics of an AC motor work? You may have some other options available to you, such as using a VFD as both a phase converter and soft starting device or changing the motor design type, depending on what exactly it is you are trying to accomplish.

As per usual, details details details... The more information you supply, the better we can help.

*Re: full starting torque setting. Many of the small cheap soft starting devices such as the one you mention are not capable of DOL starting; they rely upon the inherent current limiting function of the unit to keep from stressing their own components (SCRs). So if you have used that unit before and had to set it to Maximum to get your motor to start, that may NOT have actually been DOL, that may have just been maximum for that unit. In addition, using 2 phase control will rob your motor of torque unnecessarily (by creating negative torque due to the severe imbalance). So essentially you end up with less torque-per-amp on a 2 phase device than on a 3 phase device. A better soft starter with 3 phase control may have allowed you to start with less current.

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[waross]

I understand that this is a single phase application. Have you considered a VFD? The VFD will run a 3 phase motor with balanced 3 phase, and many may be supplied with single phase power.
" I need full load starting torque "
Do you mean full load torque or full starting torque?
repectfully

 

[briggs22]

Thanks very much for your input!
The motor is to drive a winch pulling an amphibious cargo trailer up a steep hill on an island, where system weight is significant and power is supplied by generator (to be sized by your very useful FAQ). A generator sized for DOL starts will be very hard to transport.
The motor may sometimes start at near FLA but not locked rotor current, while Vstart squared limits starting torque and current. With cable slack and a high gear ratio it may usually reach near-synchronous speed before full load.
The phase converter and soft starter are not yet specified.

A 3-phase soft starter sounds much better. A VFD phase converter would be ideal if not too sophisticated for the application. This is a 230/460 brakemotor without wye-delta terminals for one of each winding pair.

 

[aolalde]

You did not provide the motor size, however for full load torque starting I sugest a VFD single phase input with 3-phase output. Capable to ramp the acceleration with full load current.

 

[jraef]

You will want a VFD for this. With the VFD, you can attain full torque at start up if necessary, without exceeding the FLC of the motor. You can even attain full Starting torque as well, without the need for the starting current you would see with a soft starter or DOL. The best method would be to use what is called a Closed Loop Vector drive faq237-1062 which will allow full torque at zero speed, something you may find necessary on this hoisting application so that you don't allow your trailer to start slipping back down the hill when you release the mechanical brake.

As much as I like Soft starters, they are a bad idea on winches or hoists where gravity is an issue. The failure mode is potentially disastrous.

JRaef.com
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[cncad]

Here is an amateur view: Assuming it is too late to consider a Compound DC motor for this application or to oversize the winch, so maybe do the quick and dirty fix of a spring loaded take up pulley, probably dampened to avoid surprises. This would give your AC rotor a chance to hit design rpm with whatever control. (writer now takes cover from rocks hurled by EEs)

 

[jraef]

We don't throw rocks. Lightning bolts maybe, but no rocks.

Mechanical solutions are never unwelcome, we just tend to focus on electrical ones being that this is an electrical forum.

JRaef.com
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[ScottyUK]

Is the generator a single phase or three phase set? If it is a three phase set then keep away from single phase or heavily imbalanced three phase loads. If using a VFD consider that generators generally don't like rectifier loads: output voltages get distorted and alternators get hot. If your VFD has an active front end it will be a much friendlier load for the generator, so the additional money spent on the VFD offsets against the money saved on the generator.


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Sometimes I only open my mouth to swap feet...

 

[briggs22]

(response to cncad areospace):
The winch (Thern 483) is 2 hp (4000 lb) loaded <3000 lb with 120:1 worm drive and 3-phase AC motor with electric brake. Back-travel can be restrained with safety cable and large chocks, so have 1-phase 240V AC non-brake alternate motor to fit, but would rather use the brakemotor. The trailer must be re-rigged four times to zigzag up the hill. One start is steep where initial pull to new angle will limit start load, but cannot rule out full load starts after any emergency stop. Resistive starter may have insufficient torque and motor is not wired for wye-delta.
(response to ScottyUK):
By active front end do you mean pure sinusoidal output? The VFDs I have studied appear to use SCRs or Triacs switched to increase the Vmax of each cycle during the start ramp. I would use the 3-phase motor with any 3-phase supply.
The present generator is 120V 2kW but must upgrade the site anyway to about 5kW 120/240V 1-phase. There are 5kW and 10kW military units available for 120/240V 1ph and 120/208 3ph on the same unit (10kW each) but maintainability is unknown. The 10kW generators could DOL start either motor but are about 800 lb and the scenario of powering the winch for the first test from a generator on the trailer itself looks better on paper than on site.

 

[ScottyUK]

Active front ends draw a sinusoidal current from the supply, as compared with a standard rectifier which draws all its current in a short high amplitude peak starting just before the crest of the voltage waveform. The distorted current makes life tough for the generator. An active front end is about as good as it gets - the load looks resistive from the generator viewpoint.

Are you confusing soft starters and VFDs? These days SCRs are only found in really big drives (megawatt class) and in HV drives. Triacs are usually only found in small soft starts, and are inferior to antiparallel thyristors (SCRs) in almost all aspects. A VFD draws power from the 50 / 60Hz mains supply and internally synthesises a sinusoidal output voltage of controllable amplitude and frequency which allows it to maintain full motor torque down to standstill. It's a very different animal to a soft starter which basically reduces the voltage without changing frequency.


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Sometimes I only open my mouth to swap feet...

 

[briggs22]

You are correct, I was thinking of the soft starter circuits; the VFDs I have seen use an open loop V/f ramp for constant torque up to Vmax. Jraef has suggested a Closed Loop Vector drive which I will look into further. I will look for one with an active front end.

 

[waross]

Re the generators;
Most generators which will produce either 120/240 or 120/208 will not do it simultaneously. The generator must be reconnected.
The 5 KW and 10 KW will probably be 6.25 KVA and 12.5 KVA respectively. This is based on the standard power factor rating of 80%.
When reconnected for single phase they will derate from 6.25 KVA to 4.2 KW and 4.2 KVA, and from 12.5 KVA to 8.3 KVA and 8.3 KW.
100% power factor rated sets will derate 1/3rd.
The motors will be over sized when used on single phase, resulting in good surge handling capability but in your case this will not make a difference.
However overall performance will be better if you use 3 phase.
The 5 KW set is marginal for the 2 hp. motor if you end up doing any DOL starting. It may serve well with a VFD.
respectfully

 

[ccjersey]

Watch what you buy in generators. Some of the smaller used military generator sets have switchable single/3 phase capability, but do not have a neutral available in the single phase setting. This makes them only capable of 240V single phase. The best option if you need both 120 and 240 single phase on these is using single phase off the 120/208 3 phase setting.

There was some discussion a while back about the ability of various designs of drives to operate on it, but if you want a generator for both 120/240 single phase and 3 phase capabilty too, find one and connect it for 120/240 3 phase 4 wire delta.

There will be a 208V high leg you cannot use for your 120 V line-N circuits, so your single phase distribution is fed off the other 2 legs which are 120V to neutral and 240 between just like stndard single phase utility power in the US. 3 phase is 240 all around which gives you some advantage in amperage, voltage drop and wire size over 208V when runnning motors. I wuld guess you will be running only your hoist motor while winching up the hill, so voltage balance shouldn't be a problem.

 

[LionelHutz]

If you have ever compared the operation of a triac based 2-phase controlled Gavazzi version of a "soft-starter" to a real 6-SCR based 3-phase controlled soft-starter then you'd have seen that the Gavazzi will create current imbalances in the motor which equals a lot of negative torque basically making them useless as a soft-starter.

If you want phase conversion and good starting torque then a good VFD really is the ONLY way to go.

If generator size is of greater concern than the time taken then you could run the motor at a lower speed with the VFD which would also lower the generator requirements. Basically, use less power over a longer period of time.

ScottyUK - I'd like to hear about small active front-end equipped VFD manufacturers.

 

[briggs22]

Thanks to all for this very useful advice!

(reply to LionelHutz)
The Danfoss 5000 spec shows scope traces of relatively smooth sinusoisal motor current from its PWM output scheme, and does not seem to require line reactors beyond what is built in, but does not use the term "active front end". Their 2800 model (up to 3hp) accepts 240V 1-phase, while the 5000 reference lists 1-phase input but tech support says that is not tested although "one might get away with it if derated 50%" but it shuts down on a 3% line phase imbalance so I am not sure that will work.
The Hitachi L200 2hp drive accepts 240V 1-phase but wants line reactors for over 3% phase imbalance. The specs do not mention any feature to smooth line current.

Both VFDs are open-loop vector drives which accept sensor inputs for closed-loop operation, can automatically scale speed to stay under a motor current limit, and can restart the motor on the fly by resynchronizing to it.

I'm not sure whether the Danfoss PWM scheme is what ScottyUK was referring to, or whether the line reactors help much there.

 

[jraef]

No.
Neither of them are Active Front End (AFE) drives, and what LionelHutz was alluding to is that he doubts (as I do) that anyone bothers to make a drive that small in an AFE version. Active front end drives are for two main purposes;
1) to provide low harmonic distortion to the line source
2) to provide a sink for energy coming from the load during braking.

In order for a VFD to have an AFE, the conversion section (a.k.a. "front end") is not just a bridge rectifier, but rather a complete separate inverter, usually identical to the motor side's inverter. So an AFE drive is akin to having 2 back-to-back VFDs, albeit with one control system. the front end inverter looks at the waveform of line and modifies it's conversion pattern so as to keep line harmonics to a minimum (layman's version, please don't pick at it too much guys ). The price for AFE version drives then is typically twice that of a regular VFD.

For drives as small as what you need however, there are far far less expensive ways to accomplish the same results without the need for an AFE. For your purposes, reducing the harmonics so as to not overly stress the generator can be done with very inexpensive and more reliable passive components such as a harmonic trap filter or even just a reactor.

BTW, knowing what I know of ScottyUK he probably just didn't notice that your HP requirement was so low. Images of cargo hauling winches make me think of much bigger motors.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
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[ScottyUK]

jraef / lionel,

I must say I have made an error in my assumption and I'm not going to hide behind jraef's generous get-out opportunity. I thought the AFE drives were starting to emerge at low HP ratings because of the ever-tightening EU restrictions on harmonics. I guess the measures used in (e.g.) ABB's ACS600 drive are adequate to meet today's legislation, although even that range does not extend down to 5HP. It is not an active front end of the form I was thinking of, but does have some passive harmonic mitigation. I have looked long and hard and I can't find a small drive with an active front end of the type I was thinking of (and which jraef has described). My signature has rarely contained more truth!


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Sometimes I only open my mouth to swap feet...

 

[itsmoked]

Scotty; I find that interesting because they are even starting to force lowly PC power supplies into having PFC front ends. Mine has one, I paid extra for it to cut the UPS some slack. So I would guess even small motor VFDs drawing more than a PC would at least be available.

Huh..

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com