Forget it. Not gonna happen.

You want to cut the generator some slack? Get a VFD that can dish out 3ph while taking in 1ph. Replace the overwrought 1ph 5hp with a used 5hp 3ph. It will last longer, run cooler, can be speed adjusted, and the VFD can provide sublime soft starting.

Keith Cress
kcress - http://www.flaminsystems.com

What he said.

I have used such 'soft starters' for high speed 3 phase blower motors in our shop and the inrush current has always been high regardless of the voltage levels. At those low HP levels, the cost difference between a soft starter and VFD can't be too high. VFD also gives the big advantage of speed control and hence flow control of the fan, if needed.

Muthu
www.edison.co.in

Quote:

21A nominal

What is "Nominal" current"?
Use nameplate rated current.
My tables indicate a typical full load current for a 230 Volt 5 HP motor as 28 Amps.
A rule of thumb that has never let me down for motor starting DOL; The generator capacity must be equal to or greater than the existing load plus three times the rated current of the motor being started.
There may be a noticeable but generally acceptable voltage dip when the motor starts.
But, determine the nameplate rated current and if it is less than 25 Amps, then contact the manufacturer of the device for information.
Note: Standby generators and soft starts don't always work and play well together, especially if the soft start is a major load on the generator..

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Others have covered the motor side but if you're going to spend any money, there are upgrades you can make on the generator end as well. You haven't specified what your kW is but regardless, a better voltage regulator or even better, permanent magnet based excitation do wonders to keep voltage stable during big motor starts.

Soft starters for single phase motors are problematic depending on the TYPE of single phase motor. Capacitor Start motors do not fare well with soft starters and neither do the soft starters fare will with them; it’s usually a race to see which one fails first. The same is true for VFDs on single phase motors. You can use Soft Starters (and VFDs) on PSC type (Permanent Split Capacitor) single phase motors, but the likelihood that your 5HP motor is PSC is low.

Before wasting money on questionable electronics, make sure that your bower has a damper and that the damper is CLOSED when starting. It seems counter intuitive to many people so they open or remove the dampers. What that does is to immediately put load on the motor, making it take longer to accelerate. If there is an open damper, close it and try it again.



" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

If the motor is too large for the generator to start comfortably, a stable voltage may make things worse.
The overload slows the generator down.
When the generator slows down, the frequency drops.
With a stable voltage and a falling frequency the motor may saturate magnetically.
The current may be excessive and that will make matters worse.
All quality AVRs have a feature called Under-Frequency-Roll-Off. (UFRO)
When the frequency drops more than about 3 Hz, the AVR reduces the voltage in proportion to the frequency drop.
It is generally not possible to easily add a Permanent Magnet Generator (PMG)to an existing machine.
PMG helps with short circuit tripping.
For motor starting, a PMG may be overruled by UFRO.
If you are buying a new generator by all means specify a PMG if it is available.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

It may be necessary to start large motor before the smaller motors.
It may be necessary to shed load before starting a large motor.
It may be necessary to buy a larger generator.
Without seeing your complete setup, it is difficult (impossible) to give much help.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

5hp is too large for residential and in range for commercial and industrial. I'd be surprised if the standby generator is less than 50kW but OP needs to fill us in on this detail.

Waross, I have an interesting situation that occured before I started at the company. They got water in a 50kW alternaror enclosure and blew it up. A 50kW replacement was not available in the timeframe they wanted but a 75kW was available in the frame size so it was purchased. The prime mover was not replaced. They also electrified a mechanical pump with a 25 horsepower electric motor on DOL start. You have to hold the start button down until the motor is up to speed on the 50kW set due to the voltage drop not being able to hold in the contactor. The 75kW set starts it up just fine. On small sets the regulator is powered directly off the windings and any hit that causes a voltage drop also reduces the powered available for excitation which causes double trouble.

As it stands, the installation has an old existing 22 kw genset, shunt excited, that will barely start the motor (and I mean just barely) with a maximum of grunting and frequency loss. But it comes up to speed, eventually. (The fan was installed with no consideration for this).

The owners are proposing a 30 kw genset, shelf model because of time constraints and market availability problems due to current events. Auxiliary winding excitation power. UFRO is standard equipment. Stated motor starting of this model is 44 kW, and this is a code J motor, 7.99 multiplier * 5 HP = 40 kW demand to start at 35% voltage dip.

The only other significant loads here are two AHU air conditioners (2-3 tons each) plus plugs and lights for maybe a thousand square feet of offices.

Our thinking at this point would be that automatic shedding (total load current, frequency, and voltage sensing) to drop the AC units if necessary should be adequate, since plugs and lights will be well below the 4 kw surplus we show available here.

Since the original post, I have discovered Baldor offers a soft start for motors of this size (and larger), but based on advice here, that seems like a bad solution.

It may be slicing it thin to use all 44 kw of motor starting capability, 40 for motor start and 4 for house loads, but it does seem to fit into the specs (barely). And since it already starts with an existing old and decrepit genset, it should work. In an ideal world we would go larger, but they want it to be in service sooner than later.

Thoughts?

When you spec a higher KVA generator just for starting the motor DOL, isn't that an inefficient use of the DG with attendant low load wet stacking of the engine and additional fuel consumption and maintenance costs?

Muthu
www.edison.co.in

I am still interested in expanding on the initial comments on this thread.

We have also discovered Stellar makes a whole range of them for the application, but have no experience with the company.

They specifically list CS/CR 1ph 230V motors as the application for these starters.

What about them does not play well together? Are the soft start manufacturers making junk that will be problematic in the long run?

Quote:

When you spec a higher KVA generator just for starting the motor DOL, isn't that an inefficient use of the DG with attendant low load wet stacking of the engine and additional fuel consumption and maintenance costs?

I have had this discussion before.
Life is hard for a standby generator.
From a cold start, within a few seconds they may be hit with a significant overload.
Then they may run at a low load level.
If you have to start the motor, you have to do what you have to do.
My worst case was a plant serviced by a 275 KVA set that normally ran at about 10% load.
The plant used one large motor for a few hours a day.
The set worked hard for a few seconds starting the large motor and then the load dropped to about 25%.
Once started, the large motor was generally lightly loaded.
And yes, wet stacking is an issue that I have encountered several times with standby sets when there is a lot of A/C involved.
I have encountered a couple of new sets that wet stacked badly.
Several days on a load bank was sufficient to seat the rings and avoid further wet stacking.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Quote:

What about them does not play well together? Are the soft start manufacturers making junk that will be problematic in the long run?

It has to do with the current and voltage wave forms caused by the soft starter chopping the sine wave.
Many AVRs have trouble properly sensing the voltage of the interrupted sine wave.

Quote:

Our thinking at this point would be that automatic shedding (total load current, frequency, and voltage sensing) to drop the AC units if necessary should be adequate, since plugs and lights will be well below the 4 kw surplus we show available here.

That is a road that I have traveled many times.
I installed a lot of standby sets in the tropics.
Anyone who could afford a standby set generally had a lot of A/C.
A lot of sets were brought in in one order. All were sized by someone else and all were undersized.
My first learning experience:
The power failed and the set started and transferred automatically.
The A/C load caused the voltage to drop so far that the contactor in the ATS opened.
The set recovered speed and voltage.
The ATS reconnected and the A/C load pulled the frequency down again.
This repeated until the repeated arcing in the contactor burned one of the main contacts so badly that it no longer made contact.
Then at least the lights came on. (Half of the lights powered by the remaining contact and the remainder powered in series with the stalled A/C units.
Solution:
Small relays were installed in the A/C control circuits.
A push button would energize the relay and one set of contacts would seal the relay in, a second set would control the A/C.
When the power failed, all of the relays would drop out and the standby set would come on-line with no A/C load.
The owner would then select which individual A/C units to restart, within the capability of the gen-set.

There are also compressor protection relays available.
The three phase models monitor voltage, and phase sequence.
They will not operate if the voltage is too low or too high.
They will not operate if the phase sequence is reversed.
I used one of these and a lot of control relays to protect all of the refrigeration equipment in a sausage factory.
One day the utility changed out the transformers and put 416 Volts into a 208 Volt system.
They lost computers, calculators, lights, fax machines, and copiers.
The protection system saved all of the refrigeration equipment and no product was lost.

The single phase protectors were generally plug-in devices intended for refrigerators, freezers and small A/C units.
They would shut down on over voltage or under voltage. They provided automatic restart after a random time delay on the order of three minutes.
You may roll-your-own with a time delay relay that drops out on a power failure and re-closes after a suitable time delay that allows time for the large motor to start.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

In case I have not made this clear. There is no probability of all loads starting at once. The large fan does not start until controls tell it to start, and air conditioners these days all have variable delay built in.

I understand how SCR soft starters chop the waveform. And have some idea on how that can affect AVRs similar to the pulses in a VFD supply, but not a very deep understanding of what might actually happen.

But our experience has been that some more pre-packaged soft starters of a similar concept, though less industrial in nature, have been met with nothing but great success in smaller generators and standard hermetic compressors up to 5 HP. But ramp time is very fast.

Do you feel that the longer ramp time in the heavy fan will mess with the AVR more likely?

Something to keep on mind during all of this is that small genders, less than 50kW, are often not rated according to NEMA or IEC guidelines and as a result are difficult to size correctly. Many manufacturers use bogus terms in the rating such as "peak" so it's it's difficult to predict how the generator will actually behave.

If line side harmonics are a problem for the AVR you could always go old school and use a transformer based soft start.

Do you mean a "Roll-your-own" auto-transformer start, Tug?
Could work.
A 240:120 Volt transformer with the windings connected in series would develop about 66% voltage on the tap.
For continuous running you would need a 5 KVA transformer.
For starting duty I would go with a 2 KVA or 2.5 KVA transformer.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

I've still got a few magnetic soft starts in my fleet and even a pair of resistor based units.

Tugboat

Given that this high inertia fan is struggling even with full voltage (sans the line drop), I don't see how any voltage reduction methods, be it soft starter or auto trafo, helps in getting it up to speed.

Bill

With the increased capex cost, fuel cost, maintenance cost and not to mention diesel tantrums of a higher rated DG, a quality VFD (with an even VFD bypass on full speed) seems to be a no-brainer to me. Motor start and speed control tech has increased manifold from ward leonard system to VFD.

Muthu
www.edison.co.in

Auto-transformer starters were the starter of choice for high inertia loads such as fans. And, by the way the inertia of a fan was nothing compared to the inertia of a large chipper wheel.
An auto-transformer start will accelerate a fan quite well with reduced starting current.
If the inertia was too much for an auto-transformer starter the next step was a wound rotor motor. Think a large sawmill band-saw with twin 8 foot diameter band wheels.
The economics of a standby generator which may run a few hours a year have no relationship to the economics of a prime power generator or a grid connection.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Edison, there are a few issues at play. Some AVRs derate output as frequency drops to allow the prime mover to recover. Many AVR's draw power from the windings of the generator so there is a kind of cascading effect if the PM can't recover speed ans the regulator derates the regulator itself loses the power it needs to function. Even without an under-frequency roll off, sometimes the regulator itself doesn't have the capability to drive enough field current to keep itself powered during a big motor start. It's just a bad situation all around which is why I recommend separate excitation or PMG for smaller units.

UFRO is almost universal on small AVRs.
The main purpose of PMGs is to prevent voltage collapse in the event of a near in fault.
When a near in fault caused voltage collapse, there may not be enough current to trip the breaker.
Before PMGs became common, CTs were used to provide a field boost to develop enough output to trip the breakers.
A secondary purpose is to prevent AVR burn-out at slow speeds.
The old pre-uFRO sets had a switch to turn off the AVR.
It was common for mechanics running a set at idle to check valve lifter clearance to forget the AVR switch and toast the AVR.
It has been a lot of years since I have seen an AVR switch on a generator panel.
A PMG is great if you have one on the set or are able to spec a PMG on an order for a new set.
It is not generally feasible to add a PMG to an existing set.
I have installed a lot of single phase standby sets that did not have PMGs.
They were all starting as many A/Cs as they could.
As long as there was spare capacity of three times the motor rating, the ACs would start.
Actually, most times the A/Cs would start at 2 1/2 times but the voltage drop was excessive and I don't recommend using a factor of 2 1/2:1

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Quote (John McNutt)

What about them does not play well together? Are the soft start manufacturers making junk that will be problematic in the long run?

Remember, we are talking about SINGLE PHASE motors here. Soft starters work great on 3 phase motors, but lets put that aside.

On a single phase Capacitor Start motor, there is a centrifugal switch that, at rest, is closed, so that the Start capacitor is in the circuit, then it opens and removed the cap at about 80% speed. When you energize that with a Soft Starter, the capacitor charging current, which takes place instantly at the available current level, looks like a short circuit to the SCRs in the Soft Starter, which puts a type of stress called "dI/dt" (D= delta or change, I = current, t = time), so a rapid rate of rise in current. That type of stress is something that SCRs can only handle for a little while before the silicon layers inside begin to fuse, meaning the SCR shorts and can no longer be controlled; it becomes a full time conductor. At the same time, the harmonics created by the SCR phase angle firing, the so-called "chopping" of the sine wave, creates a LOT of harmonics on the circuit. Because this only lasts for the time it takes to accelerate the motor, harmonics from soft starters in 3 phase motors are not a big concern in general. But in THIS CASE (SINGLE phase cap start motor), because the starting capacitor is in the circuit during this time it will absorb those harmonics, like a trap filter, and heat up. Then because the PURPOSE of the soft starter is to EXTEND the acceleration time, the capacitor is in the circuit for LONGER than it was designed to be and eventually the capacitor swells, leaks it's electrolyte, and fails.

They work fine for PSC motors because the capacitor is not an electrolytic and the capacitance is significantly lower, so there is less stress on the SCRs

Those Stellar soft starters are brand-labeled Siemens, or at least they are both buying them from the same Chinese source. I see that they claim they are for use on CS/CR motors, but you can't change physics. The reason why most of the major players in the soft start business don't make single phase soft starters is because they used to and have been burned by multiple failures, or they were smart enough to avoid that pitfall. I cant speak as to why AutomationDestruct has decided to give it a try again, maybe they still have to learn...

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

Well, the capacitor and a motor winding are in series, so the winding limits the di/dt current due to the switching waveform on the capacitor.

Bill

I routinely test high speed centrifugal blowers of 5 to 30 HP via a continuously variable auto transformer. Never seen them reaching full speed until the voltage level is around 90% of rated. Their currents rise as the voltage is increased and drop to rated current only after they reach the rated speed. For shaft mounted fans, I had to block the fan suction for the main motor to get it up to speed even with full voltage applied to the main motor.

And excessive capital cost is excessive regardless of the duty cycle of DG.

VFD's, despite their constraints, still offer the best way to start cage motors, imo. They eliminate overrating of DG sets just for starting, staggered start of loads and other methods of aiding the start like choking the input/output of centrifugal fans, unloading the compressors, fluid coupling etc.

Muthu
www.edison.co.in

Just for my ten cents worth, there are several single phase motor formats and some just will not perform with a soft starter.

My method, which has worked successfully for years, is to only try to soft start single phase motors that you can separate a capacitor series start winding from the main run winding.
Always connect the start winding and series start capacitor to the full voltage, via the capacitor centrifugal switch unless it is a PSC, in which case there is not switch required.
Connect the soft starter in series with the run winding only. This will drop the start current to a lower value, but as always, the lowest start current is a function of the motor current and torque curves and the load torque curve.

From my experience, if you apply the soft starter to the start winding as well as the run winding, the start current will be higher and the start torque will be lower and there will be major issues. The comments of the experts above are very representative of what will happen if you try to control the voltage to both the run winding and the start winding.
Best regards,


Mark Empson
Advanced Motor Control Ltd