Soft Starter for High Inertia Fan Motor Single Phase 5 HP 2

[JohnMcNutt]

Been looking around for a product that meets specification to soft start a single phase fan motor.

5 HP, 3500 RPM, 230 V, 21A nominal, 60 Hz. Currently started DOL but would like to limit inrush to make it easier on standby generator. Current acceleration time is 2-3 seconds estimated.

This product from Carlo Gavazzi looks promising:

https://www.gavazzionline.com/pdf/MC_RGTS_DS_EN.pdf

However, on page 10 they seem to limit it to a 3 HP application.

Do you know of an alternate product that would meet specifications?

 

[TugboatEng]

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.

 

[waross]

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!

 

[jraef]

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

 

[LionelHutz]

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.

 

[edison123]

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

http://www.edison.co.in

 

[Marke]

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