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Testing 5hp converter

Don Gitzel

Aluminum
Joined
Jul 25, 2017
I assembled a 5 hp converter using a Baldor 3600 rpm motor as the idler. To test it a connected a 5 hp Hico (Korean) Motor. The Hico is 1725 rpm, four pole, 60 cycle 230 volt and considerably larger than the Baldor. After the converter is started, I connected the test load via a knife switch. The Hico turns but will not come up to speed. I then connected the Hico up to a 10 hp converter and it runs fine. What’s with that??? I welcome your responses. My learning curve increases. Ha ha
 
Usually the load should be no larger than 66% of the converter size. That rule of thumb tends to ensure there is enough current on the generated leg to spin the load motor.

The matched 5 HP load and source could not handle it, the 10 HP source could.
 
I assembled a 5 hp converter using a Baldor 3600 rpm motor as the idler. To test it a connected a 5 hp Hico (Korean) Motor. The Hico is 1725 rpm, four pole, 60 cycle 230 volt and considerably larger than the Baldor. After the converter is started, I connected the test load via a knife switch. The Hico turns but will not come up to speed. I then connected the Hico up to a 10 hp converter and it runs fine. What’s with that??? I welcome your responses. My learning curve increases. Ha ha

Ignore.. We'll come back to it...HOW a load-motor gets "up to speed'..

WHEN it has done that, it is a "consumer".. but only when actually UNDER "load".
UNTIL.. it IS "loaded', it actually functions just as much as a supplement TO the "pilot" idler as it does a "consumer" of power.

PM is full of situations where a guy turns-on OTHER 3-Phase machines to run idle.. so as to get enough total reserve online to START a heavier load than the idler alone can haul. Then once his problematic load IS up and running, shut's down those other machines and goes and does the work.

If the workload is a LIGHT one? The "load motor" is STILL contributing partly to the overall stability.

So we have situations where a 3 HP idler is powering a 5 HP machine motor. Which might only see a "real" load asked of but 1.5 -2.5 HP from it.

"Back to"...

In this case, even though your test motor was NOT loaded, it was not that easy to "start".

So it never reached the "happy place" where it was actually aiding the idler...until it DID have a heavy load demanding service. it's like water skis. FIRST you have to get "up".

JST mentioned the 66% or idler @ 1.5 times the HP for load HP .. a very common rule of thumb.

- That works more often that not.

- One can STILL add the temporary - or even full-time - contribution of another machine. Or even several. And "get by".

More often than not.

Even so, some of the longer-serving commercial makers of RPC have published examples of progressively harder-starting types of load.. that call for FAR greater multiples of RPC idler to load motor.

Four and more times oversize, even.

At which point?

Finding POWER to get the idler ITSELF up to speed may have become a "non-trivial exercise". Even with "pony start".

"Works well, and lasts a long time" RPC generalizations are still subject to "TANSTAAFL"

:D
 
Now I have two 5 hp motors. A Hico and a Westinghouse. My 5hp Baldor Converter will start the Westinghouse easily but not the Hico. How could these motors differ?
iCloud
iCloud
 
Check for different "codes" on them. The code letter indicates the range of locked rotor kVA per HP for the motor, which is an indication of the load that it puts on the RPC when being started. (NEMA motor have them, but import motors may not).

I would expect the harder-to-start motor to have a higher locked rotor kVA/HP.

The difference between motors is largely to do with the design of the rotor. Higher resistance rotors generally have lower starting currents. The difference may be due to the bar design, or the portion that connects the ends of the bars. Winding differences, and the design of the stampings for the stator can also have an effect.

OK I got the icloud photos to open, finally

Both of those are "Design B", but only one has a locked rotor code, and it is NOT the Westinghouse. The code J is average for a 5 HP.

So not much info from that.
 
Ok. The Hico has a locked rotor code j. (7.1 - 7.99 kva/hp). What , in layman’s terms does that mean? Seems like useless info to me. I don’t know how to apply it or what significance it has in choosing a motor , or determining a fit.
 
Now I have two 5 hp motors. A Hico and a Westinghouse. My 5hp Baldor Converter will start the Westinghouse easily but not the Hico. How could these motors differ?
IMG_1324.jpg
This one is an older model, only has a 1.0 SF (service factor), not listed as suitable for 208V, has a lower namplate FLA (full load amps), and has no efficiency rating or locked rotor code rating.

IMG_1325.jpg
This one is a newer design, has a 1.15 SF, listed as suitable for 208V, has a higher FLA rating, and has the efficiency rating marked and a locked rotor code.

From my field experience, higher efficiency motors have a much larger starting current, for the same load. This motor was also built with a higher input voltage variance, and a higher SF rating. Meaning it has lower resistance winding's and rotor, more mass to tolerate a wider voltage range and extra load capacity.

I've been on jobs where all the motors were swapped out for higher efficiency models in order to save energy. Hard starting loads like reciprocating compressors would not start reliably on the same circuit and motor starter as the older motor, which had been running daily for many years. The circuit supplying it could not be readily upgraded, so we had to replace it again with a less efficient motor, in order for it to run the compressor again. The contractor that replaced them had disposed of the original motor before I received the service call.

Higher efficiency, normally means higher starting currents.

SAF Ω
 
From my field experience, higher efficiency motors have a much larger starting current, for the same load. This motor was also built with a higher input voltage variance, and a higher SF rating. Meaning it has lower resistance winding's and rotor, more mass to tolerate a wider voltage range and extra load capacity.

Seconded.

Actually mentioned several times, and by many, when assessing the suitabilty of a motor for specific use as an idler.

Older, lower "efficiency" units have categorically been easier to start and easier to "balance" - also more tolerant of not being particularly fussy about any balancing at all.

"On my plate" is instrumenting two nearly identical APPEARING 7.5 HP Reliance going into the new idler "array".

Or - for one of them, maybe NOT?

- One is a moderate-efficiency classical heavy-frame "Duty Master"

- The other is a higher-efficiency unit, but also optimized for nearly constant torque applications, an "E-Master". It is dual rated as 5 HP off VFD power, 7.5 HP off pure sine-wave power.

I dont actually expect that one to make a very good idler at all! Target of opportunity, finding both "NOS", and at affordabe freight rates.

Doesn't HAVE to become an idler, though. Had been in the plan for that contributor to be but a 5 HP anyway.

"Plan B" is that it replace the marginal 7 Euro-pony-power motor in the HBX-360-BC lathe.

Stable torque optimization design is a tad closer to the Dinosaur Current the 10EE have spoilt me with!

Should keep me from getting bored, anyway..

But damn.. that's getting to be just about as expensive a goal in life as back when it was older whiskey, younger women, and faster cars and light aircraft!

"Song" weren't ever much use after being overly intimate with Nitrate explosives!

:(
 
Might be....

The code "J" is not out of line for the 5 HP, so the higher efficiency (if it really is such) is not that much more locked rotor kVA, since it is still in that range. And design "B" (which both are) is generally not a "high" start current design. It is not particularly LOW either.

It may be that the Westinghouse is a lower start current motor, even though of a nominally same/similar design. There is no code letter given for it, so we cannot compare them.

Lots of variables.

Generally, you would use the code letter to determine locked rotor amps, and therefore the starter size.
 
High Efficency Motor Starting Currents

A few articles discussing high efficiency motor designs, starting currents, and marking requirements.

Problem of motor protection and high motor inrush currents
The National Electric Code (NEC) was changed slightly in 1996 to address this problem. The problem stems from the fact that the NEC allows certain settings for HMCPs (currently 800% of full load current, 1100% for design E motors) based on the motor’s locked rotor current (LRC), which is generally 600% to 700% of full load current (FLC). However, with high efficiency motors the inrush current may exceed the 800% of FLC.

2016-DOE-NEMA-Premium-Efficiency-Rule
Beginning on the June start date, manufacturers are required to stamp the NEMA Nominal Rating on their motors’ nameplates, signifying that the motor operates at that efficiency under those "standard” operating conditions.

Protecting high-efficiency motor circuits. | EC&M
The reason for the concern about routine uses of the 1300% factors has to do with high-efficiency motors, such as those described in the question. These motors achieve their efficiency by reducing resistive losses in the motor windings, which simply appear as waste heat during operation. If a motor has very low resistance, however, that means that at standstill the motor will draw a very high current. These currents may well be appreciably higher than even the 1300% parameter, and that is what motivates these questions.

It's not a might be, it's a documented phenomenon.

SAF Ω
 
The starting inrush is not the issue. It is what is needed to establish a mag field in the iron.

After that pulse is over the remainder of the draw is going to be related to the LRA and the kVA/HP.

But that is just a detail. The real issue here is that the idler is the same size as the load, and the approximate draw of the load is going to be equal to the draw the idler would have if started on 3 phase. The idler normally does not supply as much output as it would take as starting current, due to losses.

Since the "J" designation is common at that power, both are likely to be "J" motors. But because the output is going to be lower than the draw current as a load (due to various losses) the idler simply cannot effectively start an equal sized motor.

A larger motor (rule of thumb 1.5x power) is needed to have the losses lower and have the ability to hold up voltage and provide accelerating current to get the load to full speed.

If the load motor is a higher efficiency motor, then more of the applied voltage may be made up of back EMF, and less by resistive losses. So it may be harder to get it "over the hump" and past the high current draw range.

But the kVA/HP is in the normal range for 5 HP, and this does not seem to be an initial inrush current matter, rather a matter of too much current draw to get it started, well after the initial inrush is over. The "J" is normal for a 5 HP, and I would NOT expect a regular 5 HP idler to be able to reliably start a regular 5 HP load motor.

If the OP said a 7.5 HP would not start the HICO, even that might not be too unusual, some motors with certain inertial loads are just hard to start. But the 5 HP would not, and the 10 HP did. Seems about par.

I would not consider it unusual unless the 10 HP had been unable to start it. The 2x load HP is a usual rating RPC makers give for "hard to start" loads. It's well known, for instance, that a particular lathe brand uses WEG motors made in Brazil I believe, and they are notoriously real pigs to start with an RPC.

This type story has been told before with less success reported.
 
All we know is what we see. The motor does NOT start. It sits and hums until it trips the protective device(s).

So if that is not a "starting", issue, what more politically-correct name should we be assigning to it?

........
Why is this a point of contention, then?

Because the problem is NOT what you said. The op says the HICO motor "turns but does not come up to speed".

AND therefore, per what is reported, the breaker is NOT opening on inrush, but on legitimate start current, so the idler is not quite capable of supplying sufficient generated leg to get the motor pulled up to speed. (the OP can confirm or clarify his statement as needed)

That is typical of an undersized idler, and is 100% what would be expected from equal size of idler and load motor, especially if the load motor is a bit of a "pig" to start. The Westinghouse is the same nominal "design", but we have no LRA code letter for it. Presumably it is an easier starting unit, since it DOES start. How it would RUN under load with the 5 HP idler is not known.

Confirmed by the fact that the 10 HP idler does start the HICO.
 
Yes the Hico turns slowly and I interrupt the power before the breaker trips the branch circuit. I recently tried to start the Hico and the Westinghouse wired in parallel, with the 10 hp. That tripped the breaker.
 








 
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