Limiting inrush for motor starts where POCO has a motor size limit

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fastline

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And still making no sense to me, we are having a rural electric entrance installed where the POCO has a stated limit of 10hp motors on the line yet will prove as many amps as we want. So I can get 1000A of 240/1p BUT they limit the line to 10hp motors. I have already asked about soft starts, VFDs, etc. They seem to ignore any such conversation and just say "nothing bigger than 10hp".

I have machines that are certainly bigger than 10HP and I have no choice but to do my own work to limit problems on the line and run them. I don't want to be anti-POCO but they are not making any sense to me and I would bet the EE that designs the systems knows that.

I have some machines with 15-20HP motors/VFDs. These would never start across the line. Usually 2-3sec accel rates on them. Based on some testing, I am inclined to consider the highest possible current through the VFD as the limit for a motor start. In one instance, I have a VFD that has a stated overload capacity of 120% of the 30min rating so 105A 230V/3p or 182A 230V/1p.

When I consider the inrush of a 10HP motor, the 1p FLA current should be around 55A so a hard start might be 550A.

Thoughts?


As well, I have to run rotary phase converters for now. I have a few 10HP units that I built. I tested one and got an inrush test of 128A. Now I have heard that RPCs won't have quite the inrush of a direct connected single phase motor but I will do more testing.

This brought me to my next question, if I decided to combine all my smaller RPCs into 1 bigger one, I would not want to try to start that on their line. So I was considering a pony start setup to bring the idler to full speed before energizing it. I have read where some think inrush would be a non-issue with this because the motor is already at speed BUT I know it takes a little time to magnetize the coils and produce the needed impedance. Would there be any way to determine this other than some testing?
 
let me understand this. you knew the POCO has a 10 HP limit on motor size but went and bought a bunch of equipment that exceeds that limit anyway.

why not buy equipment that has 10 HP motors? you can probably swap out the existing motors and maybe put two smaller motors in their place in a lot of cases. would be simpler than a PITA RPC.
 
ptonsparky said:
How will the POCO know, unless you invite them in if you are using vfds to ramp motors slowly?. In short, start nothing that exceeds that of a 10 HP 1Ph at start up. It causes the neighbors lights to blink. They cant more than disconnect your power.
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a VFD would be a better choice than a rpc. I hate them things.

I'd be inclined to get a 240 to 480 V xfmr and step it up so you can use 480V single phase input and 480V motors. makes the VFDs less expensive usually.
 
My guess is POCO doesn't want more then 10 HP started across the line.

Using a phase converter is still across the line starting, unless it is a VFD or other reduced voltage starting method.

Using a phase converter probably does limit the inrush some because of impedance in the converter though.

Whoever you talked to at POCO maybe don't know what the goal is with limiting inrush currents and just knows that 10 HP is the general cap on single phase services.
 
Not being said clearly here (but he said it in another forum) is that the PoCo has told him that he can have a 100kVA 1 phase service transformer, but STILL must limit the maximum motor size to 10HP, regardless of HOW MANY motors he has; in other words he can have multiple 10HP motors, but not one 15HP or 20HP motor. Makes no sense.

I think it's either a communication issue (i.e. that's not what he really meant) or the PoCo guy is just being lazy and not wanting to think about the REASON for the rule, just reiterating the rule itself as it reads.

I'll say again, you need to take this over his head for clarification, preferably to someone who actually understands WHY they have a HP limitation.
 
ptonsparky said:
We used to have a 10 hp limitation. An ice storm about ten years ago forced a sudden rebuild of POCO lines and they did not skimp.
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There is a 50 hp irrigation well just a little over a mile away from where I live that has been there for at least 30 years, maybe even longer. Supplied by a static phase converter. Largest motor I have personally seen on a phase converter.

But it is existing - today they likely wouldn't want to supply that from a single phase line.
 
kwired said:
There is a 50 hp irrigation well just a little over a mile away from where I live that has been there for at least 30 years, maybe even longer. Supplied by a static phase converter. Largest motor I have personally seen on a phase converter.

But it is existing - today they likely wouldn't want to supply that from a single phase line.
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Not entirely sure if the "50HP" is based on the motor size or what but mark it guaranteed that it ain't doing 50HP worth of work. Off the cuff, to get any reasonable life with a static converter, they may be able to do 25HP with the 50HP motor. After learning lots on the converter area, I sort of wish statics were uninvented because people sell them, they look easy and cheap, them people burn out their 3P motors over it.

I had a small hydro pump motor on a rotary converter running hot a tripping the overloads because the voltage balance was at about 6%. I retuned it to 2% and problem solved.

The main issue with rotary though is that they cannot maintain voltage balance throughout their load range thus the need for oversizing in many situations with closer tuning. People think it is just plug and play but really IMO every converter should be tuned for what it is doing. In the above example, you might consider that a +2% that would swing to -2% at higher loads. I have to make a judgement call on tuning.
 
fastline said:
Not entirely sure if the "50HP" is based on the motor size or what but mark it guaranteed that it ain't doing 50HP worth of work. Off the cuff, to get any reasonable life with a static converter, they may be able to do 25HP with the 50HP motor. After learning lots on the converter area, I sort of wish statics were uninvented because people sell them, they look easy and cheap, them people burn out their 3P motors over it.

I had a small hydro pump motor on a rotary converter running hot a tripping the overloads because the voltage balance was at about 6%. I retuned it to 2% and problem solved.

The main issue with rotary though is that they cannot maintain voltage balance throughout their load range thus the need for oversizing in many situations with closer tuning. People think it is just plug and play but really IMO every converter should be tuned for what it is doing. In the above example, you might consider that a +2% that would swing to -2% at higher loads. I have to make a judgement call on tuning.
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It is a 50 HP motor, I agree it likely is not doing 50 HP of work. My experience with static converters is you need a constant load and an oversized motor if you are going to have much success with it. Any time I have seen a static converter added to a machine with an existing three phase motor sized to the load it always had problems.
 
I recently ran into a related issue with a farmer friend. He was indicating a few specs from his pump and such and I realized he was not doing the work he thought he was due to the reduced head pressures. Basically you can have a 50HP motor and a "50HP pump", but not do 50HP worth of work because the pump was designed for more head and less volume. In my example, his pump could do more head pressure but the volume was maxed out.

Thus, it is possible that this setup is working with a 50HP pump but due to the loading, 50HP is not achievable.

I proved this to my dad a while back where he had a 2HP pump moving over 100GPM simply because there was no head pressure. He was not convinced when I told him a 1HP motor would have run the load.



In the case of the ag pump, he was indicating that when he turned on the end gun on the pivot, the pump power would actually drop. That is because he reduced head pressure and the pump volume was already maxed out. Only ways to fix that is a higher volume pump or overspeed the pump to raise the volume and bring back some head pressure.


All interesting stuff.
 
Is this possibly an issue with how you would be billed? Peak demand charges and all...

Simultaneously starting 3 10HP motors across the line would be fine with them, but one 20HP on a VFD isnt? jraef is right, that makes zero sense.
 
JFletcher said:
Is this possibly an issue with how you would be billed? Peak demand charges and all...

Simultaneously starting 3 10HP motors across the line would be fine with them, but one 20HP on a VFD isnt? jraef is right, that makes zero sense.
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Startup of those motors doesn't occur over a long enough duration to matter for peak demand billing.
 
The load on motor is irrelevant when it comes to starting inrush. If the load has particularly high inertia, it extends the duration, but not the across the line inrush. The winding impedance of a stopped motor depends on the motor size.

The very process of starting a motor on 60 Hz is similar to starting a load on a line shaft with a clutch. If the load step is large relative to the line shaft system size, it affects the other users. The electrical equivalent is flicker on other customers sharing the transformer or even onto primary if this is a rural service.

10 hp limit on a single phase service is reasonable. A 10 hp VFD will have a huge non-linear current unless it has an active PFC front-end.

An MG-set with an over-sized alternator will provide good motor start capability and you can limit inrush by taking however long as necessary to start the set which is almost all inertia meaning very little torque is needed to start and can start through a starting rheostat.
 
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