Transformer size required to start a 300HP, 460V motor

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Isaiah

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I've got a 300HP, 460VAC, Induction motor that requires temporary construction power. There is no power source nearby so I'll need to run a temp 5kV feeder from a rather distant switchgear and add a 5kV-480/277V transformer. What question is what is the required transformer size in kVA needed to start/run a 300HP motor, given the huge inrush current of app 6 times FLA?
 
This depends on: Other loads, what type of load the motor is driving and how often it will be started. For typical centrifugal pump or fans, a 500 kVA should be be adequate for full-voltage starting, as long as there are no other significant loads on the transformer. But this is just an educated guess. Not all 300 hp motors are created equal.
 
David Castor said:
This depends on: Other loads, what type of load the motor is driving and how often it will be started. For typical centrifugal pump or fans, a 500 kVA should be be adequate for full-voltage starting, as long as there are no other significant loads on the transformer. But this is just an educated guess. Not all 300 hp motors are created equal.
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500kVA = my thoughts exactly. I was just informed there'll be another 300Hp motor, 'a backup', required off the same transformer. In addition, some flood lighting and miscellaneous smaller loads, totaling around 50kVA....So now I'm thinking I need a 750kVA Xfmr?
 
Based on some rules of thumb that I've seen based on some other calcs and modeling, I'd think 500kVA would be tight for even 1 motor if it's straight across the line starting. 750kVA would be about the minimum that I'd recommend for starting (1) 300HP motor successfully across the line. Plenty of varying factors though. So results may vary.

Across the line starting can be a bear. If it were me, I'd go with 1000kVA since it sounds like you have two. With the understanding that you don't start both at the same time.
 
The IEEE Red Book has a chart that shows the voltage drop by motor size based on the transformer feeding it for across the line starting. It is very useful for questions like this one. I don't have my book handy or I would post the chart.
 
Again, it really depends on what the motor is driving. If torque requirements are low during starting, the voltage drop during starting isn't nearly as much of a concern. That's the entire basis of reduced voltage motor starting. The question will be how will the other loads deal with a temporary voltage dip down to 80% voltage. Also, NEMA contactors only have to hold in down to 85% voltage, although most will stay in down to 80%.

Also, are you sure this motor is going to be full-voltage start?
 
The rule of thumb that I was taught is to assume hp = kva and then size the transformer at 1.5 to 2.5 times the kva. So for this 300 hp motor a 450 to 750 kva transformer.
 
If the transformer is barely sized for the FLA of the motor, or has high impedance, then you 'naturally' get reduced voltage starting.

Depending upon the specific load characteristics this might actually be a benefit, reducing starting current.

Of course the load torque needs to start low because the motor output torque will be reduced, and this transformer couldn't be shared with any load that cares about voltage drop.

But it might pay to have two small transformers rather than one to feed all the loads.

Jon
 
don_resqcapt19 said:
The rule of thumb that I was taught is to assume hp = kva and then size the transformer at 1.5 to 2.5 times the kva. So for this 300 hp motor a 450 to 750 kva transformer.
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That's the rule I've always used too, serves me well.

That rule is based on trying to avoid more than a 5% VD on starting. If the transformer is feeding only the motor, you can often live with more VD, so long as your controls hold in as Dave Castor said. I started some 75HP motors on 75kVA transformers once, the voltage dropped a LOT when they were energized, but we put a UPS on the controls so they didn't drop out. If you are ever at Pearl Harbor and watch the Ford Island Bridge being opened, the hydraulic pumps doing that are the motors I'm referring to.
 
My local power company doesn’t allow across the line starting above 60-125 hp depending on the specific site.

I’ve found that soft starters are actually less expensive than across the line starters at 150 hp and above.
 
Dzboyce said:
My local power company doesn’t allow across the line starting above 60-125 hp depending on the specific site.

I’ve found that soft starters are actually less expensive than across the line starters at 150 hp and above.
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Often on temporary construction power, YOU own the transformer, not the utility, so their rules don't apply.
 
Jraef said:
Often on temporary construction power, YOU own the transformer, not the utility, so their rules don't apply.
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Agree. Actually, PoCos' concern will only be the flicker limits (IEEE 141).
There was an old article by D. A. Lentz that offered a simplified equation to compute for the %V dip (%V= Locked-rotor MVA of connected motor/available short-circuit MVA at POCC!). I hope that helps the OP.
 
Dzboyce said:
My local power company doesn’t allow across the line starting above 60-125 hp depending on the specific site.

I’ve found that soft starters are actually less expensive than across the line starters at 150 hp and above.
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You must be shopping in the wrong places.

To begin with although there is a tiny premium for it, the fact is that a soft start includes both the bypass contactor AND the electronics. So no way what amounts to a contactor alone can be less expensive. Contactors cost roughly $1 per Amp with IEC models. And you still need a breaker/MCP/MMS/fused disconnect, etc., no matter how you build it. So if you are paying more than a soft starter you overpaid at least with ATL starting. With sue delta soft starts at larger sizes can be competitive.
 
paulengr said:
You must be shopping in the wrong places.

To begin with although there is a tiny premium for it, the fact is that a soft start includes both the bypass contactor AND the electronics. So no way what amounts to a contactor alone can be less expensive. Contactors cost roughly $1 per Amp with IEC models. And you still need a breaker/MCP/MMS/fused disconnect, etc., no matter how you build it. So if you are paying more than a soft starter you overpaid at least with ATL starting. With sue delta soft starts at larger sizes can be competitive.
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But the bypass contactor is usually much smaller than an across the line starter.
 
petersonra said:
But the bypass contactor is usually much smaller than an across the line starter.
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It CAN be but not by much and you are a fool if you do that.

In IEC terms soft start bypass is an AC1 load. It’s not inductive or anything, just a switch. You MUST both soft start and soft stop to do this but the contactor will be about 20% smaller. Say 500 A vs 400 A.

Normal starting duty is AC3. And repeated starting/plugging and high current loads are AC4. All three terms also exist in NEMA. AC1 is found under DP types and AC4 starting is a separate chart in NEMA ICS 2. But IEC is easier because it’s all together on the same chart.

With stopping first the soft start has to turn on the SCRs, then open the bypass, then shut down the SCR bridge.

But if sized properly you can use the bypass contactor on its own as an emergency bypass/starter. The soft start contains its own emergency backup.

So going back to my example so an AC3 400 A contactor will run a 350 HP motor. An AC1 contactor will be about 25% smaller electrically although chances are it’s the same frame size and at roughly $1/A cost it saves the soft start company a whopping $100 on a starter package costing around $5000-10,000. Please tell me you are paying $400 vs $5000??

Don’t believe me? Here are Benshaw list prices for contactors:


Here is for soft starters:


Typically most vendors give about a 40% discount to list from these guys.

So using list prices on page 47 we find a NEMA open style starter (contactor plus overload), $1276. On page 50 is the entire starter prepackaged, $4358.

In the soft starter catalog the EMX4i is $4439, page 11, with integral bypass and all 3 phases. Flipping over to the prepackaged starters it’s $7592, page 26.

As I said these are list prices but still based on just open parts the price difference is almost 4 times for a soft start dropping to just under double as a prepackaged “combination starter” where the box, breaker, etc., reduce the difference.

These are American built starters and soft starts built in Pennsylvania, not China or Europe so the prices may look different from Siemens (India) or Schneider (France) or AB (Poland). Motortronics that does a huge amount of private branding and is also US made has very similar prices to Benshaw. And as I said these are list, not street prices that are nearly half as much and even better if you are a panel shop paying supply house prices which.

So unless you are getting ripped off for contactors there is simply no way a soft start can be cheaper. Granted I picked the Cadillac. If I picked a two phase (EMX4E) or no bypassed model you would save about $100-200 but the price differences haven’t moved the needle any.
 
The technically correct way to do this would be to model the electrical system in software to include the Utility strength, cable lengths, transformer impedance and then whatever starting means is employed. If I'm asked whether or not a motor will start I use fire pump criteria from NEC 695 which is 15% Voltage Drop (VD) on startup and 5% when running. The rule of thumb presented in previous posts is safe for initial sizing, but the analysis is necessary if money's being spent.
 
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