Hi all. This is my first post on here so bear with me if I do something wrong. I have been backed into a corner by a customer on powering a 5 axis bridge saw that he is building from scratch. Generally the saws come from Italy with specific voltage requirements to where all that is needed is to get power to the cabinet. This contraption that is being assembled is going to be manually operated and will have one 23hp 230v 3 phase motor and 5 small low speed servo motors that pull less than 1 amp and will only be operated one at the time. I've been trying to figure out what size transformer to purchase to start this motor. I was looking at a 45KVA but have concerns on the inrush amps due to him not soft starting the motor. Any input would be greatly appreciated.
Transformer For starting a motor
- Thread starter Creteus
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iceworm
Curmudgeon still using printed IEEE Color Books
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- North of the 65 parallel
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- EE (Field - as little design as possible)
First, I don't know anything about a bridge saw - five axis or 10 wouldn't matter.
Here are some questions I would ask:
1. How important is that it work first time, every time - No coughs, chokes, flickers?
Or
Can it fail to start occasionally, requiring a reset - and the customer won't get upset?
2. The five small servo motors - do they require any electronics for control? If they do:
What can the electronics take for sags, flicker, dropout.
Is there any power conditioning for the electronics?
Welcome to the world of being the Engineer of Record.
ice
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
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- EE (Field - as little design as possible)
I'm not concerned with the servo motors. All I was asked to do is make the 23hp start. However that being said the information has been changed several times and so has the design which gets me in the corner that I mentioned. There are multiple motors for multiple countries with all kinds of different requirements, but the common voltage is 230v, the biggest motor is 23hp and none of them will start at the same time.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
If starting across the line, use 3x HP = kVA of the transformer if starting the largest one first. If others start first, add them as 1x HP = kVA then add the 3x kVA of that larger one. So if the 23hP is going to start first, minimum kVA would be 69, so a 75kVA transformer. If you have 15HP steady running when it starts, it's 15kVA + 69kVA so 84kVA min, which means a 112.5kVA transformer.
The reason why they gave you oddball HP values is because these are 50Hz IEC kW rated motors being use in a 60Hz world, so they are de-rated to equivalent HP values. It also means these motors are already on the ragged edge of their tolerances. Unlike our motors that generally have a 1.15 service factor to accommodate voltage swings and such, IEC motor designs already have no such thing, they are ALWAYS 1.0 service factor in other words, then on top of that they are being de-rated for use here. So anything that increases their stresses will result in rapid failure. Under sizing the transformer and causing too much voltage drop is a sure fire way to shorten their useful life.
The reason why they gave you oddball HP values is because these are 50Hz IEC kW rated motors being use in a 60Hz world, so they are de-rated to equivalent HP values. It also means these motors are already on the ragged edge of their tolerances. Unlike our motors that generally have a 1.15 service factor to accommodate voltage swings and such, IEC motor designs already have no such thing, they are ALWAYS 1.0 service factor in other words, then on top of that they are being de-rated for use here. So anything that increases their stresses will result in rapid failure. Under sizing the transformer and causing too much voltage drop is a sure fire way to shorten their useful life.
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- Tennessee NEC:2017
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- Semi-Retired Electrician
You didn't believe this post?
Jraef is an expert in motors, controls, transformers, etc.:thumbsup:
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- Massachusetts
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- Wisconsin
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- PE (Retired) - Power Systems
He is only one of several on this forum.
The transformer appropriate size is dependent on the loading characteristics not just the motor size. For an unloaded saw blade, I am sure the OPs transformer will work. However if the saw stops while cutting, it will likely not restart.
For example: Acme, a large manufacturer of transformers, says:
The minimum transformer KVA rating required to operate a motor is calculated as follows:
NOTE: If the motor is to be started more than once per hour add 20% additional kVA.
Care should be exercised in sizing a transformer for an induction type squirrel cage motor as when it is started, the lock rotor amperage is approximately 5 to 7 times the running load amperage. This severe starting overload with result in a drop of the transformer output voltage. When the voltage is low the torque and the horsepower of the motor will drop proportionately to the square of the voltage.
Example: If the voltage were to drop by 70% of nominal, then motor horsepower and torque would drop to 70% squared or 49% of the motor nameplate rating.
If the motor used for starting a high torque load, the motor may stay at approximately 50% of normal running speed as illustrated by the graph below:
Electric-Light
Senior Member
What's the service voltage? I'd think that European motors are 400Y/230 so they'd run fine across the line on 480v at 60Hz, however you're going to spin it at 120% speed. I'd check to see if the 4' diamond blade is acceptable to use at 120% of equipment's rated speed.
Same motor makes 20% more horsepower at 480v/60Hz than on 400v/50Hz. Same torque, but spins 20% faster.
If you run the motor at 415/230 60Hz, you'd actually UNDERVOLT it. You need to maintain the V/Hz ratio.
You have to change pulleys or use a VFD if 120% rotation speed isn't acceptable.
Same motor makes 20% more horsepower at 480v/60Hz than on 400v/50Hz. Same torque, but spins 20% faster.
If you run the motor at 415/230 60Hz, you'd actually UNDERVOLT it. You need to maintain the V/Hz ratio.
You have to change pulleys or use a VFD if 120% rotation speed isn't acceptable.
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- Location
- Massachusetts
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
No, we took a poll. He is the only one.
[Snark = on]With your antipathy of engineers, I can believe you do feel that way.[/Snark]
kwired
Electron manager
- Location
- NE Nebraska
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- EC
Jraef was sizing his transformer for across the line starting, and taking into consideration possible voltage sag that may be an issue for other equipment powered from same source, he may be ok with the 45KVA if using a VFD or soft starter.
- Location
- Massachusetts
[Snark = on]With your antipathy of engineers, I can believe you do feel that way.[/Snark]
:lol:
Honestly, I do have much respect for engineers knowledge and training. I am all but certain I would have washed out had I tried to become one. I was just kidding about Jraef being the only one. You may recall I PMed you once asking how it was you knew so much about so many products.
That said I can still find humor in how an engineer could make tying a shoelace a complicated endeavor.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
That said I can still find humor in how an engineer could make tying a shoelace a complicated endeavor.
And where would we be without Dilbert and Sheldon in our lives?:happyyes:
That's why I wear loafers and my tennis shoes have velcro fasteners.:lol:
Honestly, I do have much respect for engineers knowledge and training. I am all but certain I would have washed out had I tried to become one. I was just kidding about Jraef being the only one. You may recall I PMed you once asking how it was you knew so much about so many products.
That said I can still find humor in how an engineer could make tying a shoelace a complicated endeavor.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Ex'?pert: slang mispronunciation of "Ex spurt".
Ex... a has-been
Spurt... a drip under pressure.
Engineer: main conductor on a locomotive.
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- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
Ok, I'll rephrase then.
Jraef "used to be a drip under pressure" that liked to talk about various products used to put more or less umph in things that make other things run.
milldrone
Member
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- Northern California
Ditto on the two quotes. I had the pleasure of attending along with 5 of my sparkies, his VFD 101 class. The excitement he sparked in my co workers is unbelievable. Normally when I hand out workorders for the day, the sparkies will hide out after finishing them, until quitting time. Now they are setting up "mini labs" to prove what he discussed.
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- Licensed Electrician
Jraef is available live and in person? I want to know more.
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