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Old February 20th, 2018, 07:20 AM   #1
drspanda
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Question Power rating of AC Motor as a replacement of DC Motor

Application: Lathe
Present motor: 220V, 60kW, 3000 rpm driven by 6RA70 SIEMENS DC-DC Converter.
Mode: Armature speed control with constant shunt excitation.
Point-1: Gearbox is intact

Now it is proposed to replace DC motor with AC motor, 60 KW, 2 Pole, 50 HZ driven by VFD. Is this okay?

My understanding is if a 60kW, 3000 rpm motor performs the task then 60kW, 2980 rpm AC motor run by VFD should work well. Please note that I am not changing gearbox.

Expert comments are awaited.
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Old February 20th, 2018, 08:04 AM   #2
lfe
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It seems ok to choose a motor rated 60kW at 50Hz.

Having the DC motor constant shunt excitation it seems than the AC asynchronous motor should perform fairly well.
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Old February 20th, 2018, 10:44 AM   #3
jraef
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I would do it in a heartbeat. But you will not get the same torque performance out of the AC motor/VFD unless you use Vector Control, and low speed operation may require separate cooling. So chose your AC system components wisely.
  1. Full Flux Vector Control capability of the VFD. Some will require a Shaft Encoder to attain that, there are a few that can do Encoderless Flux Vector Control. So if your application can't have a shaft encoder installed, make sure you have a VFD capable of Encoderless Flux Vector control. "Sensorless Vector Control" is a marketing term for Velocity Vector Control with improved torque capability and it MIGHT be fine for your application, but to truly match the torque performance of a DC motor / drive, you would need true Flux Vector Control. It's different, and don't let the marketers tell you otherwise.
  2. Get the best Inverter Duty motor you can afford, but understand that the term "Inverter Duty" has many different meanings, so look at how your machine operates and chose based on that.
    1. Make sure the motor "turn down ratio" is low enough to perform what your machine needs. So for example if the machine needs to run the motor at 3Hz, that is a 1000:1 turn down ratio and that will be a top level motor. If you will never go below 300RPM, then you only need a 100:1 ratio, etc.
    2. Also make sure the motor insulation is rated for a MINIMUM of 1488V, the higher the better.
    3. Lastly, make sure the motor has shaft grounding bushings or totally isolated (non-conductive) bearings to deal with stray rotor voltage created by the VFD.
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Old February 21st, 2018, 08:30 AM   #4
drspanda
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Quote:
Originally Posted by jraef View Post
[list=1][*] Full Flux Vector Control capability of the VFD. Some will require a Shaft Encoder to attain that, there are a few that can do Encoderless Flux Vector Control. So if your application can't have a shaft encoder installed, make sure you have a VFD capable of Encoderless Flux Vector control. "Sensorless Vector Control" is a marketing term for Velocity Vector Control with improved torque capability and it MIGHT be fine for your application, but to truly match the torque performance of a DC motor / drive, you would need true Flux Vector Control. It's different, and don't let the marketers tell you otherwise.
I do have incremental encoder

Quote:
Originally Posted by jraef View Post
[*]Get the best Inverter Duty motor you can afford, but understand that the term "Inverter Duty" has many different meanings, so look at how your machine operates and chose based on that.[list=1][*]Make sure the motor "turn down ratio" is low enough to perform what your machine needs. So for example if the machine needs to run the motor at 3Hz, that is a 1000:1 turn down ratio and that will be a top level motor. If you will never go below 300RPM, then you only need a 100:1 ratio, etc.
Where can i get the concept of turn down ratio?
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Old February 21st, 2018, 02:51 PM   #5
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Quote:
Originally Posted by drspanda View Post
Where can i get the concept of turn down ratio?
I just gave it to you.

Motor base speed / Turn Down Ratio = lowest safe operating speed per that design. 3000RPM motor, 1000:1 ratio = 3000/1000 = 3RPM (0.167Hz) is the lowest speed that motor can operate safely.
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Old February 21st, 2018, 03:39 PM   #6
Phil Buchanan
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Quote:
Originally Posted by jraef View Post
[*]Lastly, make sure the motor has shaft grounding bushings or totally isolated (non-conductive) bearings to deal with stray rotor voltage created by the VFD.

I have found it to be a good idea to include a shaft grounding ring even with fully insulated motor bearings if the customer will go for it because with fully insulated bearing if there is a conductive coupling (metal housing / Metal spring or chain coupling) between the motor and gear box (if there is a gear box) i have seen it cause issues with the gearbox bearings as it will take the path through the gear box bearings and through the gearbox frame.
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Old February 21st, 2018, 08:50 PM   #7
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Quote:
Originally Posted by PBuchanan View Post
I have found it to be a good idea to include a shaft grounding ring even with fully insulated motor bearings if the customer will go for it because with fully insulated bearing if there is a conductive coupling (metal housing / Metal spring or chain coupling) between the motor and gear box (if there is a gear box) i have seen it cause issues with the gearbox bearings as it will take the path through the gear box bearings and through the gearbox frame.
Yes, that's the down side of insulated bearings; it just moves the issue somewhere else, unless there is an insulating coupling.
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Old February 22nd, 2018, 09:29 AM   #8
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Quote:
Originally Posted by jraef View Post
Yes, that's the down side of insulated bearings; it just moves the issue somewhere else, unless there is an insulating coupling.
When I started at my current employer they had nothing for this and they would take out the bearings in the motor mounted encoder first and instead of replacing it they would go from FVC mode to SVC mode and run without the encoder causing process issues (film) and next they would take out the front bearing normally.

They would replace the motor because it was faster than sending out for repair and the damaged unit would go on a pallet in the warehouse meaning to get sent out but would sit until the next major cleanup before the CEO tour arrives then it got thrown out and the cycle repeated.

But yet they were amazed that I was able to save so much money on motors in my budget with a few simple changes.
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Old February 22nd, 2018, 10:54 AM   #9
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Quote:
Originally Posted by PBuchanan View Post
When I started at my current employer they had nothing for this and they would take out the bearings in the motor mounted encoder first and instead of replacing it they would go from FVC mode to SVC mode and run without the encoder causing process issues (film) and next they would take out the front bearing normally.

They would replace the motor because it was faster than sending out for repair and the damaged unit would go on a pallet in the warehouse meaning to get sent out but would sit until the next major cleanup before the CEO tour arrives then it got thrown out and the cycle repeated.

But yet they were amazed that I was able to save so much money on motors in my budget with a few simple changes.
All too common of a story, except for the part where they listened to you to save money.

Ive had several cases where they actively decided to keep doing the same dumb things even after being told there is a solution... I cant explain it other than general inertia. Weve done it this way for 25 years, were not changing now.
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Old February 22nd, 2018, 12:04 PM   #10
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Quote:
Originally Posted by jraef View Post
Weve done it this way for 25 years, were not changing now.
If I had a dollar for everytime I have heard that statement I could buy us both top tier steak dinners for at least 2 weeks.
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Old February 22nd, 2018, 06:41 PM   #11
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Full support for jraef's comments on replacing the DC motor. I would suggest the ABB ACS800 drive as one of the best for serious Sensorless Flux Vector control as needed in this application. It will probably do the job without the encoder but keep it around just in case.

jraef's response has a small typo error near the end where he wrote 3Hz when I believe he meant 3rpm to get 1000/1 turndown.

If the old system had a torque guage driven by armature amps, it will need to be replaced by a meter driven by 4020ma analog output with the output mapped to TORQUE, not current.
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