20hz to 60hz

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MBLES

Senior Member
I dont have much experience with vfd speeds.The VFD controls the speed by controlling HZ correct? Is there a chart or calc that shows the speed? If I go from 60hz to 20.9hz and lets say its a 1200rpm motor? is there a formula. I hope that makes sense.
 

Besoeker

Senior Member
Location
UK
Of course it is but we also sometimes need to remember not to swamp people with too much info and overwhelm them. I know I am guilty of doing it at times.
True but, in this case, I think it is a relevant and important bit of information to pass on to the op.
 

MBLES

Senior Member
Maybe its obvious, but voltage must lowered accordingly.

Thanks for help. If we lower the Hz we also need to adjust the voltage? The pump is a 480v 3phase small injector pump that feeds a clarifier for a water plant. The customer keeps the vfd to 20.9 hz I was wondering what other issues this causes to pump by controlling speed with Hz. Thanks for any help
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Thanks for help. If we lower the Hz we also need to adjust the voltage? The pump is a 480v 3phase small injector pump that feeds a clarifier for a water plant. The customer keeps the vfd to 20.9 hz I was wondering what other issues this causes to pump by controlling speed with Hz. Thanks for any help


Correct, the voltage at the motor should be around 160 volts for 20hz, otherwise the motor will run hotter- burning up if the voltage is high enough. The ratio V/Hz should be 7.5-8. In other words the voltage when divided by your actual frequency should always be around 7.5.



http://www.techni-tool.com/site/ART...-frequency ratio on variable speed drives.pdf
 

mannyb

Senior Member
Location
Florida
Occupation
Electrician
Correct, the voltage at the motor should be around 160 volts for 20hz, otherwise the motor will run hotter- burning up if the voltage is high enough. The ratio V/Hz should be 7.5-8. In other words the voltage when divided by your actual frequency should always be around 7.5.QUOTE]

I really do appreciate the help and answers. but how do you lower the voltage? from 480v to 160v does the vfd do that also. Like I mentioned I dont have much experience with vfd I just want to understand the process. I will read the link you shared
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
Correct, the voltage at the motor should be around 160 volts for 20hz, otherwise the motor will run hotter- burning up if the voltage is high enough. The ratio V/Hz should be 7.5-8. In other words the voltage when divided by your actual frequency should always be around 7.5.QUOTE]

I really do appreciate the help and answers. but how do you lower the voltage? from 480v to 160v does the vfd do that also. Like I mentioned I dont have much experience with vfd I just want to understand the process. I will read the link you shared



It should do it automatically. Otherwise it wouldn't be a good VFD. Jraef would know more regarding that however.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
That’s exactly what a VFD does, alter the voltage and frequency together to maintain the same ratio of V/Hz that the motor was designed for. So if the motor nameplate says 460V 60Hz, that’s 7.6 V/Hz (460/60). So as you change the frequency reference to change the speed, the BFD changes the voltage at the same time. So long as you maintain that same V/Hz ratio, the motor can produce it’s rated torque at any speed without causing over or under saturation of the windings, so no harm to the motor (in theory). There are more little nuances as you get into the details of course.
 
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jumper

Senior Member
That’s exactly what a BFD does, alter the voltage and frequency together to maintain the same ratio of V/Hz that the motor was designed for. So if the motor nameplate says 460V 60Hz, that’s 7.6 V/Hz (460/60). So as you change the frequency reference to change the speed, the BFD changes the voltage at the same time. So long as you maintain that same V/Hz ratio, the motor can produce it’s rated torque at any speed without causing over or under saturation of the windings, so no harm to the motor (in theory). There are more little nuances as you get into the details of course.

BFD? Big Freaking Device?:D

Fat fingered the key board....!
 
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