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VFD output amperage question

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
Looking at new Mitsubishi VFD
Input 200/240VAC 10.5A 3 phase,
Output 200/240VAC 7.0A 3 phase.

Too small for 3HP 3 phase motor ? Motor is on #1 Hendey 12" X 42", type using flat belt on motor with belt pulley changer in the base that connects via Vee belt to spindle.
 

atomarc

Diamond
Joined
Mar 16, 2009
Location
Eureka, CA
It could probably be configured to start and run the motor, but technically, it's too small.

The max O/L setting on the drive could possibly fall short of the max nameplate FLA(9.6a) of the 3 hp motor, so there would be no effective O/L protection for said motor.

Stuart
 

SomeoneSomewhere

Hot Rolled
Joined
Dec 24, 2019
I'd argue the opposite; the drive not being to output enough current to overload the motor makes for an *extremely* effective overload protection.
 

atomarc

Diamond
Joined
Mar 16, 2009
Location
Eureka, CA
I'd argue the opposite; the drive not being to output enough current to overload the motor makes for an *extremely* effective overload protection.

Interesting thought.:) Doesn't this mean that the motor overloads the drive and kicks it out, meanwhile the 3hp motor can't reach full potential for its intended use.

Isn't this the 'lose-lose' scenario of using a drive too small for the load..part of the 'no free lunch' syndrome!

Stuart
 

eKretz

Diamond; Mod Squad
Joined
Mar 27, 2005
Location
Northwest Indiana, USA
Interesting thought.:) Doesn't this mean that the motor overloads the drive and kicks it out, meanwhile the 3hp motor can't reach full potential for its intended use.

Isn't this the 'lose-lose' scenario of using a drive too small for the load..part of the 'no free lunch' syndrome!

Stuart

Yes. Fine idea in a pinch, but terrible idea if it's not necessary.
 

mksj

Cast Iron
Joined
Aug 10, 2010
Location
Tucson, AZ
You also need to factor in derating, if running off an RPC there would be some further derating and single phase basically these days a derating factor of around 2 +/-. Dives can also be dialed up for overload over short periods of time, say if you are using it at speeds below its base. When sizing the VFD you need to look at the motor actual amps for its rated Hp/V input. I wouldn't bother with the Mits.
 

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
One question I have before giving up on the new Mits. is the lathe I'm putting it on I never use hard. Basically is a new version of a flat belt lathe, if my memory is correct it was made in 1952.
I have Gear Drive 14" and 24" Hendey Plus the newer Model C Pratt&Whitney 16" and a 14" L&S Model X.
Some time ago I did check motor info, it's not easy to get to! If my memory is correct 240VAC was 7.2A but that would be at full 3HP load. Also think it's a 6 pole motor.
 

mksj

Cast Iron
Joined
Aug 10, 2010
Location
Tucson, AZ
VFD's you need to compare output amps to motor amps, and also apply the VFD operating mode and any derating factors. So if the motor is 7.2A you could probably get by with the Mits you have, but the typical 2 Hp VFD will have an output current in HD mode (which is what I use for lathes) in ~10A range in ND mode and ~7A in HD mode. It will turn, but if you are using the VFD for variable speed, performance may be less than expected below the base speed. A decent 3Hp VFD would run you ~$230 (Fuji FRN0012C2S-2U).
FRN0012C2S-2U | Fuji Electric |
 

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
Guess I'll pass on the Mits. I do want it for speed Change. I did remove the belt changer and rebuilding it. I thinks someone did take it apart but don't think they assembled it correctly. One side has a roller other side don't. When pushing a belt from one pulley to the other I'm thinking the roller should be on the side pushing the belt onto the larger OD pulley. Now to change speed I use the handle to lift the motor and change speed when needed with motor off. I did pick up a few NIB FVDs, name I can't remember but top USA brand but they are 480VAC VFDs, did purchase a transformer but don't have time to change voltage. Control for this lathe in in the cabinet base leg on the tail stock side. Rating is in Motor HP not amperage.
 

atomarc

Diamond
Joined
Mar 16, 2009
Location
Eureka, CA
As was noted above, a drive of the proper size is so darn inexpensive it makes basic sense to just buy one and carry on with life. Dinking around with transformers or undersized drives is just a exercise in futility..IMHO. What is your time worth?

Stuart
 

Froneck

Titanium
Joined
Dec 4, 2010
Location
McClure, PA 17059
First I have 2 machines in my shop that have 480VAC only motors so I make 480VAC. I have a few new top name (all the same) VFDs that are paid for and rated for the motor they will be used on. Only issue is they are 480VAC. I'm retired and run my shop to make spare cash and to complete ideas. The little extra time needed to connect a 480VAC VFD that I already have is better than spending more money.
 








 
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