Frequency conversion

Tony S

Senior Member
You asked what I would do.

My colleague had a Russian open cheque book, something we all dream about.

I’ve always had financial constrains on projects. I’ll admit to some “creative accountancy” to pull a project in just about on budget. Three times over on one. Once it’s working the accountants can’t snatch the money back, they complain but have to get the cheque book out.
 

Sahib

Senior Member
Location
India
That is okay, in cases the cost of VFD is higher thanthe motor itself. Is it applicable in op case?
 

Ingenieur

Senior Member
Location
Earth
i would try to run it at 50
you need a stepdown xfmr mv-lv anyways
get one that can be tapped 400-480 or as close as possible
set at 400 V so 400/50 = 480/60
Retap or replace cntl xfmrs if required

see what happens

if we knew more we could makevan informed decision
what type of machine, system, device is it?
what is the load profile
how many motors and what hp
how loaded are they? 50%, 75%, 100?
any existing vfd's
 

Sahib

Senior Member
Location
India
i would try to run it at 50you need a stepdown xfmr mv-lv anywaysget one that can be tapped 400-480 or as close as possibleset at 400 V so 400/50 = 480/60
Major condition met. Other conditions, motor cooling at 50hz, load requirement at 50 are also to be checked.
 

Ingenieur

Senior Member
Location
Earth
Cooling would be an issue
current remains ~ the same v goes down 16%, power down about 20%

but torque should remain ~ the same???
assume 100 hp at 1760 rpm T = 300 lb ft
if we drop power to 80 hp and speed to 1470 so T = 285 lb ft, 5% less
makes sense since T ~ i ~ field strength

an issue may be controls
assuming 480:120
400/4 = 100 vac, marginal
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Cooling would be an issue
current remains ~ the same v goes down 16%, power down about 20%

but torque should remain ~ the same???
assume 100 hp at 1760 rpm T = 300 lb ft
if we drop power to 80 hp and speed to 1470 so T = 285 lb ft, 5% less
makes sense since T ~ i ~ field strength

an issue may be controls
assuming 480:120
400/4 = 100 vac, marginal
Torque remains the same since the flux strength is the same. Flux strength is determined by the V/Hz ratio and 400/50 is the same as 480/60. So if torque remains the same but speed drops by 20%, that's the only reason the power rating drops by 20%. For most machines, it's torque that we need, speed is secondary. It might still be important, yes, but if it were me, I'd try it. I've done this sort of thing dozens of times and generally speed is relatively easy to deal with.

You're right about the controls however. I've found that to be trickier than the speed issues a couple of times. Separating everything out to find the frequency sensitive devices was harder than just converting it all to 24VDC and using power supplies that don't give a darn about input frequency.
 

topgone

Senior Member
A 20% reduction in output would result in case of fan/pump loads.
More like 40%.
AFAIK, when your speed changes in a fan/pump, the afffinity laws are:
1) Volume output/discharge: Q1/Q2 = N1/N2, in this case : Q2 =5/6 x Q1 = 83%Q1 (17% decrease)
2) Change in head/pressure: p1/p2 = (N1/N2)^2: p1 = 25/36 x p1 = 69%p1 (31% decrease)
Lastly:
3) Power drawn: kW1/kW2 = (N1/N2)^3; kW2 = 58%kW1 (42% decrease)
 
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