If a motor runs on 230V at 47A, how many amps will it draw at 208V?

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EEEC

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Benicia, CA, USA
First, the manufacturer said it will run fine at the lower voltage. Next, I have tried the "is / of" ratio formula and that doesn't seem to work (the result is a lower amperage which I know to be false). I also tried the "percentage" formula which I believe gave me the same answer (45A and some change). Will anyone share the formula or at least how to derive it? Thanks!
 

LarryFine

Master Electrician Electric Contractor Richmond VA
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Henrico County, VA
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Electrical Contractor
First, the manufacturer said it will run fine at the lower voltage. Next, I have tried the "is / of" ratio formula and that doesn't seem to work (the result is a lower amperage which I know to be false). I also tried the "percentage" formula which I believe gave me the same answer (45A and some change). Will anyone share the formula or at least how to derive it? Thanks!
A motor draws more current at lower voltage, not less.

I have no formula, but using the theory that a motor tends to be a constant-power load, I get 51.97a.

You can check this against a couple of motors with specs at both voltages.
 

ActionDave

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Durango, CO, 10 h 20 min from the winged horses.
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Depending on the load and the motor a 230V motor will most likely run just fine on 208V.

Until a motor is loaded it will draw lower amps at lower voltage. I know that is not what everyone says but it is true.

If your motor is not loaded to the max and the manufacture says it is ok then I would have no problem trusting what they say.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
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Henrico County, VA
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Dave, that doesn't answer the question, which is: presuming why he's asking, for circuit calculations, the max current, corresponding to the 47a at 230v rating.
 

junkhound

Senior Member
Location
Renton, WA
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EE, power electronics specialty
Assuming single phase induction motor, depends greatly on the type of load.

Constant power load? Positive displacement pump? Centrifugal pump? Fan?, etc. Reduction in speed at 208 could result in lower current as load for something like a fan drops as cube of speed.
 

kwired

Electron manager
Location
NE Nebraska
Typical three phase induction motor I had been taught long ago that 10% higher current at 208 volts is pretty typical.

If this is a single phase motor, that may still be true for a CSCR motor, but other motor types maybe not so much.

Since it is a 47 amp motor - if single phase that makes it maybe ~10 hp motor or even a specialty motor as many 10 HP only have a nameplate FLA of about 40, otherwise seems likely with that high of current it is probably a three phase motor.
 

Besoeker3

Senior Member
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UK
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Retired Electrical Engineer
Assuming single phase induction motor, depends greatly on the type of load.

Constant power load? Positive displacement pump? Centrifugal pump? Fan?, etc. Reduction in speed at 208 could result in lower current as load for something like a fan drops as cube of speed.
Good questions. But there not likely to be a significant drop in speed if the frequency doesn't change.
 

Besoeker3

Senior Member
Location
UK
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Retired Electrical Engineer
First, the manufacturer said it will run fine at the lower voltage. Next, I have tried the "is / of" ratio formula and that doesn't seem to work (the result is a lower amperage which I know to be false). I also tried the "percentage" formula which I believe gave me the same answer (45A and some change). Will anyone share the formula or at least how to derive it? Thanks!
What is the FLC of the motor?
 

drcampbell

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Location
The Motor City, Michigan USA
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Registered Professional Engineer
I'm sure the essential questions are "How much circuit capacity needs to be provided?" and "What's the appropriate OCPD size?"
If the motor nameplate doesn't specify the full-load current at 200 volts, I'd base the motor-starter overload setting on the 230-volt full-load current, and specify at least 125% of that (60 amps) for the circuit ampacity.

If this is a new installation, (and maybe even if it's not -- Do the Math) I would return the 230-volt motor and install a 200-volt motor. This is a big enough consumer that its operating efficiency shouldn't be ignored.
 

Jraef

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San Francisco Bay Area, CA, USA
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You can generalize that the motor current will increase by the same ratio of the voltage differential, so 10% is a good rule of thumb. But success really does depend on the application.

A motor only draws the amount of current that the load demands of it, and that demand is in the form of torque. A motor produces steady state running torque that is the basis of the HP rating given to it, called Full Load Torque or FLT. But it also produces short bursts of Locked Rotor Torque (LRT) when first energized and Break Down Torque (BDT) at its maximum rated slip before it begins to stall. LRT and BDT are used in many machines to initially accelerate the load or to re-accelerate it quickly after a step change in load. At a reduced voltage, the motor FTL drops at the same rate as the voltage drop, so 208/230 = .904, SO at a 10% V drop the motor produces 10% less torque. However if the LOAD still requires the same amount of torque, the motor must increase its torque, so slip increases and the motor draws 10% more current. But as voltage drops, the LRT and BDT drops at the SQUARE of the voltage change. A 10% drop in V then results in the LRT and BDT dropping to only 81% of rated. That means the motor will take longer to accelerate from a standstill and it will take longer to recover from a change in load. If the load or change in load has a lot of inertia, the motor may not be able to handle it and stall.

So if the load is something like a centrifugal pump or fan, you will likely never need LRT or BDT and just experience the 10% increase in current at a given load. But if the machine is a heavy load, like a conveyor or a reciprocal compressor, or has a lot of inertia like anything with a flywheel effect, the bigger problem may be an increases likelihood of stalling or overloading as it tries to respond.
 

Besoeker3

Senior Member
Location
UK
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Retired Electrical Engineer
If it''s the number he has to work with for 230/240v, the corresponding number at 200/208v would be use in whatever calculations he's doing.
Depends on how heavily loaded it is compared to FLC.
 
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