240 feeding 208 volt motors

[K8MHZ]

This has probably been covered before, if so, my apologies.

In a nutshell, a church as 240 volts (measured) in the panel. Air make up and range hood motors are labeled 208.

EC says it's no problem, happens all the time. I trust that it happens all the time, it's the 'no problem' thing I am suspect of.

For one thing, if a motor is rated at 17.5 amps 208 volts, won't it draw MORE amps at 240? EC says Ohm's law says more volts means less amps. I tried to show him the math, but he was really, really busy and had other things going.

It's no skin off my nose if the 240 is a problem, I am just curious. This particular EC does a lot of stuff like this so I am thinking things will work. It just makes me wonder why a 208 volt motor that can run fine on 240 volts also would not be marked as such.

Thanks in advance for answers.

 

[infinity]

The motor will draw less current on a higher voltage but if the motor is listed for 208 volts only I would guess that applying 240 volts may shorten its life not to mention possible problems with its listing.

 

[jumper]

EC&M article on the subject if you are interested:

The Highs and Lows of Motor Voltage

 

[charlie b]

EC says it's no problem, happens all the time. I trust that it happens all the time, it's the 'no problem' thing I am suspect of.

That is very wise of you!

For one thing, if a motor is rated at 17.5 amps 208 volts, won't it draw MORE amps at 240? EC says Ohm's law says more volts means less amps.

Neither of these statements is true. The first statement is untrue because the motor will draw less amps at a higher voltage. Rob has already made that point. The second statement is untrue because it is not Ohm's Law that gets you the less amps. Ohm's Law is limited to certain types of materials, resistors being one, heaters being another, but motors not being on that list. A motor behaves like a constant power machine. The product of V and I is what remains (essentially) constant. Thus, higher V means lower I.

It just makes me wonder why a 208 volt motor that can run fine on 240 volts also would not be marked as such.

If it is not so-marked, then the motor should be be used in that application. Indeed, you would have a violation of NEC article 110.4.

 

[Jraef]

Are you reading the actual MOTOR nameplates, or are you reading something on the appliances? Because it would be unusual for a motor to ONLY be rated for 208V, it makes them somewhat difficult to sell. Most small motors are dual-labeled, where the actual MOTOR nameplate will say 208/230V and provide two different FLC numbers, the higher one being the 208V rating.

NEMA design specifications call for motors to have at least a +-10% input voltage deviation specification, so a 208V motor should be able to handle 229V with no serious consequences. Just because your NOMINAL service votlage says it is 240V, doesn't mean you actually HAVE 240V at the motor terminals. So even if your MOTOR nameplates do really say 208V, measure the voltage AT the terminals before getting too excited about it.

 

[ActionDave]

Are you reading the actual MOTOR nameplates, or are you reading something on the appliances? Because it would be unusual for a motor to ONLY be rated for 208V, it makes them somewhat difficult to sell. Most small motors are dual-labeled, where the actual MOTOR nameplate will say 208/230V and provide two different FLC numbers, the higher one being the 208V rating.

I'm trying to recall if I have seen a motor with only 208V on the nameplate. 200V is all I think I have seen.

 

[ActionDave]

Neither of these statements is true. The first statement is untrue because the motor will draw less amps at a higher voltage. Rob has already made that point. The second statement is untrue because it is not Ohm's Law that gets you the less amps. Ohm's Law is limited to certain types of materials, resistors being one, heaters being another, but motors not being on that list. A motor behaves like a constant power machine. The product of V and I is what remains (essentially) constant. Thus, higher V means lower I.

I'm not arguing, I'm actually asking for some educating.........

We hook up motors wired for high voltage and run them on low voltage when we test them, 240V and even 480V tested with 120V. With the windings in the high voltage configuration they pull less amps at the lower voltage than they do at the high voltage. There is no load so I know that is part of the deal, but the point is more voltage does in fact mean more amps. What's up with that?

Out in the field our power runs a little high in my fair city so 480V can run 510-520 or so. Motors will pull a little more juice then too.

 

[K8MHZ]

Thanks everyone!

The EC is just running pipe and wire to the units. The units were supplied and set by an HVAC contractor. Both the controls and the ground mounted RTUs are labeled 208 volts.

I don't get to see the motors themselves. They are enclosed in the RTUs. I do get to see the control panel. That is marked 208. It is also marked to be supplied with 120. Seeing that is what started the conversation. I asked if this building had 120/208. He said no and I confirmed by checking the main breaker panel. It's 240.

I don't have a dog in this fight. It's just that I am so stinking curious. I have never hooked up a 208 device to 240. And I have been doing this for decades. EC says it's nearly common practice.

 

[kwired]

I'm not arguing, I'm actually asking for some educating.........

We hook up motors wired for high voltage and run them on low voltage when we test them, 240V and even 480V tested with 120V. With the windings in the high voltage configuration they pull less amps at the lower voltage than they do at the high voltage. There is no load so I know that is part of the deal, but the point is more voltage does in fact mean more amps. What's up with that?

Out in the field our power runs a little high in my fair city so 480V can run 510-520 or so. Motors will pull a little more juice then too.

I don't know every detail all that well myself, but an induction motor is going to try to acquire same rotor speed as the synchronous speed of the field. Reality is there is always some slip so a 1800 synchronous speed motor never does get much more then about 1750 or so. But lower input voltage by 10% - current will go up as slip will increase at the new voltage. Run an unloaded motor at low voltage - current will be higher then it is for same unloaded motor at rated voltage - it is only drawing what is being asked for with current conditions.

Apply over rated voltage you will get lower current, but go too far over voltage and you get over-excitation which will cause other problems.

 

[Sahib]

Voltage drop in the power cable to the motor may be considerable if the motor location is remote and this would bring down the supply voltage across the motor terminals.

 

[GoldDigger]

Be absolutely sure that the motor is a single phase rather than a three phase motor.
208v is seen pretty much only in a system with a three phase supply, even if not all phases are brought to all panels.

 

[Johnnybob]

I think (pretty sure, actually) that it has to do with EMF (electro-motive force) and CEMF (counter electro-motive force). Higher voltage (EMF) produces higher CEMF, reducing resistance, there by requiring smaller current draw than lower voltage

 

[GoldDigger]

I think (pretty sure, actually) that it has to do with EMF (electro-motive force) and CEMF (counter electro-motive force). Higher voltage (EMF) produces higher CEMF, reducing resistance, there by requiring smaller current draw than lower voltage

Uh, no!
The counter EMF reduces the effective voltage applied to the impedance of the motor windings. The impedance and resistance of the windings does not change.

 

[ActionDave]

...... Run an unloaded motor at low voltage - current will be higher then it is for same unloaded motor at rated voltage - it is only drawing what is being asked for with current conditions.....

No, it doesn't work out that way if you keep the windings configured for high voltage.

 

[Johnnybob]

Uh, no!
The counter EMF reduces the effective voltage applied to the impedance of the motor windings. The impedance and resistance of the windings does not change.

Hmm. How would that work? CEMF is the voltage induced when the magnetic field of the applied voltage collapses, and would, therefore, be of opposite polarity of the applied voltage!

 

[GoldDigger]

Hmm. How would that work? CEMF is the voltage induced when the magnetic field of the applied voltage collapses, and would, therefore, be of opposite polarity of the applied voltage!

Nope the CEMF is the voltage induced in the windings as they move through the induced magnetic field (in the case of an AC induction motor).
In the case of a motor with separate field and armature windings, the CEMF is the voltage induced in the rotor windings as they cut through the magnetic field generated by the field windings. In any case, the critical element, as you came close to correctly noting at one point, is the relative motion of the wires and the magnetic field. So CEMF can occur only when there is rotation.
If you want to really be picky, you could talk about the moving induced field lines cutting through the stationary windings, but I like to use the terminology of moving wires and stationary field lines.

The mechanism that you are trying to use to explain CEMF is just normal magnetic induction, whose effects can be predicted by Lenz's Law.

 

[mbrooke]

Isnt it where the at a certain point the voltage will cause the iron to saturate and any increase in voltage there on increases the current? My understanding is a motor has a complex voltage/torque/current curve.

 

[mbrooke]

Be absolutely sure that the motor is a single phase rather than a three phase motor.
208v is seen pretty much only in a system with a three phase supply, even if not all phases are brought to all panels.

Arent most 3 phase motors rated 230 volts and adapted to 208 volt supplies?

 

[GoldDigger]

Arent most 3 phase motors rated 230 volts and adapted to 208 volt supplies?

I cannot really say "most". But although a dual voltage motor with no wiring change is certainly common (either rated 230 with a tolerance band down to the high end of 208 or rated 208/230 with different nameplate currents, etc.) I can see specialized equipment which is designed and rated for 208 (with 230 OK at the upper end of its tolerance band.
I will let others with wider experience say whether "most" applies. Maybe for NEMA motors?

 

[ActionDave]

Arent most 3 phase motors rated 230 volts and adapted to 208 volt supplies?

I wouldn't say most. If they say 230/208V on the nameplate that's what has been done. There are plenty 230V and 200V motors though.