# 3 Phase 230V Motor Question



## PowerTrip82 (Oct 22, 2014)

*I was given wrong information*

It is single phase not three. Sorry.


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## DriveGuru (Jul 29, 2012)

Not necessarily, it depends on how heavily it's loaded. As long as you're not exceeding the nameplate current + service factor it is fine. If the current is too high, adding a buck/boost transformer in front of it will correct the issue relatively cheap.


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## Ultrafault (Dec 16, 2012)

What makes you think the frequency can be different?


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## DriveGuru (Jul 29, 2012)

Again still depends on the load wether or not you'd get away with it. At 50hz you'd be better off bucking down to around 190v. As long as you are not exceeding nameplate current your good.


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## PowerTrip82 (Oct 22, 2014)

Right. Apparently its a motor on an Ingersoll - Rand air compressor. Wouldn't it be way cheaper to get a different motor? You start talking transformers of any kind your looking at some money. He never told me the amp rating for it but I did tell him if he puts the 208 to it, it's gonna draw that amperage on up and to watch it.


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## Southeast Power (Jan 18, 2009)

PowerTrip82 said:


> Now correct me if I'm wrong please,but if I put single phase 208V on a single phase motor requiring 230V...i will burn it up after just a while right? Only the frequency can be different correct?


What kind of horsepower are you pushing?


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## chicken steve (Mar 22, 2011)

There's a world of service upgrades from 240/120 single, to 208/120 3 ph PT

We just look up the 208V in Uglys, and up the ocpd to it

There may be some specific motors sensitive to this, but generally they are a malleable animal

~CS~


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## JRaef (Mar 23, 2009)

Here's how it really works (assuming frequency stays the same, meaning 60Hz - 60Hz).

When you give a motor less voltage than it was designed to receive, it affects the _*TORQUE *_that the motor can deliver. When a mechanical engineer picks the motor he needs for a machine, let's says your IR compressor, he knows the mass / intertuial of the compressor and motor itself, he knows the friction loads, he knows the losses, and he knows the physical work that the machine will be expected to perform, in this case compressing air. From all of that, he determines the torque that he needs, and depending on how fast he needs the machine to run, the base motor speed. From those two values, he determines the "Brake HP" requirement, meaning the actual HP needed at the motor shaft. So let's say he detrm9ined that he need 14 lb.-ft. of torque at 3550RPM, he applies the torque / HP formula of tq. x RPM / 5250 = HP, so he needs 9.47 BHP. Motors come in certain sizes, so he selects a 10HP motor, but to be conservative, insists on a 1.15 service factor to allow for the possibility of temporary voltage drops. But under normal operation, that motor, rated at 28A FLC, is only going to draw the amount of current to produce 9.47HP, so around 26.5A (torque and current change at the same rate once at full speed). 

Now you come along and feed that motor with 208V. The torque from that motor will drop to 90% of normal (208/230). That means that the motor will still TRY to accomplish the same task assigned to it, but will need to pull more current to do so. Therefor instead of drawing 26.5A as it would at 230V, it will draw 29.5A. But because the ME was smart enough to insist on a motor with a 1.15SF, it can safely allow for use at up to 32.2A. So 32.2A capability, 29.5A draw because of the lower voltage, not a problem. Yes, it is above FLC and you will need to adjust your overload relay setting or heater selection to compensate, but still, it's fine, the motor was designed with this in mind.

If however, there was a different scenario, it might not be. Let's say this time the machine needed 8.3 BHP and despite the engineer's pleas, marketing wanted to keep the motor size smaller and cheaper, so they used a 7-1/2HP motor with a 1.15SF. It means they are continuously running it into the SF, because 7.5 x 1.15 works out to be 8.6HP and it needed 8.3 BHP. Still OK as long as everything is perfect, but now you come along and feed it only 90% voltage. That "fudge factor" you had that made the previous scenario acceptable has already been consumed by the compressor mfr before it even left the factory, so now you have zero tolerance in the field. THAT is when your motor burns up.


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## DriveGuru (Jul 29, 2012)

Come on now, compressor manufacturers never undersize their motors.roflmao They also always give adequate ventilation for their motors and drives...NOT,lol


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## ponyboy (Nov 18, 2012)

Isn't there as nema designated 10% variance? 10% of 230 is 207. I've never given much thought to hooking up 230 volt motors to 208 and I haven't seen any premature failures yet. I set the overloads accordingly and forget about it. There's good info here


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## DriveGuru (Jul 29, 2012)

I've seen allot of premature failures because of exactly that, JR hit it on the head, as I think back most of those were air compressors, blower manufacturers are real bad about it too. If a motor manufacturer says 1.15, don't pull 1.16, lol


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## JRaef (Mar 23, 2009)

ponyboy said:


> Isn't there as nema designated 10% variance? 10% of 230 is 207. I've never given much thought to hooking up 230 volt motors to 208 and I haven't seen any premature failures yet. I set the overloads accordingly and forget about it. There's good info here



Yes, NEMA motor design specs call for +- 10%, so 207 on a 230V design. But remember ... Utility specs allow for -5%, so a 208V system can go as low as 198V and be acceptable, but too low for that 230V motor. In 3 phase motors you can buy tri-rated motors that are nameplated as 208-230/460V. It's a special design that's basically a 220/440 design with a +- 15% tolerance. But I've never seen that in single phase. 

My point on all that however was just to back up the comment made by driveguru saying that if you watch the current and it doesn't go above the FLC + SF, it's fine.


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## wendon (Sep 27, 2010)

DriveGuru said:


> Come on now, compressor manufacturers never undersize their motors.roflmao They also always give adequate ventilation for their motors and drives...NOT,lol


My understanding is the horsepower rating on the smaller, residential type compressors is the horsepower when you stall the motor. Why, I don't know but I imagine it has something to do with selling air compressors.


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## micromind (Aug 11, 2007)

wendon said:


> My understanding is the horsepower rating on the smaller, residential type compressors is the horsepower when you stall the motor. Why, I don't know but I imagine it has something to do with selling air compressors.


The advertised HP of cheap compressors is usually the breakdown HP. This is the maximum the motor can produce before it experiences a sudden drop in RPM. It's typically anywhere from 150 - 400% of rated HP. Also, it's usually listed as torque, but your average everyday person has no idea what torque is but they know that more HP is better. Well they think they know......

If you look at the nameplate amps of these motors and do a bit of basic math, you'll find that if the advertised HP was actually true, the motor is operating at more than 100% efficiency. I'm pretty sure that if I figured out how to get more than 100% efficiency I wouldn't be building air compressors.......

The quickest easiest way to tell a quality compressor from a junk one is to look at the motor RPM. If it's 3450 (or thereabouts), it was built with low cost in mind. If it's 1725 or so, then it was built to last. Also, if the HP is listed as SPL, it's junk. 

Also, belt drive is almost always better than direct drive.


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## JRaef (Mar 23, 2009)

LOL, "SPL" is short for "Special", as in _"We're so SPECIAL that we don't have to follow the conventional engineering rules for depicting the HP rating of an AC motor, we can make up our own meaning, to there, you poopyheads... plplplplpltttt"_.

Yes, "HP" on consumer goods is usually just a marketing term, not an engineering term. Compressors are the worst offenders, but it also happens on things like power tools, pumps, vacuums etc. The term they used to use was "_*Develops *_xx HP" but a class action lawsuit in the early 2000's finally made them cease that. But then they found a new way around having to tell the truth by using the term "SPL" after HP on the label, which means whatever they want ti to mean, as long as they describe it somewhere. Where they describe it is either in marketing materials that the consumer never sees but are technically "available", or web sites now, that they bury behind so many click-throughs that 99.9% of people give up before finding it. The only place they can't lie is in the motor nameplate FLC, and you can always go backward from that to determine the true continuous HP rating of the motor.


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## garfield (Jul 30, 2009)

Finding a compressor with a standard 184t etc motor frame is also a good idea. Ingersoll Rand uses these odd voltage and frame motors so you have to go to them for replacement. (Vaseline needed).


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## Jhellwig (Jun 18, 2014)

A lot of motors have amperage for 208 volts listed in a different spot on the name plate. It may have it listed. 

Some of those compressors with starters only have overloads big enough to run the motor intermittently. If the run all the time they trip.


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## micromind (Aug 11, 2007)

Jhellwig said:


> A lot of motors have amperage for 208 volts listed in a different spot on the name plate. It may have it listed.


Also, a lot of motors that specifically state 208 operation will list the SF at 1.0 for 208 and 1.15 for 230/460.


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## micromind (Aug 11, 2007)

PowerTrip82 said:


> Right. Apparently its a motor on an Ingersoll - Rand air compressor. Wouldn't it be way cheaper to get a different motor? You start talking transformers of any kind your looking at some money. He never told me the amp rating for it but I did tell him if he puts the 208 to it, it's gonna draw that amperage on up and to watch it.


A buck-boost transformer would be considerable less $$ that a new motor. 

Just my opinion here, but anyone who has a 208 3Ø system and buys a single phase motor is going to get exactly what he deserves.


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## triden (Jun 13, 2012)

micromind, I have to say, for someone that never officially got their electrician ticket, you sure know your stuff! I commend you for that.


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## PowerTrip82 (Oct 22, 2014)

Another thing to factor is that its going to pull more amps starting up than running wide open correct?


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## micromind (Aug 11, 2007)

PowerTrip82 said:


> Another thing to factor is that its going to pull more amps starting up than running wide open correct?


A basic induction motor, capacitor start single phase or 3 phase, will draw roughly 6 times its full-load current for starting. This is also known as locked rotor current. 

A lot of factors go into this, generally speaking, the higher the efficiency the higher the locked rotor current, and voltage drop during starting will lower this current. 

As a sort of side note, lower voltage during starting will result in longer life of the motor. Provided the motor can accelerate the load within a reasonable amount of time of course.......


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## DriveGuru (Jul 29, 2012)

Not sure I totally agree with these statements...motors can draw 6-10 times rated current during startup, this is commonly referred to as inrush. Maximum inrush is equal to the motors locked rotor current determined by the motor "code" stated on the nameplate. As a motor accelerates to rated speed the rotors magnetic field is cutting the stator winding creating a CEMF(counter electro motive force) which opposes the existing current flow. As the motor accelerates the current will plateau at locked rotor then fall off when it achieves speed near rated slip and the CEMF is at maximum. It has been my experience that when reducing a motors voltage during starting care must be taken. Lower starting voltage won't necessarily equate to longer motor life. In allot of cases using a device such as a soft start in conjunction with a feature called current limit, you can easily create more heat in the motor than starting it across the line(a motors core Iron/mass has allot to do with this, and its ability to dissipate said heat). The loads will be happier such as reducing belt slippage and reduced gearbox stress, but the motor will not necessarily be happier A good rule of thumb is to try to get the load to accelerated between 3 and 6 seconds, much more than this without a specially designed motor is asking for premature failure. You may also have to start taking into consideration the number of starts per hour, heat is your enemy.


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## micromind (Aug 11, 2007)

There are 3 currents associated with motors;

1) Magnetizing.....this is an instantaneous current based pretty much on the DC resistance of the windings. No counter EMF here, it's the same as if the windings were stretched out in open air. This current can easily be 20 times the full-load current, and it lasts for roughly 1/60th of a second, depending on what part of the sine wave it started at.

1) Locked rotor. This follows magnetizing current. It occurs after counter EMF has been established. For capacitor start and 3Ø motors, it's usually somewhere around 6 X full-load current. High efficiency motors will be higher. Split-phase motors will be quite a bit higher. Shaded pole and PSC will be lower.

3) Full-load current. This is after the motor has reached 80 - 90% of its synchronous speed. It will vary with voltage, phase balance and RPM. 

The main reason that a motor will last longer if it starts on less than nameplate voltage is because of the magnetizing current. This current results in a mechanical shock to the windings. If the magnitude of this shock can be reduced, the stator will last longer. 

On the other end of the scale is too little voltage can result in thermal shock because of longer time spent with locked rotor current. 

There is no magic formula to size wire for motors, a close-coupled centrifugal pump will do quite well with 50% voltage drop on starting while a fully loaded conveyer will need more like 90%. Each application needs to be considered separately.


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## JRaef (Mar 23, 2009)

Running torque decreases linearly with voltage reduction, but STARTING torque is reduced at the square of the voltage. So at 90% voltage, your starting torque is reduced to 81% of normal. This always results in it taking longer for the motor to accelerate. But at rated voltage, if the starting torque was way more than necessary to begin with, reducing it may have no perceptible effect. It's all load dependent.

It's commonly perceived that reduced voltage (soft) starting heats up the motor more than X-Line. By the same token, some people also think that using reduced voltage starting will allow the motor to remain cooler. Studies have shown that neither statement is true. Yes it takes longer, but just as the torque is reduced, so is the current. The vast majority of motor heating that takes place in starting a motor is due to I^2 R losses, those related to current. So if torque is reduced, current is reduced at the same rate and so is the heating effect it causes. But the reduction is no more than that which is offset by the increased time. The way I teach it is to think of starting ENERGY; the power it takes to accelerate to full speed, times the time it takes for that to happen. If you look at that energy as a curve on a graph and color in the area below that curve, the calculated area of curve never changes, it is a law of physics. You can squash down the top of the curve and it will squeeze out of the end, that's what reduced voltage starting does, but it is the same amount of energy no matter what. The "less heat" team point to allowing more time for the heat to move through and get dissipated by the fans, but the counter to that is that the fans are slower too. It all washes out.

The risk of over heating comes from unsuccessfully accelerating at all, stalling the motor if the load conditions change. One other risk come specific to single phase motors with speed switches; exceeding the time that the start windings and caps can be in the circuit. They are not designed to be in the circuit continuously in most types, so failing to accelerate quickly is an increased risk to the motor. Again, totally load dependent, also design dependent in the case of single phase motors.


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## DriveGuru (Jul 29, 2012)

micromind said:


> There are 3 currents associated with motors;
> 
> 1) Magnetizing.....this is an instantaneous current based pretty much on the DC resistance of the windings. No counter EMF here, it's the same as if the windings were stretched out in open air. This current can easily be 20 times the full-load current, and it lasts for roughly 1/60th of a second, depending on what part of the sine wave it started at.
> 
> ...



Totally agreed


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## DriveGuru (Jul 29, 2012)

JRaef said:


> Running torque decreases linearly with voltage reduction, but STARTING torque is reduced at the square of the voltage. So at 90% voltage, your starting torque is reduced to 81% of normal. This always results in it taking longer for the motor to accelerate. But at rated voltage, if the starting torque was way more than necessary to begin with, reducing it may have no perceptible effect. It's all load dependent.
> 
> It's commonly perceived that reduced voltage (soft) starting heats up the motor more than X-Line. By the same token, some people also think that using reduced voltage starting will allow the motor to remain cooler. Studies have shown that neither statement is true. Yes it takes longer, but just as the torque is reduced, so is the current. The vast majority of motor heating that takes place in starting a motor is due to I^2 R losses, those related to current. So if torque is reduced, current is reduced at the same rate and so is the heating effect it causes. But the reduction is no more than that which is offset by the increased time. The way I teach it is to think of starting ENERGY; the power it takes to accelerate to full speed, times the time it takes for that to happen. If you look at that energy as a curve on a graph and color in the area below that curve, the calculated area of curve never changes, it is a law of physics. You can squash down the top of the curve and it will squeeze out of the end, that's what reduced voltage starting does, but it is the same amount of energy no matter what. The "less heat" team point to allowing more time for the heat to move through and get dissipated by the fans, but the counter to that is that the fans are slower too. It all washes out.
> 
> The risk of over heating comes from unsuccessfully accelerating at all, stalling the motor if the load conditions change. One other risk come specific to single phase motors with speed switches; exceeding the time that the start windings and caps can be in the circuit. They are not designed to be in the circuit continuously in most types, so failing to accelerate quickly is an increased risk to the motor. Again, totally load dependent, also design dependent in the case of single phase motors.



Again very well said, can't you still run into a situation where time could be a factor though? Say 4 times rated current for 20-30 seconds or 6 times for 5 seconds? I realize this is load dependent, and application specific. I've run into situations in the past where someone has limited the current enough the load couldn't accelerate to full speed no matter how long you gave it. So if I understand correctly this is the real underlying issue, not so much time, but not starving it so much it can't do its job.


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## Ultrafault (Dec 16, 2012)

These threads are the best.


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## JRaef (Mar 23, 2009)

DriveGuru said:


> Again very well said, can't you still run into a situation where time could be a factor though? Say 4 times rated current for 20-30 seconds or 6 times for 5 seconds? I realize this is load dependent, and application specific. I've run into situations in the past where someone has limited the current enough the load couldn't accelerate to full speed no matter how long you gave it. So if I understand correctly this is the real underlying issue, not so much time, but not starving it so much it can't do its job.


Yes, it is totally possible to reduce it so much that it fails to start. You can usually determine the required accelerating torque of any load if you know enough about the mass, inertia, friction etc. if you drop voltage to below that minimum level of accelerating torque, it cannot finish accelerating. Accelerating torque is the amount of torque over and ABOVE the load torque, which in most cases is dynamically changing with speed as a curve, so it can get complicated to predict without some complex (and expensive) software, such as ETAP or SKM. So if you get that wrong and starve the motor of torque at a critical moment, you fall behind the accelerating torque curve and although it may not exactly stall, it fails to accelerate faster than the thermal damage curve of the motor. That's what the overload relay is supposed to protect you against. What happens sometimes though is that people bypass the overload relay or internal protection during startup because they see that the NEC says that is allowed, but they do it without understanding the risks. That's usually when motors smoke.

I once worked for a soft starter mfr, and we were sued for $2 million by a user who claimed that our overload protection failed to protect his 500HP hydraulic press pump motor, and as a result of that failure they lost production for a week. Little did they know that we had a hidden record of the last 10 changes to the programming of key parameters with time and date stamps. So the REAL sequence was that they set it up with a class 20 OL and 350% current limit, it tripped. They reset it, extended the ramp time (which has virtually no effect if it is in current limit anyway), it tripped again. They raised the current limit to 450% and tried again in an hour, it tripped again. They raised it to 500% and after an hour they tried again and it accelerated in about 25 seconds, which is right on the ragged edge of the OL curve. But then they lowered the Current Limit back down to 350%, probably because that was what the utility said was their allowed maximum, and disabled the overload trip! That's when they smoked the motor (registered inside the soft started as a short circuit on the load). They turned the OL protection back on and turned the CL back up to 500%, then contacted us. I ran an ETAP study on their system and it told me it would start with 500% (which turned out to be true), and they claimed thats what they had set it for, after which they had their lawyer send us the summons. We sent out an independent registered PE who did forensic evaluations of electrical equipment, gave him the instructions on how to extract the hidden data, and when their lawyer got it, the suit was dropped.

The point of the story is that there are anecdotal stories of motors being damaged by using soft starters, but I have never run across one that did not have key pieces of the REAL story left out. Most low-end products don't have the type of evidence we were able to collect however, so these often go unchallenged, or the challenges go univestigated.


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## Black Panther (Mar 10, 2021)

PowerTrip82 said:


> Now correct me if I'm wrong please,but if I put single phase 208V on a single phase motor requiring 230V...i will burn it up after just a while right? Only the frequency can be different correct?


Motors can run within +\- 10% of its rated voltage. Just make sure you do the calculations for the right amperage when doing so.


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## Kevin (Feb 14, 2017)

Black Panther said:


> Motors can run within +\- 10% of its rated voltage. Just make sure you do the calculations for the right amperage when doing so.


Please take a few minutes to fill out your profile. It's required. Here's a link with instructions to assist you with this simple task.


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## paulengr (Oct 8, 2017)

To OP: 208/120 and 240 delta and 240/120 high leg delta are all pretty common. Most motors we sell list a range instead of a single voltage on the name plate. It’s not worth stocking a slightly different motor. A 200 V class motor will just run a little higher torque and about the same amps at 230 as it does at 208. Controls are less tolerant. The CPT taps should be adjusted.


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