Tips/Techniques 3 Phase motor

[Dabbler]

Now I'm confused! I'll credit it to being tired - I'm over 20 hours up right now. I have a faint impression it is inrush current caused by induction, but I'll have to sleep on this. For the moment I've stricken the assertion, and I'm going to bed.

 

[Susquatch]

Now I'm confused! I'll credit it to being tired - I'm over 20 hours up right now. I have a faint impression it is inrush current caused by induction, but I'll have to sleep on this. For the moment I've stricken the assertion, and I'm going to bed.

I don't understand. It's an over-simplication, I know, but can't I think of the windings as resistors? If the voltage is halved, won't the current be halved, not doubled?

Bottom line, no - you cannot think of the motor windings like resistors.

But this is complicated stuff and my head is grappling with what happens too. I'll give it a shot but reserve the right to strike my comments too! LOL!

Just for discussion purposes, let's assume we are talking about a 2:1 voltage difference. Say a 480V motor running on 240.

A higher voltage motor run on lower voltage will try to run at whatever speed is dictated by the frequency. But the load on the motor (friction, gear train loads, etc) does not change for a given rpm. So forgetting about other losses (just for a moment), the slip will increase (slip is the primary determinant of current) and the current will double for a given load. Simply stated I=P/V so if V goes down, I has to go up for a given load.

This can only go so far before one of two things happen. The motor stalls with saturated coils, or something starts to melt.

High load currents are almost always in the same ballpark for a given hp motor. But NOT the low load currents. The low load currents are dictated by the motor design and a plethora of other factors.

In my "opinion" the motor will likely be fine at low loads and
double the normal low load current.

However, it's condition at high loads is a whole nuther kettle of fish. I believe that slip and current will continue to be roughly double the full voltage current as the load is increased. At some point, as load is increased the motor will stall and glow, or stall and melt. Neither one is a good outcome.

Strictly speaking, I think this should all work fine in applications where startup loads and running loads are minimal (lathes have high startup loads) and where the motor mostly spins running well below its rated hp - say a mill spinning a tiny endmill. But it is a bad idea where the loads are high like a lathe starting up or a big hogging cut on a mill.

Last, but not least, consider that we always try to put a motor on a machine that is capable of doing the required work. So even if the motor doesn't melt, it will stall when used at the rated load so it's still a bad idea.

My summary - it's a bad idea. Don't do it.

Edit - I'm pretty comfy with that reading it over. But my bride and I are off on a road trip today, so I won't be able to come back and edit it till later tonight. Have fun guys!

 

[kstrauss]

Well, you can use a motor +/- 20% of rated voltage. If the voltage is half, due to lower inductance and lower resistance, the current more than doubles. This results in very fast heating of the windings and insulation failure.

For instance I have a grinder that is 3PH 550V. I can choose a voltage-doubling VFD, but I already have a rotary phase converter and 2 VFDs. I'm looking for an inexpensive 15000 VA 3ph 2:1 transformer instead. If I run it on raw 240V 3PH, I'll smell the magic smoke.

A few weeks ago I mentioned that my scrappie had some suitable 45kva (GO BIG!) transformers for cheap. Where are you?

 

[Dabbler]

@kstrauss I'm in Calgary, but will be going to Ont for the eclipse in April. Calgary-London in March, and returning to Calgary 2nd week April.

 

[Dabbler]

@VicHobbyGuy Susquatch said it better than I could.

My experience is from fixing stuff in the field. I found that on a lathe with a separate power feed motor on the carriage, that it showed signs of overheating. It turns out that a voltage conversion wiring was wrong, and that it was being supplied half the voltage, and was overheating. I fixed the wiring, but advised them it could die at any time. It took months before they called me in to replace it.

When you use 1/2 voltage, you get less than 1/4 torque out of the motor at increased amperage. I've never measured it, but the signs were clear in that case. So I was speaking out my a** when I said twice. But is enough more that the current does cause noticeable winding heating. I forgot to mention slip caused by decreased attraction due to incorrect saturation of the coils. I knew this but I was tired.

Mile Holt's Forum is a good resource for all things Electrical Engineering related. Here this problem is discussed by people in the industry that design motors and all kinds of electrical stuff. Posts #9 and #11 support my comments above. Post #5 disagrees with my assertion, but in magnitude only.

I hope this helps! [updated to include the link to Mike Holt's forum]

 

[VicHobbyGuy]

Thanks for the explanations. I did some reading online and the undervoltage and increased current situation is more clear. Most of the explanations seem to assume that the load is constant as the voltage is reduced - basically requiring the same power from the motor while supplying half the voltage in our example. So obviously the current must increase. Lots of explanations online use terms like the motor 'trying' or 'wanting' to do something, which is interesting.

 

[PaulL]

When looking at V=IR in a motor people often forget that the R term includes the work being done. That work is measured as power, and if you need the same power output at lower voltage you have to crank up the amperage.

 

[VicHobbyGuy]

When looking at V=IR in a motor people often forget that the R term includes the work being done. That work is measured as power, and if you need the same power output at lower voltage you have to crank up the amperage.

Or double the work at the same voltage?

 

[PaulL]

Or double the work at the same voltage?

Yes, and if you think about it you realize that doubling the expected work - imagine adding so much more weight on your spindle to bring up to speed - would take more current and so make a ton more heat.
As an aside, inserting voltage frequency into the thinking changes how much power you're trying to get out of the motor - lower frequency, slower work. The VFD's torque compensation curve adjusts your voltage with frequency to help manage the amperage.

[Edit: why does it hurt so much to page back in my electrical engineering class from 1987? Oh, yeah, because the class hurt so much too.]

 

[VicHobbyGuy]

Yes, and if you think about it you realize that doubling the expected work - imagine adding so much more weight on your spindle to bring up to speed - would take more current and so make a ton more heat.

So, would running a motor at half voltage and no load or much lower-than-rated-load still result in much higher current, overheating, etc... ?

 

[PaulL]

So, would running a motor at half voltage and no load or much lower-than-rated-load still result in much higher current, overheating, etc... ?

That I can't answer. I suspect that the motor internals are unlikely to like it because of all the various inductance issues that go on, but you'd have a much higher chance of not burning it out.
I really wouldn't want to pronounce on whether you could run a 440V 5HP motor as a 220V 3HP motor.

 

[cjmac]

So, would running a motor at half voltage and no load or much lower-than-rated-load still result in much higher current, overheating, etc... ?

While we are talking about what you can and can't get away with in single and 3 phase motors, I have found that some things out of spec are tolerated and other things aren't. This was based on 3 and 5 hp 220v motors From my experience over the years, you can run a 50hz (single ph) motor on 60hz and it works fine. The other way (60hz motor on 50hz) the motor overheats under load.

If the voltage on one leg of the three phases is low, that set of windings will overheat and burn out.

Also, a note of caution. The fan is designed to adequately cool the motor at the nameplate rpm. If you are running with a load on your lathe at low rpm using a VFD, you might want to provide alternate cooling for the motor because it's fan is not turning fast enough to keep up with the heat.

Chris

 

[Susquatch]

Most of the explanations seem to assume that the load is constant as the voltage is reduced - basically requiring the same power from the motor while supplying half the voltage in our example. So obviously the current must increase. Lots of explanations online use terms like the motor 'trying' or 'wanting' to do something, which is interesting.

So, would running a motor at half voltage and no load or much lower-than-rated-load still result in much higher current, overheating, etc... ?

If it takes a quarter hp to turn at motor at its 60hz speed, it takes a 1/4 hp period. The load the motor sees is related to its speed not its voltage or current. The voltage and current are what produces the power in the motor to meet its needs. But it's needs are what is happening at the output shaft.

So in order to run at that speed, current must go up in order to reach it. "Trying" really is a great choice of words, but if the motor can't actually do it, then it is stalling.

I think what you are missing here is the fact that the rated hp is the MAXIMUM the motor can do safely. But the rated hp is only reached when the load hp equals the motors rated HP. This is rarely the case. Usually, motors run at much less than their rated hp. It's a. Big mistake to assume that a 2Hp motor produces 2Hp all the time.

On a mill, the motor only runs at a fraction of its rating until you put a huge tool into it.

On a lathe, the motor sees a huge inrush current while it works to bring everything up to speed.

 

[VicHobbyGuy]

I think what you are missing here is the fact that the rated hp is the MAXIMUM the motor can do safely. But the rated hp is only reached when the load hp equals the motors rated HP. This is rarely the case. Usually, motors run at much less than their rated hp. It's a. Big mistake to assume that a 2Hp motor produces 2Hp all the time.

Yes, but you seem to be saying that the motor will draw the same current at no load, as with its maximum load: "If it takes a quarter hp to turn at motor at its 60hz speed, it takes a 1/4 hp period."
If I had a better AC ammeter, I would do the experiment with a spare fractional motor and a variac.
Anyway, thanks for the explanations - I've derailed this thread long enough!

Back to the regular program!

 

[Tomc938]

Would this motor be a good candidate?

 

[YYCHM]

Would this motor be a good candidate?

View attachment 41229

3400 RPM? What is the RPM of the installed motor?

 

[Dabbler]

Would this motor be a good candidate?

I cannot say from the plate alone.

1. Is the motor diameter and mounting plate compatible with your lathe (even if it takes a little work)?
2. 1725 vs 3400 rpm makes a difference - 3450 rpm motors have less torque at any rated HP, and the effects of slip are more profound.
2a. You can use a smaller pulley to restore your RPM settings into (approx) the right numbers., but adding a pulley won't restore all your starting torque.
3. Since you are replacing a 1.5 single phase motor with a 2.4 HP three phase motor, you are going in the right direction.
4. Your 'perfect' choice to replace a 1.5HP single phase motor is with a 2.5 HP three phase motor - at the same RPM, and inverter duty.

Your answer is a qualified yes.

You need to program you ramp-up-to-speed time to be longer - 10 seconds is a good starting point, and your ramp-down-to-stop speed to be longer as well. Then you shouldn't see any difference, except for taking deep cuts. You will also have to be careful at lower speed operation that your motor doesn't overheat when using it for extended periods of time. You will be limited to 40%-120%, with a perfectly matched pulley will get your 'effective motor speed' to 700-2000.

If you manage to get a 1725 RPM inverter duty motor, you can easily do 120-2400 rpm....

 

[Ironman]

While we are talking about what you can and can't get away with in single and 3 phase motors, I have found that some things out of spec are tolerated and other things aren't. This was based on 3 and 5 hp 220v motors From my experience over the years, you can run a 50hz (single ph) motor on 60hz and it works fine. The other way (60hz motor on 50hz) the motor overheats under load.

If the voltage on one leg of the three phases is low, that set of windings will overheat and burn out.

Also, a note of caution. The fan is designed to adequately cool the motor at the nameplate rpm. If you are running with a load on your lathe at low rpm using a VFD, you might want to provide alternate cooling for the motor because it's fan is not turning fast enough to keep up with the heat.

Chris

From personal experience and being told this by and industrial sparky: A 3ph motor can run 20% over or under the voltage on the nameplate and not be harmed. But because of the issues stated by Dabbler, that the motor wants to see watts and will get it the hard way if voltage is lowered, I have always made it a habit to double the HP of the motor I was replacing if I was running off a VFD motor. It is much easier on the motor at low rpm and heat is hard to detect. If you are putting a 5hp in place of a single phase 1.5HP that motor has an easy life.

 

[Dabbler]

If you are putting a 5hp in place of a single phase 1.5HP that motor has an easy life.

@Ironman That is why I suggest 1 HP more on a lathe. Sure 2HP more is better, but then the motor frame is so big it sometimes just won't fit.

A best practice is the largest frame that will fit (within reason). The biggest trouble in fitting a three phase motor is getting the frame to fit with enough horsepower to do the job.

*Note* I always replace a mill motor with the same HP rating - there isn't a lot of advantage to overpower a mill. But it makes installation a pain if the motor is much bigger and heavier. Particularly if the motor mounts are marginal.

 

[Tomc938]

@Ironman That is why I suggest 1 HP more on a lathe. Sure 2HP more is better, but then the motor frame is so big it sometimes just won't fit.

A best practice is the largest frame that will fit (within reason). The biggest trouble in fitting a three phase motor is getting the frame to fit with enough horsepower to do the job.

*Note* I always replace a mill motor with the same HP rating - there isn't a lot of advantage to overpower a mill. But it makes installation a pain if the motor is much bigger and heavier. Particularly if the motor mounts are marginal.

Thanks guys! The discussion has been most helpful. I now know what I am looking for. I am willing to take the time to get something that checks all the boxes. I’ve also been doing my own research online and understand better what’s involved.

Thanks again! The input is so much appreciated!