Thread: Current, wattage, HP and 3 Ph motors

Originally Posted by RayG

Hmmm... you are assuming the stall current will destroy the motor... What about stepper motors, brushless DC, shaded pole motors etc etc.

Stall of any induction motor will eliminate generation of back EMF and allow excessive current draw unless externally limited. That's why single phase motors get unhappy quickly if the start winding. cap, switching system fail.

I have come accross Steppers and Brushless DC connected to mismatched controllers that have burnt out due to being overloaded, If the contoller does not limit power to safe limits, the motor will not do it intrinsicly and can be damaged by overload.. Haven't had enough to do with shaded poles to comment.

The manufacturers power rating is a function of many factors, how the motor is wound, number of turns per lap, wire gauge used, number of poles, how efficiently the magnetic flux is transferred to the rotor and so on...

With the exception of Flux transfer, the factors that you mention are optimised to minimise heat generation and thermal losses, which is the point I am making. i.e it is easier to get current flowing in a motor than it is to disipate the heat generated by associated losses. Go into overload and it is the thermal performance that counts.

You can't expect to get 10 hp out of a 3 hp motor just by loading it.

I never suggested that you could. The point I was trying to make to the OP was that motor would attempt to meet the demands placed upon it by the load, rather than conduct a fixed current to generate a fixed power. Provided that the demands to be met were lower than rated power the unit would safely supply them, If they exceed rated power, the motors thermal performance is the critical issue unless significant external control is employed.

As you increase the load beyond the torque that the motor is capable of, the motor slows down and the hp ( the work done the motor) decreases linearly with rpm.

The motor produce zero net power (and Torque) at no load, It is merely overcoming it's own mechanical and electrical losses, which may be up to 35% of it's rated input power. As you apply a load, the motor generates torque in order to drive the load, and the motor increases it's power consumption. In an induction motor, the motor will slow below synchronous speed (slip). Torque generation and power consumption increases with slip, and there is no magic stopping point when the motor reaches it's rated output. If allowed to continue in overload, the motor will fail due to overheating unless protected by an intergated thermal protection device, external contols or supply network fuses or breakers.

The motor is effectively a torque on demand device. The torque can increase almost infinitely from the no load situation, or by a factor of maybe 500% in a situation where the motor belt drives a load, while the RPM of the motor drops by typically less than 10%.

hp= torque*rpm / 5252

Therefore your assumption below does not hold in relation to an induction motor being slowed by application of a load as variation in torque will be greater than variation (in opposite direction) of RPM.

Lower rpm = lower hp

Regards
Ray

Mal

Originally Posted by Stustoys

I have no idea what happens to Hp when you go over the motors rated rpm. My guess is you could get more power, but more power would be more heat.

Hi Stuart,

When you go over the rated 50Hz, the power stays constant for a while, but eventually slippage will limit the maximum rpm....

EDIT .. It goes without saying that since HP stays the same while rpm is increasing, the torque starts decreasing above 50 Hz.

This is for a V/F drive


Not sure what happens to the bearings as you get to higher rpm, fan cooling should be better however...

Malb,
Ignore the above graph, it's relevant to V/F drives, not what we were discussing.

I think we are converging on much the same viewpoint, I could take issue with the infinite torque comment, but either way, the end result is the same... if you overload a motor it slows down, eventually stalls and if not protected burns out by overheating... I hope we can agree at least on that.

Regards
Ray

Originally Posted by malb

Actually it doesn't make that assumption at all. It's a simple fact that if the motor cannot handle the imposed load, it either trips it's own protection, a fuse or breaker in the supply network, or destroys itself.

The point I was making is that the manufacturers power rating is an indication of the amount of heat that the motor can disipate without damage, not the hard limited input power consumption.

Not every one understands or know those simple facts
your wording "Electric motors will draw as much current as they need to to handle an imposed load." gives or may give to those who do not know and could make them assume that a 1hp could draw as much current as it want...with out detrimental effects...hence my words.

no that its all cleared up I hope Simon understands

Originally Posted by eskimo

Not every one understands or know those simple facts
your wording "Electric motors will draw as much current as they need to to handle an imposed load." gives or may give to those who do not know and could make them assume that a 1hp could draw as much current as it want...with out detrimental effects...hence my words.

no that its all cleared up I hope Simon understands

Thanks Eskimo and everyone else, I have been taking in as much of this info as possible. I have been interested about variable frequency drives and motors since I have never played with one yet. Like I said earlier, I have this motor and controller but I wanted to know what it's capabilities are in terms of its power. I guess simplistically I assumed that if I want to replace the factory 1500 W induction motor with a variable frequency drive motor, then it needed to be a similar power otherwise a reduction would be a compromise. I guess to make my point, I think I would be disappointed if I replaced the motor with a 200 Watt 3 ph motor and speed controller and expected to perform similar to the factory 1500W motor. Conversely, putting in a 2400W motor would most likely be unnecessary.... unless it was a freebie that is!

Hope I made sense.

Keep talking everyone, I'm listening!

Hi Simon,

I wouldn't be surprised if that Emerson drive and motor, were a little more sophisicated than we have been thinking. They make pretty good stuff, I've never programmed them, but I've seen Emerson drives used in some pretty slick applications.

I wonder if that controller is a vector drive? It would be the sort of characteristic that would suit a treadmill application....

If it is you will get full torque across the speed range, that's a handy feature to have.

Regards
Ray

Originally Posted by RayG

Hi Simon,

I wouldn't be surprised if that Emerson drive and motor, were a little more sophisicated than we have been thinking. They make pretty good stuff, I've never programmed them, but I've seen Emerson drives used in some pretty slick applications.

I wonder if that controller is a vector drive? It would be the sort of characteristic that would suit a treadmill application....

If it is you will get full torque across the speed range, that's a handy feature to have.

Regards
Ray

I think you may well be correct Ray. I reckon I'll have a read hot go at getting it working for me! Like I said, those inputs seem to be digital to me. Just have to come up with the correct sequence to fool the system.

The first thing I'm going to have to come up with is a simple PIC program to produce a TTL square wave output on my prototype board. That won't take long along with a simple program code.

I'm spewing I didn't take some measurements before I actually dismantled the thing! But I didn't realise what I had when I took it apart. I was quite disappointed when I saw it wasn't a DC motor so assumed it was no good to me. Was only after reading about VFD's on this forum that it clicked about what I had, by that time the treadmill was already out in the backyard for about 12 months! Everything still powers up OK, it's so well made that even out in the weather, the vital components were protected.

Cheers,

Simon

Originally Posted by simonl

The first thing I'm going to have to come up with is a simple PIC program to produce a TTL square wave output on my prototype board. That won't take long along with a simple program code.

Off topic again, but is there any particular reason you use PIC over ATMEGA? I like my Arduino's, they are great fun to play with.

Don't you have the original control board from the treadmill? I'm guessing not cause that would be the easiest way to test the motor.

Ewan

Originally Posted by Ueee

Off topic again, but is there any particular reason you use PIC over ATMEGA? I like my Arduino's, they are great fun to play with.

Don't you have the original control board from the treadmill? I'm guessing not cause that would be the easiest way to test the motor.

Ewan

Hi Ewan,

Real programmers use PIC, Arduino's are toys**...

Actually Microchip has lately been pushing it's Arduino clone system, in view of the popularity of the Arduino..

For what it's worth I use both, but prefer Hi-Tech C on the pics, but programmed pics in assembler for many years..

Regards
Ray

**Just kidding... actually I don't mind Arduino, I wonder if there is an "Emerson Shield"...

Originally Posted by Ueee

Off topic again, but is there any particular reason you use PIC over ATMEGA? I like my Arduino's, they are great fun to play with.

Don't you have the original control board from the treadmill? I'm guessing not cause that would be the easiest way to test the motor.

Ewan

Hi Ueee,

Don't want to bore you too much but I found the treadmills (yes 2) when I drove past a local gym, they were throwing them out and had them out in the carpark where they stayed for a few days. After asking permission I decided to take them home thinking there must be some good parts in them, a DC motor being one part I wanted.

Out of interest, after lugging them home I plugged them in and they both worked fine although they looked very used and the hour meter showed many thousands of hours (meant nothing to me). So, I figured I may as well take one apart and have a look. To my surprise, no DC motor but an AC one. I figured I couldn't really use an AC motor (not for a wind generator anyway which is what I had in mind) So I looked at both and decided to make one good one out of the two that would look OK. I then sold the good one on ebay and made a tidy profit which left the other and home, where it sat in the backyard for a while. It sat there until I realised that I had a 3 ph motor with a VFD, after reading and lurking on this forum. By this time I had already thrown out the top console - Bad mistake! Very bad mistake

So i wasted no time in recovering all the good stuff, including the pump, motor and controller.

Why use PIC? Well actually I use PICAXE stuff because I have never bothered to learn assembly and PICAXE stuff (as you probably know) has a bootleg program inserted that makes the programming an easier process. You no doubt do loose some flexibility with it but for me it's fine. I learnt about these after reading about them in SC magazine and I just kept using them since as that is what I have become used to.

I really should learn PIC programming and buy a PIC programming board.

Cheers,

Simon