Spec a 3PH motor 1

[ihopeitworks]

I currently have a 2HP, 3PH, 60Hz AC motor, that is connected directly to a 12" dia drum. In the past, I would pull a cable that is wrapped around it with 2 lbs of tension (torque of 1 ft-lb). I have a new project that I am working on that requires a 10.75" dia drum pulling a cable that will have 25 lbs of tension (torque of 11.2 ft-lb). My current motor is not adequate to handle this task so I am now looking for a new motor and VFD to swap out. The kicker is I'll need to be able to apply 11.2 ft-lb of torque at both 120 RPM and 1400 RPM. The reason for the different RPM is that I need to pull this cable at two different speeds, but at the same tension.

How would I go about spec'ing in a motor for this sort of application? I would hope I can get to both RPM within 10 seconds or so. I have a panel wired to step up 120 VAC to run at ~230 VAC. It seems very difficult to find a motor's torque curve for the entire RPM range, it seems that they're only included for the high RPM end.

Looking for any guidance. Thanks.

 

[ihopeitworks]

This may be a separate issue, but I just tried using my 2HP motor and tied a cable to the drum and attached a scale to the end of the cable and had it fixed. Turning the VFD to full power, the motor would only pull 3 lbs (around a 12" drum, so 1.5 ft-lb of torque) before the VFD would shut off.

 

[jraef]

HP = Tq (in lb-ft) x RPM / 5250.

You didn't state the motor design RPM, but ASSuming it's a 4 pole motor running 1750RPM, we can see that your 2HP motor SHOULD be capable of around 6 ft-lbss of torque. So something is definitely wrong with your setup. A VFD is theoretically designed to allow you to get full torque from the motor at any speed. But not all VFDs are created equal and that capability is something that gets "sloppier" as speed is reduced when using the cheapest and least capable VFDs. For something like this, you AT LEAST need what's called "Sensorless Vector Control" or "SVC" as a feature of the drive. My suspicion is that you have a simple cheap "V/Hz" drive that is ALSO set up for a "variable torque" load, like a pump or fan, so the drive attempts to save a little extra energy by decreasing the torque capability at lower speeds since in that type of machine, it's unnecessary. You MIGHT be able to fix that issue in programming if you understand all of that. But it STILL will not get you what you want...

So if you need 11.2 ft-lbs of torque at 1400RPM, you are going to need to use at least a 4HP 4 pole motor and since they don't come that way, a 5HP is what you would need to buy. A 3 HP 4 pole motor will get you only 9 ft-lbs.

Side note: I'm not checking your math on the drum force issues...



" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ihopeitworks]

jraef,

This is the nameplate on the motor I currently have.

We are using an Automation Direct GS2-22P0:

https://www.automationdirect.com/ad...ts_(115_-z-_230_-z-_460_-z-_575_VAC)/GS2-22P0

I was able to change some settings in the VFD to operate at higher startup torque. When I did this, the scale was able to read up to 18-20 lbs of tension, which correlates closer to 6ft-lb of torque.

I still get very confused at AC motors and their torque at various speeds. I have been told that the low RPM range has low torque, but when I use the HP = Tq x RPM /5250 equation, using the same HP motor, torque goes up as RPM decreases.

I'll look into SVC's as I'm not even too familiar with VFD's to begin with.

 

[jraef]

Rated torque from a motor is at the motor's rated speed. Rated speed of an AC induction motor is a simple formula: S* = 120 x Hz / Number of Poles. *S = the basic "synchronous" speed, but AC induction motors must have "slip" to function, so the Slip speed is always slightly less, expressed as a % of S. So a 4 pole motor getting 60Hz here in North America will have a Synchronous speed of 60 x 120/4 = 1800RPM, then if it is a 4% slip motor, the actual shaft speed at rated torque is 1800 x .96 = 1728RPM.

You can't make an assumption about the torque at other speeds of that motor without first defining HOW you will attain those other speeds. Using a VFD is one method, and THAT method is intended to MAINTAIN the rated torque as speed is lowered by changing the frequency, then also changing the voltage to match so that the ratio of V/Hz stays constant. The other way to change an AC induction motor speed is to change the number of poles or the pole configuration, as in using a 2 speed motor that has 2 poles or 4 poles depending on what you select via the wiring connections.

So I'm not sure what you were reading, but there are SOME methods of creating two speeds in which the motor torque DOES drop with speed, because you are lowering the speed by increasing poles in the same motor and in doing so, also putting more windings in series with each other so the effective voltage across them drops too, which loses torque. There are other ways of changing pole count however that also MAINTAIN torque the same at both speeds. That's why you can't make assumptions about torque at lower RPMs without first defining HOW you get to the lower RPM.

That AutomationDestruct drive is the simplistic cheap "V/Hz" type I was describing, it is not going to be very good at maintaining full torque at lower speeds. You likely discovered their only available "trick" which is called a "Voltage Boost", and that kind of works, but is a manual process. SVC does something similar, but does it automatically all of the time.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

For low RPM at the torque you need you will probably need auxiliary cooling. The motor fan won't be turning fast enough to cool itself.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ihopeitworks]

jraef,

Thanks for all that info. Helps a lot.

I believe I'll then be looking into 5 HP motors that can run off of my single phase input. I will plan on using the 4 poles in the standard configuration and utilize a VFD/SVC. What other brands would you consider better than AutomationDestruct? I'm hoping to keep the VFD under $1k and the motor under $1.2k.

 

[ihopeitworks]

waross,

That shouldn't be an issue. I have some blowers that I can route near the motor's fan. We should only be running the fan for 5 minutes at a time. 2 - 3 start ups per hour.

 

[ihopeitworks]

What happens to a VFD if I don’t have enough current going to it? Tech support said it needs 40A, but I’ll only have 30A. I’m looking at alternative generators for this setup, but want to see what’ll happen if I try hooking it up as is.

 

[jraef]

40A on a 2HP drive??? Or are you now referring to a different drive?

As a general rule, drives running in V/Hz Mode don’t care. Vector drives do, because they are tracking motor current anomalies in order to determine the rotor position, so below around 50% of the drive current rating the current sensor accuracy begins to drift too much. But 30A on a 40A rated drive is fine. Whomever told you otherwise is a nincompoop.

But if it is an 80A rated drive, then they may be correct because 30A < 1/2 of 80A

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Skogsgurra]

You Always learn something new on EngTips!
This time, I learned the word nincompoop. Very useful, although the lack of the second n in the early form casts doubt on this origin.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

What happens to a VFD if I don’t have enough current going to it? Tech support said it needs 40A, but I’ll only have 30A.

If the VFD draws more than 30 Amps on a 30 Amp circuit you may trip the supply breaker.
But this brings up a question Jeff or Skogs:
As the frequency and speed of the motor is dropped, does the input current to the VFD drop also?
Example:
A 5 HP motor draws 15.2 Amps at 230 Volts and full load/torque.
The motor is run at 50% speed, voltage and frequency and at rated torque. Motor draw is still 15.2 Amps but at 115 Volts.
What happens to the VFD input current?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

If you ASSume (Jeff) a constant efficiency, which is almost true, then input Power is a Little bit more than output Power and since cos(phi1) is Close to unity, the input current follows HP out.
So, what happens is that current decreases when output speed decreases and the load is constant torque type. And it decreases a lot if you have a load (fan etcetera) with parabolic torque.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ihopeitworks]

jraef,

I'm using a 5HP motor (Dayton) with a GS4-2015 (automation direction) VFD. I asked their tech support what current the VFD needed for single phase 230V, and they advised that the input is 40A.

Looking at the motor nameplate, it calls 13A at 230V. I would imagine that my 7000W generator would be able to power this thing (240 w/ 30A circuit breakers).

 

[davidbeach]

Small generators and VFDs may not play nicely together.

 

[jraef]

13A at 230V 3 phase becomes 13 x 1.732 = 22.5A from the single phase source. But for 5HP you must increase the size of the drive to be able to handle the added burden of the excessive DC bus ripple it creates inside of the drive. Some drives have DC bus inductors to help in that process, some do not, usually Asian sourced drives like yours. Plus the latest generation of smaller-cheaper drives are using capacitors that are less forgiving than those of old, so in many cases the required de-rate factor is 65%. So your 13A motor would need a drive rated at 13 / .35 = 37A. That is their likeliest route to telling you “40A”.

But that just means you need a 40A RATED drive in order to attain the correct capacitance. Your current draw from the single phase source is not going to be 40A, it’s still the 22.5A as previously stated. A LOT of inside sales people being passed off as “Tech Support” are ... nincompoops.

Odd quirk to this however. The NEC has a rule about sizing the CONDUCTORS feeding a VFD that tequires that it be done based on the VFD rated input amps. That may be 40A on the drive they have selected for you. But that again doesn’t mean the drive will DRAW 40A, it’s just the number you must use to size the wires feeding it.

Bottom line for you however is the size of your generator. Your max motor power is 5 HP which equates to 3.73kW (746 watts per HP). That applies whether it’s 3 phase or single phase. Another “rule of thumb” common to generator sizing when using VFDs is for the generator to be at least 1.5x the VFD load. So that puts you at a minimum generator of roughly 6 kW, you have 7kW, I think you are fine. But the point raised by Davidbeach is valid. Not all generators like being connected to “non-linear” loads like VFDs, especially those that are “inverter type” generators themselves. Talk to your generator supplier.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ScottyUK]

Most of the time the DC link of the drive is accessible, so adding capacitance may be cheaper than increasing the size of the drive. That said, beer can sized capacitors aren't cheap either.

 

[Skogsgurra]

Adding capacitors will reduce conduction angle in the rectifier. That means narrower and higher charging pulses with more heat in the rectifier and cables, higher RMS value. So that could make things worse. A heavy line reactor brings conduction angle back to normal. And, since you do not need to run at high speed, you will be fine. But now, there is a tiny tail wiggling a rather large dog. Drawing board again?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

Scotty,
335+ VDC wiring to the DC bus from external caps, typically not made to be separately mounted, is not something I would recommend to a novice.

Gunnar,
Never thought if that. But it’s being done every day, so it apparently works.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ScottyUK]

Good point about the conduction angle Gunnar, although being honest I've never seen it result in a failure in a small VFD.

jraef - yeah I suppose there's an element of risk there for the unwary. ;-) I've got this arrangement on an old A-B 1336 drive running my lathe, works beautifully but needs to be done safely.