VFD use on a 2-speed, constant HP motor

B

bodaver32

Active User
Registered
Joined
Dec 17, 2013
Messages
41
Hello all,

I recently purchased an Index 55 mill. It has the original 220v 3phase, 2-speed, constant 1 HP motor on it.

I wanted to put a VFD on it, to the high speed side. But, as I research it more - I find that I may not be able to do this because of the motor being constant HP.

Is this correct?

Thanks for any help, I am just about burnt out on researching the matter on the net.

Many thanks.
 
Was hoping someone would reply to your question as I am curious as to what a "constant 1hp motor" is? maybe nobody else does either?

So, could you give a bit more detail on the motor...

Many of the Bridgeport type mill motors had/have 2 speed motors in 1, 1-1/2, and 2 hp. My Enco clone has a 2hp and I have a Hitachi VFD on it. I run it in the low speed as it doesnt really like the high speed, throwing a default on ramping up and down on the speed control. If I go really slow using the pot, it will perform nicely at constant speed, but I so seldom need the high speed, I've never pursued it. If I were to use the high speed a lot, could possibly reset the parameters to handle the speed changes (I'm not sure of this..)
 
I'm with Pacer. Waiting to see if anyone recognizes a "constant hp motor."
Drive loads are commonly referred to as constant torque or constant horsepower.
Constant horsepower means that the torque increases proportionately as the rpm decreases.
There have been mechanical and electrical drives systems developed to work against a constant horsepower load, but I am not familiar with a 3 PH electric motor that would be classified as constant horsepower.

If the motor nameplate uses this term, perhaps a photo of the nameplate would help to resolve the issue.

Terry S.
 
I will try to get a photo later this week.

On the plate it indicates at low speed (rpm) the hp is 1. At the higher speed the hp is also listed at 1 hp. From what I understand this is a constant hp motor.
 
I'm no expert on motors, but for the sake of discussion, aren't some two speed motors rated for a reduced horsepower at the lower speed? I wonder if the constant horsepower is simply stating that it should have equal horsepower at both speeds. If that is the case, I would expect the current requirement to double at the lower speed. Are there any current ratings on the motor plate? I did find this when I googled constant horsepower - a link to US motors. http://www.usmotors.com/TechDocs/ProFacts/Typical-3ph-Connections/Constant-HP.aspx Isn't that the motor brand used on some Bridgeport mills? They might be able to shed some light on the question. I guess what I'm thinking is that if you set the VFD up for one speed or the other of the motor, it will probably work, but changing speeds might require different settings on the VFD to handle the higher or lower current. I suspect that if you set it up for one speed and don't try to go from DC to Daylight frequency wise it would probably work. I'm assuming you intend to use this for hobby use and not production use. I think I'd try to keep the frequency range to maybe 40 to 70 hz or even less - you would probably have to experiment to see what range the motor would tolerate and which connection gave the best results. I'm using a VFD as a phase converter on my Harrison lathe, but I don't use it to vary the speed. It has a 2 speed motor and both speeds run just fine, but I do believe it is rated for lower horsepower on the low speed.
 
bodaver32 said:
I will try to get a photo later this week.

On the plate it indicates at low speed (rpm) the hp is 1. At the higher speed the hp is also listed at 1 hp. From what I understand this is a constant hp motor.
Click to expand...

There is no such thing as a constant hp induction motor. They've just given it the same rating at both speeds. Configure the motor for whichever speed works best with your VFD and leave it there.
 
John thanks, that's what I will do.

I am out of my league with the electrical side of things.

The motor and mill manual refer to it as a constant HP motor, so that was what I was working from.
 
MY TWO CENTS:

My Enco mill has a 2 speed 3 phase motor on it. The switch turns the low speed winding on first
and after spooled up you can switch to the high speed configuration. I intend to try to start the
motor from a dead start on high speed and see what happens. My guess is that it won't start
by itself. If this is true, a VFD would probably only be usable on the low speed setting. I use
the high speed occasionally for wood or small end mills so hesitate to make the jump to a
VFD. Hopefully somebody out there has had experience in this and will supply us all with
some helpful comments. Thanks everyone.
 
Then again, some of our lathes are fitted with 3-speed constant hp motors... mine has a huge BTH Dahlander-wound motor with 4, 6 and 8-pole winding connections to give the three speeds - the current is much the same in each speed (4.5 - 5 Amps) so it gives a pretty much constant 3hp at all three speeds.

It IS connected to a VFD, but there's some extra switchery to avoid killing it when moving the Frankenstein Switch between speeds...
 
Two speed 3 phase motors are run as 4 or 8 pole motors. They'll start just fine in either mode.

I have a VFD and I just make sure to power the main power (to the VFD) down before I switch speeds. As a safety I've removed the knob from the drum switch so it's not so easy to switch it.
 
You must log in or register to reply here.
Back
Top