# VFD use on a 2-speed, constant HP motor



## bodaver32 (Dec 30, 2013)

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.


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## Pacer (Jan 1, 2014)

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..)


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## twstoerzinger (Jan 1, 2014)

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.


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## bodaver32 (Jan 1, 2014)

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.


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## SEK_22Hornet (Jan 1, 2014)

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.


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## John Hasler (Jan 1, 2014)

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.



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.


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## bodaver32 (Jan 2, 2014)

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.


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## cathead (Feb 2, 2014)

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.


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## British Steel (Feb 3, 2014)

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...


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## AlanR (Feb 3, 2014)

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.


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## master53yoda (Jun 7, 2016)

this may be a dumb question but does it have brushes, some of the old motors where wound rotor motors and they would put out constant hp at whatever speed due to the rotor windings being direct feed instead of induction feed.   The only small motors of this type that i have seen were  pre WW2  the other one that I was around was 2500hp and ran on 4200 3 phase.

Art b


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## mksj (Jun 8, 2016)

Would seem to be not your typical 3 phase motor:
*
CONSTANT HORSEPOWER*
The final type of two speed motor that is utilized is the two speed, constant horsepower motor. In this case, the motor is designed so that the horsepower stays constant when the speed is reduced to the low value. In order to do this, it is necessary for the motor's torque to double when it is operating in the low speed mode. The normal application for this type of motor is on metal working processes such as drill presses, lathes, milling machines, and other similar metal removing machines.

The requirement for constant horsepower can perhaps be best visualized when you consider the requirements of a simple machine like a drill press. In this case, when drilling a large hole with a large drill, the speed is low but the torque requirement is very high. Compare that to the opposite extreme of drilling a small hole when the drill speed must be high but the torque requirement is low. Thus, there is a requirement for torque to be high when speed is low and torque to be low when speed is high. This is the Constant Horsepower situation.

Document discusses 3 phase motor characteristics: https://www.industry.usa.siemens.co...fication/documents/nema-application-guide.pdf

You should be able to configure the VFD for one of the motor speeds, not sure which one would be better. In my experience with VFDs and most 2 speeds motors, they operate better on the low speed setting, but these where wound differently. Since this motor is constant Hp with twice the torque on the low speed setting, I would configure the VFD to run on the low speed setting, and double the operating Hz to a maximum of 120Hz. I would assume that the Hp would stay the same, the torque would drop off to about the same as in the high speed motor connection setting. There is the option to call the VFD manufacturer and/or a motor manufacturer to see what they recommend.  I have setup VFDs with sets of operating motor parameters that could be switched by one of the VFD inputs and also changed the motor winding connections.  The motor changeover switch was interlocked to shut down the VFD if the switch was operated with the motor running. I would look at a VFD that is a bit higher quality and has a range of adjustability for the motor characteristics. It would be helpful to get a picture of the motor name plate and motor wiring configuration.

You may need to run the VFD in a straight V/Hz vs. sensorless vector.


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## GT Mills (May 5, 2018)

Thanks for this.  I just bought a $79.00 Zbond VFD (which uses Mitsubishi inerds) off eBay.  I am using it to replace the NOISY, energy-hungry phase converter motor that came with my Enco 100-1525 mill.  The instructions are pretty good, but I was looking for sage advice to avoid expensive mistakes.  Things I have learned on this fine South Carolina Saturday include:  NEVER switch the mill drum switch while the VFD is powering the motor, it will cause an induction spike and fry the goodies in the VFD.  See?  I didn't know that b4 hunting advice on the net.  Here is a link to another guy who wired up a VFD to an Enco 100-1525 with a Dahlander motor. 

https://lensprojects.wordpress.com/...-odd-duck-milling-machine-motorenco-100-1525/

I also learned a LOT about Dahlander motors today.  Here is a Wiki link that explains it all quite simply. 

https://en.wikipedia.org/wiki/Dahlander_pole_changing_motor


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## mksj (May 5, 2018)

The hole idea with a 2 speed constant Hp motor running from a VFD is to not to switch the winding, and have VFD directly wired to the motors low speed windings. When you run the motor on the lower RPM windings,  you run the motor to 120Hz which would typically be the same RPM as the high speed motor winding. There are multiple benefits to running it in this configuration, and the motors runs smoother the higher the number of poles. When it is not a constant Hp motor than it is a different approach.


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## GT Mills (May 8, 2018)

OK.  So I bypass the switch, and connect the leads directly from the VFD to the motor wires as follows:  U, V, W, to 1U, 1V, 1W, and leave 2U, 2V & 2W OPEN.  That will be low speed Delta config.  High speed switch setting connects 2U-2V-2W together.   

Then program the VFD for 120 Hz, and use the listed running speed on the motor's plate, which is 3430 RPM.  No worries about damaging the motor using 120 Hz?  Do I need to start the motor in two steps, first 60 Hz, then roll up to 120 Hz?

I tried the following and it didn't work at all:  Set the drum switch to 1/2 speed.  Program VFD for 120 Hz.  I tried all kinds of things to get it to run, different start up times and speeds, etc.   It just made lots of strange noises.  Once it started and ran - very slowly - in the wrong direction.  It often  sounded like there was another small motor inside of the 2HP motor gradually revving up, but the mill motor wasn't turning at all.  WTH?  

Since doing that I can't start the motor at all this morning.  But that might be the drum switch.   I think the contacts need cleaning - again.  Even with the phase generator motor I had problems starting the mill before.  I had to start with the drum at high, then switch to low while it was about 1/2 through revving up.  Weird, I would think that it would start in low easier than high, since that lights up all 8 coils and high only lights up 4.  WTH?   Even then, first time using the mill each day it could be a bit of playing with the switch to get the motor to kick in.  I really don't mind bypassing the drum switch at, since it is not in very good shape apparently.


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## mksj (May 8, 2018)

The motor wiring sounds correct, but it is specific to each motor so go by the nameplate not online wiring diagrams. The VFD is setup for a base frequency of 60Hz, a base speed of 1750 RPM (4 poles) and the kW/Hp/current rating (4P) of the motor (i.e. 1.5kW/2Hp/?A) and motor voltage. The maximum frequency would be 120Hz, the motor is physically rated to this speed so there are no mechanical limitations to the RPM. Some motors may have some performance falloff above 90-100Hz. You do not start the motor on one speed and then switch it to another speed manually when using a VFD, it should accelerate to any speed smoothly. Your description of trying to start the motor with the manual switch suggests that one of the motor poles is not functioning, this could be the switch or the motor.  I would directly connect the VFD to the motor, if it still does not run then I would take the motor to a repair shop and have it tested.  The Z-Bond is a generic Huanyang VFD, I can find no information on it other than a stock photo.


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## GT Mills (May 9, 2018)

Good, OK.  Thanks!  I removed the drum switch completely and wired the VFD directly to the motor.  Problem solved, definitely the switch contacts were hinky.  It starts right up at any speed I dial in, and I am also able to change motor speed after it starts with the push button panel, faster or slower.  I can already see that I will want a potentiometer at some point, the push button method is a bit of a nuisance.  Not so bad if using Hz to vary speed, but then I like the RPM readout better.  Huangyang models I found on Youtube were very similar, I thought the same thing about this being a knock off from those.  If anyone ever wants a reference for the panel setups, here's what was used in this case.  BTW the 2HP motor I use is rated 6.5/7.2 A.  My shop non-load, line voltage measures 250v.  My wiring is solid 12g for only about 10 ft. from the main breaker panel, then transitions to stranded 12g in flex conduit from there for another 5 ft to the VFD, which is mounted on a swivel to the back wall so I can swing it out of the way, or leave it where the old drum switch was, and keep it isolated from machine vibration (the wiring fits into the VFD terminals w/o trimming down strands).

For non-default VFD settings, you only need to play with Pn1, 2, & 8-12 (and maybe Pn17). 

Pn1:  Set at "1" and it will display Hz.  Use any other number and it displays that chosen maximum RPM. 
Pn2:  Set the line freq to 60Hz
Pn8:  Set acceleration time to 5 seconds
Pn9:  Set deceleration time to 8 seconds
Pn10:  Set maximum running freq to 120 Hz
Pn11:  Set minimum Running freq (I used 5Hz, which is ~160RPM).
Pn12:  Set motor freq rating to 60 Hz.

(Pn17:  Set braking voltage from 1-100v.  Default is 30v.  I tried both 30v and 100v, found no difference in decel braking or fault avoidance, so I think this parameter is for the auto-reverse function which I don't have any use for.) 

There are additional setups for 7 multi-segment speeds, the one-touch stop-and-reverse mentioned, torque compensation, 2nd decel time, and choices for controls, panel vs. external etc. 

This motor does require 5 sec acceleration time for a full-speed, 120 Hz start or it will trip the internal fault.  If using electric braking, also from a full-bore stop, it requires minimum 8 seconds decel to avoid tripping the fault.  I set max Hz to 120 as recommended above, and line Hz into the panel at 60.  Apparently this allows that the freq/RPM curve is correct. 

I wish I had done this when I first got the mill.  Some guy I bought it from had jury-rigged the wiring instead of repairing the faulty drum switch.  When I repaired the switch and then put it all back together the way this guy had it wired it fried the motor.  I had to buy a very expensive, new motor, and then figure out from scratch the RIGHT way to wire up the crazy Dahlander motor.  Had I just bought a VFD in the first place I could have saved myself a lot of time, money and anger management classes.  (No, I didn't really go into anger management classes, and I still yell very loudly and cuss up a blue streak.)


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