VFD used for two speed motor

leitmotif

Member
Join Date
Nov 2004
Location
Seattle Wa. USA
Posts
3,680
Application is a lathe. The motor is 3 phase two speed ie 4 pole (1800 synch RPM) and 8 pole (900 RPM synch), rated approx 3 HP - have not seen and not sure HP rating applies to both windings. For sake of discussion and simplicity let us assume both windings are rated at same HP. This may be a bad assumption I know.

What I know is
It is possible and frequently done to run multiple motors on a single VFD that is sized to the total motor load. Overload protection must be provided to each motor.

The owner operator knows not to switch speeds unless motor is at a stop.

What I am not sure of is the effect on VFD from the "large" difference in current from the two speeds.

Dan Bentler
 
Why would you need to use both speeds on the two speed motor that is connected to a VFD?
Why not just adjust speed from the VFD and wire motor only for one speed? (High)
Maybe I am off the mark for your application, but thats what we have always done when adding VFD to 2 speed motor.

Regards,
BCS
 
The motor is 3 phase two speed ie 4 pole (1800 synch RPM) and 8 pole (900 RPM synch), rated approx 3 HP - have not seen and not sure HP rating applies to both windings. For sake of discussion and simplicity let us assume both windings are rated at same HP.

Dan are you also proposing for simplicity that the full load current for both windings is also the same? In my experience this has not been true.
 
Application is a lathe. The motor is 3 phase two speed ie 4 pole (1800 synch RPM) and 8 pole (900 RPM synch), rated approx 3 HP - have not seen and not sure HP rating applies to both windings. For sake of discussion and simplicity let us assume both windings are rated at same HP. This may be a bad assumption I know.

What I know is
It is possible and frequently done to run multiple motors on a single VFD that is sized to the total motor load. Overload protection must be provided to each motor.

The owner operator knows not to switch speeds unless motor is at a stop.

What I am not sure of is the effect on VFD from the "large" difference in current from the two speeds.

Dan Bentler


Some VSD's allow motor map change based on a digital input
Change hard wiring so that the drive must be disabled before VSD output is disconnected - possibly use an early break from the switch
 
Why would you need to use both speeds on the two speed motor that is connected to a VFD?
Why not just adjust speed from the VFD and wire motor only for one speed? (High)
Maybe I am off the mark for your application, but thats what we have always done when adding VFD to 2 speed motor.

Regards, BCS

MILLDRONE
I predict the current for low speed to be double that of high.

BEARING C
If it were mine I would run it in low speed since it is driving a lathe. I could always use 150% RPM to gain higher RPM

Dan Bentler
 
Dan, to cover the existing capabilities, you need to connect the motor to the VFD in the 8 pole configuration. Size the drive for the full load amps required in low speed for full excitation. Set up your VFD to take the motor to 120hz giving you your 4 pole speed.

You may be a little short of hp at the 120hz point but below that you should have everything the old two speed motor system had.

In this type of application, I would choose a VFD with at least 150% short-term overload current (commonly called heavy duty or constant torque rating).

I definitely would not do switching between the two motor speeds. That invariably ends up being done on the fly and a failed VFD can be the result. Plus its inconvenient when working the machine.
 
Dick

Thank you for confirming my uhh "fears". I am doing 3rd hand consultation and have not seen the machine - dont even have nameplate info.

I was thinking same use 8 pole and adjust its speed to get cutting speed desired.
Dan
 
leitmotif,
Right you are!!!
Lathe did not register with me when I replied obviously.
We were doing this on fans so I was responding from memory.
Thanks for correcting my mistake. Sorry about that.

Regards,
BCS
 
If this is going to happen can we please start with my corperate office first.
I am trying to layout a electrical design for retroffiting all of our Hyd. Winches and convert them to Electrical winches.

Corperate is throwing in there 2 cents and trying to make even attemting to do this project a HUGE MISTAKE.(at times I wonder if they are even Human now)
Project to start mid to late October so running out of time hopefully they get there $**T together.

Have a good one,
BCS
 
On the subject of 2 motors from 1 VFD , Delta do a CP2000 range which allows for 2 motors with independent settings for base speed , current and torque . They are reasonably priced and should suit your lathe nicely . Worth looking at !

Paul
 

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