Speed Surging with Teco VFD

PaulT · May 20, 2008

I've got a Teco/Westinghouse FM100 3HP VFD on my Bridgeport clone CNC mill. The mill is a step pulley type with a switchable 2 pole/4 pole 3 phase motor.

Set to 2 pole on the fastest pulley it has a max spindle RPM of 5600.

The motor control works fine on the slower pulleys, but on this fastest pulley setting under cutting load I get a "surging" in the speed. The RPM will slow down under load and then overcompensate and go up over the targeted speed in a continuous cycle. Its not a little amount, I would estimate the speed change to be at least +/- 20% or so.

The period of the surging is around 2 to 3 seconds in the full surge cycle.

The motor/spindle combo definitely has a lot more rotational inertia in this high RPM pulley setting, it takes about 5 to 6 seconds for the spindle to hit full 5600 RPM under no load.

Does anybody know of a parameter on the FM100 that can be tweaked to eliminate or reduce this surging in this situation? It appears that the high inertia of this case is messing up the speed control somehow. I tried looking in the manual but its pretty cryptic.

Thanks,

Paul T.

J Lauffer · May 21, 2008

Except for the part about overspeeding, I would think that the VFD is going into current limit, which will cause the motor to slow down...then when the load decreases, the VFD will speed up the motor again, but it shouldn't go over the commanded speed, unless there is a large decrease in load, in which case the motor will speed up a little due to the slip (about 3-5%).

John

Pierce Butler · May 22, 2008

PaulT said:
I've got a Teco/Westinghouse FM100 3HP VFD on my Bridgeport clone CNC mill. The mill is a step pulley type with a switchable 2 pole/4 pole 3 phase motor.

Set to 2 pole on the fastest pulley it has a max spindle RPM of 5600.

The motor control works fine on the slower pulleys, but on this fastest pulley setting under cutting load I get a "surging" in the speed.
Paul T.

I have not seen anything like that. Give Russ a call at Factorymation. You might have a defective VFD. Since there is a 2 year warranty on Teco VFDs, you should be covered.

PaulT · May 23, 2008

Thanks for the tips fellas. Its definitely not going into current limit, you can set the display to show motor current and its ok.

I think the warranty is 1 year on the Tecos, but its moot anyway, this ones way older than that, it came with the machine when it was built in 2000.

Apparently Teco has an 800 tech support number, so I guess I'll try contacting them and see what they say about it.

Paul T.

Pierce Butler · May 24, 2008

PaulT said:
I think the warranty is 1 year on the Tecos,

It is two years.

http://www.factorymation.com/s.nl/sc.2/category.13/.f

It is the last bullet item in the list.

Since the critter is rather old, I would have to guess and say that there is likely a bad signal cap somewhere on the control board. 2000 was the year when the "Capacitor Plague" was near it's peak.

http://en.wikipedia.org/wiki/Capacitor_plague

Forrest Addy · May 24, 2008

Your FVD may be just fine. I think your motor is running undervoltage for its overspeed and falling out of sync. It surges because the VFD is catching it and revving the motor back up to setpoint.. You may like the overspeed for your purposes but you may be asking too much from your VFD.

The solution is to secure a 460 volt rated VFD and run it from a step up transformer. Then connnect the motor for 230 and set the various VFD parameters to support 60 Hz opration at 230 volts output. Then you can run the motor to double the nameplate RPM with impunity. Naturally all this is expensive.

All my suggestions are either expensive, complicated, or time consuming. On a good day they are all three.

PaulT · May 24, 2008

Forrest Addy said:
Your FVD may be just fine. I think your motor is running undervoltage for its overspeed and falling out of sync. It surges because the VFD is catching it and revving the motor back up to setpoint.. You may like the overspeed for your purposes but you may be asking too much from your VFD.
QUOTE]

Forrest, I'm not running the motor overspeed, its at 60hz when the spindle is at 5600 RPM, this is with the motor switched to 2 pole so that its at 3400 RPM with 60 hz input.

I think the VFD is probably ok because it works fine at the other pulley settings. At this setting there is a lot of rotational inertia and it seems to be upsetting the control loop in the VFD.

Paul T.

Doug S. · May 24, 2008

A stab in the dark

What kind of "load" are you talking? A 1/4" gouge or a light surfacing pass?
I've seen drives do this with "significant" amounts of rotational mass (1,000lbs or more), but not at this size. Have you tried setting your ramp time to say 10 seconds or more? Does that help? This I'm guessing will only help initially, but not after a load cutting load is applied, but could clue you in as to weather you have a "problem" or are bouncing around synch.??

My 2¢
Doug S.

PaulT · May 24, 2008

I might have found the problem. I was playing around with it just hitting the spindle on/off button set at 5600 with no load and I noticed that both by the sound of the spindle and watching the VFD frequency output that it was overshooting the final frequency setting.

60 hz gives me 5600 RPM, but watching the VFD display when you hit the start button it would ramp up smoothly but then go up to around 72 hz before settling back down to 60, and you can hear this in the spindle also.

I have a TM100 on a manual mill also, and when set up to similar high spindle speed conditions it doesn't overshoot at all. I started comparing all the parameter settings on both VFD's and found that Parameter 76, "Motor Rated Slip" was set to 0.0 Hz on my manual mill but was set to 5.0 Hz on the CNC one. Setting it to zero on the CNC mill eliminated the overshoot during the start.

I haven't tried it with a cutting load yet, but I'm suspecting this might fix my problem, I'll post back again when I try it for real, probably next week.

Thanks for the suggestions.

Paul T.

Search

Largest Manufacturing Technology Community on the Web

Search

Speed Surging with Teco VFD

PaulT

Stainless

J Lauffer

Hot Rolled

Pierce Butler

Hot Rolled

PaulT

Stainless

Pierce Butler

Hot Rolled

Forrest Addy

Diamond

PaulT

Stainless

Doug S.

Cast Iron

PaulT

Stainless

Similar threads