Encoder requirements for new VFD system

Rebuilding P&W twin spindle gun drill and the next step is to replace the ancient spindle drive motors and the change pulley setup with dual VFD's.

Since these are basically spindle drives, I'd like the most precise RPM control. I know it is only drilling, but we are just talking about adding an encoder to a drive unit that will already accept it. At least that what I'm thinking/reading. I really don't know and haven't read anything that says "here's how stable the programmed RPM will be", without an encoder. There a reason CNC mills and lathes have spindle encoders for varying loads.

Anyhow, I'd appreciate thoughts on this.

Second issue, I haven't seen a "package" available out on the webs that checks all the boxes. Yes, there's motor/drive combos, but add a remote panel, encoder, and a motor that will accept an encoder, brake resistors, and I'm not seeing anything but parts. If that's the route, I can do it, but I'd prefer a system that has some factory support or experience putting this stuff together. If anyone can offer up a supplier, great. I'm not looking for cheap, just fair and good. Motors are 5hp 3600 rpm.

Thanks

Dave

Are the current motors still working? Are they 3 phase?

A sensorless vector drive should get you very close to perfect speed regulation without the need for encoders on the motors, at least from what I've seen! This might open up the possibility of simply using the existing motors?

I have a sensorless vector drive on one of my CNC machines and I have never heard the spindle RPM change with varying load. I have not put a tach on it while cutting, but I'm sure it's +- a couple of percent or so at all times.

Using an inverter type motor I maintain typically +/-2 RPM regardless of load on mills and lathes with sensorless vector. The RPM accuracy is typically something on the the order of 0.1%. An encoder is used for precise spindle location, with higher accuracy at low speeds or to prevent movement when locked or at very low speeds. The VFD must support use of an encoder, often this will require a more expensive VFD model and/or additional board.

Seaman said:

Rebuilding P&W twin spindle gun drill and the next step is to replace the ancient spindle drive motors and the change pulley setup with dual VFD's.

Since these are basically spindle drives, I'd like the most precise RPM control. I know it is only drilling, but we are just talking about adding an encoder to a drive unit that will already accept it. At least that what I'm thinking/reading. I really don't know and haven't read anything that says "here's how stable the programmed RPM will be", without an encoder. There a reason CNC mills and lathes have spindle encoders for varying loads.

Anyhow, I'd appreciate thoughts on this.

Second issue, I haven't seen a "package" available out on the webs that checks all the boxes. Yes, there's motor/drive combos, but add a remote panel, encoder, and a motor that will accept an encoder, brake resistors, and I'm not seeing anything but parts. If that's the route, I can do it, but I'd prefer a system that has some factory support or experience putting this stuff together. If anyone can offer up a supplier, great. I'm not looking for cheap, just fair and good. Motors are 5hp 3600 rpm.

Thanks

Dave

Click to expand...

Buy a 3HP 1750 rpm inverter duty motor with encoder something like this . . .

Premium Efficiency AC Motor: inverter duty, 3hp (PN# Y286A) | AutomationDirect

Then buy a 5HP closed loop Vector drive like this . . .

M700-03400078A10101AB100 Nidec Control Techniques Unidrive M700

Connect the 480V drive to the motor’s 230V windings and parameterize the drive with motor parameters of a rated frequency of 120Hz and rated motor speed of 3555 rpm @ 8.2Amps

This will give you 5HP at 3600 rpm with +/- .1% speed regulation from no load to full load. The prices quoted above are above industry standard pricing but should give you and idea of cost for a high performance spindle drive/motor combo that is easy to set up, gives you spindle orient capability, and allow safe rated speed at 5400 rpm all day long.

We used a similar setup using a 5HP Baldor motor running at 10hp that routinely sees 8500 rpm in a Tree Journeyman CNC milling machine. It has been in production now for 10 years without a single issue.

motion guru said:

Buy a 3HP 1750 rpm inverter duty motor with encoder something like this . . .

Premium Efficiency AC Motor: inverter duty, 3hp (PN# Y286A) | AutomationDirect

Then buy a 5HP closed loop Vector drive like this . . .

M700-03400078A10101AB100 Nidec Control Techniques Unidrive M700

Connect the 480V drive to the motor’s 230V windings and parameterize the drive with motor parameters of a rated frequency of 120Hz and rated motor speed of 3555 rpm @ 8.2Amps

This will give you 5HP at 3600 rpm with +/- .1% speed regulation from no load to full load. The prices quoted above are above industry standard pricing but should give you and idea of cost for a high performance spindle drive/motor combo that is easy to set up, gives you spindle orient capability, and allow safe rated speed at 5400 rpm all day long.

We used a similar setup using a 5HP Baldor motor running at 10hp that routinely sees 8500 rpm in a Tree Journeyman CNC milling machine. It has been in production now for 10 years without a single issue.

Click to expand...

Could you point me to some resources for sizing motors/drives for this kind of spindle use? I know that lots (all) machine tool vendors do the same thing with drives rated larger than the spindle motor, but I have no idea how to determine what the safe limit is for over-rating... I assume that the more you over-rate the motor the shorter the duty cycle is for that increased rating? How do you determine that you should connect the drives 480V output to the 230V windings and so on?

Sorry, I know there is likely a lot of theory to unpack behind your recommendation here, I just would like to make a start on understanding this for myself! Thanks!
-Aaron

You might be surprised how many CNC machines actually run open loop or sensorless spindle drives. It doesn't seem to cause any real issues.

The biggest reason to put an encoder on a spindle is if you are going to do rigid tapping.

Even if you use a VFD in a standard configuration, the motor control will be at least as good as the original straight line powered motors within the torque output limits. Motions recommendation is the best you can do for a reasonable cost.

Regarding extending the 230V wired motor to 120Hz/460V I have seen referred to supercharging the motor, below is some information I reviewed in the past but haven't tried it yet. Intriguing in particular for extending a single speed range. You are probably somewhat limited to the size 4P motor that can operate to 2X it base speed. I have been using Baldor IDNM/ZDNM series, Marathon Black/BlueMax series and Lincoln inverter/vector motors in this range with sensorless vector. These can be ordered w/wo encoders.
How to Maximize Gearmotor Speed Range | Machine Design
Explain "Supercharging" an AC motor Please ! - PLCS.net - Interactive Q & A

aarongough said:

Could you point me to some resources for sizing motors/drives for this kind of spindle use? I know that lots (all) machine tool vendors do the same thing with drives rated larger than the spindle motor, but I have no idea how to determine what the safe limit is for over-rating... I assume that the more you over-rate the motor the shorter the duty cycle is for that increased rating? How do you determine that you should connect the drives 480V output to the 230V windings and so on?

Sorry, I know there is likely a lot of theory to unpack behind your recommendation here, I just would like to make a start on understanding this for myself! Thanks!
-Aaron

Click to expand...

All induction motors have a constant torque range and constant hp range. Constant torque range is defined by the constant flux density region defined by (V/Hz) . . . a motor rotor and stator is designed for a specific flux density (V/Hz) and you can increase this flux density momentarily but stator and rotor heating goes up substantially and you need to pay attention to the motor thermal time constant when doing this.

A 230V/60Hz motor expects to see 3.83 V/Hz and as long as you can maintain this flux density, the motor will produce rated torque. Torque is reduced linearly with a reduction of flux density . . . so at 230V running at twice rated speed (120Hz) the density drops to 1.917 V/Hz or half of rated . . . So twice the speed at half the torque delivers rated hp.

The motor breakdown torque (BDT) drops at (V/Hz)^2 . . . and the BDT / FLT ratio should be maintained above 1.5 or you risk stalling the motor . . . as such there are limits on increasing speed in the constant hp range defined by the motor rated BDT. Generally a NEMA B motor has a BDT between 250 - 300 percent of rated torque.

With all that in mind, by connecting the motor 230 leads up to the 460 drive, you can now maintain the flux density at 3.83 V/Hz all the way to 120Hz . . . therefore you get rated torque to 2x base speed - slip delivering 2x rated hp.

Once you understand the underlying math, running the calcs is easy. My first effort at this made extensive use of Lotus 123 if that gives you any sense of how long this kind of application has been in use.

Most induction motors can easily operate at 2x rated speed. Many (like the black Max motor linked above) are rated at 3x base speed, and several can do 4 or 5 x rated speed. These motors can also typically operate at 2 or 3 x rated current for short periods without damage. I have operated 50hp reliance motors at 300hp for up to a minute using this approach . . . but no free lunch as I was using a 200hp ACVector drive operating in the overload region.

(And the term “supercharging” in this context is ridiculously stupid)

Many thanks to so many, just didn't know. I'll digest it a bit and ask more when required.

FWIW, existing spindle motors are 30+ years old and shouldn't be used. The original U frame oil pump motors are perfect.

Dave

The term supercharging is what others have used in the literature, but call it what you want. What I do not follow with this technique is that you are doubling the motor's designed power dissipation at 120Hz, I would think that there would be some limitations depending on the motor duty cycle. There is also the additional expense of stepping up the voltage if 460VAC is not available.

I got a 5000 count encoder for a future project.

Rotary Encoders | TAMAGAWA SEIKI CO.,LTD.

short update on the project:

Two 2hp Black Max motors with encoders will be here next week.
Two Control Techniques M700 drives will be here next week also.

I stepped down the HP from the original as I will be taking Guru's suggestion on running the high voltage feeds to the low voltage connections on the motors. Limit the current to around 6amps, and the torque will only fall off above 2400 rpm. Motors are 1200. This is ideal for a drilling machine, as less torque is needed at higher rpm.

Next step is to find an HMI and learn to program it to control two spindles and two oil pumps.

Thank you everyone.

Dave

Quick and dirty rule of thumb on what it buys you to add an encoder:

V/Hz control mode, 1-2% speed tegukation.

Sensorless Vector Control: 0.1% speed regulation (10x more than V/Hz)

Flux Vector Control with encoder: 0.01% regulation (10x more than SVC).

The question is, does that difference mean anything to your drill operation? I can’t see why it would, but that’s just my opinion.