Hurco VFD Max RPM & braking

Got a TECO N3 VFD to power the spindle motor for my old Hurco KMB1. I was using a static phase converter on this machine, but the
acceleration ramp up to speed was excessive. The TECO is a 1ph in 3ph out, and seems to work fine. I have it's I/O configured to accept the Hurco's FWD/REV signals, and it outputs overload to the Hurco control. Fine so far, but the braking from full RPM to zero is not as quick as I might like.
I was hoping to not have to rely on the air operated mechanical spindle brake, but with the N3's DC injection braking set to max., the spindle takes ~ 3 seconds to stop without the mechanical brake. I purchased the optional braking resistor, but it's not clear from the manual if this really enhances braking over DC injection, or if it's really more for situations like putting a constant drag on a play out reel, or slowing a large inertia of some sort. Normally this machine uses the mechanical brake for stopping at the bottom of a tapping cycle, before reversing.
Max RPM is the other question. The motor is a 1800 rpm, but could be dialed up to 3600 at 120 hz. This would save having to crank the vary- speed belt drive up for smaller end mills, but if it's going to kill the motor bearings I'm not going to try it. I did run it up to 2400 rpm for a few minutes and it seemed smooth and quiet. I'm guessing the bearings on 1800 rpm and 3600 rpm motors are the same, and since it's a 3ph motor there are no windings to throw. Also the Hurco control board has an analog speed command which I could wire up. This could be kinda nice, as the program could then control the spindle speed as coded, instead or the present operator adjust. Setting the vary-drive to a fixed speed, say 2500 spindle rpm @ 1800 motor rpm would allow me to cover 500 to 5000 rpm using only the VFD. So far the VFD has not spiked the 24 year old motor's insulation..
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If I'm reading this right, you bought a new VFD to run your spindle motor.
You have the resistor so hook it up and try it. It is for regenerative braking where the motor is used as a generator and the load is sinked there.
DC injection, I thought was to stop a motor that wasn't carrying a large inertial load. Like in something like a tool changer arm or a tool magazine. At least that is where I've seen DC injection used.
Wes
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...

Just one data point. I added a brake rsistor on my Hitachi VFD spindle for my Excello. Very similar mill. I was able to improve stop time from 6 seconds to 0.3 seconds. That is from 3600 motor RPM.
Karl
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wrote:

I found this tidbit, makes me think the VFD is not in the correct operation mode under braking, as the resistor is not warming up:
AC: Most AC motors do not act as generators when disconnected from the power supply. To electrically brake an AC motor requires either a DC injection brake or a variable frequency drive to provide dynamic braking. DC injection braking, applying DC voltage to the stator windings, is the simpler of the two options but is harder on the motor. Braking resistors are not involved in DC injection braking.
Dynamic braking of an AC motor is achieved by providing a slower frequency of electric current to the motor than that which would be necessary to maintain its current speed. For example, the synchronous speed of a 2 pole motor fed by a 60 Hertz power supply would be 3600 RPM. While the motor is at this speed, feeding it with a power supply operating at less than 60 Hertz creates a magnetic field in the stator which rotates slower than the rotor is rotating, and the drag produced will begin to slow the rotor down.
Although the AC motor does not have a permanent magnet in the rotor, it does have an induced magnetic field in the rotor, created by the rotating magnetic field in the stator. The energy lost in the difference between the stator and rotor speeds backfeeds into the VFD, which raises the voltage on the DC bus in the VFD. The greater the difference between the output of the VFD and the rotor's actual speed, the more energy will be backfed into the VFD. This means that if the VFD tries to dynamically brake the motor too quickly, the voltage on the DC bus will raise too high and damage the VFD. Most VFDs will shut down as a safety feature before this happens, and the motor will coast to a stop by friction alone.
This is where the braking resistors come in. The braking resistors act as an additional load on the DC bus, which helps to drain the excess voltage and keep it within safe tolerances. With appropriately sized braking resistors, the motor can be brought to a stop much more quickly without raising the voltage on the DC bus to unsafe levels.
In both the AC and DC cases, the smaller the resistance of the braking resistors, the larger the load it creates and the faster the motor can be stopped. However, less resistance means more current can pass through the resistor, and more current means more heat is produced. The extra heat must be dissipated by heat sinks, or reduced by using multiple resistors in parallel which share the load. Either option drives up the cost of the braking resistor system, so it is important to size them correctly for a given application.
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On Fri, 20 Feb 2009 21:05:54 -0800 (PST), oldjag

The braking resistor sucks up the generated hysterisis currents ...IE the load the various turning bits has when trying to stop it quickly.
Yes..its way important when trying to stop quickly.
My Gorton Mastermill, with its BIG spindle and 5 hp motor will stop in under 1 second from 3000 rpm when I hit the stop button. 5hp PC3 VFD and running from 220vt single phase, with a 200 watt ceramic braking resistor @ 140 ohms.
Gunner
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I installed the 250 watt 60 ohm resistor as per the manual. It does not seem to be adding to the brake torque, ie the decell time is the same. I suspect some kind of translation error or omission in the manual that is leaving out a parameter needed to enable the resistor braking correctly. The resistor is not even getting warm after several full rpm to zero stops. Guess I gota call Teco's help line Monday. Any thoughts on exceeding the base RPM, ie. 2X on a 1800 rpm motor?
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oldjag wrote: ...

I have an 1800 rpm motor on my lathe that I run up to 120Hz, no problem (yet). IIRC, 120 Hz is commonly used on 1800 rpm motors. On my drill press I run a 3600 rpm motor up to 90 Hz (5400 rpm). The construction of 1800 & 3600 rpm motors is probably nearly identical. Implying that an 1800 rpm motor could be run at 5400 rpm. But I don't. It would be a good experiment. <G>
Bob
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On Sat, 21 Feb 2009 17:35:25 -0800 (PST), oldjag

Id not go over 2x the base RPM. Some motors can take 3 or 4x...but they are going to be screaming!! and its got to be hard on the bearings
Check out the "Decell time" settings, often more than 2 of them, and how the various Decel times get activated. Some need Stop, others need an absense of Run and so forth.
Gunner
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On Sat, 21 Feb 2009 20:12:28 -0800, Gunner Asch

Ditto on the 2X, and that's on a 4 pole motor. I've also installed inverter duty motors with max speeds on the nameplate of only 2700 rpm, so a 90 Hz limit. Most are twice that for 180 Hz, or 5400 rpm.
I've never seen a rotor expand from overspeed and don't want to.
Pete Keillor
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oldjag wrote:

You may need to tell it that the braking resistor has been installed. Then, you need to specify a deceleration rate, usually some # of seconds (and often tenths) to stop. You may also have to specify what frequency to switch over to DC braking.
Jon
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    [ ... ]

    That may be.

    Could you measure voltage on the resistor during the stops? i would guess not.

    I would be a lot happier about that than on a 3600 RPM motor, since the same rotor is used in many motors of the same size, so it is likely built to handle 3600 RPM without problems.
    Enjoy,         DoN.
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oldjag wrote:

The braking resistor is designed to absorb the energy taken from the motor by the VFD. When the VFD resuces frequency, energy is returned to the DC bus of the VFD, and the voltage rises. When it hits about 380 - 400 V the braking resistor is turned on in pulses to get rid of the energy. You can set the frequency ramp-down rate of the VFD, and get any level of slowing you want, up to the point that the braking resistor, when on all the time can't absorb the energy, and then the drive will fault on an overvoltage condition. You need to set the slow-down time longer than when you get the fault. I have my Bridgeport 1J set for a 1 Sec ramp from current speed down to 10 Hz, at which time the DC injection turns on to complete the stop.
Jon
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