Re: Hurco VFD Max RPM & braking
- From: oldjag <msmith5242@xxxxxxxxxxx>
- Date: Sat, 21 Feb 2009 17:40:20 -0800 (PST)
On Feb 21, 6:49 am, Karl Townsend <karltownsend....@xxxxxxxxxxxxxx>
wrote:
...
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.
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
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.
.
- References:
- Hurco VFD Max RPM & braking
- From: oldjag
- Re: Hurco VFD Max RPM & braking
- From: Wes
- Re: Hurco VFD Max RPM & braking
- From: Karl Townsend
- Hurco VFD Max RPM & braking
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