Use VFD/inverter to reduce motor inrush current?
_firs...@lr_dot_los-gatos_dot_ca.us
inverter) to reduce the inrush current of a motor by ramping up the
motor slowly when it starts?
Details: I have a 6.5kVA Honda generator. It has no problem starting
and running a single-phase 3HP centrifugal water pump that draws 14.5A
at 230V when running (i.e., a 3.2kVA motor), even if there are
significant other loads already on. But then, water pumps are known
to have relatively low inrush currents (little mechanical inertia to
overcome). Yes, I know that the generator capacity should be 3 to 5
times larger than the motor kVA, but this happens to be the generator
I have available.
I tried running our air compressor on the generator, and the generator
can't start the compressor motor. The symptoms are that the generator
RPM drops way down and stays down when the compressor comes on, the
compressor runs very slowly for a few seconds, and then the 30A
breaker on the generator trips. The compressor is a belt-drive
single-stage, and is rated at 15A at 230V (the motor is rated as 3.1HP
on its nameplate, using 3.5kVA, even though the compressor is
advertised as a 6HP, but that refers to the stall rating). The
compressor does have an unloader valve on the pressure switch. The
problem occurs both with the compressor cold, and also if I first warm
up the compressor (by first running it on wall current for a few
minutes), although when warm the generator is nearly able to start the
compressor (the RPM drops way down, and then recovers nearly to the
normal 3600RPM, but then drops back again and remains down).
This all makes perfect sense: The high mechanical inertia of the
compressor makes the startup or inrush current of the compressor motor
be very high, and this momentarily overloads the generator, which
drops the RPM down, and at the much reduced voltage the compressor
motor never manages to overcome the mechanical load of the compressor
pump, which means the motor draws a heck of a lot of current, and the
generator stays down at low RPM, and the breaker goes pop. This is
unfortunately exactly what I expected.
The only problem is that I would really like to be able to run the
compressor with the generator (because for complicated reasons we need
to be able to supply compressed air in remote locations), and I don't
feel like spending lots of money on either a much larger generator, or
on a smaller compressor.
So here is my idea --- but I don't know whether it will work. Replace
the single-phase 3.1HP motor on the compressor with a 3-phase 3HP
motor of the same mechanical dimensions (it looks like a standard 182
frame). Run the 3-phase motor on a VFD (I have heard good things
about the Hitachi SJ100 inverters sold by Automation Direct). This
effectively uses the VFD/inverter to convert the single-phase output
from the generator into 3-phase. Then program the VFD to ramp up the
compressor motor speed slowly, for example ramping from zero speed to
full speed in two seconds. When I first power on the inverter, there
should be a small load surge, as the capacitors in the VFD are charged
up. Then, as the motor begins to ramp up slowly, the current draw
should remain limited, since it does take much power (in the sense of
torque*rpm = V*A) to operate the compressor slowly. The current draw
of the VFD/motor combination should then ramp up from zero to full
15A, without much of a surge (plus a little safety margin for the
inefficiency of the VFD).
Here is the question: Is this a good idea?
- First, will this work at all? What I mean is that the VFD
documentation typically requires that a portable generator capacity
be 5 to 6 times the motor kVA; but with the motor running at a
steady speed, the input kVA of the VFD should be about the same as
the motor load, which should be close the 3.5kVA the compressor
usually needs. And the slow ramp should keep the startup current
low.
- Will the motor startup current actually be reasonably low (not much
higher than the steady run current of 14.2A), if I ramp the
compressor motor speed slowly (over a second or two)?
- Will ramping the compressor slowly harm anything (I can't see how)?
- Are VFDs power-factor corrected, or do they have terrible harmonics
on their input current? Should I add an input line reactor to the
VFD to filter the input? There will be no other loads on the
generator.
- Is there something special about compressor duty motors, which would
prevent a generic 3HP 3-phase motor from working correctly (assuming
that the mounting frame size and shaft are all the correct size)?
I do understand that I will be violating the rules for running motors
off a generator (generator should have 3-5x the kVA of the motor, or
5-6x when running with a VFD), but I won't actually be exceeding the
rating of the generator when running steady. The "correct" option of
getting a 15kVA generator for this purpose is just not practical.
Advice is appreciated.
--
The address in the header is invalid for obvious reasons. Please
reconstruct the address from the information below (look for _).
Ralph Becker-Szendy _firstname_@lr _dot_ los-gatos _dot_ ca.us
it me
should be negligible because they usually have an pre-charge resistor in
series with the cap. This is bypassed by a relay, usually, after less then a
second.
It is a good way to do what you want to do from a motor stand point. I
don't know about the compressor.. not my bag!
<_firstname_@lr_dot_los-gatos_dot_ca.us> wrote in message
news:9rv9qi$1gp$1...@idiom.com...
Elliott,Jason
to the tank. Add about 6 feet of line and a secondary tank. An old 20 pound
propane tank works.
It acts like a buffer between the compressor and the tank.
I know lots of contractors that use air for roofing, and general
construction, and most of them use small 3 and 5kw generator to run there
compressors. This set up also gives enough buffer to allow the auto idle to
come up on most units..
j.Elliott
"it me" <NOs...@home.com> wrote in message
news:7mIE7.14618$wj5.6...@news1.rdc1.sfba.home.com...
Mark Daniels
the motor rating. This is hardly surprising as you point out. So it
would probably not be expected to work.
Anyway, onto some actual data.
I was setting one up yesterday and the VFD had a facility to display the
motor current, so I took the opportunity to monitor it. The rated motor
current was 3 amps.
During start the motor current ran up from about 1.7 amps to a peak of
2.1 amps, then falling to a steady current of 1.6 amps at constant speed
with no load on the motor.
The torque capability at 5 Hz was excellent, given as 150% rated torque
by the VFD manufacturer, and I found no reason to doubt this. On this
basis I suspect that your generator will be able to start your
compressor via a VFD. The motor will certainly have the torque to
overcome the inertia and friction in your compressor and this type of
starting will not cause damage to your compressor as it is softer than a
DOL start.
I have run compressors from VFDs.
In article <9rv9qi$1gp$1...@idiom.com>, _firstname_@lr_dot_los-
gatos_dot_ca.us writes
--
Mark Daniels
Dean Hoffman
_firstname_@lr_dot_los-gatos_dot_ca.us
said in part:
>Can one use a VFD (variable-frequency
> drive, also known as an inverter) to
> reduce the inrush current of a motor by
> ramping up the motor slowly when it
> starts? .....................
>I tried running our air compressor on the
> generator, and the generator can't start
> the compressor motor. The symptoms
> are that the generator RPM drops way
> down and stays down when the
> compressor comes on, the compressor
> runs very slowly for a few seconds,
>.........
Have you looked at variable speed couplings or drives? The motor runs
a constant speed and the coupling ramps up the speed of the load.
I stumbled across these while looking for something else one time. I
don't know if a variable speed coupler would be a simpler, cheaper
solution than an inverter and a 3 phase motor.
Would a centrifugal clutch/pulley help?
Good luck,
Dean
Mark Empson
not the driven load. A Motor will always draw Locked Ror Current while
starting. The driven load will determine how long that current flows for.
Typically, the Locked Rotor Current of a motor is 5.5 - 9 times the rated
current of the motor.
The starting current of the motor can be reduced by reducing the voltage
during start, but the torque reduces by the square of the current reduction.
In some cases, you can reduce the start current to a half by using a reduced
voltage starter, but this is dependent on the start characteristics of the
motor and the load.
Pressure equalising the compressor during start, will reduce the start torue
and probably alow it to start quicker in which case it may not stall the gen
set.
Alternatively, using an inverter to start and run the compressor is an
option, but you would need to change to a three phase motor and purchase an
inverter. The loading on the gen set would be higher with the inverter due
to the harmonic currents drawn by the inverter. This could cause problems
for other equipment connected to the gen set.
Best regards,
Mark Empson.
<_firstname_@lr_dot_los-gatos_dot_ca.us> wrote in message
news:9rv9qi$1gp$1...@idiom.com...
ber...@ami.com.au
which the large single cylinder oil engines were started by compressed
air. So being of an innovative mind he built his own air compressor to
charge the air receivers. It had a compression ratio of 35 : 1 and if
one was not careful would ignite the lubricating oil from the stem of
the atmospheric intake valve. The DC motor used to drive it could not
start it against the 500psi head pressure so to get it away one had to
hold down ther atmospheric intake valve until the compressor reached
full speed. The flywheel would then take over the job of smoothing out
the load fluctuations sufficiently to allow the motor to supply the
average torque. So a de-loader on your compressor and a substantial
flywheel would certainly make the job easier, even if you decide to
use a three phase SCIM and a VFD.
On 2 Nov 2001 15:21:54 -0800, _firstname_@lr_dot_los-gatos_dot_ca.us
wrote:
aeolus