AC Invertor
AC Invertor
(OP)
Hello Guys!
If I use an AC invertor to reduce the speed of a motor(900rpm) how much will the torque reduce if it is a 2hp motor? The pf i0.68 and mechanical effeciency is 78%. I donot have the torque characteristics of this motor.
The AC invertor when connected to this motor shows 70V and 1.8amp and the rpm is 148. Its an ABB ACS 350 drive. Are these values displayed on the AC Invertor true values?
If I use an AC invertor to reduce the speed of a motor(900rpm) how much will the torque reduce if it is a 2hp motor? The pf i0.68 and mechanical effeciency is 78%. I donot have the torque characteristics of this motor.
The AC invertor when connected to this motor shows 70V and 1.8amp and the rpm is 148. Its an ABB ACS 350 drive. Are these values displayed on the AC Invertor true values?






RE: AC Invertor
Assuming the values you give are RMS line-to-line voltage and line current the real power supplied at the operating point you gave is root(3)*70*1.8*.68=148W.
Mechanical power = Torque*w = 148*efficiency
w is 2*pi[rad/sec]*148/60 = 15.5 rad/sec
Torque = 148*.78/15.5 = 7.44 N*m
Since your speed is decreasing your torque rating will increase. However, since torque is proportional to armature current there is a point at which you will become current limited and you can no longer get rated power out of the motor. For induction motors this is somewhere around 1/4 to 1/6 rated speed and is based on the point at which flux weakening starts to occur.
You didn't mention the nameplate current or voltage rating of this motor. What type of load is it driving?
RE: AC Invertor
RE: AC Invertor
The torque required on the dumper is 16000in-lb. The motor is connected to a 60:1 worm gear and then 4.5:1 sprocket chain mechanism to the shaft which is rotating a cuboid shaped dumper.
I just wanted to clarify myself with this information that the way you have shown this calculation gives a very small horsepower, I calculated it the same way and it's a very small number.
Another thing is rpm is fixed by AC drive, how can the torque increase with the load?
What is the difference between AC drive and AC Invertor?
What is difference between an induction motor connected to a worm gear box and just a gear motor provided the ratio required is same?
RE: AC Invertor
RE: AC Invertor
A Variable Frequency Drive (VFD) is what you are actually discussing. A VFD puts out very complex rapidly changing waveforms to control three phase motors.
By varying this complex waveform more torque can be generated,(within limits), at specific speeds than would be allowed with just motor voltage reduction.
Keith Cress
kcress - http://www.flaminsystems.com
RE: AC Invertor
The current will be about proportional to the torque load.
This is for a reasonable range of speed reduction.
Auxiliary cooling may be needed if full torque is used at slower speeds.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: AC Invertor
RE: AC Invertor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: AC Invertor
What is it you want to know exactly?
RE: AC Invertor
RE: AC Invertor
You may consider the smallest 3 phase gearmotor available and drive it with a Variable Frequency Drive. This will give you speed control and quite a turndown ratio.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: AC Invertor
What do u think about this?
I may be able to use gearmotor with 90:1 ratio with a base 865 rpm motor whic will give me a turn down of 7.2 rpm and an 8:1 ratio on the sprocket which will give final rpm of 0.9rpm. I can use an AC drive to reduce the speed further to get my 0.5 rpm. The drive has to bring the speed down to 400rpm.
RE: AC Invertor
Use a Variable frequency drive (120 V 200 V or 240 V single phase in, Variable frequency and voltage out) to reduce the speed to 400 Hz. If the motor available is larger than you need, don't worry, the extra size will help dissipate the heat at low RPMs. Turn down ratio, if a drive is capable of runing a motor from 1800 RPM down to 18 RPM, I would call that a 100:1 turndown ratio. I use the term to describe the range of adjustability, not the fixed ratio.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: AC Invertor
"I use the term to describe the range of adjustability, not the fixed ratio". I did not quite understand this- do you mean turndown ratio.
"reduce the speed to 400 Hz" - do u mean rpm?
With your idea I will have to run the motor at 50 rpm to get 0.5rpm (50/100) at the output?(Unless I bring 18rpm to a lower value using a sprocket chain?
Is it not true that a sinf=gle phase motor draws almost the same current at any load?
Please answer each concern. I will aprreciate it?
Is it possible to talk to u?
RE: AC Invertor
"I use the term to describe the range of adjustability, not the fixed ratio". I did not quite understand this- do you mean turndown ratio. Yes, I was referring to turndown ratio
"reduce the speed to 400 Hz" - do u mean rpm? Yes, I meant RPM
With your idea I will have to run the motor at 50 rpm to get 0.5rpm (50/100) at the output?(Unless I bring 18rpm to a lower value using a sprocket chain?
OK,865RPM/90:1 ratio = 9.61 RPM. From there you may use a sprocket, but at 2 inches x 19 inches for your sprockets there may be space constrictions. but it is doable.
With a VFD at 45 RPM there may be cooling issues but it is doable.
If the budget is available I would go with about a a 3:1 or 4;1 chain reduction and run the motor at 135 RPM or 180 RPM with a VFD.
Is it not true that a sinf=gle phase motor draws almost the same current at any load? Yes no and maybe. It depends. different types of single phase motors have different characteristics, some don't drop the current much when the load is reduced. However, because of changes in the power factor, equal current does not always mean equal energy usage, and energy is what you pay for. Search this site. There have been some discussions on this topic.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: AC Invertor
I will do this then as per your idea: Run the motor at 200rpm(AC drive), gear ratio =90:1 and sprocket ratio of 4.5:1(I looked in Tsubaki catalogue)
200 * 1/90 * 1/4.5 =0.49rpm.
Do u agree?
The dia of the sprockets are 23" and 5" approximately.
RE: AC Invertor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: AC Invertor
I really appreciate it.