AC Invertor

[harrinshotmail]

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?

 

[mls1]

Torque is a function of load so your motor will produce whatever torque is necessary up to pull-out torque at which point it will stall.

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?

 

[LionelHutz]

The VFD will supply a certain current. Running at this current into the motor, the motor is capable of producing a certain level of torque - likely some level more than rated torque. Theoretically, you can get this torque at all speeds.

 

[harrinshotmail]

The rated full load current is 3.5amp and the voltage is 460V. We have used a step down tranformer from 575V to 460V.

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?

 

[harrinshotmail]

Are u guys saying that if my max torque is 2 x 5252/865 = 12ft-lb, then the torque is same at 148rpm as well.

 

[itsmoked]

The term inverter is now generally reserved for a box that converts DC to AC at a fixed frequency.

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

 

[waross]

A normal motor used within it's rating will develop about the same torque and draw about about the same full load current over a wide range of speed reduction. Horsepower will be about proportional to speed for speed reductions.
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

 

[harrinshotmail]

What kind of gearing does a gearmotor has?

 

[waross]

What kind of gearing does a gearmotor has?

worm gears, planetary gears, helical gears, spur gears, bevel gears. Check the specs. Ask the supplier.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ozmosis]

harrinshotmail
What is it you want to know exactly?

 

[harrinshotmail]

My question is the calculations shows that the HP required is so small. I can use a 1/4 hp motor for this application. The speed required is around 0.5 rpm. What is the best way to achieve it: Sprockets, worm gear, gearmotor ect?

 

[waross]

Start by looking up specs in catalogs. Find the reduction ratio that you want to use in the gear motor and make up the rest of the overall ratio with a chain and sprockets. High ratio gear motors often use worm drives, but it doesn't matter that much. Buy the drive with the ratio to suit and take what you get for the type of gearing.
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

 

[harrinshotmail]

Thanks Bill. Should I still use chain and sprockets?

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.

 

[waross]

Your cheapest motor by far will be a base 1800 RPM (base 1500 in 50Hz land). That is the synchronous speed. Actual motors will run 25 RPM to 75 RPM slower.
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

 

[harrinshotmail]

I have found motors running 865rpm for a very good price($400). I want to use a 3phase motor because 3phase is easily avialable where this dumper is going.

"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?

 

[waross]

Sorry for the sloppy writing, my bad.

"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

 

[harrinshotmail]

Sprocket will actually save us space because we donot have to mount the gearmotor axially. A 90 degree motor shaft motor can be bought and a sprocket ratio of 4.5:1 can be used.

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.

 

[waross]

It looks ok. Your sprocket ratio is actually 4.6:1 but it is close enough. Trim the speed with the VFD.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[harrinshotmail]

Thank you very much. Actually i got the ratio from the sprocket teeth which are 72 and 16.

I really appreciate it.