# Low RPM ( 100 rpm) motor without gear box

Thread Starter

#### t00t

Joined Jan 22, 2015
73
Hi All,
Have been searching online but can't seem to find any 3 phase AC motors (without gear box) that has low RPM .

I need roughly about 100 to 125 RPM motors .

Any experienced guys please advice

Thank You .

#### Hypatia's Protege

Joined Mar 1, 2015
3,226
Hi All,
Have been searching online but can't seem to find any 3 phase AC motors (without gear box) that has low RPM .

I need roughly about 100 to 125 RPM motors .
Any experienced guys please advice

Thank You .
Any experienced guys please advice
While I cannot claim to entirely fit that 'demographic' -- I'll give it a go anyway

Sounds like a job for a VFD (i.e. variable frequency drive) controller -- even so - derivation of significant torque at low AV from an induction motor could prove challenging...

Best regards
HP

Last edited:
Thread Starter

#### t00t

Joined Jan 22, 2015
73
Is there any VFD that can drop an AC frequency to 3 hertz ?

#### Hypatia's Protege

Joined Mar 1, 2015
3,226
Is there any VFD that can drop an AC frequency to 3 hertz ?
In theory, of course! - As a practical matter I rather doubt it (IOW I see your point) --- Why, if I might ask, is gear reduction undesirable in your application?

Best regards
HP

#### shortbus

Joined Sep 30, 2009
8,594
Don't think a motor can/is made to go that slow. The number of poles in the motor vs the AC cycle sets the speed of the shaft. A 'c' face motor and an inline gear box would be the only way to get that slow of a RPM. For something with a 'small' foot print, a parallel shaft motor would be what I mean. Like this - http://www.grainger.com/product/BOS.../rp/s/is/image/Grainger/49T795_AS02?$smthumb$

A VFD won't go that slow.

#### alfacliff

Joined Dec 13, 2013
2,458
we have exhaust fans that turn that slow to exhaust chemical tanks, then speed up to different speeds depending whether the tank coverf is open or not. they were previously squar d brand and now allen bradley vfd's . since the smallest one is 20 hp, might be a bit big for you.

#### MrSoftware

Joined Oct 29, 2013
2,015
How big do you need? The motors used on electric RC planes and drones are 3-phase AC variable speed. The motor controller varies the speed. I don't know how to figure out the minimum speed, but it might have something to do with the inertia of the load, so that it has enough flywheel effect to keep it rotating between poles at very low speeds. They come in various sizes, here is a random example:

http://www.hobbyking.com/hobbyking/store/__523__517__Electric_Motors-Specialised_Motors.html

#### MaxHeadRoom

Joined Jul 18, 2013
23,156
With the right style, encoder feedback for e.g. VFD's have been known to control down to 0rpm, as in elevator usage.
In this case it may be advisable to go with the highest pole count motor you can find.
Any reason you do not want to use a gear box?
Max.

#### crutschow

Joined Mar 14, 2008
27,243
What motor power do you need?

#### PeterCoxSmith

Joined Feb 23, 2015
148
Hi All,
Have been searching online but can't seem to find any 3 phase AC motors (without gear box) that has low RPM .

I need roughly about 100 to 125 RPM motors .

Any experienced guys please advice

Thank You .
A permanent magnet AC motor should be able to do what you want, for example:
http://www.electricmotorwholesale.com/BALDOR-CSPM3546T/
They also do variable frequency controllers to give the speed you want.

Thread Starter

#### t00t

Joined Jan 22, 2015
73
Thank for the replies so far ,

Reasons for not wanting to use a gear box is cost and maintenance .

I will probably be buying an 8pole motor .

Thanks

#### MaxHeadRoom

Joined Jul 18, 2013
23,156
You do not mention the motor size you need, there are AC stepper motors that run synchronous on AC at 72rpm on 60hz or 60rpm on 50hz, instant stop/start.
Max.

#### Hypatia's Protege

Joined Mar 1, 2015
3,226
Is there any VFD that can drop an AC frequency to 3 hertz ?
Inasmuch as the above implies three pole construction (given that AV @ 3Hz = 120RPM) --- I have to wonder if the motor in question is some manner of selsyn?
In any event more info as regards your application would be helpful (BTW you will be pleased to know many {most?} folks on these fora are far wiser in the ways of electro-mechanics than I)

Best regards
HP

#### tcmtech

Joined Nov 4, 2013
2,867
If you need any amount of torque at that speed I would not recomend trying to do it with a standard 3 phase induction motor and VFD unit.

I would go with a properly sized multi phase servo motor and driver unit. You will be money and time ahead.

#### MaxHeadRoom

Joined Jul 18, 2013
23,156
There are some makes of VFD which are particularly designed to operate down to zero rpm such as ABB ACS800 series.
There are some others also I do not have the number for now.
Max.