motor and vfd question

T

Tin.Man

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Sep 5, 2010
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So I made an assumption and Im coming to the realization thats the mother of all screw ups....

With that out of the way......

I purchased a 2hp leeson 3ph motor, as well as a kbac 27 vfd.

If I purchased a 3450 rpm motor, have a 5.5" drive wheel, at say 60hz what surface speed am I getting? Should I have gone with a 1750 motor?

As I understand it, a vfd can 2x the motor, but only reliably/safely 1.5.

Thank you for the help gents!
 
Belt Speed in SFM will be Dia(") x Pi x RPM /12

For your setup,

5.5" x 3.14 x 3450 / 12 = 4965 SFM

The VFD can drive the motor at whatever frequency range the designer has built into the VFD. The KBAC can go to 120 Hz.

The motor can only run safely up to the maximum speed the designer built it to run to.

Roughly half the world runs on 60 Hz mains and the rest runs on 50 Hz. The speed of a 2-pole motor on 60 Hz mains and running synchronously (no load) will be 3600 RPM (as the load increases, it wil slow down slightly until, loaded to 2 HP, your motor is running at 3450 RPM; rated speed at rated load).

Because modern manufacturing techniques maximize standardization, the same rotating assembly will be used in the 2-pole (3600 RPM), 4-pole (1800 RPM), and sometimes 6-pole (1200 RPM), variants of motor built on a given frame. It is therefore fairly safe to assume that any motor you buy will be safe to 3600 RPM (though to be absolutely certain, you'll need confirmation from the manufacturer), even if it has 4-pole windings and a 1750 RPM rating plate.

For a 4-pole motor, doubling the rated speed is therefore usually fine.

However, if you have a 2-pole motor, running at 3600 RPM on 60 Hz, doubling the speed will take it to 7200 RPM; well outside the design speed range.

It's usual to build in some sort of safety factor when designing anything. The motor manufacturer will have done this and the reality is that a 2-pole motor will run faster than 3600 RPM without self-destructing. The difficulty is that you will never know how much faster.

It's therefore best to think in terms of 3600 RPM as maximum safe speed for a motor.

Many VFDs have a relatively simple V/Hz (V/freq) relationship between Voltage and frequency. This is usually linear and is fine at frequencies above about 10 Hz. At very low frequencies, however, the voltage/frequency relationship stops being linear and the motor starts to feel "coggy", rather than running smoothly. Many drives offer slightly different V/freq curves to reduce this slightly, but I have not been able to get smooth operation below about 7 Hz on any of a number of V/Hz drives that I have tried. Sensorless Vector drives have an inbuilt system that measures the power factor and adjusts voltage on-the-fly to keep the motor running smoothly. All the SV drives I have tried have run smoothly down to 1 Hz.

The KBAC-series drives do not have SV capability, so your minimum usable speed is likely to be around 10 Hz and your maximum safe speed is 60 Hz, for a 6:1 speed range.

You should get a range of about 827-4965 SFM
 
Tin.Man said:
So I made an assumption and Im coming to the realization thats the mother of all screw ups....

With that out of the way......

I purchased a 2hp leeson 3ph motor, as well as a kbac 27 vfd.

If I purchased a 3450 rpm motor, have a 5.5" drive wheel, at say 60hz what surface speed am I getting? Should I have gone with a 1750 motor?

As I understand it, a vfd can 2x the motor, but only reliably/safely 1.5.

Thank you for the help gents!
Click to expand...

If your grinder is direct drive rather than belt driven, I say no problems


I have a 3400 rpm and a six inch drive wheel

In that vfd, the 2x speed is a jumper, so not that fast/simple to change.

I know some like to use a low rpm motor and double speed it, I haven't tried it and don't fully understand the ideas as to why that's better.
 
Thank you both for the replies!

It is direct drive. I guess the more pertinent question is which is the more durable longer lasting setup?

Would a 1725 motor that can be clocked 2x with vfd be the better option? My 3450 motor is brand new and I can return it for a 1725 motor so that's no problem.

Otherwise would I have to alter anything inside the vfd if I stay with the 3450?

817 seems still pretty fast at 10hz

So 800-5000 sfpm with 3450 10hz to 60hz

With a 1740 rpm motor, same drive wheel (5.5") and vfd I would have what 400ish at 10hz and roughly 2500 at 60hz so optimistically I would get to around 4200 at 100hz with said 1740 motor?

So much math haha.

Tim or 123 which is the more durable/efficient setup in your eyes?

Thank you again for the help!
 
When I get the choice, and assuming there are no frame size constraints, I always go with the 4-pole motor when running on a VFD and set the maximum frequency to 120 Hz.

As said, the manufacturers advice was always 10-100 Hz, but that dates back a while (I used my first VFD over 25 years ago). I have never had any problem running up to 120 Hz for around 3600 synchronous RPM.

The reason I go for the 4-pole motor is mainly down to the usable range; there is much more area under the power curve for the 4-pole.

It's not the greatest of graphs, but this is my attempt at showing the difference between a 2-pole and a 4-pole motor, both run to 3600 RPM.

The minimum speed in both cases will be at 10 Hz (unless a Sensorless Vector drive is used), giving minimum speeds of 300 RPM for the 4-pole and 600 RPM for the 2-pole.

 
Thanks man!

After researching more, 4 pole 1740 makes more sense as well because better torque throughout the range. Pretty sure you posted that a few years ago in another post but just found it.

So since I couldnt find a definitive answer elsewhere, thanks to Tim and 1234

1725/1740 rpm motor with vfd is better than 3450!
 
Durability is not really an issue.

Any industrial motor will have been designed for at least a 5-year continuous operating life at full load and maximum design speed (3600 RPM). Any quality industrial motor will have been designed for a ten year operating life under those conditions, probably more.

5 years at 8760 hrs/yr gives 43800 hrs. Divide by 40 hrs/week and 52 weeks/yr and you get 21 years from a basic industrial motor. It doubles to 42 years if you buy a quality industrial motor.
 
I was wondering why my 3450 rpm motor was jumpy at low speeds. I searched all day and messed around with my vfd to no avail. This answered my questions. Thanks.
 
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