About Induction Motor

May i know the make of drive and mobel number

There should be no problem with regards to vibration as far as running the motor from the VFD, assuming the VFD is properly programmed. Before you assume that anything else is the problem, you need to make sure the VFD is properly programmed for the motor nameplate data and the power supply.

But there are sometimes issues with mechanical resonnance when running LOADS at specific speeds, we call them "critical speeds". The VFD can usually be programmed to disallow having those specific speeds or ranges of speeds selected by operators. It responds by either staying lower or moving higher, you get to decide.

if there is a parameter on the drive for motor winding resistance,then check that one.

if u enter wrong motor resistance that time this prob may be occur

Inverter programming has vendor specific approaches ,so it would help to have the inverter type to advise. And if you are constantly working at low band like 6Hz don't expect your motor to give rated torque for serious duty cycle because of overheating risk. Anyway first check your V/F pattern's low end for programming errors like min voltage etc, stall prevention parameters, and of course motor related parameters. S.M.

Ooooh, 6Hz... I didn't do the math. You're right, that's very very slow.

What he said.

And if the VFD is not at least a "Sensorless Vector Drive", speed regulation below 4:1 is going to be very poor. You are at a tad over 8:1 turn down.

do you have a 2 pole or 4 pole motor?

to run that low you will need a 4 pole motor, if you have a 2 pole motor, the rpm you want can cause vibration as you are not achieving or at the threshold of the start Hz/rpm for the motor if under load.

OP has already mentioned 1000 rpm motor, it's a 6 pole motor. To further reduce the rpm he has to introduce a gearbox or a belt drive. here I want to ask some more information from OP, as vibration can be due to several reasons.

1. What load is coupled with motor which is is required to be run at 120 rpm?

2. How it is coupled, whether it is properly aligned?

3. Whether high vibration occurs through out the range of speed or at particular speed bands?

I see no mention of 6 poles..... are you assuming its a 6 pole motor?

I NEVER assume, as it can and has caused all kinds of screw ups...

Assumption is the Mother of all F£$% up's

Its good to remember that!

Dear brich, I am not assuming the number of poles. But, you are right, I assumed frequency as 50 Hz. It's a simple assumption as explained below:

Motor speed (rps) = 2 x Frequency (Hz) / number of poles

Motor speed (rpm) = 120 x Frequency (Hz) / number of poles

So, for 50 Hz frequency:

Motor speed (rpm) = 3000 (for 2 pole motor)

Motor speed (rpm) = 1500 (for 4 pole motor)

Motor speed (rpm) = 1000 (for 6 pole motor)

For 60 Hz frequency above figures are 3600, 1800 and 1200 rpm respectively. As OP says 1000 rpm, I assumed 50 Hz frequency with 6 pole. Hope you will appreciate my assumption.

thank you..with the greatest amount of respect.....yes I know its calculates out to be a 6 pole..but my reply was to state that one MUST assume nothing... and for the purpose of our other member who took it upon himself to teach me how to calculate, I had already completed the calculations, and my statement of "4 pole motor" was an attempt to illicit a further response from the OP, who like so many is very conspicuous by his absence, to see if he's on the ball, knows what he is talking about and he has ability to pick up on my statement AND give further info.

But and here it comes...But are the figures/facts correct from the OP? Using the OP's facts, then yes one can calculate that the motor is indeed 6 pole, on the figures given, the calculation is correct.

How many times have we sat at our respective computers, given an answer based on facts presented only to be informed by a dyslexic OP that he got the figures totally wrong..... so.. for me... no assumptions.. only facts!

The facts, the whole facts, and nothing but the facts.

Point well stated and understood.

so in your calculations you don't say what Hz is required to run the motor at 120rpm.. what is it?

If you are going to give an answer, please give a full one

My calculations is not for inventing new Hz other than 50 & 60. Getting 120 rpm is job of VFD.

Details please!

A good VFD with Sensorless Vector control or full vector control (with tach feedback) should be able to run this OK.

Is this vibration low frequency showing up in the drive train, or are you talking about a high frequency whine or buzz?

You need to give us make and model # and NP ratings for VFD and motor.

Drives like the ABB ACS800 or Rockwell Powerflex 700 series should be able to handle this type of application ie speed range. You may want a forced vent motor or TENV so that overheating is not a problem.

Hi Amith,

Why you are replying to questions asked by experts?. Many a times OP ask questions and simply disappear.

Normally it is possible to run any motor to lower speeds in the ration of 1:10 that is 100RPM theoretically.But in Practice u need to study the Torque speed curve for such low speeds.If the motor POT is below 200% of FLT then possibility of entering in the unstable (non linear)region cannot be ruled out and hence vibrations can occur.