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Changing the motor on the drive


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#1 luclandry

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Posted 30 March 2006 - 03:12 PM

Hello everyone,
I have an application where I would like to make frequent changes of my motor without changing my sensorless frequency drive.

Is it possible to run a 1/8 Hp or 1/4 Hp motor on a 1 Hp drive? If yes, how easy is it to pass from a 1 Hp motor to a 1/8 Hp motor? Do I need to re-program the whole drive? Can I get away with a small change in the program? Is it dependent on the drive company? Will it hurt the drive or reduce its life?

Thanks in advanced for all your help,
Luc

#2 jOmega

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Posted 02 April 2006 - 07:06 PM

Since you do not inform us as to name of drive manufacturer and model number, it is impossible to give you a specific answer.
Not all drives are created equal; some allow you to go one or two motor power ratings below and perhaps one motor power rating above the rating of the drive. However, that is a drive design consideration.

Best answer: Contact drive manufacturer and ask your questions.

Kind regards,

jΩ

#3 luclandry

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Posted 13 April 2006 - 12:29 PM

Thanks for your answer jOmega.

The drive manufacturer is Reliance Electric and the model is the 1 Hp from the MD65 series.

I contacted my supplier and he assured me that the drive will work with smaller hp motors.

Regards,

Luc

#4 jraef

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Posted 13 April 2006 - 10:20 PM

If they say that it can be programmed low enough then that solves that issue, but you will need to reprogram that drive every time you switch. Some drives, such as Toshiba, can store two separate operating settings so that you can use the same drive and just flip a switch to enable the different settings. An alternative is to use external Overload Relays with each motor and then turn off the OL protection in the drive (or set it only to the higher rating). Current limits are still affected however.
"He's not dead, he's just pinin' for the fjords!"

#5 jOmega

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Posted 19 April 2006 - 08:16 PM

jraef,

Drive Current Limit is for the protection of the drive; not the motor.

The electronic Motor Overload function that the drive provides is typically an inverse-time function and replicates the stand-alone hardware version Motor Overload Relay.

Current limit monitors the output current sourced to the load by the drive. In some cases, there is a timed characteristic to the Current Limit such that the thermal rating of the power devices doesn't exceed design limits. So it is that some drives -- be they 110% or 120% or 150% -- are allowed to source that much current for a predefined period of time.... i.e., 10-sec or 20-sec or 30-sec or 60-sec .... before initiating a Trip.

This is not the same type of protection affored the motor by an Inverse-Time Motor Overload device or function.

Some drives have a graduated Current limit; i.e., 150% for 1-min and/or 200% for 5-sec. before tripping off.

As you will note, nothing is mentioned about the motor; only about the current being sourced by the drive.

Some people mistakenly use current limit as a motor torque limit, and after a fashion it seems to work. However, if the value of the current is obtained from LEM's or CT's at the output of the inverter, then they are seeing the total current; i.e., Real component and the Imaginary component. Only the real componet is related to motor torque production, and Current Limit doesn't differentiate between the real and the imaginary; ergo, C/L isn't able to be a 'true' torque limiter. Close, but no cigar...

Other drives use the d-c bus current and an algorithm to approximate actual motor current; and again ...... Close... under the right circumstances,..... but no cigar.

The reason I mention this, is to first clear-up any misconceptions about the functions of Current Limit as opposed to Motor Overload protection.

Second, I have seen many applications where the user has turned down the current limit say to 120% from 150% and ended up roasting the motor because the current never got high enough to trip the Motor Overload function or relay...


Kind regards,
jΩ;




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