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VFD

VFD

(OP)
A client recently installed a screw conveyor with a Toshiba VFD, both 1HP.  The system ran fine at full load, but when turned down to 40%, the controller indicated overload.  We have a number of opinions around the plant, including the motor must be too small and that the VFD should be oversized, or that the OL was set wrong.  Does anyone have any real world practical advice for setting this straight?  We went to a DC motor, and all is well.

RE: VFD

2
By 40% do you mean 40% of base speed?? If so the check the manual. Most Toshiba Drives have a variable overload curve programmed.  At lower speeds the drive uses a lower setting since the motor fan is not providing as much cooling.  If you are confident that the motor is adequately cooled you can turn this feature off.
(If you are not confident you can still turn it off and if the motor burns the "up size or not upsize debate" will be solved.... )

RE: VFD

hi..
few questions...is the screw conveyor motor a 1 hp too? or lesser?

VFD have parameters...did you go over them and check if they are okay...critical parameters are min and max freq, accel. and deccel time, motor rated current, overtorque detection level...it should be clearly explained on the Toshiba Technical Manual..

give us more info if you have chance..

dydt

RE: VFD

The parameter that I was refering to above  is usually called "Overload Reduction Start Frequency."  

Manuals for the drives can be downloaded at www.tic.toshiba.com

RE: VFD

Most screw compressors have a relatively flat speed-torque curve down to approximately 40-50 percent of rated speed, unlike most other mechanical drives where the torque varies as some power of the speed (square, cube, etc.).  For example, with a square law curve, the torque at 40% speed would only be 16% of the full load torque, compared to 40-50 percent for a screw conveyor.  Unless the motor is sized correctly and has the correct torque characteristic, the drive can be overloaded at lower speeds as you have experienced.

RE: VFD

Jwerthman: You are absolutly right about screw compressors being a frequent case of "oops" when buying drives. A variable torque drive unless oversized would not be an appropriate choice for such an application.

But... This is a screw conveyor according to the original post. So it is much less likly that a variable torque drive is being used instead of the correct choice, a constant torque drive.

At 1HP, I would be willing to bet this is an "S" series unit, either an S7 or S9.   Both of these drive are constant torque and automaticaly reduce motor overload capacity as frequecy drops.

Feather: If you have not already done so try putting the unit into open loop vector control (Toshiba calls it TrueTorque, I think). That may solve your problem else you may need to adjust the freq. where the overload reduction kicks in.

Hit www.tic.toshiba.com for the manuals they post all current as well as legacy product manuals there.

RE: VFD

(OP)
Thanks for the help.  It will take a while to digest all the suggestions.  The motor is inverter duty, and both the drive and the motor are 1 HP.  Tho motor is rated for 3.70 A at 230V. I don´t know the Toshiba model number offhand. The overload curve will be checked.   The electrical controls man says this happens all the time for him, so I tend to believe that you are right about the overload curve.  

Keith Feather

RE: VFD

(OP)
tmahan

The unit is an S9.  Is jbartos correct when he sayts that the fan is independent of the drivespeed, so that cooling is less of an issue? If I put the unit into true torque, will that overload the motor?  They have managed to misplace the manual. Thanks for the location of the online ones.

<keith Feather

RE: VFD

Jbartos is not correct. "Inverter Duty" is a term that is thrown about and tend to mean different things to different people (or motor makers). What JBARTOS is refering to is a small secondary blower motor that provides cooling for the primary motor.

In the case of a 1HP 230V adding a blower would be very unusual (I think). Typically on a small application such as this you would simply upsize the motor frame depending on the speed range that was to be covered.The large frame would have a large thermal mass to distribute the heat generated.
There have been many threads on inverter duty motors but a brief summary would be that the motor should be able to withstand any voltage spikes produced by the inverter. I would not spen much time worrying about the aspects of inverter duty or not.  (no doubt others will disagree but at 1 HP, so long as the motor is from a good supplier like AB, ABB Toshiba... you should be fine)

With regards to putting the unit in true torque control. It will not cause any further current draw. By adjusting the magnetization current and load current open loop control may solve your problems with the overload. It will without a doubt give you much more torque for a given current level and much greater speed control.

RE: VFD

feather,
tmahan's response is further corraborated by the fact that the DC motor and drive worked OK, meaning that it isn't a mechanical problem. DC systems are true constant torque, just as the Tosh drive would be in vector (True Torque) mode. If your DC motor and drive are 1HP also, then you will not overload the AC drive and motor in vector mode.

By the way, I don't know of ANY drive manufacturer that bothers to re-rate 1HP drives for variable torque, the savings are non-existent. It might be true, however, that your drive was originally used for a VT application and was programmed as such, again lending support to tmahan's hypothesis. Check those parameters carefully, and don't ASSUME anything. A good "Clear to factory defaults" would be in order here, then start all over from scratch. I think in Toshiba's manual language you must go into "super user" mode to do this, which may require a factory password, I'm not sure.

Subvert the dominant paradigm... Think first, then act!

RE: VFD

(OP)
Thanks for the help.  I read part of the Toshiba manual about the constant torque, and the end user has to change it from the factory settings to activate it.  It seems to me you located the problem.  

Keith

RE: VFD

Suggestion to tmahan (Electrical) Aug 12, 2002 marked ///\\\
Jbartos is not correct. "Inverter Duty" is a term that is thrown about and tend to mean different things to different people (or motor makers).
///There appear to be definitions of those on web, e.g.
http://www.instantweb.com/o/oddparts/acsi/defines/tebc.htm
\\\
 What JBARTOS is refering to is a small secondary blower motor that provides cooling for the primary motor.

In the case of a 1HP 230V adding a blower would be very unusual (I think). Typically on a small application such as this you would simply upsize the motor frame depending on the speed range that was to be covered.
///If application or circumstances permit this upsize.\\\
The large frame would have a large thermal mass to distribute the heat generated.
///Yes and no, depending on the motor shaft load and motor speed for how long time.\\\
There have been many threads on inverter duty motors but a brief summary would be that the motor should be able to withstand any voltage spikes produced by the inverter. I would not spen much time worrying about the aspects of inverter duty or not.
///This is usually not advised.\\\
  (no doubt others will disagree but at 1 HP, so long as the motor is from a good supplier like AB, ABB Toshiba... you should be fine)
///It depends on application. This is why special motor designs are in existence.\\\
With regards to putting the unit in true torque control. It will not cause any further current draw. By adjusting the magnetization current and load current open loop control may solve your problems with the overload. It will without a doubt give you much more torque for a given current level and much greater speed control.
///Could you clarify how the "...magnetization current adjustment and load current open loop control solve the problem of overload and simultaneously give much more torque..."\\\

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