OT Limiting current output of VFD

rta53

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Feb 2003
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North Carolina
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We use VFDs, mainly AB Powerflex, to control pumps in our water purification systems. We try to maintain a certain pressure using a pressure transmitter as our reference. If I want the pump to have a minimum speed then I set the minimum hertz. If I want to limit the maximum speed of the pump I set a maximum hertz. We have an engineer who keeps asking me if there is a way to limit the amps that the VFD outputs. He says he is concerned about overloading the motor. I think he worries too much but he keeps bugging me about this. Can this be done?
 
See parameter A089. But explain to the engineer that the drive has overload protection and the the current is based on the load that the motor "sees".
 
Almost all VFDs have current limiting. One of the parameters available is full load amps of the motor. The details vary, and I haven't done an A-B drive for a while, but in general the drive will "clamp" the speed when current output reaches the max set for the motor. This is in addition to the overload protection Bruce mentioned.
 
brucechase said:
See parameter A089. But explain to the engineer that the drive has overload protection and the the current is based on the load that the motor "sees".

Its interesting that the engineer is concerned about the current as if the drive is somehow forcing the current ON the motor vs the reality stated above. He must be an ME. :)

My ME acts like we techs are the last practitioners of some "Black art" or witchcraft. I can't tell you how many times he's suggested to me that my program may have gone bad; although this can happen, they don't usually go a 'little bit' bad!
 
DJM said:
Its interesting that the engineer is concerned about the current as if the drive is somehow forcing the current ON the motor vs the reality stated above. He must be an ME.

Yes he is an ME. He often will not accept something I tell him the first time I tell him. I have to keep repeating myself. He's a very smart and nice guy but I once told him that dealing with him was like dealing with a teenager. (He's in his 60s)


Tom Jenkins said:
Almost all VFDs have current limiting. One of the parameters available is full load amps of the motor. The details vary, and I haven't done an A-B drive for a while, but in general the drive will "clamp" the speed when current output reaches the max set for the motor. This is in addition to the overload protection Bruce mentioned.

So lets say the FLA of the motor is 18 amps. Normally I would program this into the motor parameters. Are you saying it would be feasible to use, say 15, instead of 18. Would this achieve what he wants?
 
brucechase said:
See parameter A089. But explain to the engineer that the drive has overload protection and the the current is based on the load that the motor "sees".

For some reason I don't seem to have this parameter. This is a PowerFlex 70 drive. What is this parameter?
 
rta53 said:
So lets say the FLA of the motor is 18 amps. Normally I would program this into the motor parameters. Are you saying it would be feasible to use, say 15, instead of 18. Would this achieve what he wants?

Not knowing what he wants (assuming he does) I can't say.

If you configure the drive to limit the max motor current to 15 amps it will clamp the speed when the output current is at 15 amps. However, there is no reason to restrict the current output to less than motor FLA as long as the motor is not operating in an unusually hot location or has had ventilation blocked. The motor will give normal life if operated continuously at FLA (and if it is a 1.15 Service Factor motor it will run above FLA without problems). Most winding heating and damage occurs during motor starting. A VFD limits inrush and exends motor life for frequently cycled motors.

It is poor engineering and poor economics to size a motor above the required load, add a 1.15 SF, use a VFD, and then limit current on top of it. At some point in time you can get rediculous with saftey factors, not to mention having lousy efficiency on a lightly loaded motor!
 
Last edited:
rta53 said:
For some reason I don't seem to have this parameter. This is a PowerFlex 70 drive. What is this parameter?

Look at 147 and 148. I haven't used a PF70 yet but I have on the PF4 and PF40.

Please tell the ME that this is not necessary. Not only everything Tom said, but Powerfactor is reduced on a lightly loaded motor. If he really has a hard time understanding, suggest he call AB and talk to a drive tech. They have been helpful in the past and might shed some light on this for him.
 

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