High amperage readings on a 40 hp motor, is the motor going bad?

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Rsteenson

Member
I installed a 60 hp schnieder VFD on a 40 hp blower fan on a grain dryer (used for phase conversion) Originally there was a roto-phase running the motor but the motor was starting really hard (obviously) on an across-the-line starter (sometimes triping the 150 amp breaker). Anyways at 240 volts 3 phase the FLA is like 96 amps (104 per NEC) and im reading 120-130 at 60 HZ....when i drop to 55HZ it drops down to around the nameplate rating. On the primary side im seeing around 210 amps which seems astronomical to me. A 30 Hp I put a vfd on a month ago was only drawing around 100 on the primary side. Do you suppose im picking up harmonics....or is the motor starting to go bad??? I had the electronic overload protection set at 115 and it never even faulted in the ten minutes i had it running. Im a little apprehensive to leave a $5500 VFD on this thing only to burn up. Any insight would be greatly appreciated! Thanks!
 
I'd have the motor tested for mechanical wear, could be a bearing is starting to go. Also, one thing you need to remember about fans/pumps is then you reduce the speed, you reduce the power used by a power of 3. So reducing a fan from 60Hz to 55Hz will consume (55/60)^3 % of the power at 60Hz, or 77% of the power at 60Hz.
 

thunder15j

Member
Location
Cali
Regarding the overcurrent setting on your vfd: a couple of tech support engineering types from different drive manufactures have recomended that the OC setting be set @1.0 (nameplate amps) even if the motor nameplate shows a 1.15 service factor. I do not recall the exact reasoning.
 
I installed a 60 hp schnieder VFD on a 40 hp blower fan on a grain dryer (used for phase conversion) Originally there was a roto-phase running the motor but the motor was starting really hard (obviously) on an across-the-line starter (sometimes triping the 150 amp breaker). Anyways at 240 volts 3 phase the FLA is like 96 amps (104 per NEC) and im reading 120-130 at 60 HZ....when i drop to 55HZ it drops down to around the nameplate rating. On the primary side im seeing around 210 amps which seems astronomical to me. A 30 Hp I put a vfd on a month ago was only drawing around 100 on the primary side. Do you suppose im picking up harmonics....or is the motor starting to go bad??? I had the electronic overload protection set at 115 and it never even faulted in the ten minutes i had it running. Im a little apprehensive to leave a $5500 VFD on this thing only to burn up. Any insight would be greatly appreciated! Thanks!

It is rather difficult to comment on your description without some basic information about motor nameplate, drive catalog number and basic setting parameters, the distance and cable type used between the drive and motor and the type of 'blower fan'(although I presume this to be a centrifugal fan). Do you have any mechanical regulating capability on the airflow, like inlet or outlet vanes? Perhaps it was originally sized to be started unloaded and later on to be opened up, but only partially? The tripping of the FVNR mode sounds like to large of an inertia to start, further indicates an undersized motor. Of course it all presumes that you have checked out all insulation, wire sizing, bearings, etc. to be in proper shape and sized/selected correctly.
 

beanland

Senior Member
Location
Vancouver, WA
Motor Load

Motor Load

I had a strange overload problem with a ventilation motor. The problem was that the blower was moving a lot more air mass than expected, therefore the motor had a lot higher load than designed. The sheeves were changed to reduce blower speed and the overload was eliminated. It might have nothing to do with the electrical side of the problem, maybe the mechanical side. Try running the motor without the blower attached (remove belt) and see what happens.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I had a strange overload problem with a ventilation motor. The problem was that the blower was moving a lot more air mass than expected, therefore the motor had a lot higher load than designed. The sheeves were changed to reduce blower speed and the overload was eliminated. It might have nothing to do with the electrical side of the problem, maybe the mechanical side. Try running the motor without the blower attached (remove belt) and see what happens.

To beanland's (and others) point, LOAD on a motor in a centrifugal fan application is directly related to the air flow. If someone made ANY changes to the mechanical side of the air handler, however seemingly slight, the effects on the motor load are dramatic. As Christoff84 said, the POWER required by the fan is related to the CUBE of the flow change. So if, for example, there was an inlet guide vane being used to regulate air flow BEFORE the VFD was installed, then it was removed BECAUSE someone thought it was no longer necessary now that you have a VFD, the slight restriction of that guide vane just being there may have been limiting the air flow just a little bit. When it was removed, the higher air flow causes a 3 fold increase in motor load. If someone changed a sheave, same issue. So for example in your case with a 135% increase in loading, that could be the result of just a 10% increase in air flow (lot's of assumptions here, I'm just making a point).

Also, the single phase current will always be a little more than 1.732 (the sq. root of 3) times the 3 phase current because ALL of the HP has to be supplied by 2 legs instead of 3 on the INPUT side. So at 120A on the 3 phase side, the single phase current would be 208A at least, plus losses in the VFD. You said 210A, no surprises there.

Hopefully by the way, you used a 60HP Constant Torque rated drive (which would be a 75HP VT rated drive), because if not, the VFD is going to be overloaded. You have to over size a VFD by 2X the motor FLA when using it as a phase converter. Otherwise the diodes on the front end that are doing the conversion to DC are going to be overloaded by that 1.732 factor, and you will not have enough DC bus capacitance to smooth out the additional ripple caused by not having all 3 phases feeding the DC bus. Doubling the VFD rating takes care of both issues.
 
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Location
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Occupation
EC - retired
This may be an axial flow fan. Most of the grain dryers I see are. A couple of dryers that used centrifual fans motors would overload on cold air and had to have a minimum flame.


Did I read it right that you are using the VFD for 1ph to 3ph conversion in place of the rotophase that used to be in there?

I see jraef caught that already
 
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Rsteenson

Member
This may be an axial flow fan. Most of the grain dryers I see are. A couple of dryers that used centrifual fans motors would overload on cold air and had to have a minimum flame.


Did I read it right that you are using the VFD for 1ph to 3ph conversion in place of the rotophase that used to be in there?

I see jraef caught that already


That is correct, the 40 hp motor was starting hard so the vfd was needed for a soft start. it is an axial fan btw.
 

Rsteenson

Member
To beanland's (and others) point, LOAD on a motor in a centrifugal fan application is directly related to the air flow. If someone made ANY changes to the mechanical side of the air handler, however seemingly slight, the effects on the motor load are dramatic. As Christoff84 said, the POWER required by the fan is related to the CUBE of the flow change. So if, for example, there was an inlet guide vane being used to regulate air flow BEFORE the VFD was installed, then it was removed BECAUSE someone thought it was no longer necessary now that you have a VFD, the slight restriction of that guide vane just being there may have been limiting the air flow just a little bit. When it was removed, the higher air flow causes a 3 fold increase in motor load. If someone changed a sheave, same issue. So for example in your case with a 135% increase in loading, that could be the result of just a 10% increase in air flow (lot's of assumptions here, I'm just making a point).

Also, the single phase current will always be a little more than 1.732 (the sq. root of 3) times the 3 phase current because ALL of the HP has to be supplied by 2 legs instead of 3 on the INPUT side. So at 120A on the 3 phase side, the single phase current would be 208A at least, plus losses in the VFD. You said 210A, no surprises there.

Hopefully by the way, you used a 60HP Constant Torque rated drive (which would be a 75HP VT rated drive), because if not, the VFD is going to be overloaded. You have to over size a VFD by 2X the motor FLA when using it as a phase converter. Otherwise the diodes on the front end that are doing the conversion to DC are going to be overloaded by that 1.732 factor, and you will not have enough DC bus capacitance to smooth out the additional ripple caused by not having all 3 phases feeding the DC bus. Doubling the VFD rating takes care of both issues.[/QUOTE

I personally have no experience with this brand of grain dryer, my customer told me that this particular dryer came from the factory with "Too much restriction" on the airflow....So now that you say that i wonder if he didnt make some alterations of his own??? As far as the drive goes, Ive always ordered drives double the HP rating for scenarios like this. This is my first time installing a schneider brand VFD, so I gave my sales rep at my supplier my motor ratings and he told me i didnt need to double the size with this particular drive...hes always been a knowledgable guy so i didnt argue with him. So do you think that too much airflow could be my problem?
 

kwired

Electron manager
Location
NE Nebraska
Do you have parameters set correctly? Centrifigal fan is a variable torque load. If you have your drive set up for constant torque application this could be a problem.
 

kwired

Electron manager
Location
NE Nebraska
To beanland's (and others) point, LOAD on a motor in a centrifugal fan application is directly related to the air flow. If someone made ANY changes to the mechanical side of the air handler, however seemingly slight, the effects on the motor load are dramatic. As Christoff84 said, the POWER required by the fan is related to the CUBE of the flow change. So if, for example, there was an inlet guide vane being used to regulate air flow BEFORE the VFD was installed, then it was removed BECAUSE someone thought it was no longer necessary now that you have a VFD, the slight restriction of that guide vane just being there may have been limiting the air flow just a little bit. When it was removed, the higher air flow causes a 3 fold increase in motor load. If someone changed a sheave, same issue. So for example in your case with a 135% increase in loading, that could be the result of just a 10% increase in air flow (lot's of assumptions here, I'm just making a point).

Also, the single phase current will always be a little more than 1.732 (the sq. root of 3) times the 3 phase current because ALL of the HP has to be supplied by 2 legs instead of 3 on the INPUT side. So at 120A on the 3 phase side, the single phase current would be 208A at least, plus losses in the VFD. You said 210A, no surprises there.

Hopefully by the way, you used a 60HP Constant Torque rated drive (which would be a 75HP VT rated drive), because if not, the VFD is going to be overloaded. You have to over size a VFD by 2X the motor FLA when using it as a phase converter. Otherwise the diodes on the front end that are doing the conversion to DC are going to be overloaded by that 1.732 factor, and you will not have enough DC bus capacitance to smooth out the additional ripple caused by not having all 3 phases feeding the DC bus. Doubling the VFD rating takes care of both issues.[/QUOTE

I personally have no experience with this brand of grain dryer, my customer told me that this particular dryer came from the factory with "Too much restriction" on the airflow....So now that you say that i wonder if he didnt make some alterations of his own??? As far as the drive goes, Ive always ordered drives double the HP rating for scenarios like this. This is my first time installing a schneider brand VFD, so I gave my sales rep at my supplier my motor ratings and he told me i didnt need to double the size with this particular drive...hes always been a knowledgable guy so i didnt argue with him. So do you think that too much airflow could be my problem?

Too much airflow restriction will lessen the load on the fan. The more volume of air being moved the more current the motor will draw. You will have this problem running the dryer empty also as there is no restriction from the grain that would normally occupy the space you are moving the air through.

Slowing the motor speed down will also lessen the load which is exactly what happened when you did that. It does not decrease the load 1:1 either it. You made about a 17% reduction in speed but you reduced the load by a lot more than that. I don't recall how to calculate this but someone will probably chime in with how.

Correction: you reduced the speed by about 9%. I saw you reduced to 55 Hz after posting, I originally thought you said 50 Hz.
 
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kwired

Electron manager
Location
NE Nebraska
120 amps three phase x 1.73 = 207 amps single phase. Your input amps is not out of reason considering what the output amps @ three phase is.
 

Rsteenson

Member
Do you have parameters set correctly? Centrifigal fan is a variable torque load. If you have your drive set up for constant torque application this could be a problem.

I believe i did tune the parameters correctly, but i think i might go through them again. This particular is for a variable torque application.
 
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