vfd and motor help

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boyle78

boyle78

Senior Member
Location
new hampshire
Here's the scoop, I installed a 2hp rated vfd on a 2hp 240v 3ph motor that turns a ventury mixer. Without the drive belt, the motor draws 3.4a, 3.2a, and 3.4a. With the drive belt hooked up to the drum the motor fluctuates between 3.8a and 9.9 a....the result is kicking out my vfd. I hooked up a 3hp rated vfd and same results. I think I have a bind on my load side in either my motor under load or in the mixer drum. Am I correct in thinking this? I was told that a motor should only pull 30% of it's FLA with no load on it....if that's true, my motor is already pulling more than 50% with no load in it which backs up my hope in thinking that my motor is junk or I have a bind somewhere. Any more thoughts with this would be great! Motor is almost 50 years old to boot.
 
P

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
boyle78 said:
Here's the scoop, I installed a 2hp rated vfd on a 2hp 240v 3ph motor that turns a ventury mixer. Without the drive belt, the motor draws 3.4a, 3.2a, and 3.4a. With the drive belt hooked up to the drum the motor fluctuates between 3.8a and 9.9 a....the result is kicking out my vfd. I hooked up a 3hp rated vfd and same results. I think I have a bind on my load side in either my motor under load or in the mixer drum. Am I correct in thinking this? I was told that a motor should only pull 30% of it's FLA with no load on it....if that's true, my motor is already pulling more than 50% with no load in it which backs up my hope in thinking that my motor is junk or I have a bind somewhere. Any more thoughts with this would be great! Motor is almost 50 years old to boot.
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Mixers are tough on motors. Many times they are undersized for what is being mixed.

What do you want to bet the overloads on the old motor starter were oversized to accomodate the actual current?

You could have a bent shaft or maybe some kind of material in the bearings, or just worn out bearings. Or it could be a wornout motor. or the motor might be undersized. Get a millwright to take a look.

The 30% thing is sort of true, but it could well be more.
 
boyle78

boyle78

Senior Member
Location
new hampshire
petersonra said:
Mixers are tough on motors. Many times they are undersized for what is being mixed.

What do you want to bet the overloads on the old motor starter were oversized to accomodate the actual current?

You could have a bent shaft or maybe some kind of material in the bearings, or just worn out bearings. Or it could be a wornout motor. or the motor might be undersized. Get a millwright to take a look.

The 30% thing is sort of true, but it could well be more.
Click to expand...

The fact that you brought up oversized overloads, it makes me wonder why would something be designed so that the motor driving the mixer would be so close to it's limit? Name plate says 6 amps at 220 so one would assume that the intended load should be limited to the capability of the motor driving it right? I am limited to the max 150% overload with the vfd....but the fact that the current is fluctuating gets me. Even if I put a 5 hp rated vfd in I think I would still have the same problem.
 
John Valdes

John Valdes

Senior Member
Location
SC.
Occupation
Retired Electrician
It's got to be a load issue. When the motor goes you will certainly know it. The viscosity of the material can dramatically effect current. Are they running a new product? In most every case with these mixers it has been an engineering issue. The drive is doing what it's supposed to do, protect the motor. Until you resolve the overcurrent issue it will never correct itself. Start at the impellar and work your way back.
New impellar?
The currents you list as no load are full load values. Got another mixer to try it on?
 
boyle78

boyle78

Senior Member
Location
new hampshire
just got done swapping out a motor that tested with less current than the first problem child. After configuring the acceleration and deceleration time, things work much better! The drum in the mixer was out of balance and now spins much quieter and with much less vibration. Full load test is still to come, but already have a good feeling since I was able to run it up to full speed with good current readings...in the range of 2.3a per leg instead of 3.5a wit the first motor.
 
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