MacMcMacmac
Aerospace
- Sep 8, 2010
- 56
Good Day Everyone,
We are trying to solve an issue with a large dehydration unit. We are finding that our reactivation blower which draws heated air through a Munters CargoCaire honeycomb dehydration wheel is not creating the proper pressure drop across the wheel. The wheel is not plugged or fouled in any way.
After some investigation, we found fan is turning approximately 4300rpm, 600rpm below its rated 4900rpm. It is connected to a 7.5hp 575V 3530rpm Marathon motor via a belt drive. On the VFD, at 60Hz, the motor is pulling slightly less than 5A, so it seems there is about 2.5hp left on the table. Not a large amount, but at 50% of present draw it would seem to be worth trying for.
My direct supervisor is recommending we up the AC frequency to the 7.5hp motor to achieve 4900 rpm at the fan. Using the formula Motor rpm = 120F/p where f = freqency and p = number of poles, I rearrange the formula and calculated a frequency of 67Hz to get us to the motor rpm required to max out the fan speed at 4900 rpm. This does not seem like a very large increase, which indicates to me the motor should be happy enough at that speed. Unfortunately, there is no free lunch, and further investigation shows that max motor torque will be about 80% at that rpm. Given the load is increasing with rpm, and torque is decreasing with the higher frequency, I am assuming (dangerous I know) that we would soon be operating into the service factor of this motor, which is 1.15. All in all, it seem like a lot of messing about, and what we gain on one hand, we lose on the other.
My solution is to put an adjustable sheave on the motor, run it at 60Hz, dial in the largest diameter that results in max fan rpm or motor amp draw, and call it a day.
The VFD is an Allen Bradley Powerflex 70 series. Reading through the manual has been a somewhat frustrating experience. I would not be the one manipulating the settings, so thankfully I can remain confused to no ill effect.
We are trying to solve an issue with a large dehydration unit. We are finding that our reactivation blower which draws heated air through a Munters CargoCaire honeycomb dehydration wheel is not creating the proper pressure drop across the wheel. The wheel is not plugged or fouled in any way.
After some investigation, we found fan is turning approximately 4300rpm, 600rpm below its rated 4900rpm. It is connected to a 7.5hp 575V 3530rpm Marathon motor via a belt drive. On the VFD, at 60Hz, the motor is pulling slightly less than 5A, so it seems there is about 2.5hp left on the table. Not a large amount, but at 50% of present draw it would seem to be worth trying for.
My direct supervisor is recommending we up the AC frequency to the 7.5hp motor to achieve 4900 rpm at the fan. Using the formula Motor rpm = 120F/p where f = freqency and p = number of poles, I rearrange the formula and calculated a frequency of 67Hz to get us to the motor rpm required to max out the fan speed at 4900 rpm. This does not seem like a very large increase, which indicates to me the motor should be happy enough at that speed. Unfortunately, there is no free lunch, and further investigation shows that max motor torque will be about 80% at that rpm. Given the load is increasing with rpm, and torque is decreasing with the higher frequency, I am assuming (dangerous I know) that we would soon be operating into the service factor of this motor, which is 1.15. All in all, it seem like a lot of messing about, and what we gain on one hand, we lose on the other.
My solution is to put an adjustable sheave on the motor, run it at 60Hz, dial in the largest diameter that results in max fan rpm or motor amp draw, and call it a day.
The VFD is an Allen Bradley Powerflex 70 series. Reading through the manual has been a somewhat frustrating experience. I would not be the one manipulating the settings, so thankfully I can remain confused to no ill effect.
