VFD Motor Turn Down Ratio
VFD Motor Turn Down Ratio
(OP)
Hi
I am working on an air cooled heat exchanger for glycol. This exchanger has 12 bays and each bay consist of two fan with motors, one motor is VFD compatable. I want to find out the turndown ratio for that motor. Min and max heat exchange required is 26,268 and 1,239 KW respectively. Max inlet temp is 91 degree C and min is 46 degree C, outlet temp is 43. Any help will be appreciated.
Thanks
I am working on an air cooled heat exchanger for glycol. This exchanger has 12 bays and each bay consist of two fan with motors, one motor is VFD compatable. I want to find out the turndown ratio for that motor. Min and max heat exchange required is 26,268 and 1,239 KW respectively. Max inlet temp is 91 degree C and min is 46 degree C, outlet temp is 43. Any help will be appreciated.
Thanks





RE: VFD Motor Turn Down Ratio
RE: VFD Motor Turn Down Ratio
If your motor is TEFC or ODP, the cooling is done by shaft fan and therefore falls off as shaft speed drops. It is unlikely that you have a TENV or auxiliary blower cooled motor on a fan but if you did, the cooling would be constant over the whole speed range permitting full torque output down to near-zero speed.
Assuming a TEFC or ODP motor on your fan, you can surely go down to one-tenth speed without causing thermal problems in the motor. There is little reason to go any slower as the fan doesn't move any air at such slow speeds.
RE: VFD Motor Turn Down Ratio
RE: VFD Motor Turn Down Ratio
respectfully
RE: VFD Motor Turn Down Ratio
DickDV...you are right it's a TEFC motor and actually I was worried about the heating problem with high turndown ratios. Is there a way to calculate turndown ratio?
Thanks again
RE: VFD Motor Turn Down Ratio
So, for example, if you have an 1800rpm motor and will run it down to 450rpm, that's a 4/1 turndown.
Or, if you have a 3000rpm motor (50hz) and will run it down to 300rpm, that's a 10/1 turndown.
Overspeed range does not figure into turndown calculations. So, if we take the 1800rpm motor above and change the power train so it now now must run 2400rpm to 600rpm, that's a 3/1 turndown application (1800/600).
On inverter duty motors designed to operate down to zero speed, the turndown ratio becomes meaningless. Some manufacturers show 1000/1 turndown ratio for those motors but what they really mean is "good to essentially zero speed" without thermal problems.
RE: VFD Motor Turn Down Ratio
If the air discharge paths of the two fans are isolated from each other, you probably won't have a problem.
If the fans are dischargeing into a common plenum so that the fixed speed fan is able to apply a back pressure to the VFD fan, the torque required to maintain low flow against the backpressure may present issues not normally present.
respectfully
RE: VFD Motor Turn Down Ratio
What are air inlet and outlet temperatures?
RE: VFD Motor Turn Down Ratio
quark...Inlet air temp. is 28 C, don't know the outlet temp.