Motor running cooler on high speed - Page 1
Forum section: electronics / projects

author: justinjja
author_rank: Regular Contributor
timestamp: January 20, 2018, 11:07:31 pm
content: I have a whole house fan powered by a belt drive 1HP motor.I noticed the motor runs a little cooler on the high setting vs the low setting.That would be somewhat accounted for by the increased airflow, but I would still expect it to run warmer on high.Is it normal for a High/Low electric motor to be more efficient on the High setting?

author: Benta
author_rank: Super Contributor
timestamp: January 20, 2018, 11:13:45 pm
content: Increased airflow from the fan explains the temperature difference. Indeed, running induction motors at low speeds on a VFD often require additional forced cooling.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 21, 2018, 09:51:46 am
content: Even though induction motors designed for adjustable fan speeds are specifically designed to work with high slip, they are still less efficient when run at high slip. Combination of reduced efficiency and reduced cooling makes them run hotter.Even though the mechanical power needed is smaller, too, it doesn't help much because of the poor efficiency.Actually, in small-to-medium induction motor driven fans, the mechanical power output can be just a few watts! Those motors are often highly inefficient, so most power is converted to heat, and you can almost ignore the mechanical work done. This can be easily seen if you compare equipment fans that are available as both 12V BLDC and AC induction motor versions: the BLDC version might eat 2W, while the AC version might work at 10W, for similar airflow and pressure.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2018, 02:49:50 pm
content: What's controlling the speed of the motor? A variable frequency drive, switching the taps or a phase controller?If it's a variable frequency drive, the motor shoulnd't run any hotter at lower speeds, as the motor is driving a fan as well as being cooled by one. At lower speeds, the torque will be much lower, so the current and heating less, than at high speeds. This would be different if it were driving a high friction load, such as a conveyor belt, which will require a reasonable level of torque, at low speeds.If it's a phase controller or tap changer, then slip will increase at lower speeds, so losses will be greater, which would mean more heat, giving less cooling by the fan spinning at a lower speed.

author: John Heath
author_rank: Frequent Contributor
timestamp: January 21, 2018, 03:35:37 pm
content: Another wrench to throw into the gears. A vacuum cleaner. If I put my hand on the hose of the vacuum cleaner the speed of the motor goes up. You can hear the pitch of the motor go up. If I am stopping the air from moving why would the motor speed up? It should have slowed down as my hand is blocking the air from going into the hose making it harder for the motor to move air? Maybe this has something to do with a motor at high speed running cooler. Max energy exchange happens when the impedance of source and receiver are matched or the same.

author: justinjja
author_rank: Regular Contributor
timestamp: January 21, 2018, 03:38:09 pm
content: Ya it just has 2 taps one for high speed and one for low speed.I think the vacuum is literally making a vacuum, which makes it easier to spin due to less friction?

author: John Heath
author_rank: Frequent Contributor
timestamp: January 21, 2018, 04:21:05 pm
content: Yes less air so less air friction on the fan.Back to your problem. An easy and maybe useful bit of information would be current flow. An AC amp clamp meter on one of the motor lines. What is the amperage with the low tab vs the high speed tag? It could be less of a difference than one would expect. What comes to mind is phase lock skipping at low power of an aluminum block type motors. Phase skipping energy goes mostly into heating up the aluminum. At the higher speed it could have better phase lock , less phase skipping therefore running cooler even if it is at higher power ??

author: justinjja
author_rank: Regular Contributor
timestamp: January 21, 2018, 04:37:42 pm
content: Here are the specs to it:https://www.grainger.com/product/DAYTON-2-Speed-Belt-Drive-Whole-House-2EAX4I measure slightly higher than spec: 5A low and 9.5A high.My streets transformer is on my front lawn, so my voltage is a little high.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2018, 09:11:05 pm
content: Quote from: justinjja on January 21, 2018, 04:37:42 pmThe current flow should be less at lower speeds, but whether the lower power dissipation is enough to make up for the less cooling from the lower flow rate or not, is another thing. If the current is 5A on low, but 9.5A, when on high, the power dissipation in the winding on low will be 52/9.52= 28% of the power dissipation when on high. If the cooling due to less airflow is under this figure, compared to the high setting, then it will run hotter.

author: John Heath
author_rank: Frequent Contributor
timestamp: January 21, 2018, 09:21:35 pm
content: That an't no hand held fan. It's a 10 amp monster. Cats and small children could be sucked up into it. How much air do you need to move? At measured 10 amps times 120 is 1200 watts or 1 1/2 horse power at 750 watt per HP. Maybe 1\2 of the amperage is reactive so that would put it at 1/2 horse real power range as advertised. It is off topic but why do you need to move so much air?

author: justinjja
author_rank: Regular Contributor
timestamp: January 21, 2018, 09:55:42 pm
content: And the worst part is, that fan won't even be powerful enough in the summer.Bitcoin mining in my garage, I have my 200A service maxed out.