I putting a rheostat on these blowers that restaurants have garage doors open to keep bugs out. The units have four motors, with a current draw of 14.5 amps. They have settings of low,med,high. The rheostat will allow me turn them to low and leave them on low. The rheostat is 2.5 amp is that a problem.
Rheostat
- Thread starter guschash
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hbiss
EC, Westchester, New York NEC: 2014
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- Hawthorne, New York NEC: 2014
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- EC
I would like to see what exactly you are calling a rheostat. How does the speed control work now?
-Hal
-Hal
zbang
Senior Member
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- Roughly 5346 miles from Earls Court
You can't just use the "low" setting?
Adamjamma
Senior Member
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- Jamaica and london
We always disconnected the outputs for medium and high, so only low worked, or bypassed the switch..:happysad:.
On the face of it, yes. A 2.5A on a 14.5A load? And you maybe can't control the speed with a rheostat anyway depending on what type of motor it is.
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
I think by rheostat he means an infinite speed control-- much like a light dimmer but made for fans.
I agee that a speed control that is rated 2.5 amps is an issue. I am surprised it didn't burn up.
I agee that a speed control that is rated 2.5 amps is an issue. I am surprised it didn't burn up.
Maybe he hasn't tried it yet. And I hope he heeds what has been posted here and doesn't try it.
I'm with Zbang - just use the Low setting. Job done.
Or maybe there is some information the OP hasn't shared with us.
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
Are you saying you can't use the low setting on the fans and not have a speed control? That would be what I would do but I am not sure low will keep out the bugs.
I think we need more information from the OP.
Rheostat- adjustable speed control. Can use to adjust speed on drill press, router, fan, etc. This is a Berner air curtain. Just looking to control speed of air curtain. Have not open unit up to see specs on motor, just going by installation and maintenance instructions.
just the cowboy
Inactive, Email Never Verified
- Location
- newburgh,ny
Single phase or 3 phase.
Single phase or 3 phase.
Three phase = yes install an invertor.
Single phase = no easy way
If it is single phase you could replace the motor with a 3 phase one and install a single phase input invertor.
Single phase or 3 phase.
Three phase = yes install an invertor.
Single phase = no easy way
If it is single phase you could replace the motor with a 3 phase one and install a single phase input invertor.
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
The ones I have seen are either 208v or 120V single phase
zbang
Senior Member
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- Roughly 5346 miles from Earls Court
Not all speed controls are rheostats-
rhe·o·stat
ˈrēəˌstat/
noun: rheostat; plural noun: rheostats
an electrical instrument used to control a current by varying the resistance.
By varying the resistance you can vary the current. This works well on DC and some AC motors although it throws off the extra energy as heat. They do not work on most common AC motors.
If you want a speed control, get a proper speed control that's suitable for the motor(s) involved; a variable resistor probably isn't.
(I still think that if there's a "low" setting on the fan, just use that. If that's still too strong, then the fan probably isn't suitable for it's location/use.)
Call the manufacturer and ask them for something compatible.
If they don't have anything, don't go needlessly throwing a bunch of junk into it to try and make it do something it's not designed to do in the first place.
JAP>
Andy Delle
Senior Member
- Location
- Los Angeles CA
AC Induction Motor Speed Control
AC Induction Motor Speed Control
You can't actually control the speed of an AC induction motor by varying the voltage. The only way to change the speed is by changing the frequency hence we now have VFD's on large motors that need speed control. This is a common engineering knowledge but there's a loophole:
You can control TORQUE on an AC induction motor by lowering the voltage. And in the case of a loaded motor like a fan, the reduced torque will slow down the motor as it can't overcome the mechanical load torque requirements. Obviously this torque reduction trick will not work on heavy loads like machine tools or an HVAC compressor. In these cases the motor will drop out before any reasonable speed reduction is achieved.
The cleanest way to do this torque reduction is with a variac or autotransformer. But simple series capacitors can work just as well as in many two wire ceiling fan wall controls. An triac based light dimmer can also work but tends to make the motor buzz and some motors can overheat as the triac control does not actually lower voltage but rather varies duty cycle which is nasty with an inductive load. The "dimmer" also needs a RC snubber installed so many cheap light dimmers will not work. Low cost solid state speed controls are just light dimmers with this snubber circuit installed.
AC Induction Motor Speed Control
You can't actually control the speed of an AC induction motor by varying the voltage. The only way to change the speed is by changing the frequency hence we now have VFD's on large motors that need speed control. This is a common engineering knowledge but there's a loophole:
You can control TORQUE on an AC induction motor by lowering the voltage. And in the case of a loaded motor like a fan, the reduced torque will slow down the motor as it can't overcome the mechanical load torque requirements. Obviously this torque reduction trick will not work on heavy loads like machine tools or an HVAC compressor. In these cases the motor will drop out before any reasonable speed reduction is achieved.
The cleanest way to do this torque reduction is with a variac or autotransformer. But simple series capacitors can work just as well as in many two wire ceiling fan wall controls. An triac based light dimmer can also work but tends to make the motor buzz and some motors can overheat as the triac control does not actually lower voltage but rather varies duty cycle which is nasty with an inductive load. The "dimmer" also needs a RC snubber installed so many cheap light dimmers will not work. Low cost solid state speed controls are just light dimmers with this snubber circuit installed.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
But on MANY single phase induction motors, there is a centrifugal switch for the starting capacitor and if you lower the voltage to reduce the torque to where the fan slows down below that speed to keep the centrifugal switch closed, it opens and put the start winding and capacitor on-line again. Since those are not designed for continuous use, they burn up.
You can only use voltage control on Shaded Pole motors and some SPECIFICALLY DESIGNED types of PSC (Permanent Split Capacitor) type single phase motors, i.e. those used in some ceiling paddle fans. CSIR (Cap Start - Induction Run) and CSCR (Cap Start - Cap Run) and most standard PSC type single phase motors cannot be speed controlled with voltage only.
PSC* motors can be controlled via special VFDs, and there are a few small companies that sell VFDs for these single phase MOTORS, but you really have to want to do it; those drives are expensive.
*Technically, Shaded Pole motors can be controlled by these VFDs as well, but since they can be controlled with cheaper voltage control, there is no point.
kwired
Electron manager
- Location
- NE Nebraska
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- EC
What kind of motor is it? Jraef mentioned most of what I was going to mention. PSC, shaded pole and universal motors are about only ones you are going to vary speed with a simple control that either reduces voltage or cuts part of the current from the waveform.
You will not run for long on a 2.5 amp rated controller if you have 14.5 amp motor connected to it before the controller fails.
14.5 amp rated motor is getting high enough it is rare to find in a shaded pole, and also is somewhat rare to find on PSC blower motors. Submersible pump motors often are capacitor start when you get that high of a FLA. AC compressors is one place where you see that high of amp rating on PSC motor frequently.
In my experience very few are. A college project with DC motors maybe. In real life............
But then my experience is limited to the industrial sector with motors from a few tens of kW upwards.
kwired
Electron manager
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- NE Nebraska
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- EC
For at least 30-40 years or so, yes, solid state devices that cut off a portion of the current wave are not only less cost but waste less energy.
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