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DC motor not "suitable" for speed control

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gabeNC

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"These blowers are not recommended for use with any type of speed control devices."

From the crappy manual that bundles shaded pole, permanent split cap, dc and 3 phase motors in less than 4 pages.
Is that "lawyer" speak for it will work but we're not responsible? I could see that single speed a/c motors being a challenge to control but a d/c motor?

So I jumped the gun and ordered this fan https://www.grainger.com/Grainger/DAYTON-DC-Blower-3HMJ1 and I probably should have vetted this out first before buying it but shouldn't a PWM controller an acceptable way to control it? The data sheet is quite lacking

thanks.
 

jpanhalt

Well-Known Member
Most Helpful Member
I tried to find the manual, but couldn't. I think the answer would depend on whether there is a built-in controller.

If it is a brushed DC motor, and the leads go directly to the motor, then it should be be controllable. I suspect a brushed DC motor with a built-in controller is pretty rare. If it is brushless and has a built-in controller, then PWM on the power input to that controller may not work very well. However, in that case, if you can reach the leads to the motor itself, then it can be controlled as any BLDC motor.

The recommendation not to use a controller may be related more to the load a blower experiences than to whether the motor can be controlled per se.

John
 

nsaspook

Well-Known Member
Most Helpful Member
I don't think it's the motor that's the problem, it's in the designed power needed to operate that type of fan in a stable manner.

Figure 5b.
**broken link removed**
 

gabeNC

Member
Thanks for the replies gents. There is clearly more to this than what i've researched so far. That "fan engineering" read was interesting, but i'm not sure how to interpret that 5b graph.

I don't think it's the motor that's the problem, it's in the designed power needed to operate that type of fan in a stable manner.

What would be the result of running the fan at 50% or so? Define stable manner please. My goal was to replace an old 3 speed noisy a/c blower with (hopefully) a easy to control (pwm) d/c motor and the option of running it off a car battery during a power outage.

Attached is the spec file.
 

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nsaspook

Well-Known Member
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Thanks for the replies gents. There is clearly more to this than what i've researched so far. That "fan engineering" read was interesting, but i'm not sure how to interpret that 5b graph.

What would be the result of running the fan at 50% or so? Define stable manner please. My goal was to replace an old 3 speed noisy a/c blower with (hopefully) a easy to control (pwm) d/c motor and the option of running it off a car battery during a power outage.

The static pressure curve for that type of fan has the shape of a curve indicating that it can easily 'stall' at slower speeds. At 50% speed in your application it might work fine or self-destruct in a short time, there's just no way to know. You just have the wrong type for wide range stable operation.
**broken link removed**
https://www.wesellfans.com/pdf/underStanding-Centrifugal-Fans.pdf
 

panic mode

Well-Known Member
does blower already include any type of electronics in it?
how many wires go to motor?
 

rumpfy

Active Member
Hi gabe,
From the data sheet I guess the motor is a permanent magnet type with wound rotor.
For fan use, the output flow is proportional to speed, and the power consumption is proportional to speed cubed.
The other thing about the DC motor is that as the speed is varied, the rotor current is essentially constant at constant torque. So, depending on the application, a DC motor running at a variable speed MAY need to have an external cooling fan. You will see this with proper DC motors made for industrial applications having external cooling fans.
For some applications however, the motor cooling is obtained by a simple fan attached to the rotor shaft. These motors are mostly unsuitable for variable speed uses unless you can assure that the motor temperature rise will be within the maximum value.
The motor cooling air flow will be less at low speeds but so should the temperature rise because of the changing (reducing) load at lower speeds. To get an air flow of one tenth of the rated air flow, the rotor speed would be about one third of the full load speed. The data sheet shows that as the back pressure is increased, the flow drops (usual) but the flow is 60 CFM at a back pressure of 1 inch. So it appears that the fan motor will be OK at a flow rate of one tenth the maximum flow rate.
My approach would be to set up the fan on a bench without baffles etc, and check the motor current as the speed is varied and calculate the power dissipation over the speed range. If the dissipation reduces as the speed reduces, then you can be confident the motor will not overheat. Alternatively, a temperature probe to measure the actual temperature rise would be good.
hope this helps.
 

gabeNC

Member
Thanks for that detailed explanation Rumpfy, that was really good.

The motor arrived and there are only two wires feeding the motor, i don't see any control electronics. The initial startup amp draw for this motor has tripped my power supply (14a) so looks like i need a different p/s.

Honestly i'm beginning to second guess my initial plan and wonder if I should go back to a 3 speed a/c motor. The more I learn about good hvac design is that you need to consider all parameters involved (cfm, velocity and static pressure). The *hope* was to have a somewhat larger fan and have more of a range of speed/cfm, lower electricity consumption and easy to power during winter storms.
 

rumpfy

Active Member
Thats the problem with DC motors; they can blow all the fuses when one just 'bangs them on line'.
There is nothing wrong with your approach; you only need to ramp up the power supply voltage slowly or limit the start up current so that your power supply gently brings the fan up to speed.
To solve this we need to get inside the power supply. So I think the best is for you to supply more details of your power supply. Presumably it is a mains powered thing and has a 'output shuts down' when overloaded. (crowbar protection)
For your information, we have a 6 HP DC (11.5 amp at 440 volt DC) motor driving an irrigation pump. The start up circuit for this uses a simple transistor to shunt the input reference voltage when the motor starts up. The transistor base circuit is driven from a voltage derived from the current sensing resistor in the motor armature circuit. This arrangement means that the control electronics is floating at mains potential.
So we have to find out a bit more about your DC supply.
 

gabeNC

Member
For power i'm using a spare xbox 175w p/s that I use for test bench supply, 12V at 14.2a and 5v at 1a. My fluke 115 only goes to 10a so i'm not sure what the inrush current spike is, it's probably more than 14a since the p/s trips. I'm not going to use this for the permanent installation so i'm open to suggestions.

Here's a 15a switching supply

https://www.amazon.com/eTopLED-TM-Single-Switching-Low-cost/dp/B004OWUP5U/ref=sr_1a_1?ie=UTF8&qid=1384348507&sr=8-1&keywords=12v 15a power supply

PWM controller but only 10a
https://www.amazon.com/DROK-Steples...g-Modulator/dp/B00DVGGWC0/ref=pd_sbs_indust_7


Thanks for the help guys.
 

tcmtech

Banned
Most Helpful Member
That motor should work just fine in a variable speed application however for a fan you may want to consider going with a variable current power source instead being it's the torque you want to control in this type of setup.

Try starting your motor with a 1/2 or 1 ohm power resistor in series with it and see what happens. The odds are it will start up just fine but at a slower ramp up rate and top out at a lower RPM.
 

rumpfy

Active Member
I think the spec on yr fan was a full load current of about 10 amp at 12 volt, so in normal operation the thing looks like a 1.2 ohm resistor. At startup the DC resistance of the armature is all the power supply sees. Could be say .01 ohm. So its no wonder the power supply protests at a peak current of say 50 amps or more.
You are just going to HAVE to consider a current limiting system whatever you do.
To implement this you will have to use a current sensing resistor of say 0.05 ohm max, causing a transistor to conduct and this will drop the reference voltage in your power supply(simply put).
The use of a commercial power supply needs to be considered with reference to the isolation between mains and the 12 volt side. Some of these things have the control circuit floating at mains potential so be aware.
ps I looked at the data sheet in your reference and then I got another one with more info on it. Both were Dayton data sheets. The more comprehensive one had lots of air pressure /flow data.
 

gabeNC

Member
So between needing a soft-start mechanism, a pwm controller and a power supply i'm about to punt and buy that 3-speed a/c fan. This sure has been educational to say the least. I didn't realize the peak current could get that high, i was thinking <20a.
 

tcmtech

Banned
Most Helpful Member
You could just power it off a 15 amp battery charger that has been converted into a basic power supply. No need for high tech and fancy just to drive aDC motor. ;)
 

gabeNC

Member
Well yeah... i guess. Would be nice to have multi-speed options and tie in to the existing snap thermostat control.

You are just going to HAVE to consider a current limiting system whatever you do.

I'm looking on Mouser for power resistor options, if my power wattage math is correct i'd need a 50watt resistor. Like these?
https://www.mouser.com/Passive-Components/Resistors/_/N-5g9n?P=1z0wncfZ1z0vjjj&Ns=Pricing|0


I've got a couple of weeks before I have to send that DC motor back, might as well play around with it.
I appreciate everyone's time and responses
 

tcmtech

Banned
Most Helpful Member
Or you could just use the chrome plated wire from the spine of a notebook. ;)

.5 - 1.5 ohms @ 100+ watts air cooled.
 

rumpfy

Active Member
I think tcmtech says something quite sensible because I think your initial reaction to your power supply tripping was "what is going on here!!). I think your power supply is so clever at preventing its self destruction that it doesnt allow ANY mistake. BUT if you used a really dumb power supply like tcm says, you wont have to worry too much cos the power supply will groan a bit at the short time overload but it will get over it and as the motor spins up to speed, the current will fall to its 10 amp value. Of course this doesnt get to the end requirement of having 3 speeds (or whatever).
What I suggest is you keep your hand in your pocket until you think the matter through. You still have to work out the variable speed setting arrangement and translate this to either a DC power control, OR if you go for an AC drive, how to vary the speed of that. I still think you should stick to the original plan to use your DC fan seeing you have now purchased it. Again, tcms suggestion in post 17 of using a series resistance to control speed makes sense. Its not refined, but would work. Speed selection cold be done with a big relay. At least you would be making progress.
It is clearly the case that your existing DC power supply has a fast current shutdown design, whereas you need a current limiting design. Such a supply is likely to be better able to incorporate a variable(selectable) speed setting arrangement.
 

tcmtech

Banned
Most Helpful Member
Well I am an expert at working with dumb stuff! :D
 

gabeNC

Member
I still think you should stick to the original plan to use your DC fan seeing you have now purchased it.

Sounds good. Last night I started the sheet metal fabrication layout for this fan footprint, might as well totally commit to it.

.5 - 1.5 ohms @ 100+ watts air cooled.

This? https://www.mouser.com/ProductDetai...=sGAEpiMZZMtlubZbdhIBICNGveTnep9sFPYf/6WpdfM=

This one is cheaper
https://www.mouser.com/ProductDetai...GAEpiMZZMtlubZbdhIBICw%2bgucaajgOgwTfW15MjGQ=
 
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