Best method to slow down a 12v motor

[TURSTY]

Hi All,

I have been doing a bit of reading on the net about ways to slow a 12v motor down and have a few different options.

I have 2 12v blower fans for a boat heater and I want to be able to slow them to half their speed (so I have a high/low setting etc).

The motors have a draw raiting of 6Amps and are fused with a 10A fuse, so they are not drawing horrible amounts.

Would a simple resistor be the best way to slow these fans down, or should I be looking for a full on MOSFET type controller?

 

[dh390]

In automotive applications they use resistors to adjust the speed on their heater blower motors. You may be able to get one from a automotive parts store or a boneyard (aka junkyard) as it is common that the resistors burn out or the switch fails. But you would need to do the math to see if the resistors from one will handle the current your motor draws.

Other than that you could search E-Bay for a cheep high current DC motor speed controler. I have seen them.

 

[CDRIVE]

Pulse Width Modulation (PWM) is the prefered method for 0 to max speed control. However, since you only need half speed and both motors are identical you could switch them from parallel wiring to series. When switched in series the current should half as well.

Chris

 

[Laplace]

If these are identical DC motors with permanent magnet stators and both motors are intended to run at half-speed at the same time, then consider series connection of the motors. Each blower fan should provide the same load, each motor should run at half speed, so the series voltage should divide equally to each motor. If for some reason one blower fan becomes stalled, then the other motor will tend to run near full-speed. Series connection of the motors would not be appropriate if the individual fan loads were not the same (or nearly so).

If these are motors with stator coils, I can't seem to visualize what would happen so won't recommend series connection of the motors for half-speed operation.

 

[CDRIVE]

If these are motors with stator coils, I can't seem to visualize what would happen so won't recommend series connection of the motors for half-speed operation.

I don't think that would disqualify a series connection but I'm open to discussion of why it would.

Chris

 

[Laplace]

... I'm open to discussion ...

If the motors have stator coils instead of permanent magnets, how would a series connection of the motors perform in the case that the stator and rotor are wired in series versus parallel? I was hoping the OP would have replied "No problem, they're PM motors." rather than being forced to reveal my lack of knowledge regarding stator coils.

 

[dh390]

Ok on a very quick E-Bay search this is what I came up with & not very expensive.

The search phrase used in E-Bay search bar is: dc motor speed control

Below are links to 3 I found that may work for your application provided they are permanent magnet type motors,

This one has an input range 12 - 40VDC 10A & is only $8.49 (BIN) Free Shipping

http://www.ebay.com/itm/New-12V-40V...ltDomain_0&hash=item2a198d9320#ht_3823wt_1330


This on has an input range 6 - 90VDC 15A & is only $13.99 (BIN) Free Shipping

http://www.ebay.com/itm/NEW-6V-90V-...ltDomain_0&hash=item416e02a6ff#ht_1841wt_1142


This on has an input range of 12 - 30VDC 30A & starting bid was $31 + $8 shipping to USA. NOTE this has a cooling fan for the drivers/outputs.

http://www.ebay.com/itm/12-30V-30A-...ltDomain_0&hash=item20d5bf26c7#ht_1860wt_1330

I would go with the higher current ones to give some opperating cushion & possibly not running the controler at it's max for longer life of the controler.

Hope this helps.

 

[CDRIVE]

If for some reason one blower fan becomes stalled, then the other motor will tend to run near full-speed.

.

Hi LP. Curious, why do you say that?

Chris

 

[davenn]

Im assuming because the stalled motor will look like a short circuit and therefore the other motor will be taking full current ?

Dave

 

[CDRIVE]

Common DC brushed motors are predominantly resistive. You can approximate what they will draw using an Ohmmeter. So even if stalled it's resistive properties don't vanish.

On the other hand I would expect a 6A motor to employ an integral cooling fan. If so and if experiencing a prolonged stall I would expect it to overheat and eventually short the windings.

Chris

Edit: If intended for blower duty it may use the air intake of the blower to cool itself.