Original bus heat blower speed

[DrivingMissLazy]

Hey Richard, in that case - I'll design a PWM controller for free/open-source use and post it there (in a day or so).

I'll include the schematic, parts list (with prices and sources if I'm allowed), a board layout file (ExpressPCB - they are cheap and fast) and the source code for the PWM pulse generator Microprocessor.

The design will be for a low-side switched (that is, the ground leg of the motor is switched - not the "+" supply), 12-24VDC supply with a 20Amp-surge load.  In the microprocessor code, I'll put in some basic spread spectrum stuff (using a pseudo-random-number-generator, with optional external "entropy" via a broadband white noise generator) for better EMI/RFI performance.  For giggles, the design will support a 10K-linear potentiometer, I2C, and auto-baud-detecting multi-rate RS-485 control (the latter two for direct digital control and feedback).

Cheers!

-Tim

Tim, post it here on the main board first so that everybody sees it. I will later move it to the help board like I do a lot of the OT posts.

Richard

[Stan]

Does anyone know what the heater motor draws? I seem to remember that it it is over one horsepower.

[DavidInWilmNC]

Does anyone know what the heater motor draws? I seem to remember that it it is over one horsepower.

Doesn't the Big Blower (OTR) have a 70 amp breaker on it?  I can check mine, but I believe it's something like that.  The PWM controller would be great for the driver's HVAC, though.

David

[buswarrior]

Is the original problem that the temperature in the coach is now uncontrolable?

Restore the temp control circuitry and the fact that the duct work is gone will not be an issue.

What happened to the temp sensor in the air intake?

Is the water valve and its wiring functional?

let's fix this mouse trap, not invent a new one...

happy coaching!
buswarrior

[Tim Strommen]

...Does anyone know what the heater motor draws? I seem to remember that it it is over one horsepower...

...Doesn't the Big Blower (OTR) have a 70 amp breaker on it?  I can check mine, but I believe it's something like that.  The PWM controller would be great for the driver's HVAC, though...

Depends on what the system votage is for the give bus.  Some are 12V some are 24V.  The carrier OTR blowers in my Gillig are 24V and are breaker'd for 20Amps.  Typically, the amps double for the same ammount of work at half the voltage.

...Is the original problem that the temperature in the coach is now uncontrolable?...
...What happened to the temp sensor in the air intake?...
...Is the water valve and its wiring functional?...

In my bus, I have a shut-off valve for isolating the OTR heating from the engine, but other than that, the booster pump controls the temperature via the thermostat.  To give me more control, I replaced the dumb-switch thermostat with a digital temperature sensor and am now controlling the speed of the booster pump via PID controlled PWM to regulate the temperature (very stable).  I planed on adding a secondary PWM to the blower to give me more than an On/Off control to the blowers (which was factory on my bus) so that I could hear my passenger (I only have one... ).

I'll increase the design current capablity, but this will probably increase the cost a lot (instead of $25-40, it may be $40-120, the cheapest n-type MOSFET I'm aware of is about $12 for ~15Amps@24V or ~30Amps@12V of switching - to control a 1HP load for each voltage {1HP = ~760Watts}, you'd need about three of those in parallel so you're looking at about $36 just in MOSFETS).  I'll try to keep it as simple/cheap  as possible.

As an asside, since we are talking about these kind of currents, the diode based approach may not br practical/safe give the high currents.  Diodes which can sustain the high currents of a ~1HP motor are not cheap either, and the waste product of that approach is a lot of heat which needs to be moved somewhere (for example a 0.7 volt drop accross a diode which has a 60Amp load is 0.7 x 60 = 42Watts).  Having a lot of these will almost build you a secondary heater just from the heat you need to remove from the diodes.

Cheers!

-Tim

P.S. Richard, I'll do as you say, post it here first. -T

[Stan]

Tim: You refer to OTR blowers, so I assime your bus has more than one. MCI and GM just have one motor and the 70 amp breaker mentioned on a MC-9 at 24 volts seems to ring a bell in my foggy mind.

Motor overloads are typically 125% of full load current so that would put the MCI motor at about 1 1/2 HP and your motors at about 1/3 HP.

[Tim Strommen]

Hi Stan,

Yeah, the Carrier unit I have has two motors, one for each of the plenums that ran from the back to the front of the bus (one on the left, one on the right).  They are both about 1/3HP, and they are currently switched by a 100Amp solenoid "contactor" in the factory configuration.

Since yours is 70A @ 24V, that leads me to believe I should "go for broke" on the design and just design around a 2HP motor.  That will add some expense for those who have that much of a load, but the "stuffing" of MOSFETs will be optional so one may purchase as many as their load needs I guess.

Cheers!

-Tim

[gus]

The simplest way is to do like older auto heaters and control the amount of hot water flow with a valve without messing with the air flow. I plan to do this with my defroster water flow but on a bus the physics of this is sometimes more work than it is worth.

Modern autos use a hot/cold air blend to do the same thing.

Any fan is a fluid pump so if you obstruct the air flow either into or out of the pump/fan you increase the load on it and if you overload it the electric motor will overheat and fail. If the motor is in the incoming air flow you also decrease the cooling air to the fan motor.

[DavidInWilmNC]

My MC-8 manual mentions something about a 2-speed motor for the OTR HVAC.  There's supposed to be a resistor or something located down by the blower motor for speed control, but it's not on mine.  Perhaps it was just an option and was seldom installed.  I'll have to check, but there's supposedlya small wire (routed along with the big cable) that I assume was the solenoid to switch the resistors.  It's been a few months since I looked at it, but I seem to remember that being there.  Couldn't we just use resistors for the speed control... like many cars do?  I know they would get hot, but the blower would cool them.  I haven't a clue as to what one would use for the resistor on something like this, but it seems to be what MCI had in mind; I know they weren't using any sort of electronic controller.  Anyway, it's just a thought.

David

[Tim Strommen]

...The simplest way is to do like older auto heaters and control the amount of hot water flow with a valve without messing with the air flow. I plan to do this with my defroster water flow but on a bus the physics of this is sometimes more work than it is worth...

This is what my systm does now via the speed control of the "booster pump" (which pull warm water from the engine cooling system and pushes it into the interior heat exchanger).  The fan runs at full power all of the time when on currently, and it is possible to strike a ballance between circulated air and circulated water by controlling both the fan speed and the mixture tempurature of the heat coil.

...There's supposed to be a resistor or something located down by the blower motor for speed control, but it's not on mine...
...Couldn't we just use resistors for the speed control... like many cars do?...

This is entirely possible, and is suggested by the "rheostat" control (a variable resistance device which splits the power available to the motor and the device based on its setting).  Yes it will get hot, and running a motor designed for a specific voltage at a lower one is not really good for it (this is relative though since the implied load from the blower/fan-blades will go down with the speed).  This discussion seems to be breaking into two paths - efficiency and ease.  Efficiency goes to PWM, ease goes to the resistive voltage split option.

It's up to the individual to "do it their way..."

Cheers!

-Tim

[DavidInWilmNC]

This discussion seems to be breaking into two paths - efficiency and ease.  Efficiency goes to PWM, ease goes to the resistive voltage split option.

It's up to the individual to "do it their way..."

Cheers!

-Tim

My impression of the PWM controller was that it wouldn't handle the possible 70 amp load (likely to be actually 50-60 amps) from the big blower.  It's very possible that my impression is incorrect... it'd be nice to have a way to efficiently control the speed.  I'm going to see if I can rig this controller up to throttle back my driver's HVAC blowers.

David

[Nick Badame Refrig/ACC]

Hi Guy's,

Here is a thought. Would a Rheostat off a welder work?  My Millermatic 210 has one that goes up to 210 amps and I

think that is the DC side of the welder..

Nick-

[Stan]

Nick: A Miller Welder uses the same principle as your bus alternator. The rheostat controls the filed voltage in the generator.

That brings up another possibility for the heater motor control. Does anyone have the specs on the main heater motor. If they are shunt wound motors, they can be controlled by field voltage (low current) but if series wound they have to be armature control (high current)

[Tony LEE]

"My MC-8 manual mentions something about a 2-speed motor for the OTR HVAC. "

I think it is an extra field winding - more field flux gives lower speed and this is automatically connected when in the heating mode. It doesn't have a large effect anyway.

[gus]

Tim,

Will your heater circulation system work without the pump on?

Just curious, I can't tell what bus you have from the photo. My 4104 lists a circulation pump as optional eqpt but it circulates so well without one I'm not sure why one was ever needed.