AC motor running on DC?

[mark32]

During the storm (Sandy) a homeowner told me he hooked his furnace motor directly to his vehicle's battery, which he claims it ran for two hours before smelling something burning. Damage aside, can a 120vac motor run on 12vdc??

 

[K8MHZ]

During the storm (Sandy) a homeowner told me he hooked his furnace motor directly to his vehicle's battery, which he claims it ran for two hours before smelling something burning. Damage aside, can a 120vac motor run on 12vdc??

The newer, high effeciency furnaces use low voltage fan motors. I think the tech that put mine in said they were 12 volts.

The motors are meant for a certain duty cycle, so running continuously for a couple hours is likely what caused the motor to overheat, if the burning smell was coming from the motor.

 

[kwired]

If it is a newer ECM motor (which is what you find on newer high efficiency units, and especially on units with variable speed blowers) - they are for the most part, permanent magnet DC brushless motors. The controller necessary to operate them converts the output power to DC. It probably ran at a low speed - and 12V may even be within its operating range for low speeds, It may have developed problems as battery voltage started decaying.

If it is an older PSC motor (most common used for HVAC blowers) it will not run on DC.

 

[Jraef]

The scary thing about that is, the guy apparently tried it not knowing if it would work! Had it been a PSC motor, he might have smoked it in the attempt.

 

[mark32]

Thanks for the info guys. I've been out of the residential sector for a while now, never would have guessed newer models would have a DC motor on it. Thanks a lot.

 

[gar]

121106-1530 EST

A DC brushless motor is not a DC motor until you add a controller to it.

A so called DC brushless motor is inherently an AC synchronous motor with a permanent magnet rotor instead of a wound rotor with DC excitation to the rotor thru slip rings. It also probably includes some sensors for commutation information.

Thus, there has to be a controlled that converts a DC supply into the necessary 2 or 3 phase AC excitation to the motor stators.

A stepping motor is an AC synchronous motor but with a lot of poles. Most stepping motors are two phase. By adjusting the relative current in the two poles you can get continuous adjustment of position.

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[K8MHZ]

121106-1530 EST

A DC brushless motor is not a DC motor until you add a controller to it.

A so called DC brushless motor is inherently an AC synchronous motor with a permanent magnet rotor instead of a wound rotor with DC excitation to the rotor thru slip rings. It also probably includes some sensors for commutation information.

Thus, there has to be a controlled that converts a DC supply into the necessary 2 or 3 phase AC excitation to the motor stators.

A stepping motor is an AC synchronous motor but with a lot of poles. Most stepping motors are two phase. By adjusting the relative current in the two poles you can get continuous adjustment of position.

.

In the first year of my apprenticeship the instructor told us that all brushless motors were AC. I told him I had a DC brushless motor. He didn't believe me and told me to bring it in, which I did. It was a cooling fan off a computer and it ran off 12 VDC. Right on the label it said '12 VDC brushless motor'.

After he admitted to the class he was 'wrong' and 'ate some crow', I peeled the label off the back to show him the little circuit board inside that converted the DC to rather high frequency AC to run the induction motor.

I think I left the motor there at the class for their collection as I had several. It may still be there, for all I know.

 

[mike_kilroy]

absolutely. ZERO chance of ANY ac or dc brushless motor, new or old, whether PM or not, running on dc voltage. None. Period. Never. Won't happen.

If the guy is right and it really did rotate, then K8MHZ hit the nail on the head: the motor has a built in drive he did not see. In which case the smell was probably due to the drive electronics not being happy with 12v - either too high or more likely too low and not the motor insulation. A built in drive that makes its logic from the dc input, likely would overheat if it ran at all, if the dc input was too low as its dc-dc converter would pull extra amps from is switcher transistors to make up for the missing voltage.

 

[kwired]

absolutely. ZERO chance of ANY ac or dc brushless motor, new or old, whether PM or not, running on dc voltage. None. Period. Never. Won't happen.

If the guy is right and it really did rotate, then K8MHZ hit the nail on the head: the motor has a built in drive he did not see. In which case the smell was probably due to the drive electronics not being happy with 12v - either too high or more likely too low and not the motor insulation. A built in drive that makes its logic from the dc input, likely would overheat if it ran at all, if the dc input was too low as its dc-dc converter would pull extra amps from is switcher transistors to make up for the missing voltage.

Well I was figuring the guy in the OP just hooked the 12V battery to the furnace AC input - this would mean the motor was powered through the controller. I won't guarantee just how long or how well it would run, but it is about the only chance there is of it working at all is if it is a ECM motor.