Can an AC motor run on DC?

At work we have a small portable compressor, 120V.

Inside is a bridge rectifier and cap to convert it to DC, there is nothing that steps voltage down, so the motor runs at 120VDC.

Something broke and my co-workers put another pump unit on, smaller in size but it runs on 120VAC.

Well the genius' that they are wired it up to the old circuit and plugged it all in.

Evidently the AC motor is running on DC power as it is connected the same way through the rectifier.

Can someone please tell me whats going on? because this motor shouldn't run being powered by DC should it?

A universal motor will run on AC or DC.

An AC motor is more likely to run on DC than the other way around. But even if it does, that doesn't mean that it will run the same or that it is good for it. Depends on the design.

WBahn said:

An AC motor is more likely to run on DC than the other way around. But even if it does, that doesn't mean that it will run the same or that it is good for it. Depends on the design.

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I'd say the opposite is true. Only small universal motors, such as in vacuum cleaners, will run on both. Larger motors are usually induction type which are strictly AC.

crutschow said:

I'd say the opposite is true..

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I would have to agree, it is a bit unusual however to have one on a compressor, Universal (series) motors are notoriously badly regulated unless some kind of feedback is present.
They operate in a runaway condition and rely on load to keep the RPM from destruction.
One other type which is now common on furnace motors etc is the ECM, Electronically commutated motor, and has 3 windings like a 3ph motor but only 2 winding are energized at any one time, making it a BLDC or Brushless DC.
Max.

crutschow said:

I'd say the opposite is true. Only small universal motors, such as in vacuum cleaners, will run on both. Larger motors are usually induction type which are strictly AC.

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I won't argue. I'm thinking of some commutated designs I saw decades ago. Don't know if that sort of design is used much any more (or if it even ever was).

But, universal motors aside, it seems very unlikely that a DC motor would run on AC since it would be being told to reverse direction (or at least torque) with every change of the phase polarity.

Non of the other AC motors I have come across could run purely on DC, there have been wound rotor repulsion-induction which has a commutator that you may have seen, others are wound rotor variable speed, wound rotor with DC injection for synchronism, all AC, (apart from DC field injection).
In the Universal (AC/DC) the series field reverses in tandem with the Armature current.
They are purported to be slightly more efficient on DC.
Max.

Must be a universal motor then.

It might depend on how "good" your DC is. It may look more like just rectified AC.

WBahn said:

It might depend on how "good" your DC is. It may look more like just rectified AC.

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That still won't operate an AC induction motor.

MaxHeadRoom said:

Non of the other AC motors I have come across could run purely on DC, there have been wound rotor repulsion-induction which has a commutator that you may have seen,

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The commutator is only energized during start-up (repulsion phase) of the motor. Once it's up to speed a centrifugal clutch lifts the brushes and shorts all the commutator segments so it runs as a pure AC induction motor. They are known for generating a very high start-up torque.

Yes I realize, I had a project a few years back which required an upgrade to some 100hp compressor motors, these were synchronous motors, they were 4kv motors which were ran up to normal slip speed as induction motors, when the slip was detected to be within around 5hz, DC was injected into slip ring fed rotor windings and the motor came up to synchronism.
Max.

well, whatever, its a moot point now. they keep blowing fuses and burning out traces on the board because apparently "we replaced the motor, so we need to adjust the fuse" ..... the fuse which is in place only to protect the circuit board. I've also told them this twice, but this guy is some kind of wizkid apparently.

Long story short the thing is pretty much toast, and hopefully they will throw it out and move on.

You, or someone really need to establish what type of motor was in there!
Did the original motor have brushes?
Max.

GRNDPNDR said:

well, whatever, its a moot point now. they keep blowing fuses and burning out traces on the board because apparently "we replaced the motor, so we need to adjust the fuse" ..... the fuse which is in place only to protect the circuit board. I've also told them this twice, but this guy is some kind of wizkid apparently.

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Sounds like the wizkid is someone with just enough knowledge to make him dangerous.