AC motor with DC current and vice versa

[gar]

210529-1416 EDT

Broadly speaking I would say there are basically three classes of magnetic electric motors. These are DC brushed motors, AC synchronous, and AC induction. I do not consider a so called DC brushless motor as a DC motor, rather it is an AC synchronous motor with some sort of control to convert DC into AC. Also a stepping motor is really an AC synchronous motor.

What are called "DC brushless motors" should be called "DC brushless motor systems". The motor itself is not a DC motor.

DC brushed motors are based on a fixed magnetic field, and an integral synchronous mechanical switch (brush and commutator) to control what wire gets current to interact with the fixed magnetic field.

An AC synchronous motor has a steady magnetic field that interacts with a steady rotating magnetic field, or a pulsed magnetic field.

An AC induction motor has no steady magnetic field, but rather two oscillating or rotating magnetic fields interacting with each other with one field being generated by magnetic induction from the other. Slip of rotor vs stator determines the frequency of the second current.

Keep in mind that a North field attracts a South field and vice-versa.

See ---

Brushless DC electric motor - Wikipedia

en.wikipedia.org

.

 

[paulengr]

I never really thought about this much but I assume a light bulbs does not care whether the voltage is dc or ac. But what about a motor? I can see an ac motor working on dc but not the reverse. Anyone want to expand on this?

Also how is a dc motor built differently than an ac motor? Is a diode the only difference. That would allow voltage in one direction? Confused a bit but I thought this might make a good discussion

One of the early discoveries is when Tesla showed that Edison DC light bulbs worked on his AC distribution system.

In fact effectively the Edison dynamo was a DC generator but the only real difference between motors and generators is the direction of the power flow between electrical and mechanical so these were effectively DC motors, too. Tesla was issued patents for the AC induction motor, synchronous AC generator/motor (DC rotor with AC statue), and AC wound rotor motor. His original design was two phase and not very good. Subsequent improvements, especially Eiffel, greatly improved them into something very close to modern AC motors. DC became far more popular with Ward Leonard loops introduced in the 1930s which caused DC to dominate until the IGBT allowed much higher AC variable speed power in the 1990s. DC still dominates tools, toys, small fans, and servo drives. Even as AC continues to almost wipe out the high power end, DC continues to grow on the low end.

To make a motor work you need a rotating magnetic field. So somehow a changing voltage has to be part of the system. Fully DC motors use a commutator to create an AC voltage mechanically just as an old automotive distributor did it.

Diodes can convert AC into DC but not really the other way around, which is sort of going the wrong way. You need a device that can use forced commutation where you can turn it on and off somehow.

There are ways to do this even with pure DC. A homopolar motor is a great example because they are 100% DC and use slight phase angle differences to create a torque but they are too weak to do much with them except as scientific curiosities. You can google how to make one with a couple magnets, an AA battery, and some magnet wire.

Modern advanced motors are tending towards harkening back towards this. Reluctance motors don’t use a magnetic circuit as such. Electronically commutated motors and brushless DC uses electronics instead of traditional mechanical means which makes them highly efficient. Although most large motors are AC, the trend for high efficiency or torque seems to be moving back towards DC.

 

[gar]

210529-2248 EDT

To understand how a homopolar motor works you need to understand the critical aspects of the motor.

This kind of motor is built with a coil of wire with a DC current thru the coil. For convenience make the coil rectangular in shape. One side of this coil is aligned with an axis of rotation. With a DC current thru the coil we may approximate the magnetic field from the coil as a single magnetic vector at the center of the coil, and perpendicular to the plane of the coil. Thus, the magnetic vector is at some radius from the axis of rotation, and in a direction that can create torque around the axis of rotation, if there is another magnetic field to interact with.

If we create a uniform DC magnetic field with vectors pointing parallel to the axis of rotation, then we have a net constant force vector from the interaction of the two magnetic vectors that rotates the coil in one angular direction no matter what the angular position of the coil is.

Most critical here is that the coil net magnetic vector be offset from the axis of rotation of the coil.

If you were to make the mechanical axis of rotation pass thru the center of the coil (the center of its magnetic field), then essentially there would be no rotation.

.