AC motor with DC current and vice versa

[Dennis Alwon]

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

 

[xptpcrewx]

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?

An incandescent lightbulb does not care if the voltage is AC or DC. The whole concept of RMS voltage is to equate AC and DC voltages to an effective value.

Note: A universal motor can actually operate on either AC or DC, but most motors cannot. This has to do with motor construction - namely how the motor is intended to convert a time-varying voltage (like AC) to useful continuous torque in one direction. An AC motor cannot work on DC, but a DC motor might work with AC.

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

There are no diodes in a DC motor. The DC motor works using the concept of commutation, which is basically a mechanical switching action to prevent polarity reversal. This provides a continuous torque in one direction.

 

[Besoeker3]

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

Some motors can configured for both DC and AC. Universal machines. with commutators. My experience is limited to very small machines.
The larger ones for me have been DC machines and AC machines - and each quite different to each other.
The DC motors in my field have been commutator motors from around 22kW to 1,000kW and generally not above 600Vdc.
The AC motors I have lived with have been (squirrel) cage machines. Some have been standard 400Vac usually up to around 315kW but quite a few have been 3.3kV or above.

Then there are Static Kramers...............enough................

 

[Carultch]

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

A motor needs a direction reversal of current applied to the coil, typically every half turn. Otherwise, if you power an electromagnet with a constant current that doesn't change direction, it would spin as if it were a rotating permanent magnet subject to the same permanent magnets that comprise the stator. Half the time the magnetism would do positive work on the rotor (speeding it up), and the other half the time the magnetic force would do negative work on the rotor (slowing it down). On net, the work done by magnetism in any given rotation would add up to zero, which would make it useless as a motor. It would work like a motor in the first half of the rotation, and work like a generator in the second half of the rotation.

A synchronous AC motor reverses the current direction, by using the negative half of the AC waveform for the half of the cycle that would otherwise do negative work on the rotor. This is why such motors have a rotation rate that is directly related to the value of 60Hz. Such as 1800 rpm or 3600 rpm. 3600 rpm is the equivalent of 60 Hz. when you replace revolutions with cycles in the meaning of the units, or vice versa.

A DC motor uses a commutator to reverse the current flow direction through the coil, every half of the rotation, such that there is always torque in the same direction. This is a rotating switch contact, that reverses the polarity across the coil, so that it switches polarity to cover the second half of the cycle where it would otherwise act as a generator.

To build a simple DC motor (look up "Beakman motor", if you want to see more detail), the commutator is made by sanding the enamel off of just half of the rotating coil of wire, at the point where it contacts the paperclip supports. This turns the motor on and off, to keep the negative half of the cycle from slowing it down. Instead, it is powered through the positive half of the cycle, and coasts through the negative half of the cycle, when the enamel stops the current from flowing. This keeps it rotating in the same direction, but you can do better by having a commutator that can reverse the polarity, instead of just switching off the circuit.

 

[xptpcrewx]

Half the time the magnetism would do positive work on the rotor (speeding it up), and the other half the time the magnetic force would do negative work on the rotor (slowing it down). On net, the work done by magnetism in any given rotation would add up to zero, which would make it useless as a motor. It would work like a motor in the first half of the rotation, and work like a generator in the second half of the rotation.

A synchronous AC motor reverses the current direction, by using the negative half of the AC waveform for the half of the cycle that would otherwise do negative work on the rotor.

...so that it switches polarity to cover the second half of the cycle where it would otherwise act as a generator.

Sorry but this is not how motors, generators or physics works.

There is no negative work performed with slowing down the motor. Speeding up and slowing down will both require real power (watts) so it’s positive work either way.

The only way you will generate power (negative work) is if you apply external mechanical power to drive the shaft. Clearly that is not happening in your example.

 

[Carultch]

Sorry but this is not how motors, generators or physics works.

There is no negative work performed with slowing down the motor. Speeding up and slowing down will both require real power (watts) so it’s positive work either way.

The only way you will generate power (negative work) is if you apply external mechanical power to drive the shaft. Clearly that is not happening in your example.

This is intended to be a description of why a motor wouldn't work, if you built it this way. I.e. with a constant DC current and no commutator. Of course it isn't how motors work.

Negative work is what happens when the rotor is slowing down, due to the permanent magnet stators applying a torque to the electromagnet rotor, that is against the direction it is spinning. The energy is coming from the existing rotational kinetic energy of the rotor, that it had when it first entered the part of the rotation where the magnetic force works against its motion.

You would have to initiate the spin, and if you had frictionless everything and no electrical resistance, it would spin like this forever. Speeding up on the positive half of the rotation, and slowing down on the negative half of the rotation, with no net work done over one cycle. Obviously, that isn't part of reality, so a real attempt at building a motor like this would slow to a grinding halt.

" Speeding up and slowing down will both require real power (watts) so it’s positive work either way. "
Both positive work and negative work are real power (Watts). The power being negative doesn't make its units something other than Watts. You are thinking of imaginary numbers applied to power that we call reactive power, that give it a slightly different unit. The difference between positive and negative work is whether the object in question is the origin or the destination of the Watts. In mechanics, work done on an object is positive (i.e. work that would speed it up), and work done by an object is negative (i.e work that would slow it down).

 

[xptpcrewx]

This is intended to be a description of why a motor wouldn't work, if you built it this way. I.e. with a constant DC current and no commutator. Of course it isn't how motor's work.

Negative work is what happens when the rotor is slowing down, due to the permanent magnet stators applying a torque to the electromagnet rotor, that is against the direction it is spinning. The energy is coming from the existing rotational kinetic energy of the rotor, that it had when it first entered the part of the rotation where the magnetic force works against its motion.

You would have to initiate the spin, and if you had frictionless everything and no electrical resistance, it would spin like this forever. Speeding up on the positive half of the rotation, and slowing down on the negative half of the rotation, with no net work done over one cycle. Obviously, that isn't part of reality, so a real attempt at building a motor like this would slow to a grinding halt.

I disagree. The direction of rotation doesn’t have anything to do with power reversal. Think of it this way...

Without mechanical power input applied to the shaft, no matter if the the rotor is speeding up or slowing down, all work is performed in one direction - that is, energy conversion occurs from the electrical domain to the mechanical domain one way.

Work = [Torque • Angular Displacement]
(Force • Distance)

Consider the developed torque and the resultant angular displacement in either case of slowing down or speeding up the rotor. The magnetic fields in the machine in both cases are working to accelerate the rotor from a higher speed to a lower speed or from a lower speed to a higher speed. The torque and resultant angular displacement always have the same sign (+ • +) = +.

 

[xptpcrewx]

Both positive work and negative work are real power (Watts). The power being negative doesn't make its units something other than Watts.

Just to clarify I’m not suggesting negative real power is not watts.

 

[GoldDigger]

If you look carefully at what happens in a non-commutated permanent magnet motor when DC is applied instead of AC, you will see that as the rotor slows down in the "wrong" half cycle it is actually acting as a generator. But the exact relationship between power supplied and time is complicated by the wasted power in the winding resistance in the absence of back EMF.

 

[Carultch]

I disagree. The direction of rotation doesn’t have anything to do with power reversal. Think of it this way...

Without mechanical power input applied to the shaft, no matter if the the rotor is speeding up or slowing down, all work is performed in one direction - that is, energy conversion occurs from the electrical domain to the mechanical domain one way.

Work = [Torque • Angular Displacement]
(Force • Distance)

The power reversal has to do with the torque opposing the angular velocity, rather than acting in-line with the angular velocity. That is what happens, once it crosses its equilibrium position, and starts coasting "uphill" against a magnetic torque that tries to bring it back to the equilibrium position. Torque dot product angular displacement switches from being positive to being negative. This means energy is leaving the mechanical domain and it must be going somewhere, during this time. Neglecting friction and resistance, it would go in to the electrical circuit as a generator.

Suppose the permanent magnet's field points due north. If the electromagnet's rotor also points north, it is at a stable equilibrium. Whether you disturb it to the left, or disturb it to the right, it will return to pointing due north.

Now suppose we start it such that the electromagnet rotor's magnetic field points west. It begins to rotate clockwise, with a clockwise torque. Electrical energy is turning into mechanical energy, as is the purpose of a motor. We agree so far.

Once it passes north, it coasts past north, and continues spinning clockwise. The torque is acting against its rotation. Energy is leaving the mechanical domain, as it spins against the torque that tries to align the magnetic fields. This is where you are missing my point. Because it is slowing down, its kinetic energy has to be decreasing. That kinetic energy has to go somewhere. One destination for where the energy is going, is the friction and parasitic losses in the electrical circuit. Another destination for the energy is that it is entering the electrical domain, during a time when the motor is temporarily acting as a generator.

The rotor will come to a stopping point when it faces east. It will then reverse and begin speeding up. Energy leaves the electrical domain, and enters the mechanical domain. The cycle continues, and it swings like a pendulum from west to north to east, and back to north. If you give it enough of an initial push so it can spin all the way past the unstable equilibrium at south, it will continue rotating. It will speed up as it rotates clockwise on the west side, and it will slow down as it rotates clockwise on the east side. Or vice-versa, if we spun it counterclockwise. If friction were eliminated, it would continue spinning indefinitely, speeding up and slowing down in cycles.

This is why we have to reverse the direction of current in the half of the cycle when it spins from north to east to south. So that the torque on the rotor continues to match the direction that the rotor spins, in order that the electrical domain and magnetic fields always do positive work on rotor, rather than flip/flopping between positive work and negative work.

 

[xptpcrewx]

The power reversal has to do with the torque opposing the angular velocity, rather than acting in-line with the angular velocity. That is what happens, once it crosses its equilibrium position, and starts coasting "uphill" against a magnetic torque that tries to bring it back to the equilibrium position. Torque dot product angular displacement switches from being positive to being negative. This means energy is leaving the mechanical domain and it must be going somewhere, during this time. Neglecting friction and resistance, it would go in to the electrical circuit as a generator.

Suppose the permanent magnet's field points due north. If the electromagnet's rotor also points north, it is at a stable equilibrium. Whether you disturb it to the left, or disturb it to the right, it will return to pointing due north.

Now suppose we start it such that the electromagnet rotor's magnetic field points west. It begins to rotate clockwise, with a clockwise torque. Electrical energy is turning into mechanical energy. We agree so far.

Once it passes north, where it would otherwise remain stationary, it coasts past north, and continues spinning clockwise. The torque is now acting against its rotation. Energy is leaving the mechanical domain, and entering the domain of electromagnetism, as it spins against the torque that tries to align the magnetic fields. This is where you are missing my point. Because it is slowing down, its kinetic energy has to be decreasing. That kinetic energy has to go somewhere. One destination for where the energy is going, is the friction and parasitic losses in the electrical circuit. Another destination for the energy is that it is entering the electrical domain, during a time when the motor is temporarily acting as a generator.

The rotor will come to a stopping point, once it gets to pointing east. It will then turn around and start speeding up again. Energy leaves the electrical domain, and enters the mechanical domain. The cycle continues, and it swings like a pendulum from west to north to east, and back to north. If you give it enough of a push so it can spin all the way past the unstable equilibrium at south, it will continue rotating. It will speed up as it rotates clockwise on the west side, and it will slow down as it rotates clockwise on the east side. Or vice-versa, if we spun it counterclockwise. If friction were eliminated, it would continue spinning indefinitely, speeding up and slowing down in cycles.

This is why we have to reverse the direction of current in the half of the cycle when it spins from north to east to south. So that the torque on the rotor continues to match the direction that the rotor spins, in order that the electrical domain and magnetic fields always do positive work on rotor, rather than flip/flopping between positive work and negative work.

I appreciate your detailed explanation. The more I thought about it the more I had second thoughts about my understanding. What got me to realize your point is considering the cross product between the stator and rotor magnetic fields and how they would be flipping polarity with the rotor spinning one way and the stator going the other direction (kinda like a synchronous motor slipping poles). Looks like I am the one who needs to brush up on my motor, generator and physics theory.

 

[gar]

210527-0850 EDT

From what has been said above relatively simple concepts have been made confusing.

First, north does not attract north, and south does not attract south, rather these force each other apart. North attracts south and south attracts north. You should know this, and if you do not, then figure out some simple experiments to prove what happens.

One of the simplest motors to study is a two phase synchronous motor with a permanent magnet rotor and AC stator coils. When you make an X, Y plot of these two waveforms you find the resultant vector for all angles is a constant amplitude vector that rotates about 0, 0 as its center of rotation, So with two stator coils at 90 degree physical positions, and driven with a sine wave on one coil, and a cosine wave on the other, we produce a rotating magnetic vector of constant magnetic amplitude, and one rotation per excitation cycle. Thus, 3600 RPM at 60 Hz.

With a permanent magnetic on the rotor and no mechanical load on the rotor, then the south pole of the permanent magnet stays exactly aligned with the north pole of the rotating vector. As you mechanically load the rotor the physical position the permanent magnet lags the rotating vector, but still runs at the same speed. This phase shift is proportional to torque load until an overload point is reached,then sync is lost and the rotor may essentially stop, or pulse erratically.

More later.

.

 

[Besoeker3]

210527-0850 EDT

One of the simplest motors to study is a two phase synchronous motor with a permanent magnet rotor and AC stator coils.

Well, maybe.
For me, the simplest and the far most common motor is the cage induction machine. Simple, robust design and vast numbers of them.

 

[gar]

210527-1213 EDT

Besoeker3:

How an induction motor works is far more difficult to describe than how a synchronous motor works.

I am going to go back to a more basic concept. Take two horseshoe permanent magnets, and a sheet of Teflon 1/8" thick. As a first grader you may have done this with two small bar magnets and a sheet of paper. Put one magnet on one side of the plastic sheet, and the other one on the opposite side of the first one, and aligned so they attract each other. Move one magnet and the opposite one tracks the motion of the first one. This is what happens in the synchronous motor where one magnet is the rotating magnetic vector, and the other is some sort of permanent magnet on a bearing supported shaft.

The rotating vector is created from adding the two single axis vectors that are physically 90 degrees apart. Each of these vectors alone oscillates up and down between a + and - value. Each is not a constant amplitude vector.

Draw one vector horizontally, X axis, and add to this a vertical, Y axis, vector. This forms a right triangle where the hypotenuse of the triangle is the rotating vector. Make x = K*sin a, and y = K*cos a. This results in a composite vector = ( (K*sin a)^2 + (k*cos a)^2 ) )^.5 and using trig identities the composite vector is a constant for all values of a.

So this motor has a constant amplitude rotating magnetic vector coupled to a constant amplitude permanent magnet rotor. Thus, a motor with constant velocity, and no pulsating torque.

.

 

[Besoeker3]

210527-1213 EDT

Besoeker3:

How an induction motor works is far more difficult to describe than how a synchronous motor works.

I am going to go back to a more basic concept. Take two horseshoe permanent magnets, and a sheet of Teflon 1/8" thick. As a first grader you may have done this with two small bar magnets and a sheet of paper. Put one magnet on one side of the plastic sheet, and the other one on the opposite side of the first one, and aligned so they attract each other. Move one magnet and the opposite one tracks the motion of the first one. This is what happens in the synchronous motor where one magnet is the rotating magnetic vector, and the other is some sort of permanent magnet on a bearing supported shaft.

The rotating vector is created from adding the two single axis vectors that are physically 90 degrees apart. Each of these vectors alone oscillates up and down between a + and - value. Each is not a constant amplitude vector.

Draw one vector horizontally, X axis, and add to this a vertical, Y axis, vector. This forms a right triangle where the hypotenuse of the triangle is the rotating vector. Make x = K*sin a, and y = K*cos a. This results in a composite vector = ( (K*sin a)^2 + (k*cos a)^2 ) )^.5 and using trig identities the composite vector is a constant for all values of a.

So this motor has a constant amplitude rotating magnetic vector coupled to a constant amplitude permanent magnet rotor. Thus, a motor with constant velocity, and no pulsating torque.

.

I'm used to 3-phase cage motors. The calculation is straightforward.

 

[gar]

210527-1534 EDT

Bespoker3:

You are not presenting information on how a motor works, but rather some information on the input output characteristics of some particular motor.

.

 

[Besoeker3]

210527-1534 EDT

Bespoker3:

You are not presenting information on how a motor works, but rather some information on the input output characteristics of some particular motor.

.

Actually, it's just a fairly standard motor with nothing particular about it.
Anyway, how do you wish to provide better information on the usual standard squirrel cage polyphase motor?

 

[gar]

210527-1733 EDT

Besoeker3:

My interpretation of the original post of this thread is how do motors of various types work? The speed torque curve does not tell you how a motor works.

Relative to your question.

Polyphase means you can have a rotating magnetic field in the motor, this is in stark contrast to a true single phase motor that only has a unidirectional oscillating vector.

An induction motor, can be polyphase or true single phase, means there is no permanent magnet field present, but the second field is created by induction. Induction means you must have an AC source to create the induced current. This is a much more difficult motor to describe how it works. The operational curves of the motor operation do not describe how a motor works, but are the operational characteristics resulting from how the motor works.

I don't want to talk about induction motors yet.

.

 

[gar]

210527-2343 EDT

There is a simple experiment you can perform that can provided a simulation of the two phase synchronous motor.

This involves a compass and a permanent magnetic.

The compass is the permanent magnetic rotor of the motor.

I have already described to you that the vector sum of the two motor coils produces a constant amplitude magnetic vector that rotates in space around the axis of the motor rotor. The rotational speed is a function of the AC frequency. This frequency can be as slow as you want. Thus, I can use a permanent magnetic as that vector. If you don't believe me, then you can place two coils at 90 degrees apart and synthesize the sine and cosine currents to these coils.

In the experiment place the magnet in the optimum orientation to to produce a vector toward the center of the compass. Keep the radius of the magnet large enough to avoid demagnetizing the compass. Keep the radius constant and the magnet's vector pointing at the compass center. Make the radius short enough that the compass needle will track the magnet thru 360 degrees. Now you have made a synchronous motor.

If you make rotation speed slow enough you might call this a DC motor. But I would not do that. Rather I still want to call it an AC motor.

.

 

[Besoeker3]

210527-1733 EDT
I don't want to talk about induction motors yet.

Fairy 'nuff.
But they are the most common by far.