universal ac motor question

[Henry RancourtParticipant @henryrancourt22682]

Can three speed universal motors be run CW or CCW by reversing the leads? My motor is connected to a four position rotary switch, off, lo, med and Hi, with the hot lead from source connected to the input lug of the switch and white source wire connected to the motor white wire and it runs CCW. I want it to run CW, so I switched leads, connecting the source white wire to the input lug, and the source black wire to the motor white wire and direction of rotation didn’t change. Why didn’t rotation direction change?

[Henry RancourtParticipant @henryrancourt22682]

I’ve learned it is not a universal motor because it does not have brushes and is probably a three speed fan motor and switching leads will not change direction of rotation. But, can brushed AC motors be reversed by switching the leads?

[Henry RancourtParticipant @henryrancourt22682]

I have another AC motor with brushes and it has a built-in switch and can run CW and CCW so what type of motor is it? And, based on this motor, I thought the same could be done with all AC motors.

[Henry RancourtParticipant @henryrancourt22682]

The following are answers I got from other sites.

The different windings are only for different speeds. The motor will always run the same direction regardless of the “polarity” of the leads. BTW, is your motor really a universal motor? Those have brushes. Yours sounds like the typical 3-speed fan motor without brushes, which is not universal, but a shaded-pole motor. Same thing, though: can’t reverse it that way.
Universal motors with built-in switches and those used in corded drills and the like, reverse the leads in the switch. The switching is done by switching the two field coil leads that are in series with the armature. A brushed d.c. motor can be reversed by simply reversing the power leads.

[duncan webster 1Participant @duncanwebster1]

Not quite true. If you have a permanent magnet field, DC motor swapping the leads will reverse direction, but if it has field coils then you need to switch the field but not the armature, or vice versa.

[SillyOldDufferModerator @sillyoldduffer]

Difficult hard-to-answer questions Henry.

From the outside, electric motors look much the same, a cylinder with a shaft sticking out.   External clues include capacitors, data plates, and it may be possible to identify from terminal labels, and brushes.  Or not!

Inside is a mechanism, that by some method, arranges two magnetic fields as necessary to spin a rotor.  There are many ways of providing “some method”.  Usually fairly obvious how DC motors are arranged, less so with AC.   There are at least 6 different variants of single-phase induction motors, not counting the Ferraris disc type, perhaps 7 or 8 3-phase types.  I’ve lost count. Then we get into steppers, synchos, and other variants.

As you’re interested, I recommend a book.  Plenty aimed at kiddies, too simple.  And Electrical Engineering texts expect the reader to infer mere details like how they work from the maths. I’ve got one that’s 400 pages of formula with no mechanical drawings.

“Electric Motors in The Home Workshop“, No 24 in the Workshop Practice Series was about right for me – goes through the common types, and not too mathematical.

When asking for help with a particular motors, please provide as much info as you can – photos, data-plate, markings and – if you know – where it came from.

Dave

[Robert Atkinson 2Participant @robertatkinson2]

Hi,

Firstly a “universal” brushed motor can be identified by the presence of brushes and the absence of permanent magnets. This can be determined visually and testing for attraction of a bit of steel respectively. A permanent magnet motor is not universal.
A  universal motor has electromagnets for both the field (stationary) and armature (rotating) parts.
As a given example will run from AC or DC it should be obvious that reversing the external connections will not reverse the direction as AC power reverses evey cycle.
To reverse a universal motor you have to swap the connections of either the field or the armature (before the commutator of course). Generally this means modifying the motor.
It’s generally easier to swap the armature as this is done at the brushes which aer generally easier to access than the field connections.

Robert.

[John HaineParticipant @johnhaine32865]

The term “universal” is usually understood to mean a 2 winding commutator motor where the windings are in series. Universal because will run on ac and dc. Typically used in things like vacuum cleaners, mains electric drills etc. As Robert says you can reverse them by swapping the connections to the field or brushes but the latter is easier.  Sometimes they are designed with an angular skew between the windings around the rotation axis to optimise ac performance in one direction and one like this may not reverse so happily. My impression is that they are much less common these days with the low cost of electronic controls.

Best avoided in the workshop unless built into an appliance as their speed is mainly limited by the mechanical load.  For example, not a great idea for a grinding wheel as it could overspeed and shatter.

There are also wound field commutator motors where the field is in  parallel. Often called shunt field. More common for dc drives where the speed can be regulated by varying the field current. Some machine tools use these and quite a few motor controllers have a separate output for the field winding.

[MacolmParticipant @macolm]

A universal motors has the field coils in series with the armature; this results in a high starting torque and strong response to increased load. It normally incorporates a cooling fan which importantly helps stabilise the speed, and also limits the maximum no load speed.

[duncan webster 1]

On duncan webster 1 Said:

Not quite true. If you have a permanent magnet field, DC motor swapping the leads will reverse direction, but if it has field coils then you need to switch the field but not the armature, or vice versa.

Just a small point which is not relevant to the OP’s query. Whilst you can indeed reverse a shunt wound DC motor by reversing either the armature or the field coil polarity, it’s always best practice to only reverse it using the armature. This is because if you remove power from a field coil whilst keeping the armature energised it’s possible for the motor to run away, possibly to destruction.

The reason for this is because field-weakening is a good method for increasing the speed of a DC motor above its base speed. It’s called constant power mode – the weaker the field, the higher the RPM must go to keep outputting the same power (at the expense of motor torque).

If you configure the motor so that reversing direction is done by switching the field polarity you do risk getting into a situation where losing the field power with the motor turning and the armature energised allows it to keep running on residulal field energy where there’s enough field energy to allow the RPM to keep climbing but the field energy is so slight that the motor will just keep accellerating until it fails mechanically i.e. the rotor explodes

 

Sorry to the OP it has nothing to do with his issue of trying to reverse his 3-speed motor but worth a mention in case someone in the future is thinking about doing this.

[MacolmParticipant @macolm]

As a general rule it is not a good idea to instantly switch direction on a motor running at full speed. The supply adds to the back emf, both in the “wrong” direction, and in some cases damage is possible.

[Macolm]

On Macolm Said:

As a general rule it is not a good idea to instantly switch direction on a motor running at full speed.

Mostly I would agree, but it depends on the motor.

On my repetition lathe there are two levers on the headstock. One for forward/reverse and one for full speed/half speed. The levers are not mechanical but operate electrical switches. The (induction) motor is designed for rapid reversal. So it is perfectly acceptable to shift the lever for fast forward to fast reverse without stopping the motor. Seperately, or at the same time, the full speed/half speed lever can be shifted. So one can go from full speed in one direction to half speed in the reverse direction in one movement.

The motor does ‘clonk’ a bit, but it is designed to take it. The repetition lathe is, by definition, a production machine tool and an operator on piece work wouldn’t be happy if they had to wait to stop the motor every time they switched direction or speed.

Julie

[MacolmParticipant @macolm]

Yes, I know such designs exist, and are a good solution if well implemented. Sometimes compliance is introduced to avoid the worst of the shock loading, for example at one time rubber dog couplings were common. Electronic control easily provides for such things these days.

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