Old School Motor Speed Controls

[iwire]

Over the years I have worked on a few different older motors that used resister banks switched via rotary drum switches or contactors to control the motor speed control.

And while I have worked with them and made some basic repairs I really do not understand how they work.

How does diverting the voltage through the resistors control the motor speed? I thought cycles controlled the speed of an AC motor. :huh:

How is this voltage produced, the resistors were not directly tied into the line, they came from the armature so the motor is a generator at the same time?

Assuming a correctly sized motor could I run it continuously at a slow speed with out damage or can you just step through the speeds to get to full speed?

While running at a reduced speed will it hold a specific RPM with varying loads?

 

[GoldDigger]

Chances are the motors involved are not induction motors but rather "universal" motors with windings on both rotor and stator in a series, parallel, or compound arrangement. It would also need a commutator and brushes.
By changing the field current in such a motor you can change the rotational speed at which the back EMF balances the applied voltage producing a stable speed.

 

[ActionDave]

Over the years I have worked on a few different older motors that used resister banks switched via rotary drum switches or contactors to control the motor speed control.

And while I have worked with them and made some basic repairs I really do not understand how they work.

It sounds like you are talking about a wound rotor motor. I know that they have slip rings that go from the armature out to the resistors that limit the current. As far as I know they are meant to be used at start up only.

We had one that came out of an old gold mine in our yard and I was asking our motor guru about it. I never got to work on it. The owner was not able to get his permits to re-open the mine so he lost interest in getting the motor running.

 

[gar]

160114-2401 EST

iwire:

If the motor has three slip rings, and is an induction motor with a wound stator, then it is probably a wound-rotor induction motor.

The speed torque curve of an induction motor is very much dependent upon the resistance of the rotor. Most induction motors have fixed resistance rotors defined by the shape and material of the conductors in the rotor.

In a wound-rotor induction motor three low resistance coils are wound on the rotor and connected via slip rings to three external rheostats ganged together. This provides a means to externally adjust rotor resistance.

https://en.wikipedia.org/wiki/Wound_rotor_motor
http://www.tecowestinghouse.com/pdf/woundrotor.pdf

And my textbook reference "Alternating-Current Machinery". Bailey and Gault, McGraw-Hill, 1951. At least see p 248 for speed torque curves vs rotor resistance. I never met Bailey or Gault, but I knew Gault's wife who was a secretary in the Electronics Defense Group where I worked part time. My teacher for this course was Dr. J. G. Tarboux who was an outstanding teacher in both DC and AC machinery.

There are other places in the book where rotor resistance and motor charadteristics are analyzed theoretically.

.

 

[iwire]

It sounds like you are talking about a wound rotor motor. I know that they have slip rings that go from the armature out to the resistors that limit the current.

That describes what I worked on.

 

[iwire]

In a wound-rotor induction motor three low resistance coils are wound on the rotor and connected via slip rings to three external rheostats ganged together. This provides a means to externally adjust rotor resistance.

That seems to be what I was working on, I will look at the links when I have a chance. Off to work now.

 

[Jraef]

... As far as I know they are meant to be used at start up only. ...

There are two types of Wound Rotor Induction Motors (you will see it abbreviated as WRIM). Some were designed to just use the process of stepping through the resistances as a means of soft starting, as you said. But others were designed to run at different speeds / torques permanently. The difference was in the rotor windings, because if the intent was only for soft starting, the windings and resistors used in the controller could be cheaper. But often in things like mining hoists, they were the permanent speed/torque control versions.

Before the advent of VFDs, this was a common way to continually vary the speed on large pumps, typical of water / wastewater plants, to match flow requirements, especially where valves were problematic. On those, they used what was called a "Liquid Rheostat" controller. It had resistors connected to the rotor circuit that were lowered into a vat of electolyte. The more of the resistor inserted, the lower the resistance and the faster the motor went. There is only one company left that I know of still making the controllers called Flomatcher. I come across them still every now and then. They basically never wear out, but people don't think of them any more for new applications because it has to be a WRIM, and most people just want to use a standard induction motor and a VFD now.

 

[ActionDave]

Are wrim synchronous motors?

 

[gar]

160115-1458 EST

ActionDave:

A synchronous motor is a motor that runs at exactly a speed defined by the frequency of excitation. The shaft speed on average is exactly as defined by the excitation frequency, but this may be phased modulated as the motor loading changes, and thus there can be variation in the instantaneous speed.

Some motors that are basically synchronous AC motors are: stepping motors, so called brushless DC motors, shaded pole synchronous motor, permanent magnet rotor or DC excited conventional synchronous motors,

Basically an AC synchronous motor consists of a rotor with a constant DC magnetic field that is rotated by the rotating magnetic field from a multiphase stator. The motor can be built the other way around, but one wants the high power dissaption coils on the outside of the motor.

.

 

[gar]

160115-2006 EST

iwire:

I did not give you any references that you could see some speed vs torque curves for induction motors because I did not like the presentations.

I have copied a set of curves from Bailey and Gault that are attached. I like speed as the vertical axis and torque as horizontail.

The attached graphs are a set of curves calculated for a given motor with only the rotor resistance being adjusted. This also means supplied voltage is held constant. Note: that the maximum torque falls on a vertical line until the rotor resistance gets too high.

From these curves you can see why a high resistance rotor would be good for starting a high inertia load. One can pick a resistance that produces maximum torque at 0 RPM.




From Figure 12-6 on page 185 of the previously referenced Bailey and Gault.

.

 

[ActionDave]

Okay. So we have under discussion an induction motor but you have power going to the armature and the field like a series wound DC motor or a universal motor. So where is this induction going on?

 

[iwire]

Okay. So we have under discussion an induction motor but you have power going to the armature and the field like a series wound DC motor or a universal motor. So where is this induction going on?

Not going to the armature, coming from the armature.

 

[ActionDave]

Not going to the armature, coming from the armature.

So the power from the field, is induced into the armature, out to the resistors and back to where? IDK.

 

[gar]

160115-2352 EST

Action Dave:

In a multi-phase AC motor with stator windings that are connected to an AC multi-phase power source the flow of power is from the source to the motor when it is in motor mode as distinguished from generator mode.

Within the motor area where the motor rotor is located or is to be located there is a moderately constant amplitude, rotating in space, magnetic field produced by the electrical excitation to the stator coils. This is the same as if you mounted a permanent magnet on a shaft and mechanically rotated the shaft.

Place a closed coil that is not rotating in the space of the rotating magnetic field and a AC current will be induced in the coil. Let the closed coil rotate in the rotor space at any speed except the speed of the rotating magnetic field and current is induced in the closed coil. The frequency of the current in the closed coil is determined by the slip difference between the rotating magnetic field and the rotating closed coil.

The induced current in the closed coil produces a magnetic field that interacts with the rotating magnetic field and this provides torque to rotate the closed coil.

Most induction motors are built with rotor coils (usually low resistance conductive bars) of a fixed resistance that in turn define the motor's speed torque characteristic.

By making the rotor coils of low resistance and connected thru slip rings, not a commutator, to adjustable external resistors it is possible to change the speed torque curve of the motor on the fly by effectively changing the resistance of the rotor coils. This also moves a lot of the rotor coil power dissipation out of the motor and to the external resistors.

Power flow is from the stator windings to the rotor shaft as mechanical power output and to the external resistors as electrical power. There is some heat power loss in the rotor windings.

.

 

[ActionDave]

That makes sense. Thank you gar.

 

[gar]

160116-0826 EST

ActionDave:

The concept of a rotating magnetic field vector in a multi-phase motor is a difficult concept to visualize. I hope that my description of a fixed magniude (call it DC) magnet mounted on a rotating shaft helps visualize this.

What I did not discuss was how the stator coils produce the rotating field vector.
I won't get into that now because there may be some Internet references that could do a better job.

A single phase motor that has no starting winding or phase shift winding only produces a magnetic vector on one space axis that oscillates + to 0 to - to 0 and back to +. With no rotating in space of a magnetic vector you do not get rotation of the rotor. However, if you start rotation and the magnetic pulses are close to sync with rotation, then you can transfer energy to the rotating element much like you add energy to a person on a swing to keep them swinging.

.

 

[Jraef]

160115-2352 EST

Action Dave:

In a multi-phase AC motor with stator windings that are connected to an AC multi-phase power source the flow of power is from the source to the motor when it is in motor mode as distinguished from generator mode.

Within the motor area where the motor rotor is located or is to be located there is a moderately constant amplitude, rotating in space, magnetic field produced by the electrical excitation to the stator coils. This is the same as if you mounted a permanent magnet on a shaft and mechanically rotated the shaft.

Place a closed coil that is not rotating in the space of the rotating magnetic field and a AC current will be induced in the coil. Let the closed coil rotate in the rotor space at any speed except the speed of the rotating magnetic field and current is induced in the closed coil. The frequency of the current in the closed coil is determined by the slip difference between the rotating magnetic field and the rotating closed coil.

The induced current in the closed coil produces a magnetic field that interacts with the rotating magnetic field and this provides torque to rotate the closed coil.

Most induction motors are built with rotor coils (usually low resistance conductive bars) of a fixed resistance that in turn define the motor's speed torque characteristic.

By making the rotor coils of low resistance and connected thru slip rings, not a commutator, to adjustable external resistors it is possible to change the speed torque curve of the motor on the fly by effectively changing the resistance of the rotor coils. This also moves a lot of the rotor coil power dissipation out of the motor and to the external resistors.

Power flow is from the stator windings to the rotor shaft as mechanical power output and to the external resistors as electrical power. There is some heat power loss in the rotor windings.

.

There are also rotor energy recovery systems that, instead of burning that excess rotor energy off as heat in the resistors, use an inverter to pump it back into the line source. It has to be an inverter because the frequency of the AC that comes off of the rotor circuit is constantly changing, but you have to put a constant 60Hz frequency back into the grid.

WRIMs are actually very cool, I had one guy teaching me about them who described them as the "perfect" form of AC induction motor. I wouldn't take it that far, but I've always enjoyed any project that involved them.

One reason people often mistake then for Synchronous Motors is because of the slip rings. In some older forms of Synchronous Motors, the DC field excitation is applied through brushes as well. But one telltale difference is often the number; because on a Synch Motor, the rotor is DC, so only two circuits and on a WRIM it is 3 phase, so 3 circuits off of the rotor.

 

[mbrooke]

Jumping ahead and saying voltage can control AC motor speed via slip.

 

[Besoeker]

Over the years I have worked on a few different older motors that used resister banks switched via rotary drum switches or contactors to control the motor speed control.

And while I have worked with them and made some basic repairs I really do not understand how they work.

How does diverting the voltage through the resistors control the motor speed? I thought cycles controlled the speed of an AC motor. :huh:

How is this voltage produced, the resistors were not directly tied into the line, they came from the armature so the motor is a generator at the same time?

Assuming a correctly sized motor could I run it continuously at a slow speed with out damage or can you just step through the speeds to get to full speed?

While running at a reduced speed will it hold a specific RPM with varying loads?

To quote that song, there are more questions than answers.

The motors in question may have been DC machines.

They come in essentially two flavors. Series and shunt.
Both have two windings. An armature and a field.
For series motors, often used in traction applications, the field and armature are, not sirprisingly, in series. And an external resistance controller in series with that.
On a shunt wound motor there may be a stepped resistance starter in the armature. And, in olden times (before the Dead Sea reported in sick) when speed control was required, a variable resistor bank in the field was often used. Weakening the field mad it go faster.

Or, as some here have suggested, it could be a wound rotor AC machine. Again, two windings. A stator and a rotor. A stepped resistance starter in series with the rotor was not uncommon.

 

[iwire]

The ones I worked on where AC