Speed control for AC gear motor ?

[gar]

210304-1547 EST

You need to study DC and AC motor theory to understand various aspects of what you are trying to do.

A motor being brushless does not mean a whole lot. You need to know much more about the details of the motor.

These days when the words "brushless motor" are used it usually really means the motor is an AC synchronous motor ( meaning a motor with a permanent magnet rotor, and stator coils to make possible a rotating magnetic field ) with an appropriate DC to AC controller. And the DC input usually comes from an AC source.

A usual AC synchronous or induction motor is usually made without any brushes. So it is brushless, but it is not usually referred to as a brushless motor.

An AC synchronous motor always runs at synchronous speed. At 60 Hz this is 3600, 1800, 1200, and etc. speeds. Break sync ( overload the motor ) and you loose any usefulness. AC voltage control will do nothing to speed.

How an AC induction motor works ( characteristic curves ) is very dependent on rotor resistance. That is the resistance of the shorting coils wound on the rotor. With a high resistance rotor you can get a wide speed range by stator voltage adjustment, but speed regulation with load change is very poor. If high rotor resistance is produced in the rotor itself, then there is a major temperature rise problem. Thus, motors operated in this mode usually have brushes, low resistance rotor coils, and external load resistance, or some way to recover the large power output of the rotor at low speed. Some rather large motors are built this way to obtain speed control.

An AC induction motor wound with a fix low resistance rotor can not have much speed control from voltage control at a constant frequency. You have to change its frequency to change its speed.

Shaded pole motors are are only made in low power units, and if wound with a moderately high resistance rotor are capable of variable voltage speed control, but as I said above won't have good speed regulation with variable mechanical load.

A good DC permanent magnet motor will have an approximately constant torque continuous rating (may require an external fan for cooling at low speeds). Relatively good speed regulation without feedback from zero to full torque load, and for a short time can tolerate like maybe a 10 x torque overload.

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[LarryFine]

Shaded pole motors are are only made in low power units, and if wound with a moderately high resistance rotor are capable of variable voltage speed control, but as I said above won't have good speed regulation with variable mechanical load.

I'm under the impression he's more concerned with applied force than an exact feed speed.

The characteristics you're describing for the shaded-pole motor sound ideal for this purpose.

 

[gar]

210304-2037 EST

The speed torque curve of a shaded pole motor is still mostly controlled by rotor resistance. So certain designs can provide a reasonably constant speed ( phonograph turntable ), whereas other designs can provide a means for inefficient speed control.

The shading pole is primarily a means to provide the second pole of a shaded pole induction motor to make it a two phase motor, and simultaneously provide the phase shift of the current in the second pole.
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[Tereci]

I won't pretend I understand all the fine points of this discussion (even though I am trying to catch up - reading, watching videos) but at least I can clarify the issue I am trying to solve for.

My blade sometimes stalls in a rock - so far I was always around to turn both motors off (the shaded pole feed motor, and the motor that turns the blade) fast enough before anything got damaged. The stalling can have various causes by in my case the feed speed is for some of the material I cut just too fast. Most newer saws feed at 6min per inch speed while mine feeds at 3min per inch. I am trying to slow down the feed speed so that the saw has time to cut through the rock before more rock is pushed on it. I don't really care how fast exactly it is moving as long as it's slow enough so that the blade doesn't get stuck.
Apart from slowing the feed motor down I also want to get a smaller arbor shaft pulley to increase the blade speed (the modern blades are rated for it).

I'll be getting the solid state controller tomorrow so I can test it over the weekend. What would be the warning signs that it's damaging the motor? The motor heating to more than its usual temperatures?

And again, I really appreciate all the time you took to explain things to me!

 

[LarryFine]

What would be the warning signs that it's damaging the motor? The motor heating to more than its usual temperatures?

That's the main one. Get used to how warm it gets now, so you have a comparison. An IR thermometer would be even better.

 

[gar]

210306-1243 EST

Tereci:

You have to get to a basic understanding of how AC induction motors work.

Because a motors is labeled "shaded pole" does not define its speed torque characteristics. That is only a description of how it is built.

Being a shaded pole is insufficient information to determine whether you can get useful speed control by varying voltage, and it does not give you any idea of temperature rise at low voltage. Rotor resistance is the primary determining factor in determining the shape of the motor speed torque curve.

For a phonograph record player I want good speed regulation, ideally a synchronous motor which is different than an induction motor. For a fan motor I want poor speed regulation with to respect voltage. I want more of a constant load power vs voltage because of the characteristics of a fan in terms of power vs speed.

I took a look at a variable speed fan I have. At constant 120 V 60 Hz input---

Full speed was 1.16 A RMS and 75 W about 140 VA. Current waveform was a full sine wave with some phase shift. Much of that power input was going into the fan blade.

Minimum speed was 0.87 A and 49 W about 104 VA. Current waveform was not produced by an SCR or TRIAC. The current pulse was sort of the peak of a current sine wave centered on the voltage source peaks with time gaps between the pulses. Because of the great reduction in fan speed most of that power input was going into the motor rotor.

Without the ability to measure or model the motor rotor current I can not say for sure, but my guess is that rotor power dissipation was much higher for the low speed than for the high speed.

I don't know how your rock cutter blade works. If it does not cut but grinds, then you might be better of with a constant force actuator instead of a constant displacement device. Constant force could be obtained with a pulley and appropriate weight.

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[gar]

210308-1010 EST

I have found a shaded pole motor to run some tests on. Power meter is a Simpson 10720 electrodynamometer type.

120 V 60 Hz with spur gear box.

16.8 ohms room temperature coil resistance.
35 W no load power input at 120 V.
42 W locked rotor input at 120 V.

7 W no load at 60V. Fan was still rotating at a high speed.

By looking at the coil size I believe I would burn that motor out at 120 V and locked rotor, but it does not appear that most of the no load power input is going into the coil. I am guessing that the shading coils are large part of the no load power dissipation.

I need to see if I can create a simple means to load motor and measure load and speed. The current waveform has a fair amount of phase shift, and is not very sinusoidal.

Some additional measurements:

No mechanical load except gear box, should not be much.
15 W 0.33 A 83 V 1.8 W coil power dissipation calculated.
20 W 0.41 A 94 V 2.8 W "
25 W 0.50 A 104 V 4.2 W "
30 W 0.61 A 113 V 6.3 W "
35 W 0.70 A 120 V 8.2 W "

Locked rotor load via stopping motor fan blade.
43 W 0.77 A 120 V 10 W coil power dissipation.

I have not made any coil resistance measurements to determine what is an average coil temperature rise.

I don't know, off hand, if I can easily make some load measurements.

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[gar]

210311 -2419 EST

Have mocked up some more mechanical stuff. The motor and its gear box are clamped to the bench. I made a shaft from the motor gear box output to a fluid electromechanical dc adjustable load also clamped to the bench. I don't have a good speed measurement method yet. So I count time by 1/1000 for one revolution. It is about 6 seconds or 10 RPM. I have to lower supply voltage to about 83 V and this is 15 W into the motor at no load to get into a useful working range on the electromechanical brake.

The maximum voltage for the brake is 24 V DC. At the 83 V input I can not get the motor to start with no more than about 10 VDC on the brake.

At the present time I do not have a good way to measure shaft speed, other than by my count technique. Working on this.

With above said load that the motor can start I do not see a lot of speed change from no load to that amount of applied load.

More later.

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[paulengr]

Probably the cheapest way to measure RPM is with a timing light. Even a lot of phones can do it.

Single phase AC motors are extremely inefficient in the first place and shaded pole motors in particular are the worst. Most of the no load current is going into core and stator losses and since it does very little work, it’s almost pure reactance so expect 90 degrees lagging and pure vars. Current has almost no relationship to load. The nonlinear effect is because the shaded pole is lagging at a different phase offset from the non shaded pole. A shaded pole motor is cheap in every way except power draw. With no capacitors, no magnets, and only 2 wires it is simple as it gets if you just want something to turn but everything about it is no critical and you are trying to do more than that. So you can probably burn it up stalling it but it’s such a crappy motor it takes a lot to do it.

Voltage is torque control at best and with these motors very nonlinear. Ignoring the motor for a moment, torque in a fan varies with the square of speed plus whatever static load the bearings present so good luck doing torque control. You will be able to control it for trimming over a narrow speed range and that’s it even with a better choice of motor. There is a reason that wound rotor motors pretty much died as a dominant technology about 40 years into electric motor development...most applications are speed control.

There is a very expensive VFD that will do speed control on single phase permanent split capacitor motors only. But the vast majority of single phase speed controllers either use a DC motor or convert single phase AC to 3 phase AC (230 V) because both motor types are far easier to control, and the combination of the two is cheaper than the overpriced single phase VFD.

If you are going to continue this at least start with equipment that has a modicum chance of success. Look at kb electronics for a cheap DC soeed controller for instance or Automation Direct for some very low end AC micro drives. Or at least read up on motor theory to understand why this won’t work as is.

 

[gar]

210312-1114 EST

paulengr:

What is a "single phase AC motor"? In other words how do you want to define it, and at what operating region, starting or running?

I don't have a strobe. So I could make one, or use another technique using something I already have. I have a number of ABS magnetic sensors, but I don't know where a connector is at the moment. I will likely use a vain and an optical sensor.

 

[gar]

210311-2420 EST

paulengr:

Picking up on your suggestion of a strobe I did some digging around and found a a laser pointer I had disassembled. This had a series diode I added to it. Thus, could directly connect to a function generator. My shaded pole motor has a 5 blade fan on the motor shaft. This works nice to strobe.

A quick test produced 260 Hz at no load, and about 190 Hz at a moderately high load. Thus, 52 and 38 Hz for the shaft. Not a large adjustment range. Thus, unless Tereci has a high resistance rotor on his motor I think the best approach is a surplus DC permanent magnet motor, and suitable variable DC voltage source. For this power range a 24 V DC motor would be good.

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[LarryFine]

The Surplus Center has lots of motors cheap. (I don't know why I didn't think of it before now. )

Gearmotors: https://www.surpluscenter.com/Electric-Motors/AC-Gearmotors/AC-Gearmotors/

Their general motor listings page: https://www.surpluscenter.com/electric-motors/

 

[gar]

210312-0745 EST

Tereci:

Here is a small DC motor that could be reduced in speed by a 5 to 1 belt, and increase its torque by 5, and adjust that belt output from near zero speed to 5 RPM with an adjustable DC supply.

25 RPM 24 Volt DC Inline Gearmotor | DC Gearmotors | DC Gearmotors | Electric Motors | www.surpluscenter.com

25 RPM 24 Volt DC Inline Gearmotor, DC Gearmotors, 25 RPM 24 VDC INLINE GEARMOTOR, 25 RPM 12 VDC GEARMOTOR New, SEI model 37JB270G/32ZYT30-2468SEI,37JB270G/32ZYT30-2468,

www.surpluscenter.com

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[Tereci]

Thank you both so much! I cannot believe how cheap the motors on the surplus are.. that is great.

Meanwhile I got the solid state speed control but after all you said I don't expect it to work. I also got another shaded pole gear motor with about 1RPM that I want to test. It just has different shaft diameter so I need a new coupling and it's bigger and heavier than my current one so I have to try to figure out how to mount it. If that doesn't work I'll start ordering stuff from the surplus page

That said 2021 seems to be a year of motor issues for me I have this horizontal lapidary grinder and sometimes it slows down to zero with just a bit of pressure. The only info the company provides about the motor is "1/2" arbor and powered by a 1/4 hp, heavy-duty, ball bearing, DC motor. This motor is variable speed so you can work at whatever pace you desire, delivering speeds from 800 rpm to 3,400 rpm.". They don't sell a replacement motor. The sales rep mentioned that the motor is brushed so I wonder if an issue like the one shown on the video could be caused by bad brush contact?

The company doesn't want ppl doing repairs themselves so the only option they gave me is to send it back to them and pay them for shipping and a new motor. I am still thinking about giving it a try myself first. Are there motors where brushes cannot be replaced? The motor is mounted in the stand of the machine and has no markings on it and I have to yet figure out how to get it out of the plastic collar on top.

If you have any thoughts/insights, let me know!

 

[gar]

210317-1612 EDT

Tereci:

It appears your post #34 is on an entirely different machine.

A brushed DC motor usually has replaceable brushes.

Your stall problem with his motor needs much more clarification.

A permanet magnet field brushed armature motor will usually have a moderately good speed vs torque curve. Torque is pretty much proportional to power load. In other words armature current for a fixed source voltage is fairly close to a straight line linear curve relative to torque. Motor speed is roughly proportional to armature voltage. Basically you have a counter EMF from the motor rotor that is proportional to speed. You have a difference voltage across the rotor resistance that is proportional to load current. Counter EMF plus rotor voltage drop equals source voltage applied to the armature.

A permanent magnet brushed DC motor can be back driven to be a DC generator.

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[Tereci]

210317-1612 EDT

Tereci:

It appears your post #34 is on an entirely different machine.

You are right, the second part of the post was on another issue I am having with a different motor and I should and will move it to its own thread and keep this one on the topic of the shaded pole motor.
Just to clarify there is something wrong with the machine - I talked to other happy owners and their machines are able to handle quite a bit of pressure at different speeds before slowing down so this is not normal for that motor.

I moved it here: https://forums.mikeholt.com/threads/what-can-cause-a-dc-brushed-motor-to-stall.2559757/

 

[gar]

210318-2038 EDT

Some basic principles on magnetic motors.

Two same kind magnetic fields push each other apart. Two dis-similar fields pull each other together.

A current thru a conductor produces a magnetic field around that conductor. A permanent magnet by itself produces a constant magnetic field.

An alternating magnetic field can induce a voltage, and possibly a current in another conductor.

In a DC shunt wound field, or permanent magnet motor with a wound armature, and commutator the speed torque curve is approximately a straight line. For a fixed field excitation level the no load rotor speed is approximately proportional to armature voltage. If field is adjustable, then speed at a fixed armature voltage is inversely proportional to field current over a limited range. If a motor is running, armature voltage is maintained, and field excitation is lost, then the motor speed may increase to a high level, fly apart, and cause major damage and possibly death.

An AC synchronous motor runs at exactly one speed independent of torque load up to a breakdown point. Relative shaft angle position relative to the synchronous magnetic vector does change relative to mechanical load.

An AC induction motor speed, synchronous speed minus slip, is a function of load, and may be quite non-linear.

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