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What is the best method to control a universal motor (1/4hp) with feedback?

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bigal_scorpio

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Firstly seasons greetings to all forum members,

I am trying to determine the best way to control a universal motor without losing too much torque?

As it is a universal motor I wondered if there would be any benefit in rectifying the supply and working only with DC? I can't find any info on this but would it be better doing this and then using PWM to control the speed.

Other than that I have seen a number of articles on using SCRs and indeed I have some that would be suitable but these designs all seem to contain a DIAC as well and that is something I am lacking in.

Anyway I am open to ideas and suggestions, maybe using a PIC micro would be good?

Thanks for looking Al
 
You could rectify the AC and use PWM. It may generate less motor noise than using phase-control SCRs on the AC.

If all you need is a DIAC, just buy one. Do you mean you have all the parts to build a PWM circuit?

Don't see how a micro would particularly help.

Edit: You mention feedback in the title. Do you need to maintain a constant speed with changing motor load?
 
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Hi Carl,

I would buy a DIAC but it may be a while delivered, what with the holidays.

I will have all the parts for PWM (555 or small PIC for the PWM) depending on what I used to actually chop the power.

I mentioned feedback as the motor already has a magnet and coil fitted to the rear of the shaft and as I intend using the motor for a bench sander I thought it would be helpfull to maintain the speed, at least to a degree anyway, when the sanding disc came into contact with the workpiece. Different wood can be vastly different how it grabs the sandpaper.

So so far I may be going for the DC method but what would you recommend for the power device, the TRIAC substitute?

Thanks Al
 
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It depends upon what your priorities are: noise, efficiency, simplicity, cost?

If I wanted to keep the noise down, keep the efficiency up, and control speed, I'd be modulating a DC supply with a 16-20KHz PWM signal. An electrically isolated mount or some other means to limit current flowing through the bearings might be needed though. Better efficiency implies more torque before you stall or overheat.

If I needed simplicity, low cost, and didn't need the speed controlled, I run it with a triac or a relay connected to an AC supply. A relay won't last forever with a motor load but a triac needs a high dV/dt and a snubber circuit to switch inductive loads.

Any other combination of noise, efficiency, simplicity, and cost would depend on the application.
 
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Hi Cachehiker,

DC seemed ideal but now you have me worried! What is the current flowing through the bearings this is new to me. Now I am concerned about it, is it dangerous? Can you elaborate on the isolation you mention?

I am sorry if this seems dumb but I have very limited knowledge of motors.

Also I have some STB9NK60ZT4 fets that are rated 600v 7A 0.85rds on and 125W. Would these be ok for switching my load?

Thanks Al
 
I can't think of a common motor design that has current going through the bearings.

Please confirm it is a brushed, permanent magnet AC/DC motor, not brushless.

Light dimmers properly rated might work to control the speed, but for good torque at low RPM and constant speed, I prefer DC and PWM.

John
 
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Hi John,

The motor is an old washing machine motor. It has brushes but no permanent magnet as far as I can test (nothing attracts my screwdriver) and I know its a universal motor as when I originally took it out I tried it on 12v DC, and it ran ok even though very slowly.

So do you think there will be any bearing current through this at 230vDC?

Al
 
The is no current through the bearings.

The current in a brushed motor goes through the brushes to armature through the commutator. The field does not require moving contacts.

As for the voltage, was it attached to 220V AC (or whatever in your country)? The high voltage DC motors I have (i.e., 90 or 180 V in the USA) are quite easily controlled by rectified mains voltage. Rectified 220V produces, of course, more that 220 V DC , because the AC voltage is measured in RMS.

In my case, since I like the added bells and whistles of a commercial controller, I have just picked up inexpensive, open frame KB controllers on eBay and such. They take the AC input and output DC PWM with speed control and overload protection. I have never paid more than $120 USD for any of them and usually much less. Be careful of ads that say, "removed from working equipment" but don't warrant the controller itself. That can mean the equipment was working fine until the controller blew. I speak from experience, but fortunately, the controller was easy to fix.

John
 
The motor is an old washing machine motor. It has brushes but no permanent magnet as far as I can test (nothing attracts my screwdriver) and I know its a universal motor as when I originally took it out I tried it on 12v DC, and it ran ok even though very slowly.

Universal motors don't have permanent magnets, otherwise they wouldn't work as they do.

So do you think there will be any bearing current through this at 230vDC?

No, it was a pretty ludicrous suggestion.
 
The is no current through the bearings.

Why would motor manufacturers offer shaft grounding brushes for demanding applications if there were no bearing currents to divert?

A 16KHz PWM signal gets capacitively coupled into the rotor's lamination stack. The current, usually only a few µA to a few mA, flows from there through the bearings to the motor frame if it is grounded. I've measured it. I am paid quite a tidy sum assess motor reliability. Most cases are not extreme and don't have a dramatic effect on bearing life. However, it is worth checking if you need the motor running for tens or hundreds of thousands of hours with little or no maintenance.

Here's an example of an extreme case. At one point, our brush life fixture was upgraded to feature 1.5hp to 3.0hp PMDC motors powered with a PWM controller driving a generator whose output was also pulse width modulated through a heater coil so it was easier for us to adjust. Both motor frames were connected through ungrounded mounting brackets. It started destroying cheap Chinese bearings after 300-500 hours. Following failure mode and effects analysis, we discovered 20mA flowing through the bearings of the motor under test.

For a temporary solution, we isolated the motor being tested from its mounting bracket and then inserted a resistor (2.7KΩ) between the motor frame and the mounting bracket. Its value was chosen to match the bearing currents measured in the end product. This improved the bearing life on the fixture by a factor of 10. A larger value would've further lowered bearing currents but then it wouldn't have been a good representation of the end product application. A long term fix is being implemented using an diesel truck alternator whose field is powered by a linear amplifier so it does not contribute to the bearing current.

Completely isolating a motor connected to a high voltage PWM controller also creates problems. Try it. Then put one hand on the motor frame and the other on earth ground. A bleed resistor between the motor frame and earth ground (usually 1.0MΩ) is necessary to prevent the motor frame from being energized by the capacitive coupling and becoming a shock hazard.
 
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The motor is an old washing machine motor.

So do you think there will be any bearing current through this at 230vDC?

Yes, but washing machine motors are usually designed with bigger bearings so I wouldn't expect it to be a problem unless you intend to run it continuously for months or years at a time. If it's a pump motor, it may not even be rated for continuous operation.
 
I have never paid more than $120 USD for any of them and usually much less.

John

Hi John,

You must have a lot more ready cash about than me, if I ever spent that much on a project my missus would be at the solicitors for divorce papers before I had finished it! hehehe

Al
 
Hi Guys,
Just a follow on question. Since I have never rectified mains AC without a transformer I am not sure how to go about it. After the transformer and rectifier I usually have a smooting cap and usually a decoupler too.

With such high voltage and a load approaching .5kW I am worried about getting things right first time so if anyone can push me in the right direction as to the needs of the motor and the best smoothing values (if needed) I would be a lot more secure.

Al
 
Listen well to cachehiker ! Shaft/bearing currents can be a real problem when using high frequency PWM drives. I am currently replacing bearings every 3 months on my furnace fan motor.

pilko
 
Hi Guys,

Pilko, if I ever get the sander working it should only get a few hours use per year! Unless I start cutting things too big on a more regular basis. hehehe

Seriously it would only be used maybe a few minutes at a time, it is only for fine tolerance stuff where the belt sander is too harsh.

But I do need advice on the switching guys.

Happy Holidays Al
 
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I am currently replacing bearings every 3 months on my furnace fan motor.

I'd be embarking on a quest to find a better fan motor.

There is simply such a thing as motors that have been optimized (cost reduced) for strictly pure DC or 50/60Hz AC applications. Bearings in such motors will simply not stand up to use with inverter/PWM drives when the motor frame is grounded. There are times when even isolated mounts and shaft grounding brushes aren't enough.

The biggest thing to remember about rectified line voltages is both supply rails end up hot. With 230VAC, the positive one will settle in at +160VDC relative to ground and the negative one at -160VDC. The processor will likely share its negative supply rail with the rectified line. Pushbuttons, displays, potentiometers, and such that are connected to the processor will be shock hazards unless they're optically or otherwise isolated. Disconnecting the processor from the mains for a while (depends on how fast it discharges) before connecting it to the USB port or your computer for programming is MANDATORY unless you have and abundance of computers at your disposal. Connecting a grounded computer to a hot rail will let the magic smoke out. You might be able to salvage some drives or some memory, but the motherboard will likely be roached. This makes debugging a major pain so concentrate on writing robust or easily testable code the first time.
 
Thanks cachehiker,
I am starting a new thread so as not to hijack this one. --title "Motor Shaft Currents".

pilko
 
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