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   Home   Help Search  
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Author Topic: Reducing motor speed  (Read 7275 times)
W9FIB
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Posts: 2297




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« Reply #30 on: April 29, 2017, 06:42:32 PM »

W9FIB,

If you reduce the frequency from 60 Hz to 30Hz, provided the percentage slip stays the same do you not get half the speed? But because the winding impedance is now half, do you not need to reduce the applied voltage as well to keep the same current and torque?  It's 50 years since I went to lectures on motors and generators and crossed field machines such as Amplidynes and Metadynes, so never having needed to use them, I'm a bit rusty.

Yes. Here is some information:

.......
XL = 2 f L. Where: = 3.14, f = frequency in hertz, and L= inductive reactance in Henrys.)
Variable speed AC drives will maintain a constant volts/hertz relationship from 0 - 60 Hertz. ........

Er, XL is inductive reactance, expressed in ohms.  You omitted Π from the equation.  Should have been XL = 2*Π*f*L, where Π = 3.14, and L is the stator winding inductance, expressed in henrys.



Yep your right. That's why I included the link to what I posted so the original material can be used. Didn't notice pi didn't copy over. Had you checked the link, you would have seen that.
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73, Stan
Wisdom is knowledge you gain after you know it all.
W6EM
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Posts: 1837




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« Reply #31 on: April 29, 2017, 06:52:50 PM »

I was thinking along the line of a fan motor (like a room fan) that I have used on a fan for an amp (auxilliary fan) but likely I am talking something else here...not quite the same thing...sorry about that...

Yes there are many types of motors, some of which can have different types of speed control. It can get confusing.

Many fans have shaded pole type motors. Due to their construction, they do have the ability to have crude speed control by variable voltage.

These motors can be controlled by changing the frequency (again back to a VFD) or (depending on the load) by adjusting the voltage. Under a no load condition, changing the voltage will have very little effect. If the motor is loaded, the increased or decreased voltage will result in an increase or decrease of the strength of the magnetic field and will result in the motor speed to increase or decrease. This will usually result in an increase in coil temperatures.




Pole shading eliminates the need for a starting winding to determine rotational direction.  As a kid (who didn't know better at the time) I tried to slow down an ice cream maker (with a shaded pole motor) by using a variac.  I ended up with a cooked motor, which I disassembled and rewound to avoid parental scolding.  In short, it won't work.

However, a universal AC-DC motor, like those used in vacuums, blenders, and such, can be sped up or slowed down by adjusting voltage.  Speed is independent of frequency.  But small bench grinders can't be allowed to wind to speeds that would disintegrate abrasive wheels, so 2-pole 3600 rpm motors are usually used for them.  Not counting Dremels, of course.

  Now, if one were to instead look far and wide, perhaps a 4-pole, fractional horsepower single phase 120V 60Hz motor could be found with a shaft speed of just under 1800 RPM.

I used to get a kick out of my ME counterparts who loved to spec VFDs to adjust their air handler motors well below synchronous shaft speeds.  One day one of them wanted to slow a motor way down to about 1150 RPM, about 60% of rated shaft speed.  My suggestion to her was to call for a 6-pole motor and forget the VFD.  A little more money for the motor, but no worry about VFD motor insulation and efficiency worries.

Subjecting motor windings to endless pulse-trains from inverter-drives can indeed shorten insulation life.  In fact, most 3-phase motors today are offered with "inverter duty" options with higher voltage magnet wire insulation levels for just that reason.




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W6EM
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Posts: 1837




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« Reply #32 on: April 29, 2017, 07:03:46 PM »

what we have in the Twin Cities light rail is (from the sound of the power sheds) switching upconverters, and the sound of the motors leads me to believe it's PWM to the motors, like a hybrid car. different lines with different ages use AC and DC motors of varied voltages.  since our stuff is pretty new, it uses newer technologies.  the overhead catenaries have 550 volts on them, and one sign on a supply connection warns of 277 volts.

bench grinders are usually long-shaft AC 120 vac motors with the grind wheels (or clean-up wire or cotton wheels) attaching to an arbor.  so, no pulleys and belts.  you never see an open-shaft tool shaft overhead with belts to workstations any more.  OSHA would shoot you with automatic weapons if you tried. used to lust after one that Sears carried in the 60s.
If you're hearing a high pitched whine, then yep, it's PWM.  That commutation frequency whine gives it away.  Golf carts are also a frequently encountered example.  The 277VAC label reveals that they are using a 4-wire, 480/277 distribution system.  Meaning, the supplies are likely connected single-phase line to neutral, which would explain the 277 volt labels.  And, hopefully, with the foresight to connect the rectifier supplies to balance the three-phase supply to them.

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W6EM
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Posts: 1837




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« Reply #33 on: April 29, 2017, 07:25:07 PM »

for the bench grinder, can you just go to a softer stone? or is it a matter of physical safety holding the work?

many induction motors have a start and a run winding, and there is a centrifugal switch that cuts out the start winding when they get to speed. cut back the speed with, say, a triac speed controller, and you will burn out the start winding sooner than later because the start winding is not meant to be continuously powered, and the centrifugal switch will never cut it out.  if the motor starts up and then audibly clicks, don't try that at home.  don't know if your homebrew speed controller will work with a capacitor-start motor, haven't tried it.  I do cut speed on brush motors like drills and routers, but pay attention to their heating up.
Good catch.  Trying to use a single-phase VFD can be dangerous to the well being of a single phase induction motor with a centrifugal switch on its starting winding.  Chances are, the motors on your drills and routers are universal motors, which can be diddled with using voltage control.  Pulse trains can be rough on their insulation though.

I once had a redesign challenge to replace about 50 old DC motors which drove exhaust fan hood blowers.  No more brushes available for the motors.  The replacement cost of the hoods was exhorbitant.  So, I found a single phase to three phase VFD  designed for fractional horsepower, three-phase motors and replaced the 3/4 HP DC motors with 3/4HP 3-phase 240V motors.  And, the chemists were happy with the results.

 
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W9FIB
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Posts: 2297




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« Reply #34 on: April 29, 2017, 10:36:25 PM »

I was thinking along the line of a fan motor (like a room fan) that I have used on a fan for an amp (auxilliary fan) but likely I am talking something else here...not quite the same thing...sorry about that...

Yes there are many types of motors, some of which can have different types of speed control. It can get confusing.

Many fans have shaded pole type motors. Due to their construction, they do have the ability to have crude speed control by variable voltage.

These motors can be controlled by changing the frequency (again back to a VFD) or (depending on the load) by adjusting the voltage. Under a no load condition, changing the voltage will have very little effect. If the motor is loaded, the increased or decreased voltage will result in an increase or decrease of the strength of the magnetic field and will result in the motor speed to increase or decrease. This will usually result in an increase in coil temperatures.

Pole shading eliminates the need for a starting winding to determine rotational direction.  As a kid (who didn't know better at the time) I tried to slow down an ice cream maker (with a shaded pole motor) by using a variac.  I ended up with a cooked motor, which I disassembled and rewound to avoid parental scolding.  In short, it won't work.

As stated it depends on the load. Also, I stated that you need to monitor the coil temperature. Yes if you "play" with it too much you will fry it. But that just means you went to far, not that it does not work. Any method has limitations because it was not designed to be variable.
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73, Stan
Wisdom is knowledge you gain after you know it all.
G4LNA
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Posts: 186




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« Reply #35 on: April 30, 2017, 12:38:15 AM »

Provided you are not doing too much heavy duty grinding, then what about an electric drill grinder attachment? Nearly all drills these days have variable speed.
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W6EM
Member

Posts: 1837




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« Reply #36 on: April 30, 2017, 08:49:49 AM »

Provided you are not doing too much heavy duty grinding, then what about an electric drill grinder attachment? Nearly all drills these days have variable speed.
Best suggestion (and simplest one) yet.  I have a drawer full of drill brushes, abrasive wheels, and  rotary files.  I use them in my drill press or with the cordless drill for small jobs.

Some time back, I also acquired a very inexpensive Harbor Freight small scale bench grinder with attached flex shaft.  It has a speed control built in.  Another very handy tool.  I'm sure with its wide range of shaft speeds that it, too, has a small universal motor.

We tend to head for the complicated techie fix too often, when something simple and inexpensive can solve the dilema.

73.

Lee
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W6EM
Member

Posts: 1837




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« Reply #37 on: April 30, 2017, 09:06:20 AM »

I was thinking along the line of a fan motor (like a room fan) that I have used on a fan for an amp (auxilliary fan) but likely I am talking something else here...not quite the same thing...sorry about that...

Yes there are many types of motors, some of which can have different types of speed control. It can get confusing.

Many fans have shaded pole type motors. Due to their construction, they do have the ability to have crude speed control by variable voltage.

These motors can be controlled by changing the frequency (again back to a VFD) or (depending on the load) by adjusting the voltage. Under a no load condition, changing the voltage will have very little effect. If the motor is loaded, the increased or decreased voltage will result in an increase or decrease of the strength of the magnetic field and will result in the motor speed to increase or decrease. This will usually result in an increase in coil temperatures.




I should have commented on this before, and didn't.  With a shaded pole motor, all you might accomplish by raising the voltage would be a slight reduction in slip up to synchronous values (3600, 1800, 1200, etc.) or a slight drop in RPM, but it's dangerous to do that.

Multi-speed fan motors usually involve changing the number of poles, e.g., from 2-pole to 4-pole to 6-pole windings.  The ability to continuously vary speed is the domain of either DC or universal motors.

VFDs are both a costly and impractical solution for most fractional HP single-phase motors.  Especially those with starting windings---courting disaster.  As I said earlier, the pulse trains emanating from them can puncture magnetic wire insulation and bring an early end to motor life if the motor is not over-insulated.

Large three-phase motors don't require starting windings due to the phase sequence of supplied power.  Most of their application is in situations where the motor load is low inertia and motors can start without excessive starting current (centrifugal pumps and HVAC blowers).



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W9FIB
Member

Posts: 2297




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« Reply #38 on: April 30, 2017, 02:34:45 PM »

I was thinking along the line of a fan motor (like a room fan) that I have used on a fan for an amp (auxilliary fan) but likely I am talking something else here...not quite the same thing...sorry about that...

Yes there are many types of motors, some of which can have different types of speed control. It can get confusing.

Many fans have shaded pole type motors. Due to their construction, they do have the ability to have crude speed control by variable voltage.

These motors can be controlled by changing the frequency (again back to a VFD) or (depending on the load) by adjusting the voltage. Under a no load condition, changing the voltage will have very little effect. If the motor is loaded, the increased or decreased voltage will result in an increase or decrease of the strength of the magnetic field and will result in the motor speed to increase or decrease. This will usually result in an increase in coil temperatures.




I should have commented on this before, and didn't.  With a shaded pole motor, all you might accomplish by raising the voltage would be a slight reduction in slip up to synchronous values (3600, 1800, 1200, etc.) or a slight drop in RPM, but it's dangerous to do that.

Multi-speed fan motors usually involve changing the number of poles, e.g., from 2-pole to 4-pole to 6-pole windings.  The ability to continuously vary speed is the domain of either DC or universal motors.

VFDs are both a costly and impractical solution for most fractional HP single-phase motors.  Especially those with starting windings---courting disaster.  As I said earlier, the pulse trains emanating from them can puncture magnetic wire insulation and bring an early end to motor life if the motor is not over-insulated.

Large three-phase motors don't require starting windings due to the phase sequence of supplied power.  Most of their application is in situations where the motor load is low inertia and motors can start without excessive starting current (centrifugal pumps and HVAC blowers).

I said early on that the economic feasibility was undetermined. Only the original poster can make that determination based on his exact need. Not you, or me, or anyone else. I presented a possible alternative and then received some questions I tried to answer.

By the way, forward/reverse contactors are almost on par in price these days with VFDs that solve many control and interlocking problems. Even at fractional HP. 20 years ago, VFDs were very expensive. But like most electronics, as they became more common, the prices dropped dramatically. So again a direct conclusion can not be made for all circumstances.

Then there is the case for VFDs vs. across the line starters from the inrush current standpoint. If you have a large factory with say 50 motors that frequently start and stop, the efficiencies actually pay the extra costs. I have seen this accomplished to the bean counters great satisfaction.

Motor types, voltages, number of phases, number of poles, across the line, VFDs, and probably thousands of other things could all be debated to death. But the OP makes the final decision not any of us. Which is why the OP should be presented with multiple possibilities. But then too they should be within the realm of what the OP stated. 3 phase operation can't be accomplished when you have single phase power unless you use something to convert 1 phase to 3 phase. Damn, there is that expense again.

I think AA4HA had the best idea by shear 1 time economics. The flexible shaft idea was good too. All depends on the OP and what he is doing.
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73, Stan
Wisdom is knowledge you gain after you know it all.
K1PJR
Member

Posts: 349




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« Reply #39 on: May 03, 2017, 11:33:12 AM »

"for the bench grinder, can you just go to a softer stone? or is it a matter of physical safety holding the work?"

That's exactly what I did. I bought an aluminum oxide stone and posted the same question at a woodworking forum. All the replies were the same. You can use a grinder with a higher RPM just have some water nearby to cool the workpiece and don't leave it on the stone too long.  It wasn't about physical safety but overheating the workpiece. I'm using the grinder to sharpen knives and wood chisels. BTW it's a Harbor Freight grinder.

The oxide wheel puts an edge on the workpiece and a homemade MDF wheel with polishing rouge hones it so you can easily take hair off your arm. Razor sharp. Quick and effective. Love this setup.

Phil
K1PJR
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