electric motor control questions

[Ron Irwin]

Hello, I have a basic knowledge of electronic design, but am in need
of a little help. I have a small metal lathe with a 12 volt 4amp dc
motor. It has no capacity for variable speed and I would like to
build a motor controller for it but have no idea where to start. I
have read some about PWM (pulse width modulation) but don't really
have any idea if it is the most appropriate method for control. If
any one could help me out some I would appreciate it very much.

Thanks,
Ron

 

[hamilton]

This may help:

<http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=PWM+control+of+DC+motors&btnG=Google+Search>

 

[Baphomet]

This may help:

<http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=PWM+control+of+DC+mo
tors&btnG=Google+Search>

The circuit above might get you in the ballpark but the motor driver
transistor circuit will need beefing up to handle your 4 amp lathe motor.

 

[John Popelish]

Hello, I have a basic knowledge of electronic design, but am in need
of a little help. I have a small metal lathe with a 12 volt 4amp dc
motor. It has no capacity for variable speed and I would like to
build a motor controller for it but have no idea where to start. I
have read some about PWM (pulse width modulation) but don't really
have any idea if it is the most appropriate method for control. If
any one could help me out some I would appreciate it very much.

Thanks,
Ron

I did a google search for this key word set
[DC motor pwm speed control schematic]
and found several promising hits.

 

[Paul Burridge]

Hello, I have a basic knowledge of electronic design, but am in need
of a little help. I have a small metal lathe with a 12 volt 4amp dc
motor. It has no capacity for variable speed and I would like to
build a motor controller for it but have no idea where to start. I
have read some about PWM (pulse width modulation) but don't really
have any idea if it is the most appropriate method for control. If
any one could help me out some I would appreciate it very much.

Thanks,
Ron

I did a google search for this key word set
[DC motor pwm speed control schematic]
and found several promising hits.

There are some great circuits out there that can be built for peanuts
and perform very well, but unfortunately, none of them can provide the
degree of torque you need with a lathe operating at very low speeds. I
wish Ron luck with the project. It's a simple build and certainly
worth a try, but I suspect whatever he ends up with it will be a poor
show compared to the good old fashioned mechanical gearbox.

 

[James Meyer]

There are some great circuits out there that can be built for peanuts
and perform very well, but unfortunately, none of them can provide the
degree of torque you need with a lathe operating at very low speeds. I
wish Ron luck with the project. It's a simple build and certainly
worth a try, but I suspect whatever he ends up with it will be a poor
show compared to the good old fashioned mechanical gearbox.

You're right. Unless a tachometer feedback were added to the speed
control system, torque would drop off as the speed went down which is exactly
what you *don't* want to happen.

That motor, 12 volts at 4 amps, is only 48 watts. If I needed speed
control, I'd seriously consider finding a small 120 volt AC universal AC
(brushed) motor to replace the DC one. The motor in something like a blender or
hand drill (garage sale $1.00) would probably be ideal. Then I'd use an SCR
speed controller to vary the speed. Many blenders and drills already have that
type of speed control that compensates for changing loads and will maintain
almost constant speed built in.

Jim

 

[Roger Lascelles]

The lathe motor is most likely a permanent magnet type where speed is
roughly proportional to voltage.

The Pulse Width Modulation thing is one inexpensive way to control the
effective voltage, but you don't need to get involved with all the headaches
of making your own PWM controller.

If you buy a variable voltage DC power supply, you can simply turn the
voltage knob and control your speed.

You are looking for something with a specification something like 2 to 20
volts or 0 to 15 Volts or 1 to 12 Volts etc etc at 4 amp or 5 amp or more
rating. These are imported in cheaply from China and Taiwan. Perhaps
someone can tell us where such things are stocked in the USA.

Roger

 

[Big John]

You could try All Electronics http://www.allelectronics.com/. They sell a
0-15 volt 25 Amp DC supply for $165 USD. 25 amps is overkill for a 50 watt
motor, but the supply can be used for plenty of other things as well.

Hope this helps,
Big John


Roger Lascelles wrote in message

 

[Yzordderex]

Hello, I have a basic knowledge of electronic design, but am in need
of a little help. I have a small metal lathe with a 12 volt 4amp dc
motor. It has no capacity for variable speed and I would like to
build a motor controller for it but have no idea where to start. I
have read some about PWM (pulse width modulation) but don't really
have any idea if it is the most appropriate method for control. If
any one could help me out some I would appreciate it very much.

Thanks,
Ron

Ron,

What is present method of powering motor?

If I was going to skin this cat this is how I would do it.

Build/buy pwm controller. A simple sawtooth generator and comparator
would do. Get hold of a 115v power transformer that will get you
about 15v when rectified and filtered. Use a mosfet to pulse the
motor - will need flywheel diode for when mosfet off. Use a small
current transformer on AC side or dc shunt resistor to sense motor
current and modify speed command with current feedback signal. This
is called IR compensation. When torque goes up, current fb signal
adds to command signal and motor speed will remain (almost) fixed.

Parts probably less than 50 bucks. A neat little project. You can
Email if any questions.

 

[Paul Burridge]

The lathe motor is most likely a permanent magnet type where speed is
roughly proportional to voltage.

The Pulse Width Modulation thing is one inexpensive way to control the
effective voltage, but you don't need to get involved with all the headaches
of making your own PWM controller.

If you buy a variable voltage DC power supply, you can simply turn the
voltage knob and control your speed.

Nope. The motor will simply stall at low RPMs, even with next to no
load applied to it. PWM's the way to go. It's not hard and it may
actually be a perfectly good solution *if* as someone else has
suggested on this thread, you upgrade the lathe's motor to compensate
for the lack of torque you get without a gearbox. That'd be my
preferred option. It's only a hobby lathe so it isn't going to be
difficult or expensive to find a more powerful motor for it.

 

[Active8]

You're right. Unless a tachometer feedback were added to the speed
control system, torque would drop off as the speed went down which is exactly
what you *don't* want to happen.

That motor, 12 volts at 4 amps, is only 48 watts. If I needed speed
control, I'd seriously consider finding a small 120 volt AC universal AC
(brushed) motor to replace the DC one. The motor in something like a blender or
hand drill (garage sale $1.00) would probably be ideal. Then I'd use an SCR
speed controller to vary the speed. Many blenders and drills already have that
type of speed control that compensates for changing loads and will maintain
almost constant speed built in.

Jim

Half-wave SCR speed controls and light dimmers are frowned upon by
utility companys. I don't even think they're allowed in either US,
UK, or both. They only pass current in one direction. Might not be
a significant load but if everyone were using the stuff...

Mike

 

[John Woodgate]

I read in sci.electronics.design that Active8 <mTHISREMOVEcolasono@earth

Half-wave SCR speed controls and light dimmers are frowned upon by
utility companys. I don't even think they're allowed in either US, UK,
or both. They only pass current in one direction. Might not be a
significant load but if everyone were using the stuff...

They are not allowed in Europe, except for small kitchen machines such
as blenders. They are allowed for two and a half reasons:

- the half-wave speed control circuit with feedback has valuable
performance advantages;

- these machines are normally on for only a few minutes a day;

- in Continental Europe, there are fitted with the 2-pin Europlug or the
3-contact Schuko plug, both of which are reversible, and on average half
of them will be poled one way and the other half poled the other way, so
the net DC in the system is very low.

In Britain, they are fitted with 3-pin non-reversible plugs, but the DC
is still considered tolerable.

I've not heard that they are not allowed in USA.

 

[Active8]

I read in sci.electronics.design that Active8 <mTHISREMOVEcolasono@earth


They are not allowed in Europe, except for small kitchen machines such
as blenders. They are allowed for two and a half reasons:

- the half-wave speed control circuit with feedback has valuable
performance advantages;

over what? if you would, please.

- these machines are normally on for only a few minutes a day;

- in Continental Europe, there are fitted with the 2-pin Europlug or the
3-contact Schuko plug, both of which are reversible, and on average half
of them will be poled one way and the other half poled the other way, so
the net DC in the system is very low.

In Britain, they are fitted with 3-pin non-reversible plugs, but the DC
is still considered tolerable.

I've not heard that they are not allowed in USA.

Ok. Then you've answered that question for me. What I read said
that they were not allowed in one country and soon to be not
allowed in the other. "coming soon to a police state near you."

Maybe the controls for higher loads like motors are not allowed or
maybe the whole article was badly worded.

Mike

 

[John Woodgate]

I read in sci.electronics.design that Active8 <mTHISREMOVEcolasono@earth
link.net> wrote (in <[email protected]>
) about 'electric motor control questions', on Sun, 14 Dec 2003:

over what? if you would, please.

Full-wave phase control, for example, which is what our electricity
supply industry colleagues on the IEC working group initially wanted to
substitute. But when the circuit was analysed by one of our 'tame'
(definitely not so tame!) academics, they accepted that it was
unreasonable not to allow it to be used.

 

[Active8]

I read in sci.electronics.design that Active8 <mTHISREMOVEcolasono@earth
link.net> wrote (in <[email protected]>
) about 'electric motor control questions', on Sun, 14 Dec 2003:



Full-wave phase control, for example, which is what our electricity
supply industry colleagues on the IEC working group initially wanted to
substitute. But when the circuit was analysed by one of our 'tame'
(definitely not so tame!) academics, they accepted that it was
unreasonable not to allow it to be used.

and when I analyze this, what performance advantage should I be
looking for, I mean, is there any specific topology? Does it apply
to inductive *and* non-inductive loads? Stuff like that.

Mike

 

[Ron Irwin]

Thank you all very much for your responses. These are exactly the
answers I was looking for. I think I will go the pwm route. I will
let you know how it works.

Thanks again, Happy holidays,
Ron

 

[John Woodgate]

I read in sci.electronics.design that Active8 <mTHISREMOVEcolasono@earth

and when I analyze this, what performance advantage should I be looking
for, I mean, is there any specific topology?

Yes, but I don't have the circuit diagram any more.

Does it apply to inductive
*and* non-inductive loads? Stuff like that.

It applies to small AC commutator motors such as are used in kitchen
machines. There is a feedback mechanism that increases the conduction
angle if the motor slows under load. The IEC work was done many years
ago and I don't have the details any more.

 

[Active8]

I read in sci.electronics.design that Active8 <mTHISREMOVEcolasono@earth


Yes, but I don't have the circuit diagram any more.


It applies to small AC commutator motors such as are used in kitchen
machines. There is a feedback mechanism that increases the conduction
angle if the motor slows under load. The IEC work was done many years
ago and I don't have the details any more.

Thanks, that's enough of a clue.

Mike

 

[Frithiof Andreas Jensen]

speed controller to vary the speed. Many blenders and drills already have that
type of speed control that compensates for changing loads and will maintain
almost constant speed built in.

The Braun Multimix 700 4-in-1 has a DC motor controlled by a proper PWM
controller - no crappy half-wave SCR here!

I know because a bit of plastic broke off the old one, preventing it from
switching on, so I *had to* take it apart - find out that it is unfixable
i.e. unglueable plastic and buy a new one. The motor/gear asssembly is
good - runs very quiet - I shall have to find a use for it ;-)

 

[Active8]

The Braun Multimix 700 4-in-1 has a DC motor controlled by a proper PWM
controller - no crappy half-wave SCR here!

I know because a bit of plastic broke off the old one, preventing it from
switching on, so I *had to* take it apart - find out that it is unfixable
i.e. unglueable plastic and buy a new one. The motor/gear asssembly is
good - runs very quiet - I shall have to find a use for it ;-)

Shave the budget?

Mike