Triac motor control question

[Richard Rasker]

I'm pondering the use of triac speed control for the motor in a lifting
application, as it's smaller and cheaper than a three-phase PWM driven
motor plus controller.
The motor will be a reversible brush type with a capacitor to generate the
required extra phase -- IOW: the type used in washing machines, drills
etcetera.

However, the suppliers of motors and controllers appear to have a strong
bias towards three-phase motors and PWM controllers -- because the latter
offer more control (and are probably better for sales too).

Apart from this though, I have one point of doubt myself about triac
control: since it's a lifting application, there is the question of
braking; when lowering a load, the motor hardly needs any drive at all --
it only has to overcome the friction from the reduction gearbox. BTW: the
friction is enough to stop the motor when power is cut off, even at maximum
load.

Does anyone know where I can find more information on this latter problem?
And are there simple ways of electrically braking the motor in these cases?

Thanks in advance,

Richard Rasker

 

[PeterD]

I'm pondering the use of triac speed control for the motor in a lifting
application,

You may want to read some of the items in http://www.darwinawards.com/
if you are building a lifting type application.

as it's smaller and cheaper than a three-phase PWM driven
motor plus controller.
The motor will be a reversible brush type with a capacitor to generate the
required extra phase -- IOW: the type used in washing machines, drills
etcetera.

The type of motor used in a washing machine would NOT be a brush type
motor. A drill motor doesn't use polyphase, it is a straight universal
type motor.

However, the suppliers of motors and controllers appear to have a strong
bias towards three-phase motors and PWM controllers -- because the latter
offer more control (and are probably better for sales too).
Nonsequitor.


Apart from this though, I have one point of doubt myself about triac
control: since it's a lifting application, there is the question of
braking; when lowering a load, the motor hardly needs any drive at all --
it only has to overcome the friction from the reduction gearbox. BTW: the
friction is enough to stop the motor when power is cut off, even at maximum
load.

Please post 'after-failure' pictures. The more blood the better.

 

[baron]

PeterD Inscribed thus:

You may want to read some of the items in http://www.darwinawards.com/
if you are building a lifting type application.


The type of motor used in a washing machine would NOT be a brush type
motor. A drill motor doesn't use polyphase, it is a straight universal
type motor.

Not really relevant to the OP, but brush commutated motors are common in
domestic washing machines. Hoover, Philips, Hotpoint just to name a
few. As are electronic speed control modules.

Indeed Hoover made the motors for the "Sinclair C5"

Nonsequitor.

Probably the difference between domestic and commercial motor
applications.

Purely from a safety point of view you need a mechanical brake mechanism
that will stop things dead, particularly if there is a power failure.

 

[petrus bitbyter]

I'm pondering the use of triac speed control for the motor in a lifting
application, as it's smaller and cheaper than a three-phase PWM driven
motor plus controller.
The motor will be a reversible brush type with a capacitor to generate the
required extra phase -- IOW: the type used in washing machines, drills
etcetera.

However, the suppliers of motors and controllers appear to have a strong
bias towards three-phase motors and PWM controllers -- because the latter
offer more control (and are probably better for sales too).

Apart from this though, I have one point of doubt myself about triac
control: since it's a lifting application, there is the question of
braking; when lowering a load, the motor hardly needs any drive at all --
it only has to overcome the friction from the reduction gearbox. BTW: the
friction is enough to stop the motor when power is cut off, even at
maximum
load.

Does anyone know where I can find more information on this latter problem?
And are there simple ways of electrically braking the motor in these
cases?

Thanks in advance,

Richard Rasker

The washing machine motors that I'm aware of are brushless types (cage
induction motors) that use a capacitor to provide the extra phase. These
motors cannot safely and reliably be controlled by triac circuits using
phase proportioning. Their speed is related to the frequency of the
AC-current rather then the voltage.

Drill motors are often controlled by triacs allthough the electronics use
feedback to maintain the speed set when under load.

Lifts tend to use counterweights to keep the load more or less equal for up
and down.

petrus bitbyter

 

[Richard Rasker]

I am aware of the dangers and official regulations in this particular type
of application.

You may want to read some of the items in http://www.darwinawards.com/
if you are building a lifting type application.


The type of motor used in a washing machine would NOT be a brush type
motor.

I've been doing electronics design and repair work for some 30 years now,
and all washing machines I've seen in the past fifteen years or so have a
brush-motor, with a tacho encoder on the axle and a triac-based speed
control circuit. These are front-loaded washing machines, with a horizontal
drum and a single bearing at the back. The motor is responsible both for
the washing action and the dry spin cycle.

A drill motor doesn't use polyphase, it is a straight universal
type motor.


Please post 'after-failure' pictures. The more blood the better.

All motors selected so far incorporate a mechanical brake, which engages as
soon as the power to the motor is cut off.

Richard Rasker

 

[Richard Rasker]

Caution! Is it "friction" or is there an internal brake that comes on
when there is no power?

There is a separate mechanical brake that is released when the motor
receives power. When the power is cut, this brake engages.

Richard Rasker

 

[Richard Rasker]

The washing machine motors that I'm aware of are brushless types (cage
induction motors) that use a capacitor to provide the extra phase.

Interesting -- all washing machine motors I encountered in the past 15 years
are brush type (and I had to replace worn out brushes on quite a few
occasions) with a triac-based speed regulator (and yes, I have replaced
those triacs on a few occasions as well).

These motors cannot safely and reliably be controlled by triac circuits
using phase proportioning. Their speed is related to the frequency of the
AC-current rather then the voltage.

Drill motors are often controlled by triacs allthough the electronics use
feedback to maintain the speed set when under load.

Lifts tend to use counterweights to keep the load more or less equal for
up and down.

The application of this lift system makes the use of counterweights
impossible (I can't elaborate further on this). Most of the mechanical
system has already been designed, but we're still looking for a suitable
motor + speed controller.


Richard Rasker

 

[petrus bitbyter]

Interesting -- all washing machine motors I encountered in the past 15
years
are brush type (and I had to replace worn out brushes on quite a few
occasions) with a triac-based speed regulator (and yes, I have replaced
those triacs on a few occasions as well).


The application of this lift system makes the use of counterweights
impossible (I can't elaborate further on this). Most of the mechanical
system has already been designed, but we're still looking for a suitable
motor + speed controller.


Richard Rasker

Hmm... 15 years. Makes me realize that time flies. It has been quit some
years that I replaced my last washing machine motor. Which was a slow
turning one that has a separate spin drier. Guess newer, multi speed
machines use the motors you mentioned.

What power do you need? Can't you get either an obsolete washing machine or
a (not too expensive) drill? Both with control electronics connected
already?

petrus bitbyter

 

[David Lesher]

I've been doing electronics design and repair work for some 30 years now,
and all washing machines I've seen in the past fifteen years or so have a
brush-motor, with a tacho encoder on the axle and a triac-based speed
control circuit. These are front-loaded washing machines, with a horizontal
drum and a single bearing at the back. The motor is responsible both for
the washing action and the dry spin cycle.

These are uncommon in the US. At least over the past 30-40 years, the
vast majority of washing machines here are top loaders with a vertical
shaft. [Front loaders are again appearing on sales floors.]

Some machines use transmissions, others [Maytag] have simple belt drive
and reverse the motor to get different functions.

 

[Jamie]

I'm pondering the use of triac speed control for the motor in a lifting
application, as it's smaller and cheaper than a three-phase PWM driven
motor plus controller.
The motor will be a reversible brush type with a capacitor to generate the
required extra phase -- IOW: the type used in washing machines, drills
etcetera.

However, the suppliers of motors and controllers appear to have a strong
bias towards three-phase motors and PWM controllers -- because the latter
offer more control (and are probably better for sales too).

Apart from this though, I have one point of doubt myself about triac
control: since it's a lifting application, there is the question of
braking; when lowering a load, the motor hardly needs any drive at all --
it only has to overcome the friction from the reduction gearbox. BTW: the
friction is enough to stop the motor when power is cut off, even at maximum
load.

Does anyone know where I can find more information on this latter problem?
And are there simple ways of electrically braking the motor in these cases?

Thanks in advance,

Richard Rasker

I don't understand?

You state that the gear box is holding it back for you at full load
with no power. So why do you need a brake?

Maybe what you should be using is a Helical gear box. This will back
spin on the other side when motor isn't driving up. This may not
be applicable in your situation how ever. You may actually need that
load locking you're achieving now in case the power is lost.
I guess you could have some kind of mechanical locking pin to solve
too.

For braking to reduce down acceleration, you can use DB (dynamic
Braking) resistors that apply some DC current in the motor windings
which will cause the motor to create a drag on the drive shaft.

Normally, DB control is applied at full motor rated current and some
times higher for short periods. For long periods, 50% of the motors
rated current is normally ok.

You can also keep the motor in the forward position with a slight up
drive on it, using a Helical gear box to slowly let it down..

But then again, there's that possibility of losing control! (power)

I guess if you're not in a dangerous situation, it wouldn't matter.

 

[Phil Allison]

"Richard Rasker"

I'm pondering the use of triac speed control for the motor in a lifting
application, as it's smaller and cheaper than a three-phase PWM driven
motor plus controller.
The motor will be a reversible brush type with a capacitor to generate the
required extra phase

** ????
Brush motors use DC or single phase AC.

Reversing is by internal switching of the stator coils.

Apart from this though, I have one point of doubt myself about triac
control: since it's a lifting application, there is the question of
braking; when lowering a load, the motor hardly needs any drive at all --
it only has to overcome the friction from the reduction gearbox. BTW: the
friction is enough to stop the motor when power is cut off, even at
maximum
load.

Does anyone know where I can find more information on this latter problem?
And are there simple ways of electrically braking the motor in these
cases?

** AC motors can be held stationery by applying DC - but the motor you
have operates from DC or AC, so no luck.

A spinning brush motor will stop quickly if the supply terminals are
horted - but to hold it steady against an external force requires use of
reverse drive.

If you want good speed control over a brush motor, using SCR drive ( ie half
wave power with back emf sensing) is the way to go.


...... Phil

 

[ehsjr]

I'm pondering the use of triac speed control for the motor in a lifting
application, as it's smaller and cheaper than a three-phase PWM driven
motor plus controller.
The motor will be a reversible brush type with a capacitor to generate the
required extra phase -- IOW: the type used in washing machines, drills
etcetera.

However, the suppliers of motors and controllers appear to have a strong
bias towards three-phase motors and PWM controllers -- because the latter
offer more control

Understandable. You can drive the thing backwards between forward
pulses for braking while the load is going down, control either
torque or speed, change direction, sense current, RS232 control,
yada yada yada with a good PWM controller & appropriate motor.

(and are probably better for sales too).

I've no personal knowledge of the sales side, but it seems probable.

Ed