How to eliminate the DC motor back emf?

  • Thread starter Electronic Swear
  • Start date
E

Electronic Swear

Guest
A DC motor on starting and stopping, the back emf is very large even
it is in a very short period.

Motor supply is from a rectified 220Vac@50Hz without filtering
capacitor.
I would like to know, is there any filter (R,C) to eliminate this back
EMF?
If i place a mono-capacitor in between two motor terminal, will the
result is become better and reduce some back emf from motor?

If so, what capacitance is needed?

Thanks~
 
Electronic Swear wrote:
A DC motor on starting and stopping, the back emf is very large even
it is in a very short period.
Click to expand...
Perhaps you're confusing the inductive voltage spike with back-EMF?
Back EMF occurs because a DC motor acts like a generator, so when it's
turning it generates a voltage that reduces the drop across the windings
and hence the motor current. Or perhaps you're talking about the radio
frequency energy generated by the commutator and brushes?

Inductive kickback occurs with any coil, whether it's within a mile of a
DC motor or inside.
Motor supply is from a rectified 220Vac@50Hz without filtering
capacitor.
I would like to know, is there any filter (R,C) to eliminate this back
EMF?
If i place a mono-capacitor in between two motor terminal, will the
result is become better and reduce some back emf from motor?
Click to expand...
Do you mean a single capacitor? A large cap in parallel with the motor
terminals will reduce the inductive spike (_not_ the back EMF) when you
turn off the motor, but would consume a great deal of current on
startup. Smaller caps, however, suppress RF quite nicely.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
Tim Wescott <tim@wescottnospamdesign.com> wrote in message news:<112fn6o1006vpc0@corp.supernews.com>...
Electronic Swear wrote:
A DC motor on starting and stopping, the back emf is very large even
it is in a very short period.

Perhaps you're confusing the inductive voltage spike with back-EMF?
Back EMF occurs because a DC motor acts like a generator, so when it's
turning it generates a voltage that reduces the drop across the windings
and hence the motor current. Or perhaps you're talking about the radio
frequency energy generated by the commutator and brushes?

Inductive kickback occurs with any coil, whether it's within a mile of a
DC motor or inside.

Motor supply is from a rectified 220Vac@50Hz without filtering
capacitor.
I would like to know, is there any filter (R,C) to eliminate this back
EMF?
If i place a mono-capacitor in between two motor terminal, will the
result is become better and reduce some back emf from motor?

Do you mean a single capacitor? A large cap in parallel with the motor
terminals will reduce the inductive spike (_not_ the back EMF) when you
turn off the motor, but would consume a great deal of current on
startup. Smaller caps, however, suppress RF quite nicely.
Click to expand...

When I see the motor signal in CRO, they show out this tpe of waveform:



<-----high voltage
.....*.......................
.....*.......................
.....*.......................
.....*.......................
.....**.......................
.....**.....************....
..*...*.. *................
****...*..*.................
........ **..................
.........*...................

I don't know the peak of the instant voltage come from. It is from the back emf?
Or from other reason. I want to reduce or elminiated of that voltage.
Thanks~
 
Hi,

On Fri, 04 Mar 2005 00:18:10 -0800, Electronic Swear wrote:
When I see the motor signal in CRO, they show out this tpe of waveform:



-----high voltage
....*.......................
....*.......................
....*.......................
....*.......................
....**.......................
....**.....************....
..*...*.. *................
****...*..*.................
....... **..................
........*...................

I don't know the peak of the instant voltage come from. It is from the
back emf? Or from other reason. I want to reduce or elminiated of that
voltage. Thanks~
Click to expand...
The motor has a fairly large inductance. Inductors have the following
v~i characteristic:

V = L * (dI/dt)

In words:

"The voltage across the terminals of an inductor is
proportional to the rate at which the current through
the inductor changes".

When you turn the IGBT off you rapidly change the current through
the inductance of the motor. This makes dI/dt high, and you see a large V
across the motor terminals.

A common solution is to connect a diode across the motor terminals which
is reverse biased when the IGBT is on. The diode creates a path for the
inductor current when the IGBT is off. Diodes used in this way are often
called "flyback" or "flywheel" diodes.

Regards,
Alan


--
Alan R. Turner | Live never to be ashamed of anything you do or say.
To reply by email, remove Mr Blobby.
 
In article <910532f6.0503040018.54ecabd4@posting.google.com>,
Electronic Swear <swear_terence@hotmail.com> wrote:
[...]
When I see the motor signal in CRO, they show out this tpe of waveform:



-----high voltage
....*.......................
....*.......................
....*.......................
....*.......................
....**.......................
....**.....************....
..*...*.. *................
****...*..*.................
....... **..................
........*...................

I don't know the peak of the instant voltage come from. It is from the back emf?
Or from other reason. I want to reduce or elminiated of that voltage.
Thanks~
Click to expand...

That spike is due to the inductance of the motor windings. If this is a
moderately large motor, there can be a fairly large amount of energy
stored in the inductance. All this stored energy must be dealt with.

I assume your circuit looks like this:


V-------------
!
[Motor]
!
+------------- Scope
!
!-
!
!-
!
GND--------------

In the DC case, the easiest answer is to put a diode across the motor so
that the voltage on it can't reverse. In the AC case, you are going to
need a capacitor with prehaps a resistor in series.


If you just use a capacitor, the energy that ends up in the capacitor is
what was in the inductor so:


E = 1/2 * I^2 * L <- energy in inductor


E = 1/2 * V^2 * C <- energy in capacitor


1/2 * I^2 * L = 1/2 * V^2 * C

I^2 * L / V^2 = C <- so given a voltage limit we can find C




--
--
kensmith@rahul.net forging knowledge
 
Electronic Swear wrote:

Tim Wescott <tim@wescottnospamdesign.com> wrote in message news:<112fn6o1006vpc0@corp.supernews.com>...

Electronic Swear wrote:

A DC motor on starting and stopping, the back emf is very large even
it is in a very short period.

Perhaps you're confusing the inductive voltage spike with back-EMF?
Back EMF occurs because a DC motor acts like a generator, so when it's
turning it generates a voltage that reduces the drop across the windings
and hence the motor current. Or perhaps you're talking about the radio
frequency energy generated by the commutator and brushes?

Inductive kickback occurs with any coil, whether it's within a mile of a
DC motor or inside.

Motor supply is from a rectified 220Vac@50Hz without filtering
capacitor.
I would like to know, is there any filter (R,C) to eliminate this back
EMF?
If i place a mono-capacitor in between two motor terminal, will the
result is become better and reduce some back emf from motor?

Do you mean a single capacitor? A large cap in parallel with the motor
terminals will reduce the inductive spike (_not_ the back EMF) when you
turn off the motor, but would consume a great deal of current on
startup. Smaller caps, however, suppress RF quite nicely.



When I see the motor signal in CRO, they show out this tpe of waveform:



-----high voltage
....*.......................
....*.......................
....*.......................
....*.......................
....**.......................
....**.....************....
..*...*.. *................
****...*..*.................
....... **..................
........*...................

I don't know the peak of the instant voltage come from. It is from the back emf?
Or from other reason. I want to reduce or elminiated of that voltage.
Thanks~
that looks like the effects of both the brushes and maybe flyback
Click to expand...
effects of non filtered rectifier between the zero crossing periods.
you may want to try a small cap coupled across the + & - terminals.
 
kensmith@green.rahul.net (Ken Smith) wrote in message news:<d09u02$qfh$3@blue.rahul.net>...
In article <910532f6.0503040018.54ecabd4@posting.google.com>,
Electronic Swear <swear_terence@hotmail.com> wrote:
[...]
When I see the motor signal in CRO, they show out this tpe of waveform:



-----high voltage
....*.......................
....*.......................
....*.......................
....*.......................
....**.......................
....**.....************....
..*...*.. *................
****...*..*.................
....... **..................
........*...................

I don't know the peak of the instant voltage come from. It is from the back emf?
Or from other reason. I want to reduce or elminiated of that voltage.
Thanks~


That spike is due to the inductance of the motor windings. If this is a
moderately large motor, there can be a fairly large amount of energy
stored in the inductance. All this stored energy must be dealt with.

I assume your circuit looks like this:


V-------------
!
[Motor]
!
+------------- Scope
!
!-
!
!-
!
GND--------------

In the DC case, the easiest answer is to put a diode across the motor so
that the voltage on it can't reverse. In the AC case, you are going to
need a capacitor with prehaps a resistor in series.


If you just use a capacitor, the energy that ends up in the capacitor is
what was in the inductor so:


E = 1/2 * I^2 * L <- energy in inductor


E = 1/2 * V^2 * C <- energy in capacitor


1/2 * I^2 * L = 1/2 * V^2 * C

I^2 * L / V^2 = C <- so given a voltage limit we can find C




--
Click to expand...


I have already added a reverse biasing diode at the terminals of the
motor.
The motor is a very high power DC motor, so it has high inductance. I
am using a relay to switch on motor. And for the input signal of the
motor, I use a triac to chop in different angle to control the speed.
Because the chopped signal is in AC, then i rectify it by a bridge and
use a filter capacitor to smooth it.

I just use a small capacitor (0.015uF 1000Vdc) on ripple filtering, I
don't know the capacitor is small or not. But I want to know if I
increase the filtering capacitance of the cap. will have a better
performance or not.

Or using a R-C filter before the motor input terminial to suppress the
large surge at start-up. I am afraid the R must in a very small value
with high power range in order to not reduce the performance of the
motor.

Any suggestion?
 
Electronic Swear wrote:

I have already added a reverse biasing diode at the terminals of the
motor.
The motor is a very high power DC motor, so it has high inductance. I
am using a relay to switch on motor. And for the input signal of the
motor, I use a triac to chop in different angle to control the speed.
Because the chopped signal is in AC, then i rectify it by a bridge and
use a filter capacitor to smooth it.

I just use a small capacitor (0.015uF 1000Vdc) on ripple filtering, I
don't know the capacitor is small or not. But I want to know if I
increase the filtering capacitance of the cap. will have a better
performance or not.

Or using a R-C filter before the motor input terminial to suppress the
large surge at start-up. I am afraid the R must in a very small value
with high power range in order to not reduce the performance of the
motor.

Any suggestion?
i can only assume you must be using a DC PM or shunt more ? or maybe
Click to expand...
even a compound/universal type ?
any ways. unless your reversing the direction, you should be able to
use a large cap.
the best thing to do is use both large electro caps for filtering
and low ESR & ESL non polorized cap coupled together.
also. using a reactor is a plus.
 
In article <910532f6.0503041735.5bd1b140@posting.google.com>,
Electronic Swear <swear_terence@hotmail.com> wrote:
[....]
Or using a R-C filter before the motor input terminial to suppress the
large surge at start-up. I am afraid the R must in a very small value
with high power range in order to not reduce the performance of the
motor.
Click to expand...
I think you can do an LC filter



From rectified SCRs ---+---)))))---------+---- Motor
! !
--- ---
^ ---
! !
----+-----------------+----


At low SCR conduction angles the current from the SCRs is a bunch of think
spikes. The motor isn't really intended to have those high frequencies on
it. The inductance needs to be just enough to keep the SCRs safe from
their di/dt rating and their max current with whatever capacitor you use.

The capacitor doesn't have to be all that huge to make a fairly major
impact. You don't need to spread a pulse out that much to really lower
its high frequency content.


--
--
kensmith@rahul.net forging knowledge
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top