Re: How to eliminate the DC motor back emf?

From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 03/04/05 

Date: Thu, 03 Mar 2005 19:58:20 -0800

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. 

-- 
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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