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Tedi
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Join Date: Mar 2009
Posts: 35

Add Rectifier to AC Brushed Motors

09/27/2009 11:05 AM

I have read some stuff on AC brushed motors, in fact they are DC motors but as the direction of spinning is undependable from the polarity they work with AC as well, just slightly worse.

I wonder why they don't add 2 diodes and a capacitor eventually to the circuit except for financial reasons.

I have a very faint memory of reading about some things done to the winding of these motors in order to make them better cope with AC but I can't remember...
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mjb1962853
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Engineering Fields - Electrical Engineering - Analog and Digital Circuit Design Engineering Fields - Electromechanical Engineering - Transformers, Motors & Drives, EM Launchers Engineering Fields - Engineering Physics - Applied Electrical, Optical, and Mechanical

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#1

Re: Add rectifier to AC brushed motors

09/27/2009 11:34 AM

You are referring to series wound (DC-brushed) motors designed to run off 120Vac. They will usually run fine on full-wave rectified line voltage (4 diodes). From what I've seen, there is typically a lot less sparking at the commutator with the rectified AC.

Cost is the major factor. Even though diodes are cheaper than ever, they still add cost. If there is any improvement in the motor efficiency by using the diodes, some of it will be lost in power dissipated in the diodes. Any net performance improvement gained is usually not worth the added cost.
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Anonymous Poster
#2
In reply to #1

4 diodes or 2 diodes?

09/27/2009 11:59 AM

how are you going to use 4 diodes without a transformer, 4 diodes are used with a transformer?
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mjb1962853
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#3
In reply to #2

Re: 4 diodes or 2 diodes?

09/27/2009 12:36 PM

Isolation, grounding, and safety are separate issues. Please don't try these experiments at home if you do not understand these safety issues.

IF the motor windings required a ground reference, a transformer would be necessary to isolate the diode/motor circuit from the AC mains.

A properly insulated motor could be run directly off the 120Vac and a 4 diode bridge rectifier.

http://en.wikipedia.org/wiki/Rectifier

A transformer would also be necessary if only 2 diodes were allowed.
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Anonymous Poster
#4
In reply to #3

Re: 4 diodes or 2 diodes?

09/27/2009 1:42 PM

I admit I'm not an electrician but I can't see a 4 diodes rectifier not using a transformer.

The only ones such I see are the voltage multipliers.

P.S. why the series wound DC motor(universal motor/brushed AC motor) I am talking about would want a ground reference?
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mjb1962853
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#6
In reply to #4

Re: 4 diodes or 2 diodes?

09/27/2009 7:22 PM

I have a vacuum cleaner motor assembly that has one end of the series winding attached to the motor frame. This motor/blower assembly was designed to have the frame connected to the AC neutral/ground with the one motor lead attached to the AC hot line via an ON/OFF switch.

This arrangement should be relatively "safe" since a winding breakdown to the motor frame or armature will cause a large current surge and blow a fuse or trip a breaker. Because one motor lead is grounded, you CANNOT use the 4 diode bridge without an isolation transformer.

Almost all of the other universal motors I have were designed with full winding isolation from the motor frame. The 4 diode bridge can be used with these motors since both ends of the windings are floating with reference to ground.

Safety Note: These motors are usually mounted in plastic appliances and the end-user is never able to touch any conducting part of the motor. If a part of the winding were to electrically contact the motor iron, the whole motor frame would jump to lethal voltage potentials and CONTINUE to operate as normal. Safe if isolated inside the plastic housing, but DANGEROUS to play with on the workbench!

Side Note: Two of the diodes can be changed to SCR's to build a wood Router speed controller. Commercial speed controllers use a single TRIAC or this SCR/diode bridge.
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Anonymous Poster
#10
In reply to #4

Re: 4 diodes or 2 diodes?

09/28/2009 3:53 AM

http://en.wikipedia.org/wiki/Bridge_rectifier
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Anonymous Poster
#9
In reply to #2

Re: 4 diodes or 2 diodes?

09/28/2009 3:50 AM

DOOOOOHHHHH ! = A bridge rectifier........
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APD
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Join Date: Sep 2009
Posts: 10
#5

Re: Add rectifier to AC brushed motors

09/27/2009 4:58 PM

They are universal motors and as such will run on AC or DC
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Haajee
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#7

Re: Add rectifier to AC brushed motors

09/28/2009 12:09 AM

Regards.

AC & DC Brush Motors look similar but those have the Major difference of their BRUSHs

in that for AC are of carbon but for DC have COPPER added to the brushmaterial & in some I have noticed marked as (+) & (-), but not sure why?

Comments !!!!
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SimpleMind
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#8

Re: Add rectifier to AC brushed motors

09/28/2009 12:49 AM

Another relative issue brushed motors DC driven tend to unevenly eat up negative pole brushes faster than positive (due to more heat released on neg brush) so non revesible motors use bigger negative brush, and for reversible motors all must be oversized (comparing to AC that have balanced wear)

Also for a little better performance and less sparking a slight adjustment can be made in brush to stator angle depending on CW or CCW run.

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Haajee
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#11
In reply to #8

Re: Add rectifier to AC brushed motors

09/28/2009 5:13 AM

A GA for refined reply.
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perry
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Join Date: Dec 2006
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#12

Re: Add Rectifier to AC Brushed Motors

09/28/2009 9:40 AM

tedi; there are permanent magnet DC motors that use a full wave diode bridge to change the AC to DC to run the motor, by reversing the DC output of the bridge this will reverse motor direction. there are AC,DC universal motors, 110, or 220 volt that need the field winding reverse to make the motor run in different direction, a DPDT switch would work.perry
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Tedi
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Join Date: Mar 2009
Posts: 35
#13

Re: Add Rectifier to AC Brushed Motors

09/28/2009 10:28 AM

APD if it was such piece of cake there wouldn't be the travesty of railway 16⅔ Hz
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Users who posted comments:

Anonymous Poster (4); APD (1); Haajee (2); mjb1962853 (3); perry (1); SimpleMind (1); Tedi (1)

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