Transient suppression for bidirectional DC motor

[Franc Zabkar]

The short version: How does one suppress back emf switching
transients from a bidirectional DC motor?

Now for the full story.

I've been given an alternator regulator (B&C Specialty Products, model
LR3B-28) out of a home-built kit aircraft that occasionally trips its
circuit breaker, leaving the aircraft to run on its 24V battery. The
fault occurs when the landing gear is operated.

There are two protection devices in the regulator. The first is an
overvoltage detection circuit comprising a potential divider, voltage
comparator, and a 2.5V reference. The bottom end of the divider
connects to a tantalum cap which provides a 50ms delay (as stated by
the manufacturer). When an OV condition (32.50V) is detected, the
comparator activates a two-transistor latch which then turns on the
gate of a crowbar SCR (Teccor S6065K). This SCR trips the circuit
breaker.

The second protection device is a 1N6285A 33V 1500W Mosorb zener
transient voltage suppressor diode with a response time of less than
1ns. It is connected between supply and ground. I presume its function
is to clamp high energy transients and prevent dV/dt switching of the
SCR.

I don't know why the regulator trips the breaker, but I think the
solution involves suppressing the transients at the source. I don't
know much about the landing gear motor (I haven't even seen the
aircraft), but an auto electrician (and pilot) suggests that it could
draw 10A. AFAIK, the motor is bidirectional and is driven by two DPST
relays connected in a H-bridge configuration. I have a wiring diagram
for a 24V Taski floor scrubber that uses this same arrangement. It
shows an unidentified suppression device, drawn as two back-to-back
zeners, across the motor terminals. I'm guessing that an appropriately
rated Transzorb (eg 1N6284CA or 1N6285CA) would handle the transient.
Does anyone know of any better suppression devices or techniques? BTW,
all relays are fitted with snubber diodes across their coils.

Here are pictures of the regulator:
http://members.optusnet.com.au/~fzabkar/comp_side.jpg
http://members.optusnet.com.au/~fzabkar/solder_side.jpg

FWIW, version LR3C of this regulator replaces the SCR's 1K gate
resistor with a 10 ohm resistor. This modification has already been
done (by others).


- Franc Zabkar

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

The short version: How does one suppress back emf switching
transients from a bidirectional DC motor?

Now for the full story.

I've been given an alternator regulator (B&C Specialty Products, model
LR3B-28) out of a home-built kit aircraft that occasionally trips its
circuit breaker, leaving the aircraft to run on its 24V battery. The
fault occurs when the landing gear is operated.

There are two protection devices in the regulator. The first is an
overvoltage detection circuit comprising a potential divider, voltage
comparator, and a 2.5V reference. The bottom end of the divider
connects to a tantalum cap which provides a 50ms delay (as stated by
the manufacturer). When an OV condition (32.50V) is detected, the
comparator activates a two-transistor latch which then turns on the
gate of a crowbar SCR (Teccor S6065K). This SCR trips the circuit
breaker.

The second protection device is a 1N6285A 33V 1500W Mosorb zener
transient voltage suppressor diode with a response time of less than
1ns. It is connected between supply and ground. I presume its function
is to clamp high energy transients and prevent dV/dt switching of the
SCR.

I don't know why the regulator trips the breaker, but I think the
solution involves suppressing the transients at the source. I don't
know much about the landing gear motor (I haven't even seen the
aircraft), but an auto electrician (and pilot) suggests that it could
draw 10A. AFAIK, the motor is bidirectional and is driven by two DPST
relays connected in a H-bridge configuration. I have a wiring diagram
for a 24V Taski floor scrubber that uses this same arrangement. It
shows an unidentified suppression device, drawn as two back-to-back
zeners, across the motor terminals. I'm guessing that an appropriately
rated Transzorb (eg 1N6284CA or 1N6285CA) would handle the transient.
Does anyone know of any better suppression devices or techniques? BTW,
all relays are fitted with snubber diodes across their coils.

Here are pictures of the regulator:
http://members.optusnet.com.au/~fzabkar/comp_side.jpg
http://members.optusnet.com.au/~fzabkar/solder_side.jpg

FWIW, version LR3C of this regulator replaces the SCR's 1K gate
resistor with a 10 ohm resistor. This modification has already been
done (by others).

Those are decent pics of the PCB, easily seen. What kind of camera did
you use, or was it scanned in?

Sometimes the power wiring in autos (and other similar vehicles) gets
the currrent abruptly cut off, and the inductance causes transients and
ringing to subject anything connected to it to nasty spikes of energy.
I would try to do some low pass filtering with some RFI suppressor
sleeves and filter capacitors at the motor and relays. The snap-on
sleeves might be helpful, and are easily installed.

 

[Franc Zabkar]

Those are decent pics of the PCB, easily seen. What kind of camera did
you use, or was it scanned in?

I used a Kodak DX4530 5MP with macro.


- Franc Zabkar

 

[Peter A Forbes]

The short version: How does one suppress back emf switching
transients from a bidirectional DC motor?

- Franc Zabkar

Have you tried a decent sized Varistor? They are not polarity conscious and are
rated for AC or DC. Philips/BC Components do a good range and GE used to do
them, might be under a different name now.

Peter

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

I used a Kodak DX4530 5MP with macro.

You mean you have to purchase an additional macro adapter to get it to
do closeups? I have a Nikon 990 that does macro without any
attachments. Just curious. Like this one:
http://www.photoprojects.net/index7.html

Click on the macro light. I'm not sure why he added the closeup lens,
since the camera goes down to .8" or 20 mm by itself. If it gets any
closer it'll have to wear a condom! )

 

[Franc Zabkar]

You mean you have to purchase an additional macro adapter to get it to
do closeups?

No, the close-up capability is built-in. I'm just a point-and-shoot
guy, ie no gadgets.


- Franc Zabkar

 

[Franc Zabkar]

Have you tried a decent sized Varistor? They are not polarity conscious and are
rated for AC or DC. Philips/BC Components do a good range and GE used to do
them, might be under a different name now.

Peter

I had thought about MOVs, but for some reason I didn't feel
comfortable with this approach. However, I think this EPCOS varistor,
B72220S250K101, *may* be suitable. It is rated at 31Vdc, 39Vv(1mA),
26J, 2000A(peak).

I'll offer all three solutions (MOV, transzorb, RC suppression) to the
customer. Thanks to all for your input.


- Franc Zabkar