# Flyback Diode

M

#### Michael

Jan 1, 1970
0
Hi,

How do you go about choosing a suitable flyback diode for a 1.8kW 36V motor?
Does it have to be able to handle above the motor's stall current of 50A?

Cheers,

Michael

J

#### John Popelish

Jan 1, 1970
0
Michael said:
Hi,

How do you go about choosing a suitable flyback diode for a 1.8kW 36V motor?
Does it have to be able to handle above the motor's stall current of 50A?

You might get by with a smaller diode, but you would have to
heat sink it very well to cover the worst case situation.
At stall, something approaching 90% of motor current will
pass through the diode.
You could parallel the two halves of this one for less than
$6 from Digikey: http://www.irf.com/product-info/datasheets/data/60ctq045pbf.pdf This one costs twice as much but has more margin (both voltage and current): http://www.st.com/stonline/products/literature/ds/6728/stps80l60c.pdf M #### Michael Jan 1, 1970 0 John Popelish said: You might get by with a smaller diode, but you would have to heat sink it very well to cover the worst case situation. At stall, something approaching 90% of motor current will pass through the diode. You could parallel the two halves of this one for less than$6 from
Digikey:
http://www.irf.com/product-info/datasheets/data/60ctq045pbf.pdf

This one costs twice as much but has more margin (both voltage and
current):
http://www.st.com/stonline/products/literature/ds/6728/stps80l60c.pdf

Thanks John,

I'm actually in the UK and it seems Farnell don't sell that one, however I
used those spec's as a guide and would I be ok using this one:
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=1080069

I realise the lack of datasheet isn't helpful, but it seems to 'fit the
bill'....?

Michael

J

#### John Popelish

Jan 1, 1970
0
Michael said:
(snip)
I'm actually in the UK and it seems Farnell don't sell that one, however I
used those spec's as a guide and would I be ok using this one:
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=1080069

I realise the lack of datasheet isn't helpful, but it seems to 'fit the
bill'....?

It looks very similar, except that it is packaged in a
larger, fully insulated case, that is convenient to heat sink.

This data sheet may be pretty close:
http://ixdev.ixys.com/DataSheet/24b50478-97e5-46a7-95a3-2d9967eed46c.pdf

My only concern would be that the diode would have to handle
the voltage applied to the batteries at full charge. But if
you go with a higher voltage diode for a larger safety
factor, there, you will probably have to put up with higher
forward voltage drop, also.

P

#### Phil Allison

Jan 1, 1970
0
"John Popelish"
You might get by with a smaller diode, but you would have to heat sink it
very well to cover the worst case situation. At stall, something
approaching 90% of motor current will pass through the diode.

** Define the term " at stall " - John !!!!!!

Remember - Ambiguity is ERROR !!!

......... Phil

P

#### Phil Allison

Jan 1, 1970
0
"John Popelish"
Stall (as I am using it here) is motor standing still while being driven
at full rated current.

** That is an ambiguous definition.

The "stall current " of a motor is the current drawn at the voltage supplied
with the rotor held - it is typically a very large number and the motor
will not sustain it for more than a few seconds.

The OP has falsely equated " motor's stall current " with rated, full load
current ( ie 1800 /36 = 50) .

Maybe his PWM controller has a built in current limit of 50 amps average -
if so he needs to say that.

Otherwise confusion reigns.

........ Phil

J

#### John Popelish

Jan 1, 1970
0
Phil said:
"John Popelish"

** Define the term " at stall " - John !!!!!!

Remember - Ambiguity is ERROR !!!

Stall (as I am using it here) is motor standing still while
being driven at full rated current.

M

#### Michael

Jan 1, 1970
0
Phil Allison said:
"John Popelish"

** That is an ambiguous definition.

The "stall current " of a motor is the current drawn at the voltage
supplied with the rotor held - it is typically a very large number and
the motor will not sustain it for more than a few seconds.

The OP has falsely equated " motor's stall current " with rated, full load
current ( ie 1800 /36 = 50) .

Maybe his PWM controller has a built in current limit of 50 amps average -
if so he needs to say that.

Otherwise confusion reigns.

....... Phil

I was told by an electrical engineer that when the nameplate on the motor
states "Voltage 36V Current 50A" this is the stall current.

Michael

M

#### Michael

Jan 1, 1970
0
John Popelish said:
It looks very similar, except that it is packaged in a larger, fully
insulated case, that is convenient to heat sink.

This data sheet may be pretty close:
http://ixdev.ixys.com/DataSheet/24b50478-97e5-46a7-95a3-2d9967eed46c.pdf

My only concern would be that the diode would have to handle the voltage
applied to the batteries at full charge. But if you go with a higher
voltage diode for a larger safety factor, there, you will probably have to
put up with higher forward voltage drop, also.

Cheers John,

Michael

P

#### Phil Allison

Jan 1, 1970
0
"Michael".
I was told by an electrical engineer that when the nameplate on the motor
states "Voltage 36V Current 50A" this is the stall current.

** LOL !!

NEVER believe anything some dumb as dog shit sparky tells you !!

That 50 amp figure is the max running current the motor can sustain - needs
to be spinning fast to cool itself too.

........ Phil

J

#### John Popelish

Jan 1, 1970
0
Michael said:
I was told by an electrical engineer that when the nameplate on the motor
states "Voltage 36V Current 50A" this is the stall current.

I think those ratings would be the normal load, full speed
voltage and current ratings.

Motors driven by constant voltage (at name plate voltage)
sources have stall currents that may be 8 to 10 times rated
current, which would not be a safe stall current for very
long, and may even degauss a permanent magnet motor's
magnets, almost instantaneously.

But if you are driving the motor with a PWM drive, it may
have a built in current limit function that drops the
average voltage to the motor to limit the current if it
exceeds some settable value, and a good current to set that
limit might be the full rated current.

Do you know if your PWM drive includes a current limit?

M

#### Michael

Jan 1, 1970
0
John Popelish said:
I think those ratings would be the normal load, full speed voltage and
current ratings.

Motors driven by constant voltage (at name plate voltage) sources have
stall currents that may be 8 to 10 times rated current, which would not be
a safe stall current for very long, and may even degauss a permanent
magnet motor's magnets, almost instantaneously.

But if you are driving the motor with a PWM drive, it may have a built in
current limit function that drops the average voltage to the motor to
limit the current if it exceeds some settable value, and a good current to
set that limit might be the full rated current.

Do you know if your PWM drive includes a current limit?

Could you please explain what you mean by 'normal load'? I would have
thought that could be just about anything......

No it doesn't - I'm building the drive

Michael

J

#### John Popelish

Jan 1, 1970
0
Michael said:

I am referring to the full rated load (the torque the motor
can produce at name plate current) assuming that it is
getting enough cooling to keep it at a safe temperature.
The torque a permanent magnet or shunt wound motor produces
is proportional (roughly) to the armature current.
I would have
thought that could be just about anything......

The normal range is between no load current (the current it
takes to spin the motor with no external load) and full
rated current that drives full rated torque.
No it doesn't - I'm building the drive

Lets hope you decide to add this feature. It protects not
only the motor, but the PWM components, battery and wiring,
as well. The current sense might be based on the voltage
drop across a current shunt (a low value series resistor) in
series with the motor, or some magnetic field sensing
mechanism (like a hall effect sensor) that reacts to the
magnetic field around a motor lead, produced by the motor
current.

M

#### Michael

Jan 1, 1970
0
John Popelish said:
I am referring to the full rated load (the torque the motor can produce at
name plate current) assuming that it is getting enough cooling to keep it
at a safe temperature. The torque a permanent magnet or shunt wound motor
produces is proportional (roughly) to the armature current.

The normal range is between no load current (the current it takes to spin
the motor with no external load) and full rated current that drives full
rated torque.

Lets hope you decide to add this feature. It protects not only the motor,
but the PWM components, battery and wiring, as well. The current sense
might be based on the voltage drop across a current shunt (a low value
series resistor) in series with the motor, or some magnetic field sensing
mechanism (like a hall effect sensor) that reacts to the magnetic field
around a motor lead, produced by the motor current.

Thanks John,

I'm thinking about using a shunt resistor, with a voltage divider either
side fed into a 741 setup as a diff amp. Would that cause any problems?

Cheers,

Michael

J

#### John Popelish

Jan 1, 1970
0
Michael said:
Thanks John,

I'm thinking about using a shunt resistor, with a voltage divider either
side fed into a 741 setup as a diff amp. Would that cause any problems?
If you use a shunt resistor, the full scale signal voltage
is small, to begin with, just to keep the resistor power
waste down, so adding a pair of dividers to that, to get the
signal within the common mode voltage range of the 741 is a
step backwards. I'm not saying it can't work, but it is a
hard climb.

An integrated high side sensor chip is handy for when the
shunt is in the positive supply side of the circuit. They
convert the small differential voltage to a current to
ground, that will produce a much larger, but proportional
voltage across a grounded resistor, for the current
controller to use as a process measurement.
Here is one example at random, that you can use to pull key
words from, to look for more:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/746/

Isolated Hall effect current sensors are even more
convenient, since that don't involve the voltage of the
current carrying conductor, at all.
http://www.allegromicro.com/techpub2/current_sensing/bsp_v1_52.pdf

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