freewheel diode voltage rating for pm and shunt-wound motors?

Thread Starter

Hamlet

Joined Jun 10, 2015
252
I have a pump project that I am returning to, and after my PWM grinder project, I discovered
freewheel diodes and their importance. I killed a few of my controllers in past iterations, maybe
I can avoid that.


1) How do I select the voltage rating of the diode? Do the voltage spikes occur when
the motor is running? Are the spikes more than the psu voltage?

2) Amp rating?

3) Shall the freewheel diode be an ordinary diode, or a fast one? Is soft recovery worthwhile?

4) In a shunt motor, (the kind with rotor and field connected in parallel, not the universal type)
is there any special considerations for the freewheel diode?

The two motors I have been working with are 90v pm 1/4HP 2.5A,
and a 110v 1/3HP @ 3A shunt-wound, both 56c frame.

This is for a homestead, moving water to storage. I use a filter, but when it gets dirty,
the Amp draw goes high, so I intend to create some kind of feedback to slow the delivery
until the filter can be replaced. Anyway, the freewheel diodes are my starting point on this
project.
 

MaxHeadRoom

Joined Jul 18, 2013
19,053
What is the nature of the controller? PWM?
Those are not very large motors so I would not anticipate needing anything that large in the way of diode.
Max.
 

crutschow

Joined Mar 14, 2008
23,362
How do I select the voltage rating of the diode? Do the voltage spikes occur when
the motor is running? Are the spikes more than the psu voltage?
A Diode voltage rating of at least 150% of the supply voltage should be sufficient.
Amp rating?
Diode amp rating should be at least equal to the maximum motor current.
Shall the freewheel diode be an ordinary diode, or a fast one? Is soft recovery worthwhile?
An ordinary diode is fine. Fast diodes just turn off faster and that's not needed for this application.
In a shunt motor, (the kind with rotor and field connected in parallel, not the universal type) is there any special considerations for the freewheel diode?
Not that I'm aware of.
 

MaxHeadRoom

Joined Jul 18, 2013
19,053
3) Shall the freewheel diode be an ordinary diode, or a fast one? Is soft recovery worthwhile?
With the price of diodes there is not that much saving by going with a smaller or tight rated one.
How do you control the pump? Is there an opportunity to ramp the speed?
Are the fields connected by a steady DC supply?
What type of pump? because if a impeller style, the current would normally go down when the filter gets clogged.
Max.
 

Thread Starter

Hamlet

Joined Jun 10, 2015
252
With the price of diodes there is not that much saving by going with a smaller or tight rated one.
How do you control the pump? Is there an opportunity to ramp the speed?
Are the fields connected by a steady DC supply?
What type of pump? because if a impeller style, the current would normally go down when the filter gets clogged.
Max.
Pump will be float-switched for on/off operation. PWM for speed control. Perhaps a pressure sensor ahead of the filter, feedback
to the PWM...

I don't know if I should/need to PWM the field, probably not... that's a GOOD question!

Positive displacement vane pump, carbinator style. I have several.

I know diodes are cheap, I just want to know what I can and can't get away with in the future.
 

MaxHeadRoom

Joined Jul 18, 2013
19,053
You normally would Not PWM the field.
The field current should be set for around maximum torque required without jeopardizing the maximum rpm required. Lower field voltage= higher rpm at the expense of torque.
Max.
Correction lower field voltage!
 
Last edited:

Thread Starter

Hamlet

Joined Jun 10, 2015
252
You normally would Not PWM the field.
The field current should be set for around maximum torque required without jeopardizing the maximum rpm required. Lower field voltage= higher rpm at the expense of torque.
Max.
Correction lower field voltage!
Okay, NO to PWM on the Field. Got it.
Will a shunt-wound motor use more/less/same energy as a PWM for the same work load?
 

MaxHeadRoom

Joined Jul 18, 2013
19,053
Shunt wound just describes the nature of the field, PWM just describes the type of speed control.
Shunt field motor at a fixed field voltage varies little from a P.M. field DC motor.
Max.
 
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