Hello Group-



I have a requirement to design a bare bone, inexpensive 90 v 700 W reversable DC motor drive. I have attached the schematic showing the drive components. The prototype works well at speed, but occasionally blows R16 and or C15 at first power up. Ratings seems to be comfortably in range. Note that the diode D1 clamps reverse EMF and wind-down voltage when reversing the motor direction.



Can anyone see an issue here, or a way to improve the reliability of the design? Thanks so much for your input.

 

Putting ~320-Volts into a ~90-Volt-Motor is bound to create some excitement.

If You are trying to simply reduce that ~320-Volts down to ~90,
( and the Voltage was not just a typo ),
this will likely not work very well at all.
You need a Buck-Converter capable of
dealing with ~25-Amps or so,
and it won't be very efficient either,
when trying to drop somewhere around ~230-Volts.

With careful design and testing, You "might" be able to "tune-around" the
Inductance of the Motor a get some very questionable efficiency out of this setup.

R16 needs to be rated for at least ~20-Watts.

C15 seems to have no purpose,
in order for it to be of any use,
it would need a Resistor in front of it.
It would be a very good idea to replace C15 with a ~5-Watt-Zener-Diode
to protect your Current-Sensing-Circuitry.

Brushed-Motors have a bad-habit of putting-out tons of RF-Trash,
which can easily destroy your FET.
I would say RF-Filtering should be mandatory, and close to the Motor.

Your FET has around 1.4nF of Gate-Capacitance,
a ~2-Amp FET-Driver may not be adequate for fast, clean, Switching,
( depending upon the PWM-Frequency that You intend to use ).
( I have not calculated what the requirements might be ).

Not finding, ( and not using ), the optimum value of Gate-Resistance could lead to
Ringing, or Oscillations, during Switching.
This may, or may not, create problems, or generate excessive Heat in the FET.

If, for any reason, the PWM-Output of your Controller should stay "High" for more than
a few milliseconds, EVERYTHING WILL INSTANTLY SMOKE.
.
.
.

 

You don’t provide a complete schematic nor startup waveforms to give you an accurate assessment.
But one thing is crystal clear: whatever is driving the Mosfet driver is misbehaving at startup.
You will require to implement a power on reset and a soft start function.

EDIT: this in addition to what the previous post mentioned.

 

A PM motor will draw a lot more then it's rated current at switch on - perhaps 10 times as much. The circuit needs to be able to cope with that.
Also, if the motor is running and you simply throw S1 to reverse the direction neither the motor nor the circuit will be happy. You must remove power and wait for the motor to stop before ramping up the power again.

 

Agree with the above, the reversing SW should be Centre-OFF.
I assume this is a T.M. motor?
Take a leaf from those controllers, they ramp up very slowly, it's not desirable to go from 0 to 60 on a TM.
It does not show what is controlling the PWM?

 

Group:

Thank you so much for your thoughtful and quick replies. I appreciate the design review, no matter how brutal! I am including the entire schematic for the group's consideration.

I guess I should have already mentioned the constraints of the design. The dimensions and the cost budget requires a transformerless design running from 100 through 240 volts. The low voltage digital supply is sourced from a high voltage switching supply. If anyone has any other ideas about bringing the motor supply down from the rectified and filtered 340 volts at 240 AC input, I would appreciate it.

Yes! I know how how inefficient the design must look. I am driving the MOSFET into saturation so that it works at its maximum efficiency. The driving waveform at the input of the FET looks nice and square swinging between 0 and 12 volts. I am considering selecting a beefier gate driver to drive the high capacitance gate, However, if anyone has a suggestion as to an alternate FET, I'd welcome that input. The current FET runs very cool, temp rise is only 5 degrees C.

I have increased the power rating of R16 in the FET source to 20 Watts. The purpose of C15 is to smooth the sampled voltage to the buffer and micro A to D. Good suggestion to add zener and resistor to sampling circuit

The direction switch is center off and could require the user to dwell at center position before switching directions. Was hoping that D7 would have snubbed the reverse voltage, Looking for a reliable way to let the controller know that the switch has been flipped to allow for a delay and a soft-start. Would love to have the space and budget for a H-bridge!

Yes, as the group notes, excitement happens on power-up. Bringing input voltage up slowly-everything good. No issues and very little heat during steady state runs. Would love to know if anyone has a suggestion on "soft start.

Microcontroller controls the 1023 step PWM signal which should limit the motor voltage. R14 should hold gate of FET low during power up.

EMI filter is supplied at the motor terminals and common mode choke should reduce EMI out the line.

All thoughts are most welcome, THANKS AGAIN FOR THE DESIGN REVIEW

 

If the PWM is Micro controlled then it should be simple to provide a ramp up, just as the micro in a TM control does.
What I did recently for a PWM ramp was to use the PWM timer to increment from 0 to 255 over a set delayed time.

 

Hi Max! Yes, that should work at power up, and it’s actually already implemented in code. The issue is how to detect the DIRECTION switch has been switched to cause the same delay. Thanks!

 

A small relay fed off one of the outputs?
The other issue is to detect when the motor is at zero RPM in order to swap direction.

 

A small relay fed off one of the outputs?
The other issue is to detect when the motor is at zero RPM in order to swap direction.

 

Max- yep, I like your second idea and that what I think I will try.

 

If the motor switch is ON and the FET is conducting at turn-on, then for that first instant the motor at rest generates no back EMF, and so the current is limited by not much resistance, except for the mains resistance. So that instant current may be possibly 20 amps, or maybe 100 amps, although I doubt that. Thus instant destruction of a 7 watt resistor followed by a 50 volt capacitor.
Turn-on transients often are invisible demons that cause problems.

 

@JRTech Using the method in #7 mimics the accel method used in TM's and results in Very low current from 0 up to the set RPM.

 

All:

Implementing a design that allows slow ramp-up in FET drive PWM duty-cycle. I certainly agree that the 7 watt source resistor gave up the ghost at motor start-up due to excessive current. gate PWM signal looks nice a square, so hoping that the 2A drive potential of the gate driver is sufficient to handle the 1200pF gate capacitance (any opinions?) I may lower the gate pull-down resistance.

Opinions welcome and THANKS!

 

A small relay fed off one of the outputs?
The other issue is to detect when the motor is at zero RPM in order to swap direction.

I had bad experience wehere relay used to get spot welded

 

In my application, I switch the relay with little or no current through the contacts. Guess you could add snubbing network to reduce arcing

 

I had bad experience wehere relay used to get spot welded

Usually happens when immediate reverse is carried out without a detection of zero RPM.
Or at least allow time for zero condition.