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PWM Motor Speed Control

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pbency

New Member
Hi,

I have build a PWM to control a 3HP DC motor. The problem is that the MOSFET is getting damaged because of high return current/voltage. I used a Diode at the motor end but that diode is also getting burnt.

Main DC 380 Volt
PWM used to control a 220 DC 3HP motor

I found out that a snubber circuit is need to take up the spike generated by the motor when the MOSFET is switched off. Can anyone help me with the exact calculation of Capacitor and Resistance needed for Snubber?

Regards,
Bency
 

dknguyen

Well-Known Member
Most Helpful Member
There are a few kinds of popular snubbers. All are disipative snubbers that burn off the energy somehow:

A C snubber (just a capacitor)- limits the rate of voltage rise for both switching off time, but ALSO comes into effect for the switching on-time, which isn't good because it slows down the turn-on time and more losses happen and isn't needed because there are no inductive voltage spikes during turn on since the inductor is not releasing any stored energy (it's in the process of storing energy during turn-on)

The RC (series resistor-capacitor) snubber- used to dampen ringing from the parasitic inductances during switching

The RCD Snubber (resistor-capacitor-diode, wired in the voltage rise limiting version)- used to limit the voltage rise during switch turn off (it basically works just like the capacitor snubber except the resistor and diode make the capacitor only affect voltage rates during turn-off, and not during turn on.)

THere are other snubbers too. One is an RCD snubber where the components are wired differently to clamp the voltage rather than to control the voltage rise. THere are also snubbers that use inductors to control current rise as well as lossless snubbers that return the energy to somewhere else in the system rather than burning it off.

Simple design article for RC snubbers and RCD snubbers:
https://www.electro-tech-online.com/custompdfs/2008/01/design.pdf

In-depth article on many kinds of snubbers and their operation:
https://www.electro-tech-online.com/custompdfs/2008/01/slup100.pdf

You still need a fast-recovery, low voltage diode however to actually clamp the voltage if the snubber is not able to do the job entirely on it's own (the snubber will help take the load off of the diode to help it from burning out). If this is a H-bridge drive the diodes go cross each power MOSFET to give the inductive flyback current a path to flow. If it's just a unidirectional, single switch drive then the diode goes across the motor. The RCD snubber goes across the switch. The RC and C snubbers goes across the switch, but I don't see a reason why they can't go across the motor terminals as well to reduce ringing. I would think the C snubber across the motor rails would also act just like a flyback diode, except it would be much faster and reduce the intial spike, but it would probably shut off sooner than the diode since capacitors only conduct transients that are fast enough. I also think the capacitor's current capability is less than a diode. So you may still need a diode too- the capacitor will do the job during the period where the diode hasn't turned on yet.
 
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Ubergeek63

Well-Known Member
sounds to me like you are not getting your numbers right.

3HP*750W*1.3/220V=13A continuous in the motor. At low speeds most of this 13A goes through the diode and at high speeds most of it goes through to FET or IGBT. These two parts need to dissipate 26W each plus switching losses without over heating.

Not only that, both of these parts need to run at relatively high frequencies so generic parts won't work since the inexpensive ones are only good for around 1KHz.

Dan
 
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