Re: Which MOSFET / IGBT can afford 5A current normal operation and 40A current at 1-2 seconds?

From: Electronic Swear (swear_terence_at_hotmail.com)
Date: 11/29/04 

Date: 28 Nov 2004 17:18:29 -0800

The DC motor is operated at (rectified) 220Vac.
It is for food juicer. We don't want to use current
sensing because the motor start-up current is very large.
Similiar to what the current at lock rotor. As a result,
current monitoring is not a very good method. Even the motor
will be over-loading for a short moment when the user is putting
a very hard / big food for juicing at the beginning.
The current will be very large at the beginning but will drop
to a suitable value. However, we cannot cut-off the motor source
because of over-current. We will allow the motor operating at a
very high current for a short time, several seconds.

The motor is for masss production and look for safety protection
as well. If use fuse on protection, it is very inconvenient for
exchange. We want some active protection rather than passive components.

I will try for using the SCRs at the bridge rectifying.
However, the current rating of the SCR just 25A is enough or not?
And any detail on snubber capacitor?

nico@puntnl.niks (Nico Coesel) wrote in message news:<41a8862e.308874387@news.planet.nl>...
> swear_terence@hotmail.com (Electronic Swear) wrote:
>
> >Thank you for your opinion.
> >
> >Basically, I am doing over current protection when the motor is going
> >to lock rotor. I am not monitoring the current but monitoring the speed
> >of the motor. I use the power Mosfet /IGBT to be a switch for cut-off.
> >
> >Of course, in normal operation, motor is running and the transistor need
> >to handle 5-6A current. However, if there are rotor locking, the current
> >will rise to 40A. When circuit detect the speed is zero, it will trigger
> >the transistor to cut-off.
>
> I think this is not very reliable, because you assume the motor will
> be running at a certain speed in a few seconds after switching it on.
> Why not use a small series resistor and sense the current? If it goes
> over a specific current, you shut the circuit down or turn the
> transistor off for 1 AC cycle. The latter also provides a crude
> soft-start mechanism.
> Also, you might be able to get away with 1 FET or IGBT because you can
> determine the current handling limits of the transistor more
> precisely.

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