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Guidance requested for Universal Motor Controller

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Heretic

Electrical
Feb 3, 2003
5
Hello All

I am about to redesign a microprocessor based controller for a 240V 5HP Universal motor and I have a few questions before I go ahead.

The current controller uses a TRIAC as the switching element, in a pretty standard chopping configuration, and I am considering changing to DC mode (full wave rectified AC) with an IGBT and PWModulating the drive signal at a frequency of about 20k.

1) Why do some IGBTs available for motor control have an integrated reverse diode and some do not? I have found some very similar circuit examples which do use IGBTs with the diode -
(PAGE 1)
and others which do not
(BOTTOM OF PAGE 3)
What is the purpose of it and why would some circuits not show it?

2) I am considering 20kHz as the PWM frequency. I know that the choice of IGBT has a lot to do with the switching speed. But is there some practical limit for switching speed imposed by the motor? Is faster better for EMI performance?

3) The current TRIAC controller has a fairly chunky common mode choke to control conducted emissions; given that the rising and falling dV/dT can be controlled by the gate drive, would it be possible to do away with this? Would other filtering still be needed?

4) Some IGBT circuit examples show a capacitor across the rails after the bridge rectifier at the mains input. Are they talking about a 1uF X2 style filter cap for example or a much larger bulk storage cap?

Sorry there are so many questions but I am having trouble finding the answers and I have read as much related documentation online as I could find.

Thanks in advance for any input.
 
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In re: 1)I have seen many transistor diagrams that did not show the diode in it. Go to a manufactures data book to see the full diagram of the device.
2) That diode has a few names, free-wheeling is the most common. It's purpose is to shunt the reverse voltage caused by field collapse of inductive devices (motors, relays, etc)around the transistor. Purely protective. If you use a transistor without an integrated diode, install one!
4) That cap. is used to convert the rectified Dc to something closer to pure DC. There is a formula used to size it in relation to your load. Can't remember it off hand though. It will need a diode also in order to protect your bridge.

Why do you want to change your control scheme?

 
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