Thyristors or FETs for motor drive 3

[BobM3]

I'm working on a new drive for a universal motor. Previously we have used triacs for simple turn on/off and speed control. It was suggested to me that MOSFETs or IGBTs may be more economical and produce less heat. Any opinions out there on which to use? Our motor draws about 20 amps at 115 VAC and produces a lot of dv/dt.

 

[itsmoked]

Universal motor huh... Do you have any interest in converting to a non-universal? Ditch the brushes? In that case would seem like IGBTs would make a whole lotta sense. But if you are going to stick with the universal I wouldn't bother with a design change. Both the FETs and the IGBT would probably cost a little more without a whole lot of advantage. Not to mention the learning and development curve.

An induction motor would let you reduce the dv/dt a bunch if that has any advantage for you.

 

[Skogsgurra]

Triacs are the best choice if you are going to stick with the universal motor. Simple, small, low losses, low dv/dt (in comparison to what you get with PWM and IGBT and MOSFET) and low frequency switching. Universal motors also have a favourable torque characteristic and they do not cost much.

Disadvantages of universal motors: Not very reliable, maintenance needed, commutator sparks a lot sometimes, considered low-tech.

So it is more a question of deciding which motor to use. Throwing in something else than a triac without changing motor type will not get you anywhere.



Gunnar Englund

http://www.gke.org

 

[BobM3]

We like the universal motor because of its power/weight ratio. That's important for our product. I'm told that brushless DC motors also have a high power/weight ratio but the controls (electronic commutation) are more costly.

Sound like sticking with the triacs is the way to go.

 

[Warpspeed]

For size, cost, and simplicity triacs would be difficult to beat. They are also a lot more electrically rugged. They will withstand short term current overloads rather well, and voltage spikes just typically cause momentary spurious turn on.

FETs and IGBTs on the other hand tend to go pop fairly quickly if either the maximum rated voltage or current of the device are exceeded even for an instant. This can be a problem with mains connected motors, unless some sort of fairly elaborate protection is provided. That all adds to cost and complexity.

My own experiences with something similar, were with a range of laboratory centrifuges. Long term reliability of both motor and the controller were never very good. I had more luck when I had a chance to redesign and up rate the controller. The single triac was replaced with back to back SCRs, but the motors still had a short life.

 

[BobM3]

Thanks guys! You addressed the issues I was concerned about.