EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: XaviPacheco on May 29, 2018, 03:29:28 am
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I attach a picture found on the web as further reference. It's from a motor controller, the AC input goes through a relay and feed the rectifier. The rectifier output goes to capacitors and then to PMDC motor. The point is that I'm making my own controller. I've made several modifications, for example, instead of relay, I'm using solid state device (triac) with zero crossing detection. My question is regarding the big capacitors needed in the rectifier output. According to the image attached, they are two capacitors 1500uF/200V in parallel and a tvs diode. I don't know how they chose them. As I'm making my own design, I want to determine the capacitors that I need. How can I determine the capacitance needed? I will control a PMDC motor of 4 HP @120VDC using PWM. So I know the capacitors voltage rating should be at least 200V. Also, is that TVS diode relevant there?
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To determine bulk capacitance you need to account the current consumption and your allowable ripple, with those two you can choose the value you need. The type, for the size you are pretty much left with electrolytic ones, it doesn't make much sense anything else.
The TVS is there for protection, what is it protecting against, I need to see more of the circuit, but some transient or PS failure probably.
In parallel with all this also can go an extra power transistor and a braking resistor to discharge the caps when the voltage goes over the specified limit, as it's braking the current will flow from the motor to the driver overcharging the caps. I don't thing the TVS has something to do with this as it seems to have a much higher voltage and they are usually good for very short high current spikes, rather than dissipating a lot of energy from slowing down a motor.
JS
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I agree with JS.
The usual rule of thumb for bulk input capacitors is 8200uF per (amp of current)/(peak-to-peak voltage ripple) based on a line frequency of 60Hz and a full wave rectifier which produces 120Hz.
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You don't need the caps at all. The PMDC motor will be perfectly happy running on rectified AC alone. Only downside is that your AC supply is 110 V and the motor is rated 120 V, so you'll lose ~9% speed.
With the caps, you'll be supplying the motor with ~150 V.
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You don't need the caps at all. The PMDC motor will be perfectly happy running on rectified AC alone. Only downside is that your AC supply is 110 V and the motor is rated 120 V, so you'll lose ~9% speed.
With the caps, you'll be supplying the motor with ~150 V.
The motor might be but the power transistors not so much, one walk-around would be to control the level with SCR instead of power transistors on DC. Still you need to be careful of over-voltage as the kickback of an inductive load can be enough to break the coil insulation.
JS