Скачать или смотреть What is a Motor Drive?

The definition of an amplifier, or drive, is it takes a low-level signal in and produces a high output power to a load. It could be a motor, could be a linear or rotary, could be a fan. It could be a valve, could be a lot of different things, but it's usually in response to some sort of controller or terminal or PC or something that has logic to it. If you're used to somewhat more sophisticated sound equipment, you might be familiar with the tuner and an amplifier which then powers the speaker. The tuner brings in a radio station, for example, or whatever the music is, and then that goes through an amplifier which puts out enough power to the speakers. Speakers are essentially one-phase motors. It's very similar to what we were talking about with electric motors in the last episode. The analogy here is a controller to a drive to a motor. So, we use the terms “drive” and “amplifier” interchangeably. Now, some people talk about the drive in terms of the mechanics. What's the mechanical transmission? Or maybe the motor? But really when we talk about a drive in motion control, it's the amplifier, it’s the electronics.

There are different types. There's the linear amplifier which is shown over there on the right, which has an output amplitude that is controlled as a linear function of the input, and has a great result of electrically quiet and non-distorted output, but they are larger and less efficient, so we don't see them very much in industrial automation. They do exist, but we just don't see them very much. They're much better for lower power stuff, so we typically use PWM, or pulse width modulated, amplifiers which have the output consisting of a series of constant amplitude pulses whose width is a linear function of the input. I’ll come back to that in a moment. The result is more efficient, but with some electrical noise and distortion. If you want to know more about this, go back to my Episode 9 on electrical noise and then how to deal with that. When I started in this industry twenty-three years ago or so, the analog drives were definitely predominant. There were very few digital drives on the market. I didn't use any until pretty much the next year with the manufacturer that I was working with. The analog type drives are basically all the electronics are made by linear circuits and the adjustments are made with potentiometers, so those can drift a little bit and you have to use the little screwdriver to adjust it periodically. The digital type drives have their functions all created by microprocessors via firmware, and all those adjustments are made via computers and software. It doesn't drift, so it's nice and compatible, and it could be used with a lot of different communication bus structures and protocols. 8 to 20 kilohertz is the typical chopping frequency for the PWM. Now, I've worked with drives up to 40 kilohertz, which are in particularly needed for low inductance motors. We’ll come back to that in just a second as well, but 8 to 20 kilohertz is typical. I've seen 4 also, and 16, but those are the typical ranges.

Here's what a power circuit looks like for PWM drives. You have the AC power coming in on the left. It goes through a rectifier and then it gets filtered a little bit to help with the electrical noise and then it goes back through an DC to AC inverter. Notice we have AC coming in on the left, then to DC and then back to AC and it's AC that goes out to the motor. What you see there under the inverter is essentially a series of H-bridges where the transistors are turned on and off in a certain sequence. What I show here right in the middle is one coil of a motor. So when this is turned on and this is turned on, the current goes through there, and then when it is switched and these are off, these are on, the current will go back the other way and that's done per phase of each motor. So, it looks something like this where you have the bus voltage is chopped into little pulses. Notice the voltage is the same, you get the positive and you have the negative but it changes the current as it goes through, so the amount of current is adjusted by the width of these pulses. If we look at it a little more closely, it has to do with the voltage equaling the inductance times the change in current over the change in time and it's using a bus voltage. When 120 volts AC comes in, it gets rectified up to 170 volts DC. 230 volts AC gets rectified up to 340 and so on. So, that's what the bus voltage is that we talk about. That's the potential and that is this voltage here. So for at 0 volts here, that's a positive 170. Here's a negative 170.

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