Episode #37: What is a Motor Drive?

Contact Valin today for more information at (855) 737-4716, or fill out our online form.

The Motion Control Show

A lot like the topic I discussed in the last episode about electric motors, there isn’t a whole lot new and really earth shattering in the world of drives.  But it is an important topic to understand: what they are and what a drive is, in the context of motion control, is what we're going to talk about this episode.  I'm going to use a lot of old graphics because, really again, nothing much has changed.  But if you have any questions or applications you'd like to discuss, reach out to us at Automation Support at Valin.com or TheMotionControlShow.com.  We're happy to help.  I'm Corey Foster.  Let's see what we can learn.

Definition of an Amplifier or 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. 

Types of Drives:
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.

Basic Power Circuit for a Pulse-Width-Modulated Inverter:
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. 

What is PWM?
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.

A lot of people will make the mistake of troubleshooting a drive and motor problem by putting a multi-meter between the motor and drive and seeing what the voltage is, but you can see the voltage is constantly changing from 170 to 0 to -170 so they get some weird voltage.  Now you can see that voltage if you put an oscilloscope on there, but again, that's not what you want to look at if you're troubleshooting between a motor and drive, what you want to put on there is a current clamp and look at the current as it's changing.  Now, if the motor is not moving you're stuck at whatever that current is at in that location, so this is one location of the motor.  As it moves, though, it approximates an AC sine wave, so again, there's some terminology that tends to confuse people.  Is that an AC motor?  Well, a servo motor is a type of electric motor, and the AC servo drive is a type of drive, but a servomotor approximates an AC sine wave as the motor is moving, so this is one electrical cycle of the motor and you can see here as the transistors in the H-Bridge are changing you can see the current kind of going up and down as it's controlled.  How quickly it’s controlled has to do with the inductance of the motor and the chopping frequency of the drive.  So, if you have a low inductance motor, that current can change very quickly and you need to have a high frequency drive.  That's why I worked with a 40 kilohertz drive because that motor had a really low inductance and that was very important for matching them. 

I hope this helps you to understand the basics of what a drive is.  We're going to be talking more about drives and the different types in the future.  If you want to get some help from us, reach out to us here at this email address and our website.  I'm Corey Foster at Valin Corporation.  I hope that helps.

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.