Energy management with variable speed drives, Part 1

Scott Sullivan walks through the basics of motors and safety, variable frequency drives, the three sections of a drive and how pulse width modulation works.

By CFE Media February 15, 2023
Courtesy: Advanced Energy

Motor and Drives Insights

  • Measuring the lockout or taking the fuse out of anything before working on a power terminal is a simple precaution that can prevent major injuries.
  • The drive is composed of three separate sections, an input converter, a dc bus and an output inverter, which is how power is outputted.
  • Pulse width modulation is what’s used to control how fast the frequency is and the overall average voltage sent out to the motor.

Variable frequency drives provide effective speed control of ac motors by manipulating voltage and frequency. Controlling the speed of a motor provides users with improved process control, reduced wear on machines, increased power factor and large energy savings. The most significant energy savings can be achieved in applications with a variable torque load. Reducing a fan speed in a variable torque load application by 20% can achieve energy savings of 50%. For most motion control applications, reducing motor speed is often the easiest way to achieve large energy savings.

Scott Sullivan, a field service engineer at Electronic Drives and Controls or EDC, a certified system CSIA system integrator, gave a presentation on Energy Management with Variable Frequency Drives with CFE. Sullivan specializes in the application of variable frequency drive technology and onsite field service of ac drives. Since joining EDC in 2016, Sullivan has served on EDC’s field service support team performing repairs, preventative maintenance services, startups, training, and much more for ac and dc drives, PLCs and factory automation. Sullivan is a graduate of the University of Rhode Island with a bachelor’s degree in electrical and electronics engineering.

The presentation has been edited for clarity.

Understand electric safety basics

For safety basics, it should probably go without saying, but never work on live equipment. If you’re not trained to work on live equipment, it’s not safe to do so. Always follow proper lockout/tagout procedures and every person working on a machine should have their own lock on there and their own key on there. Always check for external power sources. Some older machines especially may use 120-V logic and even with the main power off, it could still have live 120 volts. Make sure to measure the lockout or take the fuse out of anything that you work on before you touch it.

Drive power terminals is where you feed the power into the drive and ultimately hope to power the motor. Commonly, it’s either 230 V or 480 V. Under no circumstances should these ever be touched while the drive is running. The drive changes energy and converts frequency output, but in its intermediate stage, the drive uses an input bridge to rectify this to a higher voltage. So internally, it’ll be anywhere between 320 and 650 depending on the type of voltage input you use.

Drives have power terminals. Common input power is 3 phase 230 VAC or 480 VAC.

Drive power terminals. Common input power is 3 phase 230 VAC or 480 VAC. Courtesy: Electronic Drives and Controls, CFE Media and Technology.

This is very dangerous and should never be touched while the drive’s powered on. In fact, whenever you power the drive off, you should measure these terminals to make sure it’s safe to touch. It’s important to note that brake terminals are not the same as dc terminals. If you see a brake terminal and you try to measure it, it may not tell you exactly what the voltage is on the inside, so do not confuse dc terminals with brake terminals. The capacitors are used to store the energy inside a drive. Once you power it off, these will remain charged for a short time.

Most drives will do this and should be measured with a meter to make sure that it’s okay to touch. The general rule that we recommend is, if it’s under 50 V, it’s okay to start touching. Most drives have a charge light to indicate that a charge is still present. I, personally, don’t trust this light. Even if the light is on, I still measure it and I recommend you do, too. Just because the light’s off, it doesn’t necessarily mean anything, for all you know the light could have burnt out, so always measure it before you touch the drive.

Understanding motors and drives fundamentals

For motor basics, there are two main things to focus on, which are the stator and the rotor. The stator is stationary, hence the name, and the rotor rotates, hence the name. For motor rotation, take a simplified example of a single-phase motor. The top of a wave is known as the peak. When the voltage is at its peak, the poles are going to be lined in a north south fashion.

Motor Basics.

Courtesy: Electronic Drives and Controls, CFE Media and Technology

The poles represent the stator and the middle part represents the rotor. The rotor will rotate and push against the north and south poles. A short time later, this wave’s going to propagate and when it reaches the bottom of the wave or the trough, these poles are going to reverse and cause the motor to continue to rotate. Because of this, the rotating speed of a motor is dependent on how often these peaks and troughs come across. Peaks and troughs coming across at different speeds is known as frequency. Frequency modulation is how you control the speed of a motor.

While there are simple single-phase examples, most motors you see in any industrial process or HVAC process have three-phase motors. Three-phase motors are similar, each individual wave on a three-phase motor looks similar to the previous example. The three of them are overlapped on three incoming lines. Because of this, you can use them to more precisely control how fast the motor is turning.

Motor 3 phase.

Courtesy: Electronic Drives and Controls, CFE Media and Technology

The drive is composed of three separate sections, an input converter, a dc bus and an output inverter. Sometimes, drives are referred to as inverters because that’s the part they care about, but it’s not technically correct. Other transfer drives you may have heard are variable frequency drives, adjustable speed drives, adjustable frequency drives or just drives in general. If we’re being technical, though, VFD is the accepted term because you’re actually going to be varying the frequency out to change the speed.

An important thing to note is nothing’s perfect. When a drive tries to emulate a wave, the output is going to be a little messy. If you have a nice clean sine wave that you’re supplying to the drive, you’re going to see the sine wave that the drives actually output into the motor. The drive is going to be chopping up dc voltage to mimic ac voltage at desired frequency, so they end up getting a jagged mess.

Because a motor is mostly inductive, these voltage spikes, peaks and troughs get smoothed out, as far as a current’s concerned, you get something that approximates a more similar sine wave. If you ever hear ringing in motors, or you hear people who’ve been in the industry awhile talk about VFDs burning up drives or motors, this is something that was a problem a long time ago. We’ve since come to address it and minimize it, but the real problem was how the sine wave is outputted.

A diode works like a one-way valve for electricity. For example, if each phase has a diode facing up and a diode on the lower part, it’s actually going to be working in the backwards direction because it’s negative. Since it only allows current flow in one direction, we end up with a wave that’s almost entirely positive, and a wave that’s almost entirely negative.

It is bumpy, and it’s not clean dc, but this is the bulk of what a drive does to convert to dc. The dc bus is the middle section of a drive. In the middle part you’ll see things that look like jagged lines, those are known as resistors, a flat line and curved line underneath it is the electrical symbol for a capacitor. That’s what actually is storing the energy and filtering it out. Capacitors can only charge up so fast. They can’t instantaneously charge or instantaneously discharge. Because of that, the peaks and troughs on a wave form get smoothed out.

The peaks and troughs on the positive and negative side can be as high as 50 volts, whereas on the output side, it’s going to be one volt or less. At this point, it’s almost entirely clean dc, and it can be used to chop up to send out to the motor.

The last part of drives is the inverter. In this example, a single phase is used because it’s easier to conceptualize. Coming in from the previous phase, there would be clean dc, a tiny bit of ripple, but nothing really to worry about. There would be a positive and negative one. For a 480-V system, this will be about 325 V and -325 V. This will pass a signal along to a symbol for a diode and a symbol for a transistor, specifically called an insulated-gate bipolar transistor. Transistors can do a lot of things, but in this example, they are used as an on-off switch.

Once the transistor is switched on, it’ll pass along the +325 V to the output. If at a later time the transistor is shut off and the other one is then turned on, this will bring it from +325 down to -325. If someone later shut off the negative one and turn on the positive one, that would bring it from -325 back to +325, and it repeats adding an item or as long as we really need it to. A square wave going out is what’s actually getting put out to the motor. So, the earlier mentioned jagged sine wave is really what this is seeing. Because the motor is largely inductive, the current gets smoothed out and we have a nice reasonable sine wave.

Pulse width modulation’s role in motor control

Pulse width modulation is what we use to control how fast the frequency is sent out to the motor. The previous example I mentioned was just a simple on-off example. But really what happens is that we turn the positive transistor on and off repeatedly. The longer we turn it on, the more voltage is sent out and the more positive it becomes. We eventually gated on almost 100% of the time to meet the peak of the wave, then, we start to slowly bring it back down. After it is back to neutral, we start slowly gating on the negative until it becomes almost entirely negative then we slowly bring it down, bring it back up to zero, and it causes the wave to repeat.

Motors and drives with pulse width modulation.

Courtesy: Electronic Drives and Controls, CFE Media and Technology

This process of how fast these pulses come out is called carrier frequency, and it’s called pulse width modulation because we modulate the width of the pulses to actually change the frequency. This will allow us to change not only the frequency, but the overall average voltage that gets sent out to the motor.

Author Bio: Since its founding in 2010, CFE Media and Technology has provided engineers in manufacturing, commercial and industrial buildings, and manufacturing control systems with the knowledge they need to improve their operational efficiency. CFE delivers the right information at the right time around the world through a variety of platforms.