Transcript
Introduction to Motor Soft Starters
Good morning, everyone, and welcome to today’s white paper webinar, and today we’re talking about motor soft starters. And I have John Ellis with me here to talk about why soft starters are needed for induction motors, how they work, and how they improve power quality. How are y’all doing today? I’ll just walk through the paper and hit the highlights.
How Induction Motors Work
An induction motor is just an asynchronous motor, generally brushless. They use these type of motors when you have a lot of high torque requirements. You can operate them off single phase or three phase. In this case here, we’re primarily talking about a three phase.
The way that those motors work is that you’ve got an electric field that moves around with the three phases, and it changes the field so that the rotator spins within the housing, and so that generates the torque. But to get those motors moving and get the loads moving, these type of motor can have a really large inrush current, which causes extended problems for power quality. Now, those motors really don’t have a great power factor anyway, but this will help minimize issues that come along when the motors actually start up.
Startup Current Behavior
On this sheet here, we’re talking about just how the current can behave when you power up a motor. You can see on that one plot there on figure four, it shows kind of a simplified startup current plot, so that when you initially apply power to the motor, you can have a 600 plus percent inrush current over your steady state. And so once the motor gets moving, everything becomes more stabilized and your current consumption will drop down to 100% of your normal level.
An example of how you might control the situation is to ramp up the voltage to the induction motor so that it can gradually apply force as opposed to a sudden burst where it has the huge inrushes.
Soft Starter Technique Using Triacs and Diacs
In figure five, we go through a simple example of a soft starter technique where they’re using triacs or dual diacs with contactors in parallel with each phase. You have a microprocessor that can, if you look at the figure six, actually when the waveform is red, that’s when it’s actually conducting. So you can gradually over time ramp up the voltage by phase delaying when you start the triac conducting. And so as you gradually ramp that up, you minimize the amount of torque required to get the motor turning. And so it creates a much more stable ramp-up current.
The way that the diac works, or the triac diac combination, on the induction motor is that in this particular instance, they use solid state devices to control the voltage, which is what generates that controlling voltage on the right plot. But then once the motor reaches a steady state, then they will activate a contactor which will bypass each of the solid state circuits, and that just relieves the current requirements for those solid state devices, ’cause a lot of these devices can handle a big inrush current, but they just can’t take a lot of steady state continuous current. So they’ll overheat and break. You basically use the electronics until you get it steady state, and then you switch the contactors in and bypass the electronics.
Benefits of Soft Starters
The soft starter really has two functions. One, it gets rid of that inrush current. It can also improve the power factor. But those voltage sags caused by the inrush currents are really one of the key problems with hard start induction motors. By reducing that inrush current, you reduce the sags, where that inrush current causes an extreme voltage drop.
ProVision Example of a Hard Start Motor
And I can show an example of that here in ProVision. So here we have PQ data file, and we have a hard start motor. This is a three-phase motor, and we see here in purple the current spikes. In dark green are the voltage sags. So every time we have this motor start, we have an inrush current and then a voltage sag at the same time.
If we look at the waveform capture that’s been triggered, we can see that this is a hard start motor. The current in blue starts from a small level and just abruptly jumps up to a much higher level, and that’s a hard start motor. And this current will stay high, as John said, until the motor ramps up to speed.
If we look at the RMS version of this instead of the instantaneous waveforms, we can again see that it’s even more clear here where we’re looking at a sliding RMS window that the current jumps up immediately, probably because of that motor and the voltage sags right along with it just from Ohm’s law.
Transformer Sizing and Voltage Drop
Typically transformers are sized not for the inrush current of a motor or for the total load, but for the steady state thermal demand from the load. So you’ll have an abnormally high voltage drop just during the inrush current because from a steady state thermal point of view, that high current doesn’t last long enough to actually cause any transformer heating or wire heating. So everything’s sized for thermal reasons, the wire and transformer, because these current spikes aren’t sustained.
But because of that sizing to the steady state current, which like John said is five, six, eight times less than the inrush current, you have a really high voltage drop during that inrush current that causes the deep voltage sags, and that’s what the soft starter is for.
Mechanical Benefits and Soft Starter Failure Modes
There are also other detrimental effects. That hard start on a motor is a mechanical shock to the motor. A soft starter can reduce limitations on the number of starts per hour a motor can have because when it’s a hard start, that’s a lot of mechanical stress on the motor as well. So the soft starter helps electrically and also mechanically to reduce wear and tear on the motor itself.
Soft starters themselves can fail. Going back to the paper, it’s a little bit complex. Under the hood, there’s contactors and semiconductor switches, diacs or triacs. If a contactor fails, then that would expose the semiconductors to steady state continuous current, which they may not be sized for. They may fail much earlier because they’re not supposed to have steady state current going through them. Or conversely, one of these semiconductor switches could fail, and if that happens, the motor would still start because the contactor would eventually kick in, but you don’t have a full soft start anymore, so it’s back to a hard start. So there’s multiple ways for the soft starter itself to fail over time, given its complexity.
Variable Frequency Drives as the Ultimate Soft Start
Now, the ultimate soft start is a variable frequency drive. A soft starter helps reduce inrush currents by controlling the duty cycle of the voltage waveform as the motor starts, but it’s not perfect. Essentially a perfect soft starter is a variable frequency drive, which brings its own problems to the picture because a VFD presents harmonics to the utility.
So the trade-off there is a perfect soft start with a VFD brings harmonic currents, which increases transformer heating in a different way on a steady state basis. So a regular soft start can be helpful. VFD is an ideal soft starter, but brings a whole host of other problems to the picture.
Contact Information
If you have any questions about the paper, give us a call anytime at 1-800-296-4120 or send an email to support@powermonitors.com. Everyone have a great day, and thanks for attending.