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Learn how to choose the right power supply for your stepper system. Includes discussion on linear unregulated and regulated supplies
Automation Direct offers a variety of Stepper System Power Supplies, so how do you know which one is the right one for your application? Before answering that, let’s take a look at how the stepper system works together. Stepper systems have a power supply, a drive, and a motor. A PLC or some other kind of controller tells the stepper drive how to take the current and voltage from the power supply and chop it up to send positioning pulses to the motor. The Drive monitors the current consumed by the motor and constantly adjusts it to provide precise positioning of the motor. In general, the more voltage you provide, the better the motor will perform. You can see that on the motor curves. In this example with a 32 volt supply this motor is limited to 1200 rpm and the torques falls off real fast. But with a 70 volt supply, the motor can run out to 1800 rpm and still have about triple the torque. The bad news is the more Voltage and Current you supply, the more heat you have to dissipate. So normally, you will usually want to choose the supply with lowest voltage rating that will achieve the speed and torque you need. Beware that the voltage of an unregulated supply tends to float higher with lighter loads, so make sure your un-regulated supply stays within the drives tolerance. Of course, you don’t need to worry about any of that if you are using an appropriate AutomationDirect unregulated SureStep supply with an AutomationDirect drive– they are designed to work together. You can use this chart in the user manual to figure out which AutomationDirect Sure Step power supply goes with which drive. These are the drives, these are the unregulated power supplies. Why doesn’t this supply work with these drives? Well, this is a 70 volt supply, and these drives have a max input rating that is lower than 70 volts so we don’t want to use this supply with those drives. That’s also why none of these will work with this guy … their voltages are too high. But what about these guys? There are three possible supplies for each drive. Now which one do you choose? We recommend two different ways to decide which power supply to use. For a quick rule of thumb, if you select a power supply with 2/3 the current output of your motors max phase current, you’ll be in good shape. For example, this Stepper Motor from AutomationDirect has a max phase current of 6.3 amps. Take two thirds of that – which is a little overt 4 amps – and we quickly see that this 5 amp supply will handle the worst case load. If you are powering 3 motors, then you will need three of these which is a 2 x factor. Etc. If you don’t think you will be anywhere near the worst case load or maybe you are going into production and need to absolutely minimize the cost of the end system, the second way to choose a power supply is to get one power supply using the 2/3 rule – because you KNOW that will work - Then power up your system, run it at it’s full load and simply measure the current out of the supply. To choose the voltage you need, just look that motor curve and choose a voltage that will get you the motor speed you need. Easy. So far we have been talking about using unregulated linear supplies. Can you use a switching or regulated supply? Sure! But, there are a couple you need to worry about. When you first power up a drive system or quickly accelerate a heavy load, there is a large current draw as the motor stator establishes its magnetic fields. Regulated supplies may think that is a short circuit and they’ll reduce – or fold back – to a lower voltage to protect themselves. Of course, the Rhino PSB series regulated supplies from AutomationDirect are designed to work with the 48volt SureStep drives and will help reduce these issues when compared to other supplies that weren’t specifically designed to do this. The other issue with switching regulated supplies is regen. That is, if you try to slow down a heavy load too quickly – maybe you turn the power off while it is still ramping down – the motor now becomes a generator and pushes current back up to the drive. The drive then tries to dump that back into the power supply which can boost the voltage beyond what the supply expects to see. This can trip the overvoltage protection of a switching power supply, and cause it to shut down. The best defense when using regulated or switching supplies is to add a regen clamp. It goes between the power supply and drive. Its job is to take that extra energy and re-route it to a large resistor to dissipate the power. We call that a braking resistor. This regen clamp from AutomationDirect has a built in braking resistor that can dissipate 50 watts of power continuous and up to 800 watts peak. If you need to dissipate more power than that you can just add an additional braking resistor. This one from AutomationDirect adds another 100 watts of continuous power dissipation. Remember, large current surges at startup and large regen voltages coming back to the supply are only issues with switching or regulated supplies. None of that applies to linear unregulated supplies. Which explains why most folks like to stick with the unregulated supplies – they just do the job without the extra concerns or hassle. AutomationDirect’s SureStep supplies are all un-regulated so you don’t have to worry about any of that stuff. And one more important thing to keep in mind – avoid the temptation to power stepper drives in a daisy chain. Wiring them in a star configuration with all power wires starting at the power supply will help ensure that all drives get the same power input and one drive doesn’t affect the others. If you have any questions about AutomationDirect’s Stepper Systems, please contact AutomationDirect’s free award winning support team during regular business hours. They will be happy to help. And don’t forget the forums! There are lots of experience automation professionals there that love to share their years of experience. Just don’t post any questions directed at AutomationDirect’s support team there, they don’t monitor the forums on a regular basis.