Selecting the Best Power Supply for Your Stepper or Servo Motor Application

Motion control applications have some unique requirements compared to most applications; two are particularly unique: 1) they have a peak power demand that is typically very high relative to the average demand and 2) the motors often act as a generator rather than as a load, and pump current into the power supply rather than drawing from it (regenerated energy or “regen”).

If you need a DC power supply for your stepper or servo motor application, you have three types to choose from: 1) unregulated bulk linear supplies; 2) regulated PWM switching-mode power supplies (SMPS or PWM switchers); or 3) hybrid, regulated resonant mode supplies.

This article discusses the technical considerations unique to motion control and compares the three types of power supplies.

It’s important to consider the unique demands of a motion control application when selecting a power supply. During accelerations, motor drives can quickly draw large amounts of power. Additionally, motors can create regen and push current back into the power supply during deceleration (i.e. they act as generators), which means the power supply needs to handle the resulting increase in voltage. Highly dynamic motion applications (ones with large inertial loads, fast accelerations/decelerations, and high peak velocities) place large and rapid demands for current on the power supply.

There are many other important factors to consider when choosing the best power supply that are not specifically related to motion control. Some of these are especially important to OEM machine designers looking to minimize the cost of their product and provide reliable operation over a wide variety of operating conditions.

Required Power (Peak and Average):A pumping application that generally runs at a fixed or slowly varying speed and torque uses a peak power fairly close to its average (continuous) power. A pick-and-place machine, on the other hand, with lots of starts and stops at high acceleration has a much higher peak than average power demand. For a well-designed system, you’ll need to consider the peak and average power draw for all axes combined (which usually is not simply the sum of the individual axes’ requirements). A multi-axis machine with axes that have overlapping motion profiles (i.e. the axes may accelerate at the same time) will likely require much more peak power than machines where only one axis moves at a time.

DC Output Voltage Level: Assuming you want the lowest overall cost for the mechanical power your application needs and you are using motors in the 100 to 750 watt range (fractional horsepower), there is a sweet spot around 65-85 volts DC. Many people want to use a 24-volt power supply because they are very easy to find or because their application already needs 24 volts (for sensors and other components). Many motors can be run from a 24-volt supply, so why not use 24 volts? The main reason is that increasing the motor’s supplied bus voltage (to a point) is the lowest-cost way of getting more motor power. The mechanical power from the shaft of a motor at any given speed is the speed multiplied by the torque. The maximum speed of any motor is directly related to its supplied voltage. The amount of torque you can get from a motor is proportional to the current you push through its windings, which in turn is also limited by the power supply voltage. So, the way to get the most power out of any given motor (speed × torque) is to increase its supplied voltage. For any given voltage, a motor will spin faster if the motor’s stator coils have fewer turns of copper wire. And with fewer turns, you can use heavier gauge wire, which will have lower resistance and provide more current per volt.

Conversely, why not use really high voltage? For motors larger than 1-2 horsepower, it would be impractical not to use high voltage but for fractional horsepower motors, the use of high voltage brings a number of safety and regulatory issues that increase project complexity and increase cost. When using a supply in the 75 VDC range, the current required to achieve motor power up to 1-2 horsepower is not high enough to worry about the resistive losses and copper fill issues described above. And at 75 VDC, it’s pretty easy and inexpensive to meet electrical safety regulations. You might be able to use a lower than optimal voltage and still get the mechanical power you need but you’ll probably have to use a larger and more expensive motor.

Load Regulation/Output Voltage Stiffness: The voltage output of power supplies will droop to some extent as the supply’s load increases. The amount of droop depends on the type of supply, the quality of the design, and the amount of load. As the supply output voltage decreases, the maximum available motor shaft power decreases. Motor drives act as power converters, so some amount of droop can be momentarily compensated for by the drive because it will simply draw more current to provide the required power to the motor drive. But, because the droop reduces the voltage supplied to the motor, the maximum motor power will also be reduced.

Power supplies that experience too much voltage droop can cause motor position and velocity errors. With high enough droop, a stepper motor will lose steps and a servo may issue a shutdown due to excessive instantaneous error.

Line Regulation: There is a wide range of nominal AC voltages across the world; they vary by location, time of day, and power grid load. Regulated power supplies generally handle any reasonable change in AC line voltage well — they typically have very little output voltage change. But the output of an unregulated supply, like a bulk linear supply, will change proportionately with the change to the input line voltage. If your machine needs full output voltage to reach its target motor speed and was tested at full line voltage, you may be in for a bad surprise when the machine is operated in low line conditions.

A high line AC condition can also be a problem for applications without line regulation. Most motor drives protect themselves from over-voltage conditions but if the DC bus voltage on an unregulated supply increases due to a high AC line, your drive will now operate closer to its over-voltage limit. This decreases the design margin with respect to regenerative energy because regen will also act to increase the voltage seen by the motor drive.

Regeneration Control Support: All electric motors generate a reverse voltage (back-EMF) when producing torque against the direction of the motion (e.g., during deceleration). This regen pumps current back into the supply and increases the total bus voltage. There are various ways of dealing with this reverse flow of current.

You can add capacitance in parallel with the supply output to act as a reservoir that absorbs this regen energy and stores it for reuse later when power needs to be drawn from the supply. A large output capacitor takes up space, is relatively expensive, and has a relatively low lifespan compared to other electronic components. The lifespan issue can be mitigated by choosing a capacitor with a significantly higher voltage rating than the nominal supply voltage. If your axis or machine produces significant regen, you may have to consider a dedicated regen circuit to shunt current through a load resistor to burn off the excess energy. You can also incorporate a separate output capacitor with associated “blocking diode” and/or a regen control circuit with its associated load “braking” resistor. The capacitor will function passively to absorb some amount of regenerated energy (and provide it back as needed).

These extra components (Figure 1), along with the requisite wiring, add expense, cabling complexity, and take up more cabinet space. Additionally, the braking resistor can get hot enough to be a safety hazard and may require steps to prevent user injury. The output capacitor may need some additional circuitry in order to prevent in-rush current from tripping your circuit breaker upon power up as well as circuitry to dump the stored energy upon power down.

Size/Footprint: Size and/or form factor are important for most machine builders and the motion control power supply is often one of the largest components found within an electrical cabinet. Electronic cabinets or enclosures (especially if rated for a harsh environment) are expensive, so smaller supplies and fewer components decrease space requirements and costs.

In-Rush Current Protection: In-rush current is the initial, instantaneous current that a component draws when it is first powered on. Uncharged capacitors will draw lots of current as they begin charging. In-rush current for a DC power supply can be many times greater than steady-state input current. Without inrush current limiting circuitry, power supplies can trip correctly sized circuit breakers or blow fuses when powered on.

Cost: OEM machine builders can be particularly sensitive to costs because they become significant as machine volume increases. It is important to consider the cost and labor associated with integrating ancillary electrical components (such as an external regen control circuit, blocking diode, braking resistor, or additional capacitors).

Unregulated, Linear Power Supplies – One of the simplest power supplies around, the bulk-linear unregulated power supply has three main components:

Transformer: The primary transformer converts the input AC line voltage to an alternative AC voltage (typically the final, desired DC voltage level). They take up a lot of space and their steel laminations and copper windings are heavy.

Full-Wave Bridge Rectifier: The bridge rectifier is an array of diodes (typically in one package) that converts the negative half cycle of the transformer’s AC output voltage into a positive voltage. The output of the rectifier has twice the frequency of the input AC but only positive polarity.

Capacitor: The capacitance stores energy so that even if you pull current from the supply during the phase of the AC input where the voltage is low, the output voltage won’t droop too much.

Bulk-linear power supplies have a number of advantages. They are simple, electrically quiet, and provide a readily available source of current. Disadvantages include more voltage ripple than most designs because the AC input voltage is well below the DC output voltage for a relatively long time. If the voltage droops too far, the stepper or servo motor will not have enough voltage to spin at the required speed.

These power supplies are relatively big and heavy, which makes it harder to fit them into a compact machine. Not all bulk-linear power supplies are well-suited to handle regen. The returned energy from a motor will charge up the output capacitor, increasing the DC output voltage.

Bulk-linear unregulated power supplies (Figure 2) are typically “bare bones” devices, and don’t have a variety of useful features found in other supplies such as diagnostic LEDs or discharge of stored energy upon power down. Also, most bulk-linear supplies are not enclosed, so you will need to fabricate an enclosure of some sort if user shock protection or mechanical protection of the circuitry is important in your application.

Regulated, Switched Mode Power Supplies – A regulated switching power supply (Figure 3) includes control electronics designed to maintain the output voltage at the specified level regardless of load. Switchers incorporate active circuitry and are more complicated than their bulk-linear unregulated counterparts. Switchers actively regulate their DC output voltage using a technique called “pulse width modulation” (PWM) and feedback.

The advantages of switching power supplies are that they produce a nearly constant voltage regardless of load because they have active circuitry to regulate the output voltage. As long as you use them within their specified current range, you won’t see much voltage droop under load. This can provide a notable performance advantage over unregulated supplies.

Switchers have a smaller volume and are lighter than unregulated power supplies. Their transformers are significantly smaller and the output capacitance is much lower. Most switching power supplies directly accept a wide range of AC input voltages, usually 100 to 240VAC, with line frequencies of 50 to 60 Hz. A properly specified switcher will generally not trip circuit breakers when power is turned on. Most switchers also have some form of overload protection; they automatically shut down if the load is too demanding and will not supply DC output power until you first cycle AC input power.

There are several disadvantages of switching power supplies. Switchers generally have little peak capacity. Motion applications require peak power for the duration of the acceleration of the load; this typically takes much longer than the amount of time that a switching power supply can provide peak power. As compared to the output regulation of a switcher, the voltage droop of an unregulated bulk-linear supply is normally considered a disadvantage; however, it does enable you to pull substantially more power for brief periods of time (well-suited for acceleration portions of a move profile).

All electric motors will generate regenerative energy when they deliver torque of an opposite sign to the direction of motion. The motor returns this energy to the power supply’s DC voltage output, increasing the voltage. Switching power supplies do not have sufficient output capacitance or a separate regen circuit to absorb and/or dissipate this energy.

So, although PWM switching supplies have a number of drawbacks for use in motion applications, they can be successful, particularly in applications that have more continuous loading (e.g., pumps and mixers) as opposed to applications with more peak demands (e.g., multi-axis machines with frequent motor accelerations and decelerations).

Hybrid Power Supplies Designed for Stepper or Servo Motor Control – Switchers need to be significantly oversized to handle the typical peak loads. Switchers also almost always require some additional user-supplied circuitry to work reliably.

Bulk-linear supplies can supply the high peak power usually required for motion applications (albeit, with output voltage droop) and they have a moderate amount of regen capacity due to their typically large output capacitance.

An RDFC supply could be designed to handle large peak loads. In an RDFC, the switching transistors only turn on and off when they are in a “no-current” or “no-voltage” state. In the more common PWM switcher, the transistors will switch at full power, and their silicon die have to dissipate a lot of heat. Because of this, any amount of power usage in excess of the continuous rated power will quickly heat up the transistors to a damaging level.

In the resonant mode switcher, the transistors dissipate much less power, so the thermal limit in this type of supply is the transformer’s thermal limit. The transformer has much more thermal mass than the silicon inside the transistors, and thus can absorb much higher peak loads and longer duration peaks.

If you combine a resonant mode switcher with a healthy amount of output capacitance and a regen controller, you have a hybrid power supply that is ideal for motion control applications. This hybrid design (Figure 4) combines all of the advantages of the PWM switching supply with the advantages of the bulk-linear supply.

This design also allows your application to draw significantly more than even the rated peak current without causing a shutdown. Exceeding the peak current rating will cause some droop but voltage droop can be acceptable if it allows you to get more current for those moments of high torque demand at lower speed.

Hopefully, you now have a good understanding of the different types of power supplies along with their pros/cons for motion control applications. Unregulated bulk-linear and regulated switched mode supplies are both common but have some drawbacks for motion control applications.

The hybrid resonant mode architecture combines the best features of the other supplies and is ideally suited to providing DC power for servo and stepper motor drives.

This article was written by Abe Amirana, Director, Teknic (Victor, NY). For more information, visit here .

This article first appeared in the December, 2020 issue of Motion Design Magazine.