DC Power Supplies

Spinning motors act like generators; they produce a voltage whose polarity induces a decelerating or braking current. So in order to accelerate a motor or maintain constant velocity, a motor drive must apply a voltage in excess of this generated voltage so that the net voltage produces the desired current. When the motor drive wants to decelerate the motor, it can let the regenerated energy flow back into the power supply slowing down the motor in the process. Ideally, for efficiency, the regenerated energy will be stored by the power supply to be used again for the next acceleration phase.

Most switching power supplies can not handle this reverse current and will shut down (or operate erratically). A power supply designed for servo and stepper motor drive applications can losslessly store a large amount of regenerated energy.

If the “regen” power climbs to a level beyond what can be stored (e.g., during aggressive deceleration of large inertial loads, lowering of gravitational loads, etc.), the ideal power supply will shunt the excess energy into a load resistor to safely dissipate it. This is not as efficient as storing and reusing the energy, but it’s much better than applying excessive voltage to the drive causing it to shut down or get damaged.

Motion control applications typically have periods of acceleration, periods of constant velocity, and periods of deceleration. They often also have significant periods of standstill. Acceleration (and to a lesser degree, deceleration) takes significantly more power than running at constant velocity and standing still. But because the accel and decel periods are typically a small fraction of the overall move profile, the average power required of the motor (and hence the power supply) over the entire motion cycle is typically much lower than the peak power. (This is less true in a stepper motor system than a servo system because steppers use a relatively large amount of power even when moving at constant speed or standing still.)

If a power supply can not adequately supply large amounts of peak power, the supply will need to be sized such that its continuous power rating is sufficient to meet the peak motor demands. This leads to a large and expensive power supply. A motor drive power supply is designed to supply large peaks of power without problem, and therefore can be physically smaller and less expensive.

There are many electronic components used in machine automation that run on 24 volt power, which is one reason why 24 volt power supplies are readily available from dozens of suppliers. This makes it tempting to use 24 volts to power your DC-powered motor drive. But the economics of the semiconductors (and other electronic components) used in servo motor and stepper motor drives have caused most drive manufacturers to make their low-voltage drives run all the way to 75 volts (or maybe a little higher).

If you run a 75 volt drive from a 24 volt supply, it will typically work, but you are literally throwing away two-thirds of the mechanical power you could easily get from your system. Moving to a larger motor to get more power is much more expensive than simply supplying more voltage. This is why power supplies for low-voltage motor drives are usually in the 60 to 80 volt range. The same thing holds true for higher voltage drives which are typically designed to internally run off of 300 volts DC.