Motors have windings that are electrically just inductors, and with inductors comes resistance and inductance. Winding resistance and inductance result in an L/R time constant that resists the change in current. It requires five-time constants to reach nominal current. To effectively manipulate the di/dt or the rate of charge, the voltage applied is increased. When traveling at high speeds, there is less time between steps to reach current. The point where the rate of commutation does not allow the driver to reach full current is referred to as Voltage Mode. Ideally, you want to be in Current Mode, which is when the driver is achieving the desired current between steps. Simply stated, a higher voltage will decrease the time it takes to charge the coil and therefore will allow for higher torque at higher speeds.
Regeneration current (Back EMF)
Also, a characteristic of all motors is Back EMF, and though nothing can be done about back EMF, we can give a path of low impedance by supplying enough output capacitance. Back EMF is a source of current that can push the output of a power supply beyond the maximum operating voltage of the driver and as a result could damage the stepper driver over time.
How steppers use power
Bipolar chopping steppers are very current efficient as far as the power supply is concerned. Once the motor has charged one or both windings of the motor, all the power supply has to do is replace losses in the system. The charged winding acts as an energy storage in that the current will re-circulate within the bridge, and in and out of each phase reservoir. While one phase is in the decaying stage of the chopper, the other phase is in the charging stage; this results in a less than expected current draw on the supply.
Stepper motor drivers are designed with the intention that a user’s power supply output will ramp up to greater or equal to the minimum operating voltage. The initial current surge is quite substantial and could damage the driver if the supply is undersized. If a power supply is undersized, upon a current surge, the supply could fall below the operating range of the driver. This could cause the power supply to start oscillating in and out of the voltage range of the driver and result in damaging either the supply, driver or both. There are two types of supplies commonly used, regulated and unregulated, both of which can be switching or linear. All have their advantages and disadvantages.
An unregulated linear supply is less expensive and more resilient to current surges. However, the voltage decreases with increasing current draw. This can cause serious problems if the voltage drops below the working range of the driver. Also of concern is the fluctuations in line voltage. This can cause the unregulated linear supply to be above or below the anticipated voltage.
A regulated supply maintains a stable output voltage, which is good for high-speed performance. They are also not bothered by line fluctuations. However, they are more expensive. Depending on the current regulation, a regulated supply may crowbar or current clamp and lead to an oscillation that as previously stated can lead to damage. Back EMF can cause problems for regulated supplies as well. The current regeneration may be too large for the regulated supply to absorb and may lead to an over voltage condition.
Switching supplies are typically regulated and require little real-estate, which makes them attractive. However, their output response time is slow, making them ineffective for inductive loads. IMS has designed a series of low-cost miniature non-regulated switchers that can handle the extreme varying load conditions which make them ideal for stepper motor drivers and DC servo motors as well.
Rules of thumb
- Type: Unregulated Linear
- Ripple Voltage: ± 5%
Cabling best practices
- Use shielded twisted pairs.
- Do not run power cable along side signal cablings such as communications and I/O.
- Earth the cable shield at the power supply end of the cable.
10-25′ (3 – 7.6 m)
|MDrive Plus NEMA 14||+12 to +48||0.75 A||20|
|MDrive Plus NEMA 17||+12 to +48||2.0 A||20|
|MForce MicroDrive||+12 to +48||3.0 A||18|
|MDrive Plus NEMA 23||+12 to +75||2.0 A||20|
|MDrive Plus NEMA 23 (Quad length)||+12 to +60||3.5 A||18|
|MDrive Plus NEMA 34 (85 mm)||+12 to +75||4.0 A||18|
|MForce PowerDrive||+12 to +75||4.0 A||18|
|Lexium MDrive NEMA 17 / LMM||+12 to +48||2.0 A||20|
|Lexium MDrive NEMA 23||+12 to +60||3.5 A||18|
|Lexium MDrive NEMA 34||+12 to +70||4.0 A||18|