training - power supplies
How to Choose Power Supplies
Which is better, Linear or Switcher Supplies?
Since Servo Motors are inductive they may run highly dynamic motion profiles. As a result, their current demand can vary widely. Surge currents from stand-still to maximum load may be extremely high, yet steady state curret demand over time may be relatively mild. As a result, proper care should be taken when selecting power supplies.
Animatics offers two basic types of power supplies.
The below chart gives a brief comparison of the two types of supplies.
|AC Input||Field selectable
|Power Factor Corrected||NO||Yes|
|Relative Size||Big and bulky||Lightweight|
|Cooling||Ambient Convection||Fan cooled|
|Voltage Regulation||15% Drop over range||0%, fixed8|
but not typically
|In most cases,
1See Shunt Section for more information!
Voltage Drop Comparison
As seen in the graph to the right, Linear (Unregulated) suppllies can handle large surge current loads. This is because Linear Supplies typlically contain large output capcitors to handle those surges well.
Voltage regulations: Switchers are highly regulated supplies. They will maintain fixed voltage until they reach maximum load and then will "crowbar" to zero volts to protect the output stages. Linear supplies will slowly drop in output voltage while supplying more and more current.
This is the most funamental difference between switchers and unregulated supplies.
Even though a switcher cannot handle the higher current surges, if it can ouput as much current as you would expect for a given servo application, then they will actually help the servo accelerate much faster because system voltage will be maintained at maximum level.
However, if your servo application requires surge currents in excess of 50 Amps or more, the switchers may not be cost effective. Getting 50 amps from an Animatics 20 Amp supply is easy. Getting 50 Amps from Animatics switchers would require placing multiple units in parallel, so it may not be cost effective to do so.
Intro to Shunts
Animatics Corporation offers several shunt options for use with DC input servo motors.
Shunts are needed to protect the servo controller and drive stages from over voltage.
Over voltage sources originate from the following:
- Back EMF due to back driving the motors
- Sudden or hard decelerations
- Hard stop crashes (immediate deceleration to zero speed)
- Vertical load drops
The shunts actually add an additional load to the DC bus automatically when voltage exceeds the trigger level by connecting large load resistors across the bus. Trigger voltage is typically 49.5VDC. As a result, the shunts will work with any of the supplies we offer.
The switcher supplies have an adjustable output trim pot. If used with our shunts, the output voltage MUST BE adjusted to <+48vdc to="" insure="" the="" shunts="" do="" not="" stay="" gated="" on="" span="">
- Shunts cannot be placed in parallel with each other to increase capacity. The shunt with the slightly lower trigger voltage will trigger first while the other shunt never triggers at all. Please consult factory for information on how to deal with larger shunt requirements.
- Shunts should always be placed between the motor input and any disconnect or e-stop relay to insure protection of the motor when power is not applied or e-stop relay contacts are open.
The Real story about Back EMF:
Generally speaking, back EMF is the voltage generated in a motor when it spins. This voltage is typically propotional to speed. However, this is a general rule. The truth is that the back EMF voltage is proportional to the rate of change of magnetic flux in the windings of the stator. As a result, consttant speeds produce contant and predictable voltages. However, sudden changes due to decelerations or hard stop crashes cause an immediate change in magnetic flux or even a total instantaneous collapse. As a result, voltages can go 5 to 10 times higher than spinning the motor at its maximum speed.
For this reason alone, it is highly recommended to use a shunt in all vertical load applications or any case where the motors could be stopped quickly or back driven suddently.
We offer both open frame and enclosed shunts in 100Watt and 200Watt capacities. The shuns are all automatic and get their power from the DC bus they are attached to. They simply need to be placed in parallel with the DC bus.