Motion Control - ElmoMC

Quality & Performance


Power Supply Quality

Many times an amplifier is blamed for poor (or strange) performance while the reason stays at one of the peripheral units: a power supply or a transformer.
The following paper includes a detailed discussion about the effect of "Power Supply Quality" on the servo performance.
It analyzes Motoring problems and Regeneration problems in two separate sections.
When using a discrete amplifier and power supplier design, it is rather simple to perform tests similar to thefollowing examples. However, while using integrated amplifier/power supply drive, it is quite difficult (for the user) to perform such investigation. It does not mean that a packaged unit is immune from these problems. It just requires good servo understanding and a lot of experience in servo systems.

Motoring: Using a properly connected oscilloscope, the DC bus voltage can be monitored during the motoring cycle and compared with the motor speed. The DC bus voltage during motor's standstill is compared with the DC bus voltage during the last portion of the motor(s) acceleration. If the DC bus voltage is not stiff enough (see Figure 1) it indicates that: Either.
  • The power supply's capacitance is not sufficient for this application, Or
  • The transformer is under-rated for this application.


Figure-1: Voltage during acceleration

Solving the above problem can be done by either,

  • Adding capacitance to the filtering stage. Or,
  • By replacing the transformer with a higher power one. Or By both above actions.

Regeneration: During regeneration condition, the DC bus voltage is being charged by the amplifier (the kinetic energy in the system is converted to electrical energy in the capacitors). If the system is sized properly and the power supply includes a shunt regulator, the following plots will be recorded on the oscilloscope. (Fig 2).

Vr - Regulating voltage

t1 - Beginning of deceleration

t2 - Beginning of shunt operation

t3 - End of shunt operation

Figure-2: DC bus voltage vs. Motor speed with correctly sized shunt regulator

 

 

If the selected power supply does not have enough regeneration capacity, the motor's deceleration will be slowed and will sound as an erratic braking. This case is plotted in the following figure (Fig 3).

OV - Amplifier's over voltage hysteresis are

Vr - Regulating voltage of the shunt

t1 - DC bus value reaches over voltage point

t2 - Over voltage condition is released

t3 - DC bus value reaches over voltage point

Figure 3: DC bus voltage vs. Motor speed with a weak shunt regulator

 

 

Solving the above problem can be done by either,

  • Adding capacitance, Or
  • By paralleling the power supply with an additional PSS unit.