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Can someone explain the Armature Output Waveform from a Parker 514C Controller?

Here is a trace from my oscilloscope of the armature output of a 514C controller.  The controller is powered by 245V single phase mains.  There are voltage pulses at 60Hz that reach a maximum of 342V and a minimum of 158V and and average of about 280V.  I expected more of a flat DC like waveform from the controller.  Why are there pulses and not a flat voltage output?

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  • Hi John.  This is actually quite normal for a drive like the 514C.  The 514C (and all of Parker's brushed DC motor drives) use silicon-controlled rectifiers (SCRs), also known as thyristors.  These devices are like diodes that are need an initial pulse to turn on.  Once turned on, they conduct current in a single direction until the voltage reverses, like a one-way valve.  There is a diagram of how this works on Wikipedia here:

    Thyristor - Wikipedia

    This is very different from how a typical DC power supply works.  A DC power supply typically uses a transistor (IGBT or MOSFET) in conjunction with capacitors, inductors and feedback to produce a constant, smooth DC voltage.

    DC drives, on the other hand, produce a very rough and harsh DC supply that resembles a sawtooth wave (more apparent when using a DC drive running on 3-phase).  The average voltage is close to the the desired output voltage, but the voltage swing is massive and in-step with the mains frequency.

    Modern industrial DC motors (well, relatively modern) use higher-quality insulation than DC motors of prior decades so that they can be used with DC drives.  The better insulation helps them stand up to the higher heating and voltage transients that DC drives can generate on the armature.  Nearly all industrial brushed DC motors in service today can be used with an SCR-based drive.  The exceptions are motors with very low inductance (usually billed as DC servos in their literature) and the occasional ancient motor found still running in a motor-generator setup.

    The long and short is that DC drives can run DC motors because the motors don't care that the voltage is all over the place as long as the average is correct.  However, you definitely do not want to use a DC drive in place of a true DC power supply for electronics -- that will destroy the electronics.

  •   The exceptions are motors with very low inductance (usually billed as DC servos in their literature) and the occasional ancient motor found still running in a motor-generator setup.

    That has me a bit worried as that is exactly the type of motor I am trying to drive.  A 1942 3HP Reliance DC motor.  Mostly used in elevators, but in my case a metal lathe.

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