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Phase Controlled Rectifier - SCR Drive

Phase Controlled Rectifier - SCR Drive

Phase Controlled Rectifier - SCR Drive

Application: 240 VDC Armature Motor
             415 VAC 3-Ph. SCR Phase Controlled Drive
             (max output 500 VDC)

I am aware that when using a PWM type DC controller, the DC Motor's Armature Voltage should be rated at, or above, the DC Bus Volts of the controller. This is necessary to prevent premature brush ware, as the motor is exposed to voltage spikes peaking at the DC Bus Volts during switching.

With a Phase Controlled Rectifier or SCR type DC drive, should the motor be rated for the maximum DC Voltage Output of the drive as in the above case ?

I believe that the motor can be rated bellow the maximum DC voltage output of the controller, since the output voltage is not generated from a DC Bus, but rather by clipping the input line voltages.

If anyone can offer any input or comments on the above, please do so.

Thanks in Advance.

RE: Phase Controlled Rectifier - SCR Drive

When sizing a DC drive, the voltage output of the drive should equal the rating of the motor if the motor is rated for that type of controller and if the motor and controller are NEMA standard. Specifically, most modern motors are rated for a S6R, or six pulse DC rectifier and take into account in the ratings any motor heating caused by the harmonics of the drive. There is a specific NEMA term for the "power supply class" (ie. DC generator, S6R, S3R=class I, II, or III...don't have it in front of me right now) that is included in the motor specs to address this question. I can define the NEMA power supply classes tomorrow if anyone is interested (reference at work).

Assuming that the motor is rated for the controller (in terms of power supply class), the next issue is the rating of the drive versus the motor in terms of current output. More specifically, a 10hp, 500VDC motor will have an armature current of approx. 18 amps, where a 10 hp, 240VDC motor will require 38 amps. If you are sizing the drive on armature current, you are ok. However, if you are sizing the drive by HP, you need twice the HP rating of the motor to ensure proper current rating of the drive.

Another issue may be power quality. A dc drive running at less than full voltage output has a poor power factor. Associated with the poor power factor is an increased level of 5th and 7th harmonics on the AC line supplying the drive. If the drive is big and/or your facility is small, this may be an issue.     

RE: Phase Controlled Rectifier - SCR Drive

Suggestion: The DC bus may need an additional treatment, e.g. by filtering.
Additional products, e.g. ABB DCS 400, etc., could be checked since they claim that the IGBT switching increases carbon brush lifetime. Autotuning for armature and field currents could be found useful.

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