Silicone Controlled Rectifier VS. Pulse Width Modulation 1

[stabmaster]

Can someone please help me understand the practical meaning of these two "terms" or "methods"? Let's put it into context:

In order to control a, say BLDC 1/10HP motor one needs a DC motor drive. In my case 115VAC power source and an output DC voltage of 0-90VDC usually, for a 1/4 HP motor drives. The rating is usually 1/4 for SRC motors and 1/3 for PWM motors. Not entirely sure what that means, or why the difference in HP range.

The control signal can be by way of "signal following input voltage range 0-5DC" or by way of potentiometer. This is the example here:

http://www.electrosales.com/kb/pdf/kbws.pdf

I can't seem to figure out if PWM is actually "performed" by the driver, and why the driver is specified as for both "SCR or PWM" motors. Any clarification would help me with my scrambled brainsauce. Thanks!

 

[ScottyUK]

The drive is a PWM drive. It is declared as suitable for motors intended for use with either SCR or PWM sources. Neither an SCR drive nor a PWM drive give 'perfect' DC as would be delivered by a battery because both contain an AC ripple component. The additional ripple superimposed on the DC when either an SCR or PWM drive is used requires a derating of the motor compared with its rating on a pure DC source. PWM drives produce lower amplitude ripple at a higher frequency than a SCR drive because they switch much faster, and thus are closer to the 'perfect' DC source than an SCR drive, so a motor running on a PWM drive requires less derating than the same motor running on a SCR drive.

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Sometimes I only open my mouth to swap feet...

 

[stabmaster]

Thank you! So PWM requires less derating but "all else" are not equal over the entire range of frequencies, correct? In other words, are there trade-offs in design when deciding on the operating speed of the motor, or is PWM always a better signal, or more "perfect"?

 

[cswilson]

I think it is fair to say that PWM is always a better signal, so it becomes a question of comparing price, and if the PWM solution is significantly more, whether it is worth the premium. At this power level, the PWM transistors are going to switch at 10 kHz or above, whereas the SCR thyristors can only switch at 100 or 120 Hz (for 50 or 60 Hz input, respectively). So the current ripple will be on the order of 100 times higher for SCR than PWM, which is why the motors need extra derating.

Also, a PWM solution has an inherently linear relationship between duty cycle and resulting time-averaged voltage, but the SCR solution has a distinctly non-linear relationship. I don't know how well inexpensive SCR drives compensate for this non-linearity.

Curt Wilson
Delta Tau Data Systems