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I am designing a DC motor driver circuit with an H-Bridge and capability for PWM speed control.

The input DC voltage is coming from rectifying the mains voltage (127 Vrms), which gives me about 178 DC Volts to play with.

This circuit should be able to drive various types of motors, including ones rated for 180 V, 150 V and 90 V.

My question is if I need to include a buck converter to supply the correct voltages for the lower voltage rated motors, or if I could get away with using the PWM capability to drive them without exceeding their power rating.

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    \$\begingroup\$ You can drop the power via PWM as you describe. What you can't do is drop the absolute maximum voltage. Possible the motor internally can't handle high voltages w/o some kind of dielectric breakdown (i.e. punching through the varnish on the wires). Note also that 127VRMS is not a constant... Different times of day it'll be a different voltage. Meaning your motor will spin at different speeds unless you have some kind of regulator. \$\endgroup\$
    – Kyle B
    Nov 22, 2021 at 18:40

1 Answer 1

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The input DC voltage is coming from rectifying the mains voltage (127 Vrms), which gives me about 178 DC Volts to play with.

With full-bridge, you get about 127V DC equivalent, and the peak at about 178V, considering possible max loading.

My question is if I need to include a buck converter to supply the correct voltages for the lower voltage rated motors, or if I could get away with using the PWM capability to drive them without exceeding their power rating.

In short, you do not need a buck converter, normally. PWM is simpler, easier to control, more energy efficient, cost effective.

Snake lags (Redundant):
DC Linear motor model is a resistor in series with a Voltage source (BEMF). Power rating is from the design factors of the motor, electrical and mechanical. In order to meet the mechanical spec (speed, torque, operating temperature, etc.), electrical parts are designed to be capable of driving the mechanism sufficiently, both with margin. Most of the time, dielectric insulation capability far exceeds the voltage spec. What can break with the voltage is to heat up the internal impedance (and junctions) by the current flowing through (thus the power).


Edited.
I have used direct bridge rectified 120V line to control 68V DC motor with PWM, decades ago. The "thumper" is still regarded as professional, not gonna break easily. What prevents motor damage is the control circuitry.

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  • \$\begingroup\$ You need to use a capacitor after the bridge rectifier, so the OP is correct that there would be about 178 VDC (if the input is 127 VRMS). Operating without an output capacitor means having raw rectified output, not recommended for any type of PWM. \$\endgroup\$ Nov 22, 2021 at 23:33
  • \$\begingroup\$ @JackCreasey , Thanks for checking. OP will never see 178V once load has been applied, regardless of how much capacitor he has, aside the cost. It is not practical. What OP concerns is possible high voltage on lower voltage spec-ed DC motor. Meantime, you would see where the OP is heading here. The OP is going on a right track, needs only a little guidance. \$\endgroup\$
    – jay
    Nov 23, 2021 at 0:46
  • \$\begingroup\$ I agree there will always be ripple at load ...but having no caps WILL NOT work ...the 127 VDC average you mention is quite misleading since it cannot be used to power the PWM driver. \$\endgroup\$ Nov 23, 2021 at 4:13
  • \$\begingroup\$ @JackCreasey , I did not lie, "I have used direct bridge rectified 120V line.. decades ago", in my answer post. The product is still selling at the best price, It uses PID controlled PWM, no capacitor at all. I received an award for the design from a notable magazine of those days, getting emails from all over the world. Take time, and give some thoughts. It is simple after all. If you'd like, we can talk about it offline. \$\endgroup\$
    – jay
    Nov 23, 2021 at 4:43
  • \$\begingroup\$ I don't doubt you may have done this in the past, but with most of todays PWM controllers it would not work. If you know of any PWM controller that you can feed with simple rectified DC and work, I'd be most interested to learn of it. I'd be fascinated to see your magazine article. \$\endgroup\$ Nov 23, 2021 at 5:09

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