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AC motor drives are pretty well documented; topologies, inverters, frequency control, volts-per-hertz, space vector modulation. It's all out there in a dozen places.

But what is a DC motor drive, and how does it control the motor?

Stephen Collings
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    Motor driver use Current sense for torque or acceleration feedback and Vavg sense for RPM as no load RPM/V is voltage controlled, precise speed requires other RPM feedback, hall sense Tach , or shaft encoder etc. This is just general theory. details depend on specs. – Tony Stewart EE75 Apr 04 '17 at 14:16
  • You surprise me that you think AC motor drive info' is plentiful but that DC motor drive info' isn't. It's all out there, well written up. – TonyM Apr 04 '17 at 14:49
  • It controls it by adjusting the input/output power. The feedback associated control strategy depends on what you are controlling: acceleration, speed, location, torque... – user3528438 Apr 04 '17 at 14:51
  • " topologies, inverters, frequency control, volts-per-hertz, space vector modulation. It's all out there in a dozen places." - why all that stuff for AC motors but not DC? Because they don't need it! – Bruce Abbott Apr 04 '17 at 19:26

4 Answers4

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It doesn't drive the motor. I assume you're talking about a brushed DC motor. Brushed DC motors have the commutation built into the mechanics of the motor - the brushes are part of that. You can just apply a voltage and it'll go. But since voltage relate to speed and current relates to torque then you can start doing fancy things to control those variables to have the motor behave in a desired way. Brushed DC motor controllers are there simply to help with one or both of those parameters but are not involved in commutating it.

kkemper
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I assume that "DC Motor" refers to a commutator motor and not a brushless DC motor.

Most motor speed control is accomplished by providing a means by which the motor's torque vs. speed characteristic curve can be adjusted so that the speed vs. torque demand curve of the load can intersect it at any desired speed. The intersection of the motor's speed vs. torque capability curve and the load's speed vs. torque demand curve is the speed and torque operating point.

With a commutator motor, that can be done by changing the voltage supplied to the armature, the current supplied to the field, or both, either separately or simultaneously. The most common method is to provide a constant field flux either by using a dedicated power supply for the field or by using permanent magnets. The speed vs. torque curves shown below illustrate this method.

enter image description here

Note that only the curve for the lowest voltage intersects the Y axis at the locked-rotor torque point. Curves for higher voltages result in current that would exceed the capability of the controller and perhaps also the motor. Controllers for all but the smallest DC motors have a current limit feature that prevents the motor from operating on the dotted portion of the curve. Since the current is limited by regulating the voltage, operation on a curve of a higher voltage is prevented until the motor has accelerated to a speed at which the current is below the limit.

Better Performance

For better performance, closed-loop armature current control can be used to control the torque. The torque reference for speed control would be speed error from either an armature voltage controller or tachometer feedback. If tachometer feedback is not used, a means for estimating armature IR voltage drop can be used to improve speed regulation.

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DC motors' drivers use PWM (pulse width modulation) to Drive Motor and control their speed you can make PWM using 555 IC OR any microcontroller and a Mosfet, but If you want to control it to change the direction of the spinning shaft you need an H-bridge H-bridge is some transistors working together to bias motor in forward direction and also backward direction by reversing the polarity So if you use both PWM and H-bridge you will have a really good driver that helps you control the speed and also the direction of motor Spinning

AHIVO
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  • Not all DC drives are PWM, in the sense you are thinking. Almost anything over 375 W (1/2 HP) uses an SCR bridge with relatively simple triggering. Analog SCR phase controlled drives are fully capable of nearly infinite variations in armature voltage or current control and speed. Digital drives, with an SCR power end, are not capable of such resolution due to the finite step value of whatever word size you are using. – R Drast Apr 05 '17 at 11:59
  • Good Point I did not know that Thank you – AHIVO Apr 06 '17 at 05:31
  • My 2kw servo uses pwm. And coming 15kw will use pwm too. –  May 07 '17 at 17:24
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On motors using 3 phase AC supply in the US of 5 hp or larger there are 2 D.C. Control approaches. Smaller systems may use diodes to rectify the AC voltage then use SCRs or IGBT transistors to pulse width modulate this rectified supply to the motor.

The more common approach for larger horse power motors (20hp to 200hp) is to use a "stack" of SCRs. Non regen drive systems can only supply torque in one direction and will have 3 SCRs one for each of the 3 phases. Regen drives will have 6 SCRs, two for each phase. The control of the SCRs will use some type of phase angle firing on the sine wave to control motor speed and torque.

Most D.C. Drives have another control board dedicated to field control. Field control is usually in 2 flavors - either constant voltage or constant current- with constant current being more common as field weakening for operation above base speed uses reduction in field current.

Tinkerer
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