Understanding DC Drives

one of the most primary function of a dc drive is to convert ac voltage into variable dc voltage.which is needed to control the dc motors speed. A thyristor (SCR) is one device used to convert ac into dc.

DC DRIVES - PRINCIPLES OF OPERATION

DC drives, because of their simplicity, ease of application, reliability and favorable cost have long been a backbone of industrial applications. A typical adjustable speed drive using a silicon controller rectifier (SCR) power conversion' section, common for this type unit The SCR, (also termed a thyristor) converts the fixed voltage alternating current (AC) of the power source to an adjustable voltage, controlled direct current (DC) output which is applied to the armature of a DC motor.

SCR's provide a controllable power output by "phase angle control", so called because the firing angle (a point in time where the SCR is triggered in to conduction) is synchronized with the phase rotation of the AC power source. If the device is triggered early in half cycle, maximum power is delivered to the motor; late triggering in the half cycle provides minimum power, as illustrated by Figure 3. The effect is similar to a very high speed switch, capable of being turned on and "conducted" off at an infinite number of points within each half cycle. This occurs at a rate of 60 times a second on a 60 Hz line, to deliver a precise amount of power to the motor. The efficiency of this form of power control is extremely high since a very small amount of triggering energy can enable the SCR (Silicon Controlled Rectifier) to control a great deal of output power.

AC DRIVES - PRINCIPLES OF OPERATION

Adjustable frequency AC motor drive controllers frequently termed inverters are typically more complex than DC controllers since they must perform two power section functions, that of conversion of the AC line power source to DC and finally an inverter change from the DC to a coordinated adjustable frequency and voltage output to the AC motor. The appeal of the adjustable frequency drive is based upon the simplicity and reliability of the AC drive motor, which has no brushes, commutator or other parts that require routine maintenance, which more than compensates for the complexity of the AC controller. The robust construction, and low cost of the AC motor makes it very desirable for a wide range of uses. Also, the ability to make an existing standard constant speed AC motor an adjustable speed device simply by the addition of an adjustable frequency controller creates a very strong incentive for this type of drive.

Greetings,

This is my first post to this forum. I found this post going through our visitor logs.

This info came from our website, joliettech.com which has a lot more information regarding AC Variable Frequency Drives and DC Variable Speed Drives. I'm glad to see that people are finding the information and using it for it's intended propose, to learn about VSD's and their advantages.

If you would like more information about AC and DC Drives go to the information section of our site, http://www.joliettech.com/information.htm.

John Gierich

Welcome to CR4. It's good to know that there is somebody out there that understands motors. Be warned however you are likely to get at least one lunatic a month that recons they have invented a new type of electromagnet drive system that works at better than 100% efficiency. I have spent many hours trying to explain to people why it doesn't work the way they think it dose. Motor theory was never my strongest subject so it tends to be a bit of a strain but now we know who to ask in future.

Welcome aboard.