Programming parameters associated with DC drives are extensive & similar to those used in conjunction with AC drives. An operator's panel is used for programming of control setup & operating parameters for a DC drive.
DC drives with motor field control provide coordinated automatic armature & field voltage control for extended speed range & constant-horsepower applications. The motor is armature-voltage-controlled for constant-torque, variable-horsepower operation to base speed, where it s transferred to field control for constant-horsepower, variable-torque operation to motor maximum speed.
Controlled bridge rectifiers are not limited to single-phase designs. In most commercial & industrial control systems, AC power is available in three-phase form for maxi mum horsepower & efficiency. Typically six SCRs are connected together, to make a three-phase fully controlled rectifier. This three-phase bridge rectifier circuit has three legs, each phase connected to one of the three phase voltages. It can be seen that the bridge circuit has two halves, the positive half consisting of the SCRs S1, S3, & S5 & the negative half consisting of the SCRs S2, S4, & S6. At any time when there is current flow, one SCR from each half conducts.
Armature voltage-controlled DC drives are constant torque drives, capable of rated motor torque at any speed up to rated motor base speed. Fully controlled rectifier circuits are built with SCRs. The SCRs rectify the supply voltage (changing the voltage from AC to DC) as well as controlling the output DC voltage level. In this circuit, silicon controlled rectifiers S1 & S3 are triggered into conduction on the positive half of the input waveform & S2 & S4 on the negative half.
DC drives vary the speed of DC motors with greater efficiency & speed regulation than resistor control circuits. Since the speed of a DC motor is directly proportional to armature voltage & inversely proportional to field current, either armature voltage or field current can be used to control speed. To change the direction of rotation of a DC motor, either the armature polarity can be reversed, or the field polarity can be reversed.
Non-regenerative DC drives, also known as single-quadrant drives, rotate in one direction only & they have no inherent braking capabilities. Stopping the motor is done by removing voltage & allowing the motor to coast to a stop. Typically nonregenerative drives operate high friction loads such as mixers, where the load exerts a strong natural brake. In applications where supplemental quick braking and/or motor reversing is required, dynamic braking & forward & reverse circuitry, may be provided by external means.
DC drive technology is the oldest form of electrical speed control. The
speed of a DC motor is the simplest to control, & it can be varied
over a very wide range. These drives are designed to handle applications
such as:
Winders/coilers - In motor winder operations, maintaining tension is very important. DC motors are able to operate at rated current over a wide speed range, including low speeds.
Winders/coilers - In motor winder operations, maintaining tension is very important. DC motors are able to operate at rated current over a wide speed range, including low speeds.
1. List three types of operations where DC drives are commonly found.
2. How can the speed of a DC motor be varied?
3. What are the two main functions of the SCR semi conductors used in a DC drive power converter?
4. Explain how SCR phase angle control operates to vary the DC output from an SCR.
5. Armature-voltage-controlled DC drives are classified as constant-torque drives. What does this mean?
6. Why is three-phase AC power, rather than single phase, used to power most commercial & industrial DC drives?
2. How can the speed of a DC motor be varied?
3. What are the two main functions of the SCR semi conductors used in a DC drive power converter?
4. Explain how SCR phase angle control operates to vary the DC output from an SCR.
5. Armature-voltage-controlled DC drives are classified as constant-torque drives. What does this mean?
6. Why is three-phase AC power, rather than single phase, used to power most commercial & industrial DC drives?
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Mentoring, a valuable source of learning and maturing as an engineer.
Lead Acid Battery Charging Question
Distributed Generation (DG)
Circuit breaker failure protection
HART Communication with Siemens PLC
Estimate ROI for BIPV (building integrated photovoltaic)?
How to properly test directional relays?
Mentoring, a valuable source of learning and maturing as an engineer.
