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Direct current motors are used throughout much of industry because of their ability to produce high torque at low speed, and because of their variable speed characteristics. DC motors are generally operated at or below normal speed. Normal speed for a DC motor is obtained by operating the motor with full rated voltage applied to the field and armature. The motor can be operated at below normal speed by applying rated voltage to the field and reduced voltage to the armature.
In SECTION 33, resistance was connected in series with the armature to limit current and , therefore, speed.
Although this method does work and was used in industry for many years, it's seldom used today. When resistance is used for speed control, much of the power applied to the circuit's wasted in heating the resistors, and the speed control of the motor isn't smooth because resistance is taken out of the circuit in steps.
Speed control of a DC motor is much smoother if two separate power supplies, which convert the AC voltage to DC voltage, are used to control the motor in stead of resistors connected in series with the armature ( Ill. 034-1). Notice that one power supply is used to supply a constant voltage to the shunt field of the motor, and the other power supply is variable and supplies voltage to the armature only.
The Shunt Field Power Supply Most solid-state DC motor controllers provide a separate DC power supply, which is used to furnish ex citation current to the shunt field. The shunt field of most industrial motors requires a current of only a few amperes to excite the field magnets; therefore, a small power supply can be used to fulfill this need. The shunt field power supply is generally designed to remain turned on even when the main (armature) power supply is turned off. If power is connected to the shunt field even when the motor isn't operating, the shunt field will act as a small resistance heater for the motor. This heat helps prevent moisture from forming in the motor due to condensation.
The Armature Power Supply The armature power supply is used to provide variable DC voltage to the armature of the motor. This power supply is the heart of the solid-state motor controller.
Depending on the size and power rating of the controller, armature power supplies can be designed to pro duce from a few amperes to hundreds of amperes. Most of the solid-state motor controllers intended to provide the DC power needed to operate large DC motors convert three-phase AC voltage directly into DC voltage with a three-phase bridge rectifier.
The diodes of the rectifier, however, are replaced with SCRs to provide control of the output voltage ( Ill. 2). Ill. 3 shows SCRs used for this type of DC motor controller. A large diode is often connected across the output of the bridge. This diode is known as a freewheeling, or kickback, diode and is used to kill inductive spike voltages produced in the armature. If armature power is suddenly interrupted, the collapsing magnetic field induces a high voltage into the armature windings. The diode is reverse biased when the power supply is operating under normal conditions, but an induced voltage is opposite in polarity to the applied voltage. This means the kickback diode will be forward biased to any voltage induced into the armature. Since a silicon diode has a voltage drop of 0.6 to 0.7 volts in the forward direction, a high voltage spike cannot be produced in the armature.
Voltage Control Output voltage control is achieved by phase shifting the SCRs. The phase shift control unit determines the output voltage of the rectifier ( Ill. 4). Since the phase shift unit's the real controller of the circuit, other sections of the circuit provide information to the phase shift control unit. Ill. 5 shows a typical phase shift control unit.
Ill. 5 Phase shift control board for controlling SCRs.
Field Failure Control As stated previously, if current flow through the shunt field is interrupted, a compound wound, DC motor will become a series motor and race to high speeds. Some method must be provided to disconnect the armature from the circuit in case current flow through the shunt field stops. Several methods can be used to sense current flow through the shunt field. In SECTION 33, a current relay was connected in series with the shunt field . A contact of the current relay was connected in series with the coil of a motor starter used to connect the armature to the power line. If current flow were stopped, the contact of the current relay would open, causing the circuit of the motor starter coil to open.
Another method used to sense current flow is to connect a low value of resistance in series with the shunt field ( Ill. 034-6). The voltage drop across the sense resistor is proportional to the current flowing through the resistor (E = I R2). Since the sense resistor is connected in series with the shunt field, the current flow through the sense resistor must be the same as the current flow through the shunt field. A circuit can be designed to measure the voltage drop across the sense resistor. If this voltage falls below a certain level, a signal is sent to the phase shift control unit and the SCRs are turned off ( Ill. 7).
Current Limit Control The armature of a large DC motor has a very low resistance, typically less than 1 ohm. If the controller is turned on with full voltage applied to the armature, or if the motor stalls while full voltage is applied to the armature, a very large current will flow. This current can damage the armature of the motor or the electronic components of the controller. For this reason, most solid-state, DC motor controls use some method to limit the current to a safe value.
One method of sensing the current is to insert a low value of resistance in series with the armature circuit. The amount of voltage dropped across the sense resistor is proportional to the current flow through the resistor. When the voltage drop reaches a certain level, a signal is sent to the phase shift control telling it not to permit any more voltage to be applied to the armature.
When DC motors of about 25 horsepower or larger are to be controlled, resistance connected in series with the armature can cause problems. Therefore, another method of sensing armature current can be used ( Ill. 8). In this circuit, current transformers are connected to the AC input lines. The current supplied to the rectifier will be proportional to the current supplied to the armature. When a predetermined amount of current is detected by the current transformers, a signal is sent to the phase shift control telling it not to permit the voltage applied to the armature to increase ( Ill. 9). This method of sensing the armature current has the advantage of not adding resistance to the armature circuit. Regardless of the method used, the current limit control signals the phase shift control, and the phase shift control limits the voltage applied to the armature.
Speed Control The greatest advantage of using direct current motors is their variable speed characteristic. Although the ability to change motor speed is often desirable, it's generally necessary that the motor maintain a constant speed once it has been set. E.g., assume that a DC motor can be adjusted to operate at any speed from 0 to 1800 rpm. Now assume that the operator has adjusted the motor to operate at 1200 rpm. The operator controls are connected to the phase shift control unit ( Ill. 10). If the operator desires to change speed, a signal is sent to the phase shift control unit and the phase shift control permits the voltage applied to the armature to increase or decrease.
DC motors, like many other motors, will change speed if the load is changed. If the voltage connected to the armature remains constant, an increase in load will cause the motor speed to decrease, or a decrease in load will cause the motor speed to increase. Since the phase shift unit controls the voltage applied to the armature, it can be used to control motor speed. If the motor speed is to be held constant, some means must be used to detect the speed of the motor. A very common method of detecting motor speed is with the use of an electrotachometer ( Ill. 11). An electro tachometer is a small, permanent, magnet generator connected to the motor shaft. The output voltage of the generator is proportional to its speed. The output voltage of the generator is connected to the phase shift control unit ( Ill. 12). If load is added to the motor, the motor speed will decrease. When the motor speed decreases, the output voltage of the electrotachometer drops. The phase shift unit detects the voltage drop of the tachometer and increases the armature voltage until the tachometer voltage returns to the proper value.
If the load is removed, the motor speed will in crease. An increase in motor speed causes an increase in the output voltage of the tachometer. The phase shift unit detects the increase of tachometer voltage and causes a decrease in the voltage applied to the armature. Electronic components respond so fast that there is almost no noticeable change in motor speed when load is added or removed. An SCR motor control unit's shown in Ill. 13.
1. What electronic component is generally used to change the AC voltage into DC voltage in large DC motor controllers?
2. Why is this component used instead of a diode?
3. What is a "freewheeling" or "kickback" diode?
4. Name two methods of sensing the current flow through the shunt field.
5. Name two methods of sensing armature current.
6. What unit controls the voltage applied to the armature?
7. What device is often used to sense motor speed?
8. If the motor speed decreases, does the output volt age of the electrotachometer increase or decrease?
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