DC Motors - Advantages and Hazards of running DC Motors
A DC Motor usually consists of: An armature core, an air gap, poles, and a yoke which form the magnetic circuit; an armature winding, a field winding, brushes and a commutator which form the electric circuit; and a frame, end bells, bearings, brush supports and a shaft which provide the mechanical support. See the DC Motor cutaway near the bottom of this resource page.
Advantages of DC Motors
Types of DC Motors
Hazardous Conditions for DC Motors
- Speed Variation
Speed variation is accomplished by changing either the armature voltage or field voltage, or a combination of both. For example, a motor with a base speed of 1750 RPM and armature voltage of 500 VDC will run at 875 RPM with a 50% reduction in armature voltage (to 250 VDC).
- ARMATURE VOLTAGE CONTROL - For this type of speed control the armature voltage is varied while maintaining constant shunt field excitation.
Output torque of a DC motor is proportional to the product of the main pole flux, armature current, and a machine constant that is a
function of armature windings. Therefore, with armature voltage speed control and constant shunt field excitation, the torque is dependent
upon the armature current only. In other words, at rated armature current the torque is constant.
A DC motor, operated with armature voltage control and fixed field excitation, will develop rated torque at rated armature current independent of the speed. This is commonly called constant torque operation.
- SHUNT FIELD CONTROL - With speed control by field weakening, the voltage applied to the shunt field is adjusted by a variable resistance
rheostat in series with the shunt field circuit or by varying the voltage of the shunt field power supply.
Reducing the shunt field voltage decreases the field current, which in turn reduces the field flux allowing the speed of the motor to increase. Increasing the field voltage to obtain a speed below base speed cannot be used, as the field will overheat at higher than rated current. DC motors operated at constant armature voltage and with field weakening have a constant horsepower capacity over their speed range. Field control speed values range from 1:1 to 6:1.
- COMBINATION OF ARMATURE VOLTAGE AND SHUNT FIELD CONTROL - Utilizing both methods of speed control will give wide speed ranges. Armature voltage control is used for speeds below base speed, resulting in a constant torque capacity. Shunt field control is used to obtain speeds above base speeds resulting in a constant horsepower capacity.
- ARMATURE VOLTAGE CONTROL - For this type of speed control the armature voltage is varied while maintaining constant shunt field excitation. Output torque of a DC motor is proportional to the product of the main pole flux, armature current, and a machine constant that is a function of armature windings. Therefore, with armature voltage speed control and constant shunt field excitation, the torque is dependent upon the armature current only. In other words, at rated armature current the torque is constant.
The definition of an electric motor is a device that converts electrical energy into mechanical energy. In reality, a motor could be better defined as a "torque generator". Torque can be defined as a turning force that tends to produce rotation on a shaft. The primary advantage of the DC motor is that it can develop constant torque over a wide speed application.
Power supply is an important consideration in the application of DC motors. The most common way to provide DC voltage to a motor from an AC line is through the use of an electronic drive. Depending on the construction, the drive will provide a pulse wave form similar to the perfect voltage from a battery. These pulses are characterized by a form factor that is defined by NEMA (National Electrical Manufacturers' Association) as a power supply code. Codes are based on the quality of the power output. Application concerns include drive cost, operational cost (efficiency), reliability, and output power quality.
- NEMA POWER CODE A - This power supply is a pure DC power supply such as a battery or a generator. High frequency PWM power supplies will approach NEMA power code A. All NEMA rated DC motors may be operated off this type of power supply without the use of external reactors.
- NEMA POWER CODE C - This power supply is close to being pure and consists of six silicon controlled rectifiers (SCRS) connected in a three phase, full-wave bridge configuration. All NEMA rated DC motors may be operated off this type of power supply without the use of external reactors.
- NEMA POWER CODE D - Power code D contains slightly more distortions than code C and consists of three SCRS and three diode style rectifiers connected in a three phase, full-wave bridge configuration. A freewheeling rectifier is used across the armature terminals. Motors rated 250 HP or less may be operated on this type of supply without the use of external reactors. Motors rated 300 HP or greater may require the use of external reactors when operated on this type of power supply.
- NEMA POWER CODE E - This power supply has average quality and consists of three controlled rectifiers (SCRS) connected in a three phase, halfwave bridge configuration. Most DC motors will require some derating or the use of an external reactor when used on this type of power supply. Consult the factory when this type of power supply is used.
- NEMA POWER CODE K - This power supply has limited applications and consists of two controlled rectifiers (SCRs) and two diode style rectifiers connected in a single phase full-wave bridge configuration. A freewheeling rectifier may be used across the motor armature terminals. This type of power supply is normally used for motors rated up to 7-1/2 HP.
- SHUNT MOTORS - A shunt motor has its armature and field in parallel or it may have separate field and armature supplies. In either case, this type of motor has good speed regulation (5% to 10%) and is capable of delivering 300 percent of its full load torque for a very short period of time.
- SERIES MOTOR - As its name implies, the series DC Motor has its armature and field connected in a series circuit. This type of motor is used where the load requires a high breakaway torque such as locomotive, crane, or oil drilling rig applications. The starting torque developed by a series motor can be as high as 500 percent of its full load torque rating. The series motor is able to deliver this high starting torque due to the fact that its field is operated below saturation. Therefore, an increase in load will result in an increase in both armature and field current. As a result the armature flux and field flux increase together. Since the torque developed in a DC motor is dependent upon the interaction of armature and field fluxes, torque increases by the square of the value of current increase. Therefore, a series motor will yield a greater torque increase than a shunt motor for a given increase in current. The speed regulation of a series motor is inherently poorer than that of a shunt motor. If the load on the motor is reduced the current flowing in both the armature and the field circuits is reduced causing a reduction in their flux densities. This results in a greater increase in speed than would be realized in a shunt motor. If the mechanical load were to be removed from the motor entirely the speed would increase without limit until the centrifugal forces generated by the armature would destroy the motor. For this reason, series DC motors should always be connected to a load.
- COMPOUND WOUND MOTORS - These motors are used whenever it is necessary to obtain a speed regulation characteristic not obtainable with either a series or a shunt motor. This type of motor offers a fairly high starting torque and delivers constant speed under load. This characteristic is achieved by placing part of the field circuit in series with the armature circuit. This configuration is not to be confused with interpoles that contain only a few turns of wire for the purpose of neutralizing armature reaction. When a load is applied, the increasing current through the series winding increases the field flux, thus increasing the torque output of the motor. As a result, this increase in field flux will yield a greater reduction in speed, for a given load change, than a shunt motor.
- PERMANENT MAGNET MOTORS - Permanent magnet motors are generally used where response time is a factor. Their speed characteristic is similar to the shunt wound motor. They are built with a conventional type of armature, but have permanent magnets in the field section rather than windings. Permanent magnet motors are considered less expensive to operate, as they require no field supply. They can, however, lose their magnetism with age and as a result produce less than rated torque. Some permanent magnet motors have windings built into the field magnets for re-magnetizing purposes.
- OVER-TEMPERATURE - Overload is only one cause of over-temperature problems. High ambient temperatures or improper cleaning of filters on the machine itself contribute to short service life by increasing operating temperatures. This in turn causes abnormally high differential expansion stress resulting in cracks in the insulation which usually propagate through to the bare conductor, opening the circuit to contamination failure. In addition, the commonly known effect is the more rapid degradation of the insulation materials that shrink and harden, then gradually lose both strength and insulating characteristics. Ambient temperatures greater than 40 Degrees C are also detrimental to grease, cables, brushes, and commutation.
- CONTAMINATION - Nonconducting contaminants such as factory dust and sand gradually promote over-temperature by restricting cooling air circulation. In
addition, these may erode the insulation and the varnish, gradually reducing their effectiveness.
Conducting contaminants such as metal dust, carborundum, carbon, and salt, in addition to promoting over-temperature, also provide immediate conducting paths for shorting or grounding leakage currents wherever the electrical circuit is contacted. Normal differential expansion, rotational stresses, and thermal expansion of trapped air in voids within the insulation system eventually open the insulated circuit at unpredictable locations. Depending on the severity of the operating voltage, service life may be measured in years, months, days, or hours.
Oil deposits promote easy adhesion of contaminants to the internal insulated and exposed un-insulated surfaces to promote early service life problems.
Water from splashing or condensation seriously degrades an insulation system. The water alone is conducting. Nonconducting contaminants are readily converted into leakage current conductors. Intermittent or occasional wetness ultimately causes service failure because successive leakage situations gradually deposit a permanent path for continuation of the damaging shorting or grounding currents.
- VIBRATION - High vibration promotes service life problems by subjecting the shaft to stress, which finally results in actual shorting of conductors between turns or between layers. In addition, the severe stress causes fissures and cracks in the conductor insulation exposing the electrical circuit to contamination failure. Another important factor is the work hardening effect that this vibration has on the conductor itself, resulting in an open circuit by conduction or cracking. Commutation problems may arise because of brush bouncing. Continued severe vibration fatigues metals and could cause failure in casting or bearings.
- ALTITUDE - Standard motor ratings are based on operation at any altitude up to 3300 feet (1000 meters). All altitudes up to and including 3300 feet are considered to be the same as sea level. High altitude derating is required because of lower air density that requires a greater amount of cooling. DC motors are derated by 3% per 1000 feet above the 3300 feet. In some cases, a blower will be sufficient to cool the motor instead of using a larger frame motor.
- AMBIENT TEMPERATURE - Motors for use in abnormally hot places are usually designed to accommodate the higher ambient by having a lower winding
temperature rise. If the ambient temperature is above 50 Degrees C, special consideration must also be made of the lubricant. Although it's possible to operate in
ambients above 50 Degrees C, application should be referred to the manufacturer to determine what steps must be taken.
In general, the simplest method of derating for high ambients is to derate the horsepower rating and operate the motor at field weakening. In this way, both the armature and field will be operating at reduced current. For ambients lower than 40 Degrees C, a standard 40 Degrees C machine is normally used at rated load. In the case when the ambient is maintained well below 40 Degrees C, a standard ambient motor may be used at overload, provided the following factors are known:
- That the ambient is known always to be low.
- Shaft stresses, bearing loading and commutation are approved by the factory.
- That overload protection for the motor from an over load or stalled condition is available and used.
Operation of motors in ambients below O Degrees C results in severe duty on the machine component parts. Of major concern are the lubrication system and the insulation system.
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