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DC Machines and Controls

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Construction of a DC Motor

• Frame
• Shaft
• Bearings
• Main Field Windings (Stator)
• Armature (Rotor)
• Commutator
• Brush Assembly




Types of Motors

The types of motors are series, shunt, compound, permanent magnet (or electromagnet). Here is a brief description of each:

Series - In a series motor, the field is connected in series with the armature windings.


Shunt - In a shunt motor, the field is connected in parallel with the armature windings.


Compound - In a compound motor, a field is connected in series with the armature, and they have a separately excited shunt field.


Permanent Magnet - In a permanent magnet motor, a magnet is used to supply field flux. It is also the only motor that you can change line polarity to change the direction of the armature.


Nameplate on a Motor

The following specifications are generally indicated on the nameplate:

• Manufacturer’s type and frame designation
• Horsepower at base speed
• Maximum ambient temperature
• Insulation class
• Base speed at rated load
• Rated armature voltage
• Rated field voltage
• Armature rated load current
• Winding type (shunt, series, compound,permanent magnet)
• Enclosure




Horsepower

The horsepower rating of a motor refers to the horsepower at base speed.



Winding


The type of field winding is also listed on the nameplate. See below for an example for each common motor.




Insulation Class

The National Electrical Manufacturers Association (NEMA) insulation classes are A, B, F, and H.
The NEMA standardized air temperature is 40°C. An example would be if a motor with Class F insulation has a maximum temperature rise of 105°C. Add the 40°C ambient, which gives a maximum winding temperature of 145°C.


Anything past the 145°C is called the motor's hot spot. For each 10°C temperature increment above the 145°C (as an example), the life expectancy of the motor will be cut in half.


Armature Voltage

The following armature voltage equation demonstrates the various principles of a DC motor. This equation can be used to demonstrate how armature voltage, CEMF, torque, and motor speed interact.

Va = (KtΦn) + (IaRa)
Where:
Va = Applied Armature Voltage
Kt = Motor Design Constants
Φ = Shunt Field Flux
n = Armature Speed
Ia = Armature Current
Ra = Armature Resistance

CEMF

Rotation of the armature through the shunt field, induces a voltage in the armature (Ea) that is in opposition to the armature voltage (Va). This is counter electromotive force (CEMF).
CEMF is dependent on armature speed (n) and shunt field (Φ) strength. An increase in armature speed (n) or an increase of shunt field (Φ) strength will cause an increase in CEMF (Ea). Ea = KtΦn or Ea = Va - (IaRa)