Elec467 Power Machines & Transformers Electric Machines by Hubert, Chapter 11 Topic: Characteristics of Shunt, Compound and Series DC motors

Compound DC motors Compound motors have an extra winding in series with the armature and positioned so its flux field is additive with the shunt field winding. Thus two windings, shunt and series, combine to form a single mmf seen by the armature. Shunt motor were covered in Chapter 10

Pole showing shunt and series coils

Series DC motors A series motor eliminates the shunt winding. The series winding suffices to operate the motor. It consists of a heavy winding to handle high current flow. If the load on a Series motor is lost, it will accelerate to destructive levels because TD > Tload

Characteristics of motor types This graph is actually two graphs that share the x-axis in common. Speed and torque characteristics of the three DC motor types are compared. A Series motor is the most non-linear but are capable of extremely high torque at low speeds (with high current). Whatever a shunt motor can do…a compound motor can do better (slightly). In common are the crossing points for Series, Compound, and Shunt motors at 100% speed and 100% torque. These crossings occur at 100% armature current. At these two points all three motors are essentially the same.

Adjustable voltage drive system Diesel Electric The advantage of a diesel electric drive is the diesel can be set for optimum performance as its load is constant. The connection between the generator and the motor is by electrical cable. This can eliminating a transmission box on the output and long shafts. Ships and trains now use this type of power train.

Dynamic braking In (a) contacts M1 & M2 are closed thru while M3 is open. This is the normal motor operating position and the current flow thru the armature is the source for the mechanical rotation of the motor in the CW direction. When braking if the current flow is reversed the motor will rotate in the opposite direction. By opening M1 & M2, closing M3 in (b) and not changing the field current the rotation of the motor will now generate a current in the opposite direction. This means the motor new rotation will oppose the momentum that is creating the opposing mechanical torque.

Plugging operation Plugging is a little different than the dynamic breaking in the last slide. Here the voltage supply is not disconnected letting the motor run on its own but actually reconnected with the voltage reversed causing the current to force the motor to turn in the opposite direction. This stresses the mechanical equipment attached to the output shaft because of the high torque differential. In Fig. 11.13, you have to supply the diagonal open/closed marking for the contacts.

How to change direction of rotation for a Compound motor

How to change direction of rotation for a Series motor

NEMA terminal markings Terminal markings on Shunt and Compound motors to enable selection the rotation direction.

Magnetization curves The shunt field coils produce the flux seen by the armature. This flux is a product of mmf (N*I). Since the turns are set, flux becomes a variable of the field current. The flux created affects the counter emf created in the armature which opposes the incoming armature current. Magnet saturation creates a non-linear relationship between the field current and flux density. The higher the flux density will result in a lower armature current and is the best choice for motor efficiency.