Electric Machines (7) Flashcards
What is the equation for back emf constant, Ke’, using flux per pole, Φ?
What is the equation that relates Torque and Volume of a DC motor? Describe their relationship.
What other factors are present in the equation
Relationship between motor torque and volume
* A motor’s torque capability is proportional to its volume.
* Torque equation can be shown to relate the volume. B is the flux density (or magnetic loading). A is the electric loading, the equation and definition are shown below. Vr is the volume of the rotor, the equation is shown below.
What is the equation for power of a DC motor, relating to torque?
P = Tω
True or False:
It is desirable to speed up a machine using a gearbox to keep size small.
True. This means we can use a gearbox to keep motors small, so they have high torque that can then generate high speeds. Or vice versa.
What is meant by a wound field machine?
One that doesn’t use a permanent magnet. Instead it uses coils to generate an electromagnet instead.
* Wound fields can be further classified in how the field coil and the amature coil is connected.
* A wound field machine requires high currents. As you need to power the field and armature coils.
What are the names of the four types of wound field connections?
- Separate excitation: We have a DC source for the field coil to set up the magnetic field and another to set up the current for the armature coil. This means the two currents are separate. Once the field current is fixed, this acts like a permanent magnet DC machine.
- Series excitation: The field coil is in series with the armature coil so the current is the same for both.
- Shunt connection: the field coil and armature coil are in parallel.
- Compound connection: essentially the combination of the shunt and series connection.
- Each one has a different torque and speed relationship.
What is the equation relating torque and speed for a seperate excitation wound field DC motor?
Think about the equivalent circuits and equations.
- The relationship is close to the permanent magnet DC machine. Therefore we can use the same equivalent circuit as learnt before (shown below)
- We can use the torque equation and voltage equation for a DC motor, to then re-arrange and solve for an equation relating torque to speed, shown in equation 4.
What does the graph of speed against torque for a seperate excitation DC motor look like? Note any important points.
We can plot a graph of Speed against torque. Shown below.
* No-load speed: When no load is connected to the motor. There is zero torque on the motor (no force on it at a distance). Therefore there is the highest rotational speed. This is found using the equation shown.
* Stall torque: If you keep increasing the torque, at a point, the motor won’t be able to rotate anymore. So rotational speed is zero. This torque is shown in the equation.
How do the other field connections (such as shunt, composite, and series) vary in their speed-torque characterisitcs?
Torque-speed characteristics for other connections (not including seperate excitation):
* As you can see the rest aren’t linear, shown in the diagram on the right.
* They each have some benefits though depending on the load characteristics they are used for:
->For example, a series connection might be beneficial for a train, where high torque and low speed are required for starting the train’s motion, then less torque and higher speeds are required in motion. The same can be stated for different applications for the rest of the connections.
