Electric Machines (8)

Question 1

Name some ways to control the speed of a DC motor.

Answer 1

These are some ways we can control the speed of a DC motor:
* Change the voltage, V, applied to the motor
* Controlled resistor – to change R in the circuit
* Field regulator – by changing field current, i_f, therefore changing the flux and flux density, B

Question 2

Using the question below. How do you use a variable resistor to control the speed of a DC motor?

Answer 2

• A variable resistor will change R in the equivalent circuit. This means we can add resistance to result in lower voltage going to the DC motor or reduce to increase the voltage to the DC motor, thus affecting the only non constant factor, the speed of the motor.
• See slide for method for answer.
• Note: that adding a variable resistor to control the speed, the power consumed by that variable resistor is very high. This means it is very inefficient due to high power loss.

Question 3

How does a DC chopper work to control the speed of a DC motor?

Answer 3

MOSFET or thyristor is used to control the average voltage supplied to the motor. This is controlled using very high frequencies, such as 15KHz.
* The gate voltage is controlled at a high frequency to turn on and off the MOSFET.
* Once complete time period, T, is equal to the time on, t_on, + time off, t_off.
* We can get a new equation called the Duty ratio, shown below, which shows that it will be equal to 1 when time on is maximum and 0 when time on is minimum.
* The voltage supplied to the DC motor will be equal to whatever the supply voltage, Vs, is. Minus the voltage used up to run the MOSFET, V_MOSFET, which is usually around 0.7v.
* The average voltage supplied to the DC motor is shown in the equation in red. This shows we can control the speed by controlling the time on on the gate of the MOSFET.
* MOSFETs are more efficient then using a resistor and use little power being much more efficient at controlling DC motor speeds.

The diode is added to protect the circuit when the circuit suddenly switches off. This is because there is an inductor, La, effectively in the circuit, which can cause a sudden large voltage through the MOSFET resulting in it breaking, as an inductor tries to resist this sudden change. This diode is called a free wheeling diode and is a necessary safety measure.

Question 4

How can you control the direction of a DC motor using MOSFETs?

Answer 4

We can use 4 MOSFETS to be turned on and off to control the flow of the current to then change the rotation of a DC motor.

Question 5

What are the benefits of brushless and stepper motors?

Answer 5

• Commutators are relatively inefficient, also require periodic maintence because they use a brush, which degrades due to friction and arcing.
• We can have brushless machines which use semiconductor switches, which are becoming more popular. This obviously doesn’t degrade to friction.
• Stepper motors work on driving the motor in fine steps using many different coils. This is useful for fine control, such as in CNC machines