Starting a DC motor:

I. Overview:

Starting: The process of a DC motor reaching a stable speed from standstill.

At startup: n=0 Ea=Ceфn=0

U=Ea+IaRa

Ia=(U-Ea)/RaS

If direct movement (that is, direct closing, add U amount to the motor)

Then the starting current Iq=(U-0)/Ra=U/Ra is very large

1. The consequences of a large Iq:

( 1) Strong spark phenomenon.

( 2) The current is proportional to the torque, and an excessive torque will bring a great impact.

( 3) Voltage fluctuations affect the stability of power supply.

2. Measures:

Larger capacity DC motors are not allowed to start directly. The reasonable starting requirements are as follows:

( 1) There must be a large enough starting torque.

( 2) Iq should be limited within a certain range

( 3) The starting equipment should be simple and reliable.

Common startup methods:

( 1) Buck start

( 2) Series resistance start

Second, step-down start (lower power supply voltage start):

   Reduced voltage starting is to limit the starting current by temporarily reducing the power supply voltage of the motor during starting. Step-down starting requires a DC power supply with variable voltage. At present, thyristor circuits have been used as adjustable power supplies for DC motors in many occasions. The armature circuit does not need to be connected in series with the starting rheostat, and is directly connected with the adjustable power supply. Raise the voltage from zero to the rated value to complete the starting process.

   Note: The voltage to the excitation winding must be added to the rated value at the beginning, otherwise the starting current is still very large, but the starting torque is very small, so that it cannot start.

   Features: The equipment is relatively complex, does not consume energy, and is suitable for motors that are always started, such as motors on lifting and transportation machinery

Three, armature circuit series resistance start (limit Iq) :

   For general DC motors, a rheostat is connected in series in the armature circuit to limit the starting current when starting. The rheostat specially used for starting the starter is called a starter. The starter is essentially a variable resistor. When the speed gradually increases and the back electromotive force of the armature gradually increases, the resistance of the starter can be cut off step by step. Until the speed of the motor rises to a stable value, all the resistance of the starter is cut off, and the starting process is completed. Features of series resistor startup:

   Advantages: The equipment is simpler and the price is lower.

   Disadvantages: During the starting process, the starting resistor has energy loss. It is suitable for low-power DC motors and motors with slightly larger capacity but which do not need to be started frequently.

Speed regulation of DC motor:

1. Speed regulation method:

∵ n=(U-IaRa)/ Ceф

Change the supply voltage

Change the armature circuit resistance Ra

Change the excitation current to change ф

Second, change the power supply voltage speed regulation:

main feature:

1. Wide range of speed regulation, called stepless speed regulation.

2. There is no additional energy loss, and the hardness of the mechanical properties remains unchanged after the voltage is reduced, and the stability is good.

3. The speed can only be adjusted down, not up.

4. The required equipment is more complicated and the cost is higher.

Three, change the armature circuit resistance speed regulation:

main feature:

1. The required equipment is relatively simple and low in cost, and it is widely used in low-power DC motors.

2. The speed can only be adjusted down, which is a step-by-step speed regulation, and the characteristic curve is soft.

3. There is a large energy loss in the speed regulating resistor, and the economic performance is poor.

Fourth, weaken the magnetic field speed regulation:

main feature:

1. The speed regulation is carried out in the excitation circuit, the energy loss is small, and the control is convenient.

2. Smooth stepless speed regulation, but the speed can only be adjusted upwards from the rated speed, which is often used as an auxiliary speed regulation.

3. The range of speed adjustment is narrow, and when ф decreases too much, it is difficult to change direction and the spark is large.

Reversing of a DC motor

To change the direction of n, that is to change the direction of the electromagnetic torque T T = CMфIa

1. Change the direction of ф (ie If), that is, reverse the excitation winding

2. Change the direction of Ia, that is, reverse the armature winding (shunt excitation motor)

Braking of a DC motor:

   Braking: Add a torque on the motor shaft opposite to the direction of rotation to achieve a quick stop or deceleration speed limit. If the tram goes downhill, the heavy object is lowered.

1. Regenerative braking:

   That is to say, the motor is running in the generator state and sends the generated electric energy back to the grid .

   Motor: Ia=(U-Ea)/Ra U>Ea . Ia and U are in the same direction.

   If Ea>U, Ia is opposite to U. The motor outputs electric energy to the grid, because T=CmфIa, then T is opposite to the direction of rotation of the motor, so it acts as a brake.

   For example: the electric motor drags the electric car downhill, the acceleration of gravity will increase the speed of the car, and it is necessary to brake and limit the speed for safety. When the motor speed rises and the increased armature induced electromotive force is greater than the grid voltage, the motor will become a generator, and the direction of its armature current and electromagnetic torque will be reversed, which limits the further increase of the speed and plays a role. Braking action. The direction of the armature current is reversed, and the electric power is fed back to the grid, so it is called regenerative braking. The electric power fed back comes from the potential energy released by the electric locomotive when it goes downhill.

2. Resistance braking (energy consumption braking):

   Cut off the armature winding from the power supply, and connect the armature to the resistor at the same time. The armature continues to rotate by inertia to generate an induced potential to supply power to the braking resistor. The direction of the armature current is opposite to that during operation, so T is the braking torque . .

main feature:

( 1) Energy loss cannot be utilized on resistance.

( 2) It is not easy to stop the machine quickly, but the deceleration is smooth without big impact.

3. Reverse braking:

The reverse of T becomes the braking torque

T= CmфIa

( 1) Change the voltage direction applied to the armature winding, that is, reverse the armature winding.

( 2) Change the direction of the excitation current. (to reverse Φ)

illustrate:

1. When the armature winding is reversed, a resistor should be connected in series in the armature circuit to limit the armature current.

2. When n drops to close to zero, cut off the power immediately to avoid the reverse rotation of the motor.

   The reverse braking method is suitable for occasions requiring strong braking or rapid reversal. The disadvantage is that the mechanical impact is greater during braking and consumes more energy.