When the separately excited DC motor is running stably, what factors are related to its armature cur

There is an equation of mechanical characteristics of a DC motor, and when the separately excited DC motor is running stably, the armature current is only related to the load torque, the total resistance of the armature circuit and the excitation current, and is not directly related to other parameters. When the load torque is constant, the armature current will be affected when the total resistance of the armature circuit is changed, and the relationship between the two is inverse. Or changing the excitation current can also achieve the purpose of changing the armature current, and the two are also inversely correlated, but in practice, changing the excitation current is to adjust the excitation current to a small place, so the magnetic flux should be reduced, and the armature current should be increased at this time. As for changing the power supply voltage, it only changes the speed n of the electric motor, and has no effect on the armature current.

While changing the parameters of the motor, the motor will automatically adjust according to its own mechanical characteristics, which is reflected in the mechanical characteristic curve of the motor, that is, the motor will maintain balance in a very running state.

Based on the above analysis, it is not difficult to understand several starting methods of separately excited DC motors, such as armature loop string resistance start, step-down start, etc. Step-down start can not be simply understood as reducing the armature current start-up, it is fundamentally different from the armature loop string resistance start-up, although the step-down start also has the effect of reducing the armature current at the initial time, but when the motor is operating normally, and then boost, the armature current of the motor is unchanged, which is reflected in the mechanical load characteristic curve is that these curves are a cluster of parallel straight lines, and this is the core of the step-down start-up, which is also fundamentally different from the armature loop string resistance. In addition, adjusting the excitation current is generally only used in the speed regulation of the motor.

It is suggested that lz carefully analyze the mechanical characteristic equation and mechanical characteristic curve of the DC motor, and then the change relationship of each quantity will be very clear. I hope it helps, and if so, be sure to give extra points. o(∩_o~

When the separately excited DC motor is running stably, its armature current is related to the mechanical loss, copper loss, iron loss and load size of the motor.

When running stably as an electric motor: U=ES+ISRS Back EMF ES+U- ISRS=190V

When operating stably as a generator: U=ES-ISRS potential ES-U- ISRS UF(U)= ES+ISRS=210V

Due to the power p=mw

Electromagnetic torque m=p (nv. 30)=30x17x1000 3000 3·1416=54·11 n·m

Efficiency = 17000 220 88·9 = 0·869

Excitation current 220 181·5=1·21a

Rotor current 88·9-1·21=87·69A

Back EMF E1 = 220-0·114x87·69=210V at 3000 rpm

Back EMF E2 at 2800 rpm The back EMF is proportional to the rotational speed, so there is:

e2/e1=2800/3000

Therefore e2 = 2800xe1 3000 = 196v

The string resistance r1 = (220-e2) 87·69-r0=0·159 ohms.

It is composed of two-terminal electronic components made of resistive materials, which have a certain structural form and can play a role in restricting the passage of current in the circuit. If the resistance value cannot be changed, it is called a fixed resistor. Those with variable resistance are called potentiometers or variable resistors.

The ideal resistor is linear, i.e., the instantaneous current through the resistor is proportional to the applied instantaneous voltage.

Some special resistors, such as thermistors, varistors, and sensitive components, have a non-linear relationship between voltage and current. Resistors are the most widely used components in electronic circuits, and are usually selected by power supply circuit designers in different series according to power and resistance.

Summary. There is a certain relationship between the armature voltage and the rated voltage of the separately excited DC motor. Normally, the armature voltage of the separately excited DC motor should be at least greater than 75% of the rated voltage, and its maximum armature voltage should not exceed 105% of the rated voltage.

The relationship between the armature voltage and the rated voltage in a separately excited DC motor.

There is a certain relationship between the armature voltage and the rated voltage of the separately excited DC motor. Normally, the armature voltage of the separately excited DC motor should be at least greater than 75% of the rated voltage, and its maximum armature voltage should not exceed 105% of the rated voltage.

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Is the number of phases of the stator winding of an asynchronous motor equal to the number of phases of the cage rotor winding?

No, the number of phases in the stator windings tends to be more than in the cage rotor windings.

Other-excited DC motors are generally not allowed to start directly, and the reason for this is that under Pena:

DC motor direct start you first have to know how much power this thing is, how much V voltage needs to be input, generally the indoor voltage is 220V, if your DC motor is 100V you start it directly and burn it, you must use the corresponding transformer to convert 220V into the voltage required by the DC motor.

It is not dry to say that the DC motor can not be started directly, but if it is started directly, the starting current will be very large, which is not conducive to the commutation and temperature rise of the motor.

Cage asynchronous motor, generally the capacity is below the can be started directly, the capacity of the large also need to start the equipment.

Unless the capacity of the transformer supplying power to the motor is much larger than the capacity of the motor, it can be started directly. Generally speaking, DC motors are not allowed to start directly, except for motors with very small capacity.

Because the armature resistance Ra is small, the starting current is very high when starting directly, usually up to 10 to 20 times the rated current. Excessive starting current will cause the grid voltage to drop and overmatch, affecting the normal electricity consumption of other users on the power grid; worsen the commutation of the motor, or even burn out the motor; At the same time, excessive impact torque can damage the armature winding and transmission mechanism.

When the separately excited DC motor is running stably, the main influencing factors of armature current are as follows:

1.The current is smaller when the speed ** speed is high).

2.The magnitude of the load resistance.

3.Fluctuations in the voltage of the power supply.

4.The temperature of the coil.

Answer]: C For separately excited DC motors, the change of the terminal voltage including the friend cover u does not affect the excitation flux, and the same excitation current is not affected, that is, if ifn constant, different artificial mechanical characteristics are obtained by changing the terminal voltage u. By talking about the formula of mechanical characteristics.

It can be seen that when the armature voltage decreases, the rotational speed will decrease accordingly if it is a constant torque load. Again.

That is, the effect of this item related to the load is small, so when the load of the motor increases, the rotational speed decreases very little, which is manifested as a stiffer mechanical characteristic of the bridge, therefore, the artificial mechanical characteristics of the armature voltage decrease are straight lines parallel to the inherent characteristics, as shown in the solution diagram.