This article covers various types of starters for the DC motors including:
- Manual Starters
- Automatic Starters
- Definite Time Starters
- Counter EMF Starter
- Current-Limit Starter
Each type of starter along with its circuit diagram is described in detail in this section.
Recalling the equivalent circuit of a DC motor, shown in Figure 1, there is no counter EMF (Ea) when the motor is at rest. Thus, when the voltage is applied to the terminals, all of the voltage drops will appear across the armature resistance, Ra.
Because Ra is designed to be small, applying full voltage to it could result in current several times the rated current of the machine. Such a high current could cause serious commutation problems because the commutator must reverse the current in a coil during the time the brushes are shorting out the coil.
If the current is too high, it will not reverse during the commutation interval and arcing will occur at the brushes.
In general, the motor starting current should be limited to twice the rated full-load current to prevent excessive commutator arcing.
FIGURE 1: Equivalent circuit of a DC shunt motor.
One way to limit the inrush current is to reduce the voltage that is applied to the motor and then bring it up slowly to rated voltage. Of course, that requires a variable-voltage DC supply. Although such a supply is frequently available in the laboratory, it is usually not in the workplace. Thus, we must limit the current by other means.
In particular, we can change the voltage applied to the motor by placing the resistance in series with it, as shown in Figure 1.
By placing a large resistance in the circuit at starting and reducing it in steps as the speed increases, the motor may be started quickly without excessive armature current. Both manual and automatic starters are used to do this.
Figure 2 shows a manual starter circuit diagram. The arm is spring loaded and is rotated in the clockwise direction, gradually reducing the armature resistance as the motor accelerates. This starter is a so-called three-point starter.
The electromagnet that holds the starter in the run position is in the field circuit. This type of starter can be used for shunt and compound motors, and if the field is lost, the starter drops out, protecting the motor against runaway.
FIGURE 2: Three-point manual DC motor starter circuit diagram
The disadvantage of this type of starter is that it may drop out if field resistance control is used to weaken the field for increased motor speed. This type of starter cannot be used for a series machine.
Figure 3 shows a four-point starter circuit diagram. Here, the electromagnet is connected directly across the line voltage. It does not drop out at low values of field current on the shunt machine and thus does not protect against runaway. However, the fuses in the motor circuit would probably blow before the machine reached dangerous speed. It can also be used for starting series motors.
FIGURE 3: Four-point manual DC motor starter circuit diagram
An automatic starter operates in a similar fashion, except that automatic relays short out sections of the starter resistance either by a time sequence or when the armature current drops to a selected value.
Automatic DC starters
Figure 4 shows the automatic DC starter circuit diagram. Contactor M is closed to start the machine. After suitable time delays, contactors 1A and then 2A close, reducing the armature circuit resistance in steps. More than two steps of starting resistance could be included by adding additional contactors.
Note that the field rheostat is shorted out until the final starting resistor is shorted. This keeps the field current at a maximum value to help create torque to accelerate the motor and load. There are three common techniques used to short out the individual resistors in the starting resistance.
FIGURE 4: DC motor with starting resistors in armature circuit.
Definite Time Starters
Figure 5 shows definite time starter circuit diagram. Relays TD1 and TD2 have independently adjustable time delays.
When relay TD1-1 closes, power is applied to coil 1A, which closes 1A in the armature circuit (shown in Figure 4), eliminating R from the circuit.
After the second time delay, relay TD2-1 closes, energizing coil 2A, which closes 2A in the armature circuit, shorting out R2.
This type of starter would be suitable if the motor was always expected to start the same load, which would allow the calculation of the proper time delays for shorting out the resistors.
FIGURE 5: Definite-time DC motor starter circuit diagram
Counter EMF Starters
One way to ensure the armature current is held to a reasonable level is to measure the counter EMF and remove starting resistors at predetermined values of EMF. Figure 6 shows the counter EMF starter circuit diagram.
The coils PI and P2 are selected to have pickup voltages at the desired values of counter EMF. When the armature terminal voltage reaches the pickup value of PI, relay PI closes, applying the voltage to coil 1A, which closes relay 1A, shorting out resistance R l, etc.
FIGURE 6: Counter EMF DC motor starter circuit diagram
This type of starter uses the actual current in the armature circuit to determine when to remove the starting resistors. Figure 7 shows the current limit starter circuit diagram.
Current sensors are placed in series with each of the starting resistors. When the armature current drops to a low enough value, the current sensor S1 causes relay S1 in the control circuit to close, thus energizing coil 1A and shorting out resistor Rl.
The process repeats for the next starting resistor until all segments of the starting resistance are shorted out.
FIGURE 7: Current-limit DC motor starter circuit diagram