|Home | Articles | Forum | Glossary | Books|
There are conditions where it is not possible to start a motor across the line due to excessive starting current or excessive torque. Some power systems are not capable of providing the high initial currents produced by starting large horsepower motors. When this is the case, some means must be employed to reduce the amount of starting current. Two of the most common methods are resistor starting and reactor starting. Although these two methods are very similar, they differ in the means used to reduce the amount of starting current.
FIG. 1 Resistors are connected in series with the motor during starting.
Resistor starting is accomplished by connecting resistors in series with the motor during the starting period (FIG. 1). When the Start button is pressed, motor starter coil M energizes and closes all M contacts. The three M load contacts connect the motor and resistors to the line. Since the resistors are connected in series with the motor, they limit the amount of in-rush cur rent. When the M auxiliary contact connected in series with coil TR closes, the timer begins its timing sequence. At the end of the time period, timed contact TR closes and energizes contactor R. This causes the three R contacts connected in parallel with the resistors to close. The R contacts shunt the resistors out of the line and the motor is now connected to full power.
The circuit in FIG. 1 uses time delay to shunt the resistors out of the circuit after some period of time.
Time delay is one of the most popular methods of determining when to connect the motor directly to the power line because it is simple and inexpensive, but it is not the only method. Some control circuits sense motor speed to determine when to shunt the resistors out of the power line (FIG. 2). In the illustration, permanent magnets are attached to the motor shaft, and a Hall effect sensor determines the motor speed. When the motor speed reaches a predetermined level, contactor R energizes and shunts the resistors out of the line (FIG. 3).
Another way of determining when to shunt the resistors out of the circuit is by sensing motor current.
Current transformers are used to sense the amount of motor current (FIG. 4). In this circuit, the current sensor contacts are normally closed. When the motor starts, high current causes the sensor contacts to open.
An on-delay timer provides enough time delay to permit the motor to begin starting before contactor coil R can energize. This timer is generally set for a very short time delay. As the motor speed increases, the current drops. When the current drops to a low enough level, the current sensor contact re-closes and permits contactor R to energize.
A reactor starter is the same basic control except that reactors or choke coils are used to limit in-rush current instead of resistors (FIG. 5). Reactors limit in rush current with inductive reactance instead of resistance. Reactors have an advantage in current limiting circuits because of the rise time of current in an inductive circuit. In a resistive circuit, the current will reach its full Ohm's Law value instantly. In an inductive circuit, the current must rise at an exponential rate (FIG. 6). This exponential rise time of current further reduces the inrush current.
Some resistor and reactor starters use multiple steps of starting. This is accomplished by tapping the resistor or reactor to provide different values of resistance or inductive reactance (FIG. 7). When the Start button is pressed, M load contacts close and connect the mo tor and inductors to the line. The M auxiliary contact closes and starts timer TR1. After a time delay, TR1 contact closes and energizes S1 coil. This causes half of the series inductors to be shunted out, reducing the inductive reactance connected in series with the motor.
Motor current increases, causing the motor speed to increase. The S1 auxiliary contact closes at the same time, causing timer TR2 to start its timing sequence.
When TR2 contact closes, contactor S2 energizes, causing all of the inductance to be shunted out. The motor is now connected directly to the power line.
Some circuits may use several steps of starting, de pending on the circuit requirements.
1. What two electrical components are commonly connected in series with a motor to limit starting current?
2. What advantage does a reactor have when limiting in-rush current that is not available with a resistor?
3. Refer to the circuit shown in FIG. 1. Assume that timer TR is set for a delay of 10 seconds.
When the Start button is pressed, the motor starts in low speed. After a delay of 30 seconds, the mo tor is still in its lowest speed and has not accelerated to normal speed. Which of the following could not cause this condition?
a. The Start button is shorted.
b. Timer coil TR is open.
c. Contactor coil R is open.
d. Timed contact TR did not close after a delay of 10 seconds.
4. Refer to the circuit shown in FIG. 7. Assume that each timer is set for a delay of 5 seconds.
When the Start button is pressed, the motor starts at its lowest speed. After a delay of 5 seconds, the motor accelerates to second speed. After another delay of 5 seconds, the motor stops running. During troubleshooting you discover that the control transformer fuse is blown. Which of the following could cause this condition?
a. TR1 coil is shorted.
b. S1 coil is open.
c. S2 coil is shorted.
d. TR2 coil is open.
5. Refer to the circuit shown in FIG. 7. Assume that each timer is set for a delay of 5 seconds.
When the Start button is pressed, the motor starts at its highest speed. Which of the following could cause this condition?
a. The Stop button is shorted.
b. TR1 timer coil is open.
c. S1 auxiliary contact is shorted.
d. TR2 timer coil is shorted.