M-G Set – Construction, Working, Advantages, Disadvantages and Applications
What is a Motor Generator Set?
Motor Generator set is a device used to convert electrical power to another form of energy (mostly AC to DC). It consists of a motor and a generator connected mechanically on the same shaft. The electrical supply is given to the motor. The mechanical output of a motor is used to rotate the generator. The type of generator depends on the applications.
In other words, a motor-generator set is a converter consisting of a three phase AC motor and a DC generator (generally compound DC generator) which is used to convert the AC supply to the DC supply and vice versa (not always the case). Both motor and generator are mechanically coupled. The motor is connected to the AC supply and the coupled generator provides DC voltage as output. The output of the DC generator is independent of the Input of the motor. The output of the generator is further regulated using a field regulator. A synchronous motor can also be used instead of an induction motor with a generator, if so, it can be additionally used to improve the power factor.
The motor-generator set is used to isolate the electrical load from the supply line. It is also used to change supply frequency or voltage. It is also used to eliminate spikes and variations in the industrial power supply. A motor-generator set is used to convert one type of electrical energy into another type of electrical energy. It cannot be used to generate electrical energy.
Motor generator set consists of different types of motor and generator coupled together in a single unit. In most cases, the motor is driven by the input electrical power from a commutator and the output of generator coils is used for the desired applications.
Working Principle of M-G Set
A typical connection diagram of a motor-generator set is shown in the figure below.
Here, two electrical machines are connected on the same shaft. Out of these two machines, an electrical supply is given to machine-1. Hence, machine-1 behaves as a motor and it will start rotating when we give electrical input.
As the motor runs at rated speed, the mechanical energy produced by the motor is used to rotate the machine-2. Therefore, machine-2 behaves like a generator. The generator behaves as a load to the motor.
Now, the mechanical energy produced by the motor is again converted into electrical energy by machine-2. The machine-2 is selected according to the application where we will use the electrical energy.
Here, we have seen both machines in separate units. But for a small motor-generator set, it can be placed on a common unit. The input of this set is electrical energy and the output of this set is also electrical energy. But in some cases, the type of electrical power we have is not suitable for our load. In that case, we can use a motor-generator set to get electrical energy which is suitable for our load with the help of a motor-generator set.
Conversions Process of Motor-Generator Set
The motor-generator set is used for electrical energy conversion. It means it is used to convert one type of electrical energy to another type of electrical energy. Here, we will discuss a couple of examples.
Let’s say, we have a DC supply. But the load needs AC power to operate efficiently. In this condition, we can use a motor-generator set that contains a DC motor and an AC generator. The available DC power is given to the DC motor which is a couple on the same shaft with an AC generator. As the DC motor rotates, the AC generator rotates and generates AC power. (Nowadays, this application is easily done with the help of inverters. But before the power electronics revolution, this type of arrangement was used to convert DC power into AC power.)
For the second example, we have an AC power with a 50 Hz frequency. The load we have operates efficiently on 60 Hz frequency. In this condition, we will use both an AC motor and a generator. But the motor is designed on 50 Hz while the generator is designed on 60 Hz. So, 50 Hz electrical supply is given to the motor-generator set and it gives an output of 60 Hz electrical energy. Hence, it is also used for frequency conversion.
Here, we have listed some of the other conversions that could be possible with the help of a motor-generator set.
- AC to DC: The AC power can be converted into DC power with the help of a motor-generator set. In this case, AC motors (synchronous motor or induction motor) are coupled with DC generators.
- DC to AC: In this case, the DC motor is coupled with AC generators or alternator (special cases) .
- AC to AC: One frequency of AC power to another frequency of AC power.
- DC to DC: One voltage level of DC power to another level of DC power.
- Fixed AC voltage to variable AC voltage.
- Single-phase AC power to three-phase AC power.
- Creating or balancing a three-phase DC system.
Applications of M-G Set
The applications of a motor-generator set are as listed below.
- The motor-generator set is used to remove transients in power supply.
- This set gives a smooth power cut from the main supply.
- It can be used instead of semiconductor devices for the same applications. This set can deal with large overloads for a short time compared to modern semiconductor devices.
Advantages & Disadvantages of M-G Set
Following are the some advantages of a M-G set.
- It is a self-starting device.
- It can also be used as a power factor improvement device.
- It is reliable with a simple design.
- A wide range of DC voltage regulations can be achieved using the M-G set.
- There is no risk of reverse polarity and flash-over in the motor generator set.
- The DC output of the generator is constant and won’t be affected by the change in AC supply voltage to the motor. If the change in supply voltage to the motor and load connected to the generator is large enough, the process of synchronization may be lost.
- The DC output can be easily controlled by adjustment of the shunt field regulator.
- The synchronous motor can be directly connected to the supply voltage without installing a separate transformer (which is necessary in case of rotary converter).
There are some disadvantages of using a motor-generator set that is listed below.
- In the motor-generator set, two machines are used. Hence, some losses occur that are associated with machines. These losses are very high as compared to other techniques.
- The entire system is noisy and it requires regular maintenance as it contains more rotating parts.
- During interruptions, the machine will slow down. It may affect the output voltage and frequency.
- It is costly as well as heavy and occupies more space as compared to the rotary converters.
- The efficiency is noticeably lower than other devices used for the same purpose.
- It has a low power factor due to the induction motors, hence compensation is required.
- The main con of the M-G set is that it can only be used to convert the AC into DC (i.e. it can’t be used to convert the DC into AC).
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