# Synchronous motor coupled to a DC Compound Motor Problem

• Engineering
• Loz30
In summary, the DC generator delivers power at a current of 20A and the efficiency of the AC motor is 91.67%.
Loz30
Hi All. I'm having trouble working out the solution to this problem.

A Synchronous motor is coupled to a DC Compound Motor.
The DC Machine is rated for 220 V DC. Its Armature Resistance is 1.0 Ω .
The Synchronous Machine is a 4 pole 400 V 50 Hz 3 Phase AC Machine with an efficiency of 80%.

Since Synchronous Motor is not self starting , the DC Machine is run as a Motor first with a Terminal Voltage of 220 V DC. When both the Machines are up to speed AC Power and DC Power (For Field ) are switched on to the AC Machine to Run as Synchronous Motor.

Now the DC Machine starts to run as DC Generator and delivers power at a current of 20 Amps.

1) Determine the input Active power to the Synchronous Motor
2) Determine the Efficiency of DC Generator

I know I can calculate the nominal speed of the AC machine given the frequency and number of poles N=(120f)/p which works out to be 1500RPM but I don't have the torque so I can't calculate the power from that.

I've also tried starting from the DC machine side when acting as a generator, but I'm only able to calculate the armature power from it's resistance and the output current which is P=I2R=202X1=400W..

I'm not looking for the whole solution, but I would greatly appreciate a little help to get me on the right track so I can work it out on my own.

Thankyou very much!

Welcome to PF!

What is the field resistance? Is the motor long-shunt or short-shunt?

When you say the generator delivers power at a current of 20A, is it armature current or load current?

cnh1995 said:
Welcome to PF!

What is the field resistance? Is the motor long-shunt or short-shunt?

When you say the generator delivers power at a current of 20A, is it armature current or load current?

Thanks.

The problem doesn't mention what the field resistance is, nor whether the motor is long-shunt or short-shunt..

As for the 20A of current, I would say that the Compound DC motor is separately excited as initially when both the DC motor and AC generator are up to speed it states that the DC Power (field) is switched onto the AC machine. So the armature current would then be equal to the load current I think..I have what I think may be a possible solution as I've been thinking a lot on this problem..

Considering that the armature current = load current = 20A, We have the power across the armature as PA=I2R=202x1=400W.
Since the Synchronous AC motor is run up to speed using 220V supply on the DC. The DC machine when acting as a generator would be give out a terminal voltage of 220V? So the output power would be POUT=IV=20x 220 = 4400W.

The input power would then be equal to the armature loss plus the output power. So PIN= PA+POUT=400+4400=4800W.

The efficiency would then equal (POUT/PIN) X100]=(4400/4800) X 100=91.67%
Since the AC motor has an efficiency of 80% the active input power would be equal to the Input power of the DC Generator/.8=4800/.8=6000W

I not exactly 100% on this solution as the output voltage of the DC generator at 220V is giving me doubts..[/SUB]

## 1. What is a synchronous motor coupled to a DC compound motor?

A synchronous motor coupled to a DC compound motor is a type of motor system that combines the features of both a synchronous motor and a DC compound motor. This allows for better control and efficiency in certain applications.

## 2. How does the coupling of these two motors work?

The synchronous motor is connected to the DC compound motor via a mechanical coupling, which allows for the synchronous motor to control the speed and direction of the DC compound motor. This coupling also allows for energy to be transferred between the two motors.

## 3. What are the advantages of using a synchronous motor coupled to a DC compound motor?

Some advantages of this motor system include improved control over speed and direction, increased efficiency, and the ability to handle heavy loads. It also allows for the use of both AC and DC power sources.

## 4. What are some common applications of a synchronous motor coupled to a DC compound motor?

This motor system is often used in industrial applications such as in paper mills, steel mills, and power plants. It is also used in transportation systems such as trains and ships.

## 5. How do you troubleshoot problems with a synchronous motor coupled to a DC compound motor?

If you are experiencing problems with this motor system, it is important to check for any loose connections, worn parts, or damage to the motor. You can also consult the manufacturer's manual or contact a professional for assistance.

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