- #1
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In cumulative cascade connection of two induction motors, why does the net speed reduce? Also, how are the torques of the two motors in the same direction?
Cascade connection of induction motors
- Thread starter cnh1995
- Start date
In summary, the net speed reduces as the number of poles in the motor increases. This is because the synchronous speed of the two motors is reduced. The torque of the two motors is in the same direction as long as the load torque is greater than the motor's rated torque.
- #2
anorlunda
Staff Emeritus
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Explain more about your cascade.
Are the cascaded electrically or mechanically on the same shaft?
Are two motors connected to provide torque in the same direction, or opposing connections?
How much mechanical load is applied relative to the ratings of the motor?
Edit: a drawing would be very helpful.
Are the cascaded electrically or mechanically on the same shaft?
Are two motors connected to provide torque in the same direction, or opposing connections?
How much mechanical load is applied relative to the ratings of the motor?
Edit: a drawing would be very helpful.
- #3
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They are mechanically coupled, so both run at the same speed . One motor is driven by 50 Hz mains supply and its rotor emf is given to the stator of the other motor. How does this assembly work? How are the connections made to provide torques in the same direction and in the opposite direction? There are mathematical formulae about this in my book but I want to know what's happening in there physically.anorlunda said:
- #4
anorlunda
Staff Emeritus
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I never heard of an induction motor pair connected like that. I don't know.
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anorlunda said:
It looks like this.
- #6
Babadag
- 590
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If the rotor rotates synchronously with stator rotating magnetic field no EMF is induced.
The voltage [EMF] and the frequency in rotor are directly proportional with the slip.
The slip it is the difference between synchronous velocity and the actual velocity ,divided by synchronous speed.
The slip increases linearly [approximate] with the motor torque up to maximum torque.
More torque= more slip= less speed.
At start, in order to accelerate, the motor torque is more than load torque. At steady state motor torque is equal with load torque. Then Tqload=Tq1+Tq2.
In my opinion,the load torque is shared by the two motors according to their torque-speed curve and the supply voltage[see the fig.no.1].
In this example I took motor A of two poles and motor B of 4 poles.
Since p*nsyn/120=f where p=no.of poles, nsyn=synchronous rpm, f=frequency[Hz]
the first motor synchronous speed is 3000 rpm[50 Hz] and for the second 1500 rpm.
The first motor stator supply voltage is constant. The second motor stator voltage depends on speed.The both rotors are rotating at the same speed but both stators are standing.
Curve 1 and 2 represent the torque as speed function for rated voltage and frequency.
Motor A-the brushes are up and the slip rings are short-circuited.
Motor B is squirrel cage rotor.
Curve 3 it is for motor A when the slip rings is connected with an impedance equal with
Zext [impedance equivalent with of stator and rotor of motor B –see fig.no.2]
Curve 4 for motor B when the supply voltage is V2=kr1*s1*V1 where:
kr1=Vstator/Vrotor for motor A at rated supply ;s1=slip of motor A at actual speed.
V1 =the motor A rated voltage and the frequency will be f2= s1.frated [1/3 in this case].
In assemble the synchronous speed will be nsyna*p2=120 *f2=120*s1*frated.
If nsyna[actual]=x and nsyn1=n and 120*frated=p1*n then
x*p2=p1*n*s1 s1=(n-x)/n x*p2=p1*(n-x)
x(p2+p1)=p1*n
nsyna=p1*nsynch1*p1/(p1+p2)=2/(2+4)*nsynch1=nsynch1/3.
The voltage [EMF] and the frequency in rotor are directly proportional with the slip.
The slip it is the difference between synchronous velocity and the actual velocity ,divided by synchronous speed.
The slip increases linearly [approximate] with the motor torque up to maximum torque.
More torque= more slip= less speed.
At start, in order to accelerate, the motor torque is more than load torque. At steady state motor torque is equal with load torque. Then Tqload=Tq1+Tq2.
In my opinion,the load torque is shared by the two motors according to their torque-speed curve and the supply voltage[see the fig.no.1].
In this example I took motor A of two poles and motor B of 4 poles.
Since p*nsyn/120=f where p=no.of poles, nsyn=synchronous rpm, f=frequency[Hz]
the first motor synchronous speed is 3000 rpm[50 Hz] and for the second 1500 rpm.
The first motor stator supply voltage is constant. The second motor stator voltage depends on speed.The both rotors are rotating at the same speed but both stators are standing.
Curve 1 and 2 represent the torque as speed function for rated voltage and frequency.
Motor A-the brushes are up and the slip rings are short-circuited.
Motor B is squirrel cage rotor.
Curve 3 it is for motor A when the slip rings is connected with an impedance equal with
Zext [impedance equivalent with of stator and rotor of motor B –see fig.no.2]
Curve 4 for motor B when the supply voltage is V2=kr1*s1*V1 where:
kr1=Vstator/Vrotor for motor A at rated supply ;s1=slip of motor A at actual speed.
V1 =the motor A rated voltage and the frequency will be f2= s1.frated [1/3 in this case].
In assemble the synchronous speed will be nsyna*p2=120 *f2=120*s1*frated.
If nsyna[actual]=x and nsyn1=n and 120*frated=p1*n then
x*p2=p1*n*s1 s1=(n-x)/n x*p2=p1*(n-x)
x(p2+p1)=p1*n
nsyna=p1*nsynch1*p1/(p1+p2)=2/(2+4)*nsynch1=nsynch1/3.
Attachments
- #7
Related to Cascade connection of induction motors
1. What is a cascade connection of induction motors?
A cascade connection of induction motors is a method of connecting two or more induction motors in series, where the output of the first motor is connected to the input of the second motor and so on. This allows for a higher output voltage and speed than a single motor.
2. What are the advantages of using a cascade connection of induction motors?
The main advantage of a cascade connection is the ability to achieve a higher output voltage and speed compared to using a single motor. This can be useful in applications where high speeds and power are required, such as in industrial machinery or large appliances.
3. How do you determine the output voltage and speed in a cascade connection?
The output voltage and speed in a cascade connection can be determined by multiplying the individual voltages and speeds of each motor in the series. For example, if two motors with voltages of 120V and 240V are connected in a cascade, the output voltage would be 120V x 240V = 28800V. The output speed can be calculated in a similar manner.
4. Are there any disadvantages to using a cascade connection of induction motors?
One potential disadvantage of a cascade connection is the added complexity and cost of connecting multiple motors together. This can also make troubleshooting and maintenance more challenging. Additionally, if one motor in the series fails, it can affect the performance of the entire system.
5. What are some common applications of cascade connection of induction motors?
Cascade connections are commonly used in large industrial machines, such as pumps, compressors, and fans, where high power and speed are necessary. They can also be found in large appliances, such as refrigerators and air conditioners, to achieve higher efficiency and performance.
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