Electrical Machines
Induction Machines
Practice questions from Induction Machines.
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IncorrectThe operating region of the developed torque and speed () of an induction motor drive is given by the shaded region OQRE in the figure. The load torque characteristic is also shown. The motor drive moves from the initial operating point O to the final operating point S. Which one of the following trajectories will take the shortest time?
The shortest time to reach the final operating point S happens when the motor accelerates with the maximum possible torque.
In the given characteristics, the maximum developed torque corresponds to the uppermost boundary of the shaded region, the line .
So, the fastest trajectory will follow: .
Hence, correct answer is option (a).
A 3-phase, pole, 50 Hz star connected induction motor has the following parameters referred to the stator:
Stator resistance, magnetizing reactance and core loss of the motor are neglected. The motor is run with constant control from a drive. For maximum starting torque, the voltage and frequency output, respectively, from the drive, is closest to,
Given:
- Rotor resistance referred to stator:
- Stator + rotor leakage reactance at
- Constant V/f control:
Key Insight:
For maximum torque at start, the starting slip (s=1) must equal the slip for maximum torque:
Where is the total reactance at the new frequency .
Step 1: Set for Maximum Starting Torque
Step 2: Find New Frequency
Since reactance scales with frequency:
Set :
Step 3: Compute New Voltage
Using constant :
Line-to-line voltage:
A three phase, pole induction motor runs at . The stator copper loss, core loss, and the rotational loss of the motor can be neglected. The percentage efficiency of the motor is
Losses are ignored but we cannot ignore rotor copper losses. Since there are no friction and windage losses the mechanical developed power is same as output power.
Since, stator losses are ignored the input power is same as air gap power.
A 3-phase star connected slip ring induction motor has the following parameters referred to the stator:
The per phase stator to rotor effective turns ratio is . The rotor winding is also star connected. The magnetizing reactance and core loss of the motor can be neglected. To have maximum torque at starting, the value of the extra resistance in ohms (referred to the rotor side) to be connected in series with each phase of the rotor winding is ________ (rounded off to 2 decimal places).
Only rotor resistance load
For start, s=1
For maximum torque we must have maximum air gap power that is power across rotor resistance should be maximum.
By MPT,
Referring the external resistance to the rotor side,
The following columns present various modes of induction machine operation and the ranges of slip
(A) Mode of Operation | (B) Range of Slip |
(a) Running in generator mode | (p) From 0.0 to 1.0 |
(b) Running in motor mode | (q) From 1.0 to 2.0 |
(c) Plugging in motor mode | (r) From -1.0 to 0.0 |
The correct matching between the elements in column A with those of column B is
a-r, b-p, and
a-r, b-q, and c-p
, and
a-q, b-p, and c-r
Torque-slip characteristics of Induction machine are shown below.
A 10-pole, , single phase induction motor runs at while driving rated load. The frequency of induced rotor currents due to backward field is
Synchronous speed,
Since, rotor rotates opposite to backward field
Sup,
Rotor induced frequency due to backward field
= sf = 1.9 × 50 = 95 Hz
A three phase , 6-pole, squirrel cage induction motor drives a constant torque load at rated speed operating from rated supply and delivering rated output. If the supply voltage and frequency are reduced by , the resultant speed of the motor in RPM (neglecting the stator leakage impedance and rotational losses) is _________ (Round off to the nearest integer).
if both voltage & frequency are reduced by 20% then
Motor drives a constant Torque Load, Ns – Nr = constant f1 = 50 Hz
Ns1 – Nr1 = Ns2 – Nr2
1000 – 960 = 800 – Nr2
Nr2 = 760 rpm