EE6601 SOLID STATE DRIVES ANSWER KEY
1. Draw the block diagram of electric drive system.
2. Mention the different factors considered to select a motor to drive the load.
1) Steady state operation requirements.
2) Transient operation requirements.
3) Requirements related to the source.
4) Capital and running cost, maintenance needs, life.
5) Space and weight restriction.
6) Environment and location.
7) Reliability.
3. Define load equalization.
Load equalization is the process of smoothing the fluctuate load. The fluctuate load draws heavy
current from the supply during the peak interval and also cause a large voltage drop in the system due
to which the equipment may get damage. In load equalization, the energy is stored at light load, and
this energy is utilized when the peak load occurs. Thus, the electrical power from the supply remains
constant. The load fluctuation mostly occurs in some of the drives. For example, in a pressing machine,
a large torque is required for a short duration. Otherwise, the torque is zero. The problem of load
fluctuation can be overcome by using the flywheel. The flying wheel is mounted on a motor shaft in
non-reversible drives. In variable speed and reversible drive, a flywheel cannot be mounted on the
motor shaft as it will increase the transient time of the drive. If the motor is fed from the motor
generator set, then flywheel mounted on the motor generator shaft and hence equalizes the load on
the source but not load on the motor.
4. Factors which affect the stability.
Electrical -- Inductance (L) & Mechanical -- Inertia (J)
5. Distinguish DC and AC drive.
6. What causes poor input power factor in phase controlled DC drive?
The input power factor in the phase controlled rectifier is low when the output voltage is less than the
maximum, which is when the firing angle is large. When the firing angle increases, the converter draws
more lagging reactive power. Therefore, the input power factor is low.
7. Significance of multi-quadrant operation.
The motor operates in two modes: motoring and braking. In motoring, it converts electrical energy
into mechanical energy which supports its motion. In braking, it works as a generator, converting
mathematical energy into electrical energy and thus opposes the motion. Motor can provide motoring
and braking operations for both forward and reverse directions.
8. Drawbacks of converter fed DC drives.
Distortion of supply.
Low power factor.
Ripple in motor current
9.
Part -B
10. A. Classes of duty
Continuous duty
Short time duty
Intermittent periodic duty
Intermittent periodic duty with starting
Intermittent periodic duty with starting and braking
Continuous duty with intermittent periodic loading
Continuous duty with starting and braking
Continuous duty with periodic speed changes
Continuous Duty:
This duty denotes that, the motor is running long enough AND the electric motor temperature reaches
the steady state value. These motors are used in paper mill drives, compressors, conveyors etc.
Short Time Duty:
In these motors, the time of operation is very low and the heating time is much lower than the cooling
time. So, the motor cooks off to ambient temperature before operating again. These motors are used
in crane drives, drives for house hold appliances, valve drives etc.
Intermittent Periodic Duty:
Here the motor operates for some time and then there is rest period. In both cases, the time is
insufficient to raise the temperature to steady state value or cool it off to ambient temperature. This is
seen at press and drilling machine drives.
Intermittent Period Duty with Starting:
In this type of duty, there is a period of starting, which cannot be ignored and there is a heat loss at
that time. After that there is running period and rest period which are not adequate to attain the steady
state temperatures. This motor duty class is widely used in metal cutting and drilling tool drives, mine
hoist etc.
Intermittent Periodic Duty with Starting and Braking:
In this type of drives, heat loss during starting and braking cannot be ignored. So, the corresponding
periods are starting period, operating period, braking period and resting period, but all the periods are
too short to attain the respective steady state temperatures, these techniques are used in billet mill
drive, manipulator drive, mine hoist etc.
Continuous Duty with Intermittent Periodic Loading:
In this type of motor duty, everything is same as the periodic duty but here a no load running period
occurs instead of the rest period. Pressing, cutting are the examples of this system.
Continuous Duty with Starting and Braking:
It is also a period of starting, running and braking and there is no resting period. The main drive of a
blooming mill is an example.
Continuous Duty with Periodic Speed Changes:
In this type of motor duty, there are different running periods at different loads and speeds. But there
is no rest period and all the periods are too short to attain the steady state temperatures.
10.b
The drive is said to be in equilibrium if the torque developed by the motor is exactly equal to the load
torque.
If the drive comes out of the state of equilibrium due to some disturbance, it comes back to steady state
for stable equilibrium but for unstable equilibrium the speed of the drive increases uncontrollably or
decreases to zero. When the drive coming out of the state of equilibrium preserves it steady state at
different speed (lying in small range), it is said to be in neutral range.
The stability of the motor load combination is defined as the capacity of the system which enables it to
develop forces of such a nature as to restore equilibrium after any small departure therefore.
Equilibrium state of the drive mainly disturbs because of the following two types of disturbances,
1. Changes from the state of equilibrium takes place slowly and the effect of either the inertia or the
inductance is insignificant Steady state stability.
2. Sudden and fast changes from the equilibrium state so effect of both inertia and inductance cannot be
neglected- Dynamic or transient stability.
Criteria for steady state stability: -
Let the equilibrium of the torques and speed is TM, TL and and the small deviations are Tm, TL and
. After the displacement from the equilibrium state the torque equation becomes,
Considering the small deviation, changes can be expressed as a linear function of change in speed,
From the torque equation, where all quantities are expressed in terms of their deviations from the
equilibrium,
Where, ()0 is the initial value of the deviation in speed. For the stable system the exponent must be
negative, so speed increment will disappear with time. The exponent will always be negative if,
Criteria for the steady state stability is for a decrease in the speed the motor torque must exceeds the
load torque and for increase in speed the motor torque must be less than the load torque. To check the
stability at an operating point of the motor, if an increase in speed brings greater increase in load
torque than the motor torque, the speed will tend to decrease and return to its original value, so
operating point will be a stable point else operating point will be an unstable point.
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