Operation of three phase induction motors (12hrs)
Induced rotor currents are made up of two parts: 1) a component in phase w/ the induced voltage called _____. 2) a component that is 90 degrees out of phase w/ the induced voltage called _______.
1) active rotor current 2) reactive rotor current
A 4 pole, 60hz motor has a no load speed of 1798 r/min & the full load slip of 3.2%. Calculate the percent speed regulation
3.19%
A four pole, 60hz SCIM has a full load speed of 1740 r/min. What is the percent slip at full load?
3.3%
CEC allows over current protection for motors to be as high as _____% of FLA for start up process.
300
A two pole, 60hz motor has a 5% slip at full load. What is the full load speed of this motor?
3420 r/min
What's the full load torque of a 10 Hp motor w/ a full load speed of 1750 r/min
40.69 Nm
Calculate the full load torque of 7.5kW motor w/ full load speed of 1740 r/min
41.16Nm
From no load to full load most induction motors have a small speed variation of 2 - __% of synchronous speed?
5-8
What's the efficiency of a fully loaded 10HP motor w/ input power of 10kW
74.6%
Calculate percent efficiency of a 5 hp motor w/ the following losses: Rotor copper 200w Stator copper 300w Hysteresis 95w Eddy currents 125w Friction 40w Windage 20w
82.27%
Calculate the percent efficiency of a 10 Hp motor that has total losses of 1017W at full load?
88%
When would each NEMA rotor design be used?
A & B machine tools, fans C conveyor, loaded compressor D punch press, pump jack
NEMA rotors starting current is what for each design?
A high B normal C normal D low
Breakdown torque for each NEMA rotor is?
A high B normal C low D low
Full load slip for each NEMA rotor is ?
A low B normal C normal D high
NEMA rotors starting torque is what for each design?
A normal B normal C high D very high
Maximum torque or breakdown torque is achieved when?
Active rotor current equals reactive rotor current
Rotor current is at a max when? What happens to rotor current as the rotor accelerates?
At the instant of start Decreases
Speed regulation is most related to the percentage of: a) slip the motor has at full load b) rotational frequency change from NL to FL speeds c) motor efficiency at full load d) motor efficiency as compared to torque output
B
Which NEMA rotor design is the most common
B
Resistance of a wound rotor is adjustable using external resistors connected through ____& slip rings? This enables the operator to _____ starting current while _______ torque. After running the external resistors can be ______ to increase ______ & _______.
Brushes Reduce Incresase Bypassed Rotor speed & efficiency
Which NEMA rotor design has the double squirrel cage
C
Rotor resistance (Rr) of a SCIM is ________?
Constant
Increasing inductance (rotor reactance) decreases starting _____.
Current
Rotor power factor ______ with slip increase.
Decrease
Rotor impedance ___ with rotor a acceleration. Therefore it would _____ if slip increased
Decrease Increase
As a rotor speeds up rotor frequency & reactance ________, causing torque to increase to its max value, then ______ to the value needed to carry its load.
Decrease Decrease
Rotor impedance increases with slip though not linearly. Is at a max value at the instant of start ( blocked rotor) & ____ as the rotor accelerates.
Decreases
Rotor voltage is _________ to slip? What's the value of rotor voltage at synchronous speed & instant of start (blocked rotor)
Directly proportional 0 v minimum value Maximum value at start
Formula for rotor voltage?
Er= s x Ebr
Speed control of a wound rotor is suitable from 0 r/min to ______?
Full load speed
The slip speed of a rotor _____ as the load is added to the shaft of a motor?
Increases
Active rotor current (IRr) ____ with slip until it equals reactive rotor current & then it ____. Active rotor current is the only quantity that increases & then decreases as the rotor accelerates.
Increases Decreases
A slip of 1.0 (100% slip) occurs when A slip of 0 (0% slip) would occur when
Instant of start If rotor speed equals synchronous speed
What's the formula for motor current
Ir = Er / Zr Current equals voltage at that time divided by impedance at that same time
Efficiency can equal Pout/Pout + P losses What are these losses?
Iron losses - core of the stator & rotor Copper losses (I2R) Friction - bearings. Between brushes & slip rings Windage- fraction of air movement by the rotor
What's the formula for reactive motor current
Irx = Ir x sin r
Motors with higher full load rotor speeds have ______ percent speed regulations & _______ efficiencies?
Lower Higher
Reactive rotor current (90* out of phase) does not produce torque. Reactive rotor current Irx & stator current are necessary to establish the _______ field that a motor needs to operate.
Magnetic
The rotor speed always lags the stators magnetic fields speed, allowing rotor bars to cut _____ lines of _____ & produce _______.
Magnetic Force Torque
Rotor power factor is at a ________ (max angle) at the instant of start. Because Xr decreases Zr decreases, the power factor ________ (angle decreases) as the rotor accelerates.
Minimum Increases
What's the formula for synchronous speed?
Nsyn = 120(f)/N 120 is a constant f is frequency B is the # of poles
What's the formula for rotor power factor (cos r)
Pf r(cos r) = Rr / Zr
Active rotor current (in phase) (IRr) equals rotor current at the same time multiplied by rotor _______ ______ at the same time.
Power factor IRr= Ir x cos r (pf)
When a motor is at normal running speed, slip speed & torque are________?
Propirtional
SCIM can imbed rotor bars further into the iron increasing the flux produced by the current in the bars which increases rotor ________
Reactance Xr
Rotor impedance (Zr) is the phase sum of rotor resistance & rotor ______? What is the formula for rotor impedance?
Reactance Xr Zr = square root of: (Rr2 + Xr2)
Increasing rotor _______ increases starting torque but decreases ______.
Resistance Efficiency
SCIM can increase rotor _________ by building motors w/ thinner rotor bars.
Resistance Rr
Breakdown torque or max torque & max active current are produced when: Rotor reactance = _______. Rotor power factor angle is? Rotor power factor is?
Rotor resistance Xr = Rr 45 0.707
Within an induction motors range of running speeds, torque is directly proportional to _______?
Slip
Slip (s) is the ratio of ______ speed to ______ speed, expressed as a decimal or %
Slip Synchronous
What's the best indicator of power output for a motor
Slip & slip speed
Rotor reactance (Xr) is proportional ______? Rotor reactance is at what value at synchronous speed & instant of start (blocked rotor)
Slip (s) 0 ohms minimum value Max value at instant of start
Rotor speed is less then synchronous speed. The speed difference is referred to as _______ ______?
Slip speed (ns)
A stator's magnetic field is constant in magnitude & speed. This constant speed is referred to as______?
Synchronous speed
Once a motor is at running speed, if load counter torque becomes greater then breakdown torque what would happen?
The rotor stalls
Active rotor current produces _______. As torque increases it becomes proportional to _______ rotor current
Torque Active
If the load is increased beyond rated & full load. Slip increases at a higher rate. This increase lowers the rate that rotor current increases & results in?
Torque that does not increase directly with slip
A motor with low percent speed regulation is more efficient then one with high percent speed regulation? T or F
True
Which rotor performs the best in terms of producing high starting torque, low starting current, low percent speed regulation & high efficiency?
Wound rotor
Formula for rotor reactance?
Xr = s x Xbr
Is Rotor frequency is directly proportional to slip? What's its value at synchronous speed & at instant of start (blocked rotor frequency)
Yes 0 hz minimum value 60hz (line frequency) maximum value
Formula for rotor frequency?
fr= s x fbr
What's the formula for slip speed?
ns = Nsyn - n ns = slip speed Nsyn = synchronous speed n = rotor speed
What is the formula for slip (s)
s = ns / Nsyn