Operation of three phase induction motors (12hrs)

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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


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