NEMA Constraints on Voltage and frequency

• A voltage variation of up to +/- 10% rated voltage while operating at rated frequency

• A frequency variation of +/- 5% rated frequency while operating at rated voltage

• A combined variation in voltage and frequency, with the sum of the absolute values of the respective variations not exceeding 10% providing the frequency does not exceed 5% of rated frequency.

Effect of Off-Rated Voltage and Off-Rated Frequency on Induction Motor

Performance

• Effect on running torque

2

0.3

2

2

0.3

21.12

(120 )D

s

Ds

V sT f

Rp

V sT

f

Effect on Locked-Rotor Current

2

2 22

1 1 2

2 2 21.0

1 2 1 2

( ) ( )

( ) ( )

" "

slr

lr

VI

RR X X

sV V

IZ R R X X

Z locked rotor impedance

Locked-Rotor Impedance (s=1)

1 2

1 2

1 2

21.0, 75

1 2 1 2

arctan( )

75

2 ( )z

z

z

lr

s

X XR R

If

Z X X

V V VI

X X f L L f

Locked-Rotor Current

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75

1 2

2

1 2

1.0, z

o

o

lr

lrs

I I I

I I

I I I

VI

f

Effect on Locked-Rotor Torque

1 2

1 2

2

2

D stator rotor

stator stator D stator rotor

lr

lr

lr lr lr

T B I

B I T I I

T I I

I I I

VT I T

f

Operating 60-Hz Motors on a 50-Hz System

• Operation below rated frequency causes– a significant decrease in magnetizing

reactance XM

– an increase in magnetization current– resulting overheating

• To prevent overheating,– reduce the applied voltage, keeping the ratio

of volts per hertz constant

Operating 60-Hz Motors on a 50-Hz System

• General-purpose 3-phase 60-Hz NEMA-design induction motors with 2, 4,6, or 8 poles operate satisfactorily on 50-Hz systems if the horsepower and voltage ratings at 50-Hz are 5/6 of the corresponding ratings at 60-Hz.

• The locked-rotor and breakdown torques at 50-Hz will be essentially the same as for 60-Hz operation.