motor undervoltage and effect on current and torque

• 10-10-2012, 09:02 AM
IgnatiusJReilly
motor undervoltage and effect on current and torque
I'm having trouble reconciling a couple different situations:

1) Huge induction motors use reduced voltage autotransformer starting, resulting in lower current and lower torque. Does this essentially result in a new motor speed/torque curve (new LRA torque, new pull up torque, new breakdown torque)?

2) Are multi-horsepower (multi-tap) single phase motors - common for small evaporators - doing the same thing with their speed taps as in #1? The current and torque both decrease.

3) We know that when induction motors receive undervoltage conditions, the current increases. What's the difference between this and #1 and #2?

Thanks,

IJR
• 10-10-2012, 11:04 AM
DPinst
Usually small evaporators use fractional horsepower psc motors. When you reduce the HP by changing the taps you are not reducing the input voltage.
Get out your ohm meter and ohm out a PSC motor between common and the various incoming line(speeds/Horsepower). You will notice that common to high will give you the lowest resistance and common to low will give you the highest resistance.
All you are doing is increasing resistance by increasing the amount of windings that the current must flow through. When that happens less current flow ,weaker magnetic field and less torque less HP produced by the motor. Exact opposite for highest HP settings same voltage, lower resistance,more current flow, stronger magnetic field more torque ,higher HP .
Hope that makes sense .
• 10-10-2012, 05:25 PM
jayguy
#1: yes. you get new LRA values and torque values

#2: no. multi-hp (NOT multi-speed) motors do not do the same thing as autotransformers. motors that use autotransformers, change the resistance (sorta) but then after it is up to speed, it will bypass the autotransformer and the motor is connected across-the-line. multi-hp motors do not start in 1 configuration and then change to a different configuration later.

#3: no difference. both motor types act the same during a low voltage condition.
• 10-11-2012, 07:58 AM
IgnatiusJReilly
Right, eventually the autotransformer on #1 is taken out of the circuit, but I should have specified that I'm just talking about before that happens (in order to make the comparison), are #1 and #2 the same effect?

And more importantly, why are they different than #3? That is, why does a reduced voltage in #1 and #2 result in less current and torque, and a reduced voltage in #3 result in more current and torque?

IJR

just for clarification to DPinst, for #2 to run in low speed I believe that you are reducing the input voltage to the main winding in fractional multi-hp motors, see AOSmiths "basics of multispeed motors":

http://www.centuryelectricmotor.com/...pe=1033&id=766
• 10-11-2012, 02:47 PM
jayguy
Quote:

Originally Posted by IgnatiusJReilly
Right, eventually the autotransformer on #1 is taken out of the circuit, but I should have specified that I'm just talking about before that happens (in order to make the comparison), are #1 and #2 the same effect?...

they are not the same thing (if i understand you correctly). a multi-tap motor and an autotransformer (which also has taps) are very different things.

i think that you and i (probably mostly me) are talking about 2 different things under 2 different circumstances...but i think that i see your question in there.

a multi-hp motor essentially has several windings within it. you pick the hp that you want for your application. with a lower voltage you would get a higher amperage WHILE RUNNING.

a motor STARTING UP using an autotransformer uses the autotransformers windings for starting. this inserts extra inductance (AC resistance) into the circuit making the motor see lower voltage which causes it to use less amperage at START UP (this is the key) than it would if it were STARTING ACROSS THE LINE. both starters have lower voltage at start up and this causes the amperage to be higher...the difference is how is how much higher. the motor using the autotransformer DOES have a higher current at start up than running because it is starting...that part is the same as a multi-hp motor.

multi-hp motors are not using different taps like autotransformers do...although both use the same terminology.
• 10-12-2012, 01:39 AM
Russ57
Also.....lower voltage cause higher amperage "if" torque is required. In other words torque is proportional to square of voltage. Motors that are being started at reduced voltage are usually seeing little to no torque requirments....compared to what the see when fully loaded at line voltage.

So conditons for them are very different than what you are suggesting in #3.