Motor Operation - Speed Torque

[W@ttson]

Hello,

Please confirm the two scenarios below, each of which is regarding a NEMA A motor with across the line starting:

Scenario 1:
NEMA A motor with the following speed torque characteristics:
Locked rotor = 180% FLT
Pull up torque = 70% FLT
BDT = 250% FLT

Load Profile with the following characteristics:
Starting Load Requirement = 150% FLT
Acceleration Load Requirement = 120% FLT
Constant Velocity Load Requirement (moving at the rated slip speed) = 70% FLT

I assume in this scenario the motor will start, begin to accelerate, get bogged down because the pull up torque availability from the motor is less than the Acceleration Load Requirement of the load. The motor will slow down head toward locked rotor torque. Since locked rotor torque is higher than the acceleration requirement the load will continue to move but very very slowly. This will cause a lot of heat generated in the motor and eventually fail. The motor will never get to full rated speed.

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Scenario 2:
NEMA A motor with the following speed torque characteristics:
Locked rotor = 125% FLT
Pull up torque = 70% FLT
BDT = 250% FLT

Load Profile with the following characteristics:
Starting Load Requirement = 120% FLT
Acceleration Load Requirement = 130% FLT
Constant Velocity Load Requirement (moving at the rated slip speed)= 70% FLT

I assume in this scenario it would be similar to scenario 1 where since the pull up load capability of the motor is less than what the load requires, the motor will slow down to try and get to the locked rotor torque. Since in this scenario the locked rotor torque capability of the motor is less than the acceleration load requirement, the motor will come to a complete stop and draw locked rotor current until it burns itself out.

 

[ptonsparky]

You may be correct but supplying either with proper overcurrent protection should prevent smoke escape.

Is this purely an academic exercise?

 

[W@ttson]

You may be correct but supplying either with proper overcurrent protection should prevent smoke escape.

Is this purely an academic exercise?

yes, its an academic exercise. I was trying to explain the importance of understanding load profile and motor pull up torque capability to someone. I pulled out the typical torque speed curve of a NEMA A motor and showed how the motor traverses through the chart based on the applied load.

 

[Julius Right]

It is still unknown the motor behaviour. If the load torque-speed curve is like no.1 is, then, it will be o.k. If the load torque curve will be like no.2, then, the motor will rotate in point A and as in this point the current is still very high the protection will act-I think- and it will shut-down.

 

[Tip DS]

It is still unknown the motor behaviour. If the load torque-speed curve is like no.1 is, then, it will be o.k. If the load torque curve will be like no.2, then, the motor will rotate in point A and as in this point the current is still very high the protection will act-I think- and it will shut-down. View attachment 2572715

I think you're right JR, but OP stated the motor is NEMA A.

 

[Jraef]

Constant Velocity Load Requirement (moving at the rated slip speed) = 70% FLT

This part of your stated problem is going to depend on the NATURE of the load involved. If for example it is a centrifugal load, such as a pump or fan, there is a large difference in the load torque requirement at different speeds. So typically, at the speed at which the Pull Up Torque "dip" occurs, the load needs far less torque than the motor is providing at that point, so that 70% number doesn't really matter.

I mention this because Design A motors are used almost exclusively on centrifugal loads like pumps and fans (if at all, because MOST people just apply Design B motors due to their ubiquitous presence in the marketplace).

 

[W@ttson]

This part of your stated problem is going to depend on the NATURE of the load involved. If for example it is a centrifugal load, such as a pump or fan, there is a large difference in the load torque requirement at different speeds. So typically, at the speed at which the Pull Up Torque "dip" occurs, the load needs far less torque than the motor is providing at that point, so that 70% number doesn't really matter.

I mention this because Design A motors are used almost exclusively on centrifugal loads like pumps and fans (if at all, because MOST people just apply Design B motors due to their ubiquitous presence in the marketplace).

the load profile is more like a Constant power load. High Low speed torque. Low High speed torque. Its for a hoisting application where it takes a lot of torque to begin the lift, and then accelerate it. Then the torque tapers off in the constant velocity region as the load is moving steadily. The design A motor can be replaced with a design B motor. The torque speed curve of A and B should be similar enough for this exercise.

 

[W@ttson]

It is still unknown the motor behaviour. If the load torque-speed curve is like no.1 is, then, it will be o.k. If the load torque curve will be like no.2, then, the motor will rotate in point A and as in this point the current is still very high the protection will act-I think- and it will shut-down. View attachment 2572715

I don't understand how it would be OK in scenario 1. It would really depend on which speed the motor hits the 70% pull up torque to the exact load demand. The load demand is going from 150% to 120%. How long that acceleration to full speed is, it is not given in the problem statement. If it lingers at that 120% level, and the motor drops down to the 70% Pull up torque, I imagine the motor will start slowing down.

 

[Julius Right]

You are right W@ttson. However, if the load minimum torque is slightly less than the motor minimum torque then the motor can pass this point without reducing speed and start accelerating afterwards.