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Motor stall during start and speed switch setting

Motor stall during start and speed switch setting

Motor stall during start and speed switch setting

(OP)
High intertia motors have actual starting time in excess of the allowable starting time. Hence, to sense that the motor is accelerating a speed switch input is wired to the motor protection relay and the motor tripped prematurely if the feedback is available that the motor is not accelerating.
I have worked with the above arrangement in no. of projects. But never paid attention to the subject of speed switch setting i.e. at what speed we really conclude that the motor is accelerating. To me, it was just a potential free contact to be wired to the relay. Now, with electronic speed sensing schemes, there are options to select like 1%, 3% or 23% of rated speed for example.
Can some one share their knowledge / experiences please!
Thanks in advance.  

RE: Motor stall during start and speed switch setting

First of all I think you have to read "Starting High inertia loads" FAQ237-1131: Starting High inertia loads or:
http://www.eng-tips.com/faqs.cfm?fid=1131.
For electric motors with constant acceleration torque, acceleration time is: tac=WK^2*(N2-N1)/308/Tacc
Where: WK^2=WK^2M+ WK^2L   WK^2M=motor inertia moment WK^2L= load inertia moment [lb.ft^2].
Tacc acceleration torque =Tmotor-Tload [lb.ft]
N1 starting rpm N2 required rpm.
From this equation one will get N2= tacc*Tacc*308/wk^2+N1
N1=0 at start. N2%=N2/rated rpm*100
Let's take an example:
An ABB motor of 3 KV 1000 rpm 710 kW rotor wk^2= 871 lb.ft^2 load wk^2= 14200 lb.ft^2 Tmrated= 5028 lb.ft
Max.stall time=20 sec. Max start time 8 sec.
Tacc=Tmotaverage-Tload. Tmotaverage=0.45*(Tstart+Tmax) Tload=Tmrated.
Tacc=[0.45*(1.3+2.6)-1]*Tmrated= 3800 lb.ft
We can take 3 sec from start.
N2=3*3800*308/(871+14200)= 234 rpm
N2%=234/1000*100=23.4%
 

RE: Motor stall during start and speed switch setting

I think I super estimated the acceleration torque. Actually, for such small slip the motor torque could be still Tstart.
As ABB calculating the total start time did, we shall take also a 0.85 factor as the supply voltage could be less than rated.
ABB calculated the total starting time with from rpm=0 to rated rpm taking Tload=(1-s)^2*Tmrated where s=slip
  slip=(synchro rpm-actual rpm)/synchro rpm
This Tload corresponds to a pump or a ventilator.
For a lifting machine, for instance, the torque is constant -close to motor rated torque.
So the starting time from still stand up to a certain slip depends on a lot of factors as load type, load wk^2, motor torque/rpm diagram and other.
From the torque-slip [or rpm] diagram you have to pick the Tacc up-as the difference between motor torque and load torque and then you have to calculate the rpm when to free the velocity switch
 

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