Time for start-up between two motors
Time for start-up between two motors
(OP)
We have two motors on a conveyor belt (Each 50kW) How do you determine the time-delay between the start-up time of the two motors? (DOL-starters) I don't want to start both imediatelly (due to the high start-up current), but I also don't want the time-delay between the start-up of motor1 and motor2 to be too long. (overloading of motor1, especially when there is load on the belt)
Is 0.5 seconds too long/short?
thanks
regards
RCC
Is 0.5 seconds too long/short?
thanks
regards
RCC





RE: Time for start-up between two motors
On the electrical side, if some form of reduced voltage starters are used, (auto-transformer, SCR, VFD,etc.) they should be started together. I do not know what DOL - starter means, so maybe this last statement is invalid.
If the motors are across the line started, I would be more concerned about belt slippage or damage than overheating the first motor.
Good luck,
Podo
RE: Time for start-up between two motors
RE: Time for start-up between two motors
1- Combined inrush currents (DOL) will increase the voltage drop of the line, reducing the accelerating torque and extending the time to accelerate.
2- The substation and lines will suffer overheating (r*i^2) due to the combined inrush currents.
Measure the inrush current of the first motor to know the time when it drops to the steady load current (3 to 5 seconds), and then start the second motor.
RE: Time for start-up between two motors
I much prefer the suggestion of having fluid couplings, or magnetic couplings, to allow a cushioned start. Because these couplings will significantly reduce the period of maximum in rush current, it should be possible to stagger the starts without overstressing the belt. The actual delay will be a factor of the design of the belt, but I would try to keep it as short as possible. The 0.5 second that you suggest should be enough. Monitor the start sequence with recording ammeters to ensure you are getting what you want.But without the couplings, you will probably just have an additional 0.5 second of locked rotor current on the first motor, which won't help your situation.
RE: Time for start-up between two motors
The motors are not on the same pulleys. (2 pulleys on one belt) There are fluid-couplings between the motors and the gearboxes. The problem is that one motor can't take a fully loaded belt away on its own. (Without any material on the belt there is no problem) With a long delay between the motors (3seconds) the first motor eventually trip on overload.
Macmil, what will happen with two motors on one pulley? (Both with fluidcouplings) Will there be any difference?
Regards
RCC
RE: Time for start-up between two motors
RE: Time for start-up between two motors
We have used fluid couplers with extended times before they start to transfer torque to the load. They are called double-delay couplers. They are made by all of the fluid coupler people. They will allow for a short delay between motor start-ups.
Also, 3 seconds seems like a very short period of time for an overload to react, especially with a fluid coupler drive system. You may want to look a little deeper. Does this circuit include any type of "jam" protection? This may be part of the problem.
Podo