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Two Identical Motors - One Invertor

Two Identical Motors - One Invertor

Two Identical Motors - One Invertor

(OP)
We have a lift (elevator) application with two mechanically
coupled 7.5 HP 460V motors.  I Want to control both motors
with 1 Siemens 15HP invertor.  The system will include a
large regen resitor for breaking during the down cycle.

Will This work.  Does anyone have experience doing this.

Thanks

RE: Two Identical Motors - One Invertor

It can be done using PWM-type drives.  You will need independent overload protection for each motor.

But whether or not it should be done is another question.  If this elevator is used for moving people, I would use the equipment provided the elevator manufacturer and nothing else. Elevators have a unique set of code and safety requirements.  This has, unfortunately, resulted in the standardization of some very antiquated approaches to motor control, but elevator control should be sole responsibility of elevator manufacturer.  



RE: Two Identical Motors - One Invertor

I agree. My company makes PWM inverters and dpc's post is right on. It can be done, but if you are moving people, leave it to those who have liability insurance that covers them (and you) should anything go wrong. We stay out of the elevator business for that reason.

If, however you are speaking of a PRODUCT elevator, just make sure that you have individual overload relays for each motor as dpc said, either of which will shut down the drive on overload. Another tip is do NOT put separate output contactors on the motors in an attempt to drop off only the motor that overloads. Should you reset that overload and close the contactor while the drive is running the other motor, it may damage the drive.

Also, be aware that even though your header mentions that they are identical motors, any slight variation in slip  will result in the motors not sharing the load equally. This is common and usually not too bad, but be aware of it. Get a true RMS reading ammeter and measure current draw at full speed. If they are different by more than 2% you may have a problem with one overheating.

RE: Two Identical Motors - One Invertor

Suggestion: Considering the inverter rating, it may be somewhat small since you have to consider motor efficiencies and the inverter may have certain overload duty only. Often, the elevators are overloaded. If it is your own private elevator, with your own responsibility for it to yourself only, you will probably try to lift yourself by it. The elevator safety depends on its design. The elevator propulsion is a part of it only. The elevator controls and safety features are another aspects. Theoretically as well as practically, the elevator for things may be engineered and designed safer than the one for people. However, I do agree with the elevator manufacturer responsibility for the elevator and will all applicable safety code pertaining to passenger elevators.

RE: Two Identical Motors - One Invertor

Suggestion: Considering the inverter rating, it may be somewhat small since you have to consider motor efficiencies and the inverter may have certain overload duty only. Often, the elevators are overloaded. If it is your own private elevator, with your own responsibility for it to yourself only, you will probably try to lift yourself by it. The elevator safety depends on its design. The elevator propulsion is a part of it only. The elevator controls and safety features are another aspects. Theoretically as well as practically, the elevator for things may be engineered and designed safer than the one for people. However, I do agree with the elevator manufacturer responsibility for the elevator and with all applicable safety code pertaining to passenger elevators.

RE: Two Identical Motors - One Invertor

A couple of things that I have found out with elevators and hoists.

1.) Like Jbartos posted, really look at inverter rating. I have always rounded to the larger drive if the current output rating was close to the total of all the motors being driven.

2.) Individual motor protection is also a necessity. With the new style MSP's (motor starter protectors) I include aux contacts to trip the drive and a shunt trip to remotely e-stop both motors (the e-stop of course engages your mechanical brake.)

3.) The type of inverter is also critical. I only use vector drives on lifting applications. The 150% or so torque at 0 speed allows for a smooth engagement of the mechanical brake. You can use a sensor-less vector or volts per hertz drive...but it's a task getting the brake engagement correct without tripping the drive, especially when the design is close to the minimum load requirements.

Just a few points I look at.

Carl

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