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Topic Title: Motor power larger than VSD power
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Created On: 04 February 2012 08:00 AM
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 04 February 2012 08:00 AM
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HI all,

Please i want to know what happen when a 11kW VSD drive a 30kW motor? Does the VSD have any fuse inside that protect them if more current is required? All the installation is made for 30kW motor except the VSD because of a mistake.

Thanks in advance,

 04 February 2012 10:05 AM
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It's just not going to work is it? Worst-case is that you could cause a fire/explosion depending on the design. In the UK specifically, but also many other countries, Health & Safety Legislation makes it an offence to power this up.

One or more of the following will happen:

- Overcurrent protection will operate if fitted in the VSD

- Power electronics is damaged. If not protected by Overcurrent Protection because of the design for 30 kW and none internally, there may well be a fire/explosion as a result.

- "crowbar" or similar protection if included in the design of the VSD will prevent it from driving the motor.

EUR ING Graham Kenyon CEng MIET TechIOSH
Principal and Proprietor,
G Kenyon Technology

 07 February 2012 07:23 AM
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Are you sure about Graham? It is pretty hard to damage a modern VSD. They are very well internally protected. VSDs are specifically designed to limit the amount of power they put into a motor. I think the VSD will put up to 11kw into the motor as required by the power demand on the motor output shaft and then current limit. If the VSD does not think it can drive the motor safely it will just refuse to have anything to do with it. It is possible that because the motor is oversized the VSD will decide that it is seeing an output short circuit and will just not start. The drive will not be very efficient because the motor has been designed to run with more magnetising current than it can get from the VSD.

It is not unusual to somewhat oversize motors used with VSDs to avoid overheating.

The VSD manufacturer ought to be able to give you a more specific answer. I would give it a go.
 14 February 2012 01:00 PM
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It depends on the type of load. I would be very suprised if you damage the drive, in built protection on modern drives is very good. If you require 30kW load power then the chances are the drive will stall and shutdown on current limit. With applications such as fans/pumps you may be able to drive the motor upto a certain speed without tripping. I assume this is running in simple V/F mode other modes such as "vector", it may refuse to autotune on such a large size descrepancy.
 08 March 2012 11:19 PM
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I have used a siemens 0.5kw VSD on a 0.75kw motor and put the parameters for the bigger motor in and it ran for 10 months ok which is a 50% overloading but the drive ran at about 35Hz maximum so i dunno. If the motor is running at half load you may get away with it for a short period but i wouldn't recommend it.
 09 March 2012 04:45 PM
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From past experience, the drive will run up too a certain speed. But will not be able to deliver full load current and torque to the 30 KW motor.

Also I agree that it won't do any damage to the motor or inverter, due to not being able to deliver excess current to the drive. If it was the other way round, a 30 KW inverter powering a 11KW motor then you would have to be careful what parameters you put in the settings.

Hope this helps

Fail to prepare...Prepare to fail!

 21 May 2012 11:05 AM
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The VFD will give somewhere in the region of 26 Amperes @ 400 volts, and depending on the type of load will run the motor, but will not be able to provide the 30kW of work.
The VFD protection will trip if current exceeds the factory setting, and the speed which is achieved before it trips will depend upon the load, is it constant torque like an auger drive, or variable torque a pump or a fan. If the latter then the speed may be higher before the VFD runs into 'current limit', at which point it will back off the frequency to protect itself.

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