2 HP AC motor revere controller


Thread Starter

Pete Lefferson

I'm out of my electronic field. I promised to design the AC controller for a mechanicl engineer friend. He has designed a new boat lift for large boats. His new lift system is on each side of the boat. Each side requires a 2 HP AC cap start motor. They always run together. They must run in either direction.
Must I select the very expensive 2 HP reversing contacors for each motor? Can I use a 5 HP reversing contactor for both motors? It seems that I must price each contactor near $500. It seems that there must be a less expensive method.

Thanks for your thoughts and comments.
Pete Lefferson
A consult of the 1999 National Electrical Code (NEC) is fruitful. My 2002 version is in the mail.

Look at the NEC article 430-87 for number of motors served by a controller. Then look at article 555 for marina installations.

Al Boake P.E.

Bob Peterson

An IEC reversing contactor can be had for far less then $500. Personally, I'd be inclined to use a single contactor with a seperate OL for each motor that would trip the single contactor.

Are these 220V motors (I'd surmise from the capacitor start thing that they are single phase)? Even 2 HP motors at 220V will each eat about 12 A. That means you will need a 30A 220V supply. Is that something that is available?

Bob Peterson

Curt Wuollet

There are an awful lot of relays around that are rated for 2hp and don't cost $500.00. Suggest you talk to a relay company rather than an automation company. This simply isn't a very exacting task. The reversing function can be arranged so that the contacts transfer when the motor is stopped. ie stop between forward and reverse. A $100.00 Garage door opener works for years, I doubt they are using $500.00 contactors. Logic would suggest $10.00 relays. SSRs are good in this range also.



Peter Lefferson

Thanks Al,

That is a big book!! I am still back on 430-52 where it seems that I must use a seperate fuse for each motor (430-42 B) that is rated at
300% (430-33)above the full load current. I would assume that I can put these fuses inside the control box on the dock. What protects
the wire between the control box and the motor? The wire size is rated to be at least 125% (430.22 A). Common sense seems to say that I wire this short run (less than ten ft.) with wire rated at 300% of full load current. I can do that.

The boat lift will have a key lock. The contactor will have a low voltage coil. Can the key lock control just the low voltage system?

430-74 seems to say that I also need a mains power switch for the motors and another for the controller (the low voltage transformer and
contactor). I don't see that on the docks. I see up and down buttons and possibly a lock.

Any more thoughts?

Thank, Pete Lefferson

Pete Lefferson

Thanks Bob,

I assume that it will be available? You raise a good question and I will check on it.

Thanks, Pete Lefferson

Pete Lefferson

Thanks ccw,

In the Square D catalog I find the three pole mechanically interlocked contactors. The one rated for 2 HP has a cost of $421.

I think that you are right. There is a cheaper way but I must operate within the NEC book and I can't stand to be sued ten years from now
because of a poor design.

Any more thoughts?

Thanks, Pete Lefferson

I commend you on your desire to do things right. However the contactors you are looking at are probaly NEMA rated. This is overkill for almost every application. Except for mobile equipment like cranes and mining equipment. Take a look at some IEC rated contactors. Allen-Bradley makes a great set for around $100. I'm sure other companys do also. As for your mains disconnect.
Yes, you must have one. My suggestion would be to use a simple fused safety switch disconnect. Put it in the line above your control box. This will be above your control transformer, so it will drop out your control power also. As for protecting the wire from the control box to the motor. This depends on the distance. If it is less than six feet use seal-tite "liquid tite flexable metal conduit. If it is over six feet use rigid or emt conduit. Personally I hate emt but if you do not have a hydraulic bender handy rigid can be a pain. I would use the money saved on the contactors to install anti-plugging timer to prolong the life of your motor. There are some low cost timers on the market, I recommend solid-state anywhere it is possible. Also if you have not yet purchased the motor, and you have three phase power. Then get a three phase motor. Three phase motors are more efficent, last longer, and are easier to work with.

Good Luck