ECN Forum Forums Technical Discussion Technical Reference Area AC Dynamic Lowering Hoist Control

Attached drawings of an AC Dynamic Lowering Hoist Control, used on a Hoist with a Wound Rotor AC Induction Motor.

Schematics of complete system and close-up details are included.
Also included are text notes for this system and text notes regarding Wound Rotor Motors, which was submitted to be by an ECN member.
It was quite some time ago and I cannot remember the member and when this was submitted to me. If that member would send me a message I would give credits.

Also, I'll try to setup a discussion thread for this topic in the Electrical Theory area.

Here's the drawings!



Fig. 1-1: Overview of the complete system schematic wiring diagram for AC Dynamic Lowering Hoist Control.



Fig. 1-2: Detail of the Disconnect and Directional contacts sections.



Fig.1-3: Detail of the control circuit, DC power supply and mechanical brake sections.



Fig. 1-4: Detail of the Wound Rotor Motor.



Fig. 1-5: Detail of the Secondary Resistors network, time-delays and Dynamic brake systems.



Fig. 1-6: Detail of Resistor network and Motor as connected.



Fig. 1-7: Description and detail of collector bar and collector shoe

Text For This System;

AC Dynamic Lowering Hoist Control

A typical AC Dynamic lowering Hoist control power circuit with Rectifier-operated DC shunt Magnetic brake and power limit switch is shown in this Schematic.
The Motor, brake, and limit switch are mounted on the Trolley. The brake relay BR and the Rectifier are mounted on the control panel.
Such an arrangement requires 10 collector rails, as shown.

In hoisting, contactors H and M and the brake relay BR close on the first point of the master switch to release the brake and apply hoisting power to the motor with all secondary resistance in the circuit.
On the second point, 1A closes without delay; and on the third, fourth, and fifth points, 2A, 3A and 4A, respectively, close in sequence under control of relays responsive to variation in Rotor Frequency or motor speed.
On entering the limit switch, the two normally closed contacts LS open to remove power from the motor and to permit the brake to set.

On the first point lower, contactors L, DB and 1A close, tying motor terminals T1 and T2 together and applying single phase voltage across T2 and T3. The brake is also released, allowing the load to lower (if Overhauling) against Dynamic braking or retarding torque developed by the motor as it gains speed. The closure of 1A gives a secondary resistance value which provides a retarding torque of approximately full-load torque at synchronous speed.
On the point lower, DB opens and M closes to apply balanced 3-phase lowering power to the motor, and at the same time 1A opens to provide low torque. This point is used in "Inching" a light load downward.
If the master switch is held on the second point, 2A, 3A and 4A will close in frequency-controlled sequence.
If the master switch is moved to the third point from the "Off" position, 1A closes instantly to provide higher driving torque in the lowering direction, and 2A, 3A and 4A follow in frequency-controlled sequence.
With the secondary resistance short circuited, and with an overhauling load, the motor runs at a speed somewhat in excess of synchronous speed, the speed being limited by regenerative braking.

On returning the master switch to the second point, 1A opens without any effect, but when the master switch is returned to the first point, M opens and DB + 1A close, while 2A, 3A and 4A open to set up the Dynamic braking connections.
If the master switch is then moved to the "Off" point, DB, L and 1A remain closed and the brake sets. Dynamic braking assists the brake in bringing the motor to rest, at which time one of the frequency-responsive relays 2AR or 3AR opens its contacts to open L, DB and 1A.
This is reffered to as "Off Point Dynamic Braking".
The retention of Dynamic braking during the setting time of the magnetic brake prevents the load from dropping during this short time interval and greatly reduces brake-lining wear.

-----end of notes text-----

FYI: I'll try to insert speed-torque curves of a Dynamic braking AC Hoist controller built in accordance with the wiring diagram / Schematic shown here, once I compile the data and make it a Raster Image.

S.E.T. 10/13/2002 - C:\Pc2\website\images\gif\hoist_text.txt no rev.

end. new as of 10.17.2002 by S.E.T.

editted to add text! DOHHHH!!!

[This message has been edited by Scott35 (edited 10-18-2002).]

Text for above drawing.

AC Dynamic Lowering Hoist Control

A typical AC Dynamic lowering Hoist control power circuit with Rectifier-operated DC shunt Magnetic brake and power limit switch is shown in this Schematic.
The Motor, brake, and limit switch are mounted on the Trolley. The brake relay BR and the Rectifier are mounted on the control panel.
Such an arrangement requires 10 collector rails, as shown.

In hoisting, contactors H and M and the brake relay BR close on the first point of the master switch to release the brake and apply hoisting power to the motor with all secondary resistance in the circuit.
On the second point, 1A closes without delay; and on the third, fourth, and fifth points, 2A, 3A and 4A, respectively, close in sequence under control of relays responsive to variation in Rotor Frequency or motor speed.
On entering the limit switch, the two normally closed contacts LS open to remove power from the motor and to permit the brake to set.

On the first point lower, contactors L, DB and 1A close, tying motor terminals T1 and T2 together and applying single phase voltage across T2 and T3. The brake is also released, allowing the load to lower (if Overhauling) against Dynamic braking or retarding torque developed by the motor as it gains speed. The closure of 1A gives a secondary resistance value which provides a retarding torque of approximately full-load torque at synchronous speed.
On the point lower, DB opens and M closes to apply balanced 3-phase lowering power to the motor, and at the same time 1A opens to provide low torque. This point is used in "Inching" a light load downward.
If the master switch is held on the second point, 2A, 3A and 4A will close in frequency-controlled sequence.
If the master switch is moved to the third point from the "Off" position, 1A closes instantly to provide higher driving torque in the lowering direction, and 2A, 3A and 4A follow in frequency-controlled sequence.
With the secondary resistance short circuited, and with an overhauling load, the motor runs at a speed somewhat in excess of synchronous speed, the speed being limited by regenerative braking.

On returning the master switch to the second point, 1A opens without any effect, but when the master switch is returned to the first point, M opens and DB + 1A close, while 2A, 3A and 4A open to set up the Dynamic braking connections.
If the master switch is then moved to the "Off" point, DB, L and 1A remain closed and the brake sets. Dynamic braking assists the brake in bringing the motor to rest, at which time one of the frequency-responsive relays 2AR or 3AR opens its contacts to open L, DB and 1A.
This is reffered to as "Off Point Dynamic Braking".
The retention of Dynamic braking during the setting time of the magnetic brake prevents the load from dropping during this short time interval and greatly reduces brake-lining wear.

-----end of notes text-----

FYI: I'll try to insert speed-torque curves of a Dynamic braking AC Hoist controller built in accordance with the wiring diagram / Schematic shown here, once I compile the data and make it a Raster Image.

S.E.T. 10/13/2002 - C:\Pc2\website\images\gif\hoist_text.txt no rev.

post 10/17/2002 S.E.T.

Wound Rotor Motor text.

Wound Rotor Motors:

For the purpose of discussing the Theory of operation and Troubleshooting, the Three-Phase Squirrel-Cage Rotor Induction Motor may be thought of as an Isolated Transformer whose Primary Winding is the Stator of the Machine, and the Secondary Winding is the Rotor.

"Standard" Squirrel-Cage Rotor versions of this type Induction Motor have Short-Circuited bars on the Rotor, and all Speed Control is accomplished on the Stator (Primary) side.
The Wound Rotor version, as the name suggests, incorporates a Wire-Wound Rotor, on which various "Tap Points" allow fixed Resistances to be connected externally to the Motor.
These Fixed Resistances allow the Motor to achieve Speed Control via the Rotor (Secondary).

Normal setup of the Wound Rotor Motor is to connect both the Stator and the Rotor Circuits in 3 Phase Closed Wye
Configuration.

When the Rotor is open Circuited, almost no Current will flow in the Rotor Circuit.
Complete Isolation is impossible, as there will always be some Capacitive and Inductive coupling, causing a small amount of Current to flow through the Rotor Circuits.

When the Secondary (Rotor) Current is low, the Primary (Stator) Current will also be low.
These low Current levels in the Coils result in very little Magnetic Flux being developed in the Stator and/or
Rotor Poles.
With minimal Flux developed, the Motor has limited Torque capabilities, and the Rotor will be very easy to hold steady (keep from rotating).

With the Rotor stalled and open Circuited, the Machine basically becomes a 3-Phase Transformer, with the Voltage developed at the Secondary (Rotor) dependent to the Primary-to-Secondary Turns Ratio of the Stator/Rotor Windings, with a slight loss attributable to the Air Gap separating the Primary and Secondary cores.

The most common scenario is for the Rotor to contain
fewer turns than the Stator; so the Open Rotor Circuit Voltage will usually measure less than the Line Voltage applied to the Stator Windings.
With the Rotor open Circuited and stalled, the Rotor Voltage will be at its maximum value, and the Frequency of the Rotor Voltage equal to Line Frequency (-60 HZ - for standard applications in the USA). This is a "100% Slip" condition.


For example, a Machine having a Turns Ratio of 3:1.
With 480 volts on the Stator, the stalled Rotor Voltage (Esr) would be 160 Volts
(480v/turns ratio or 480/3)

If we short the Rotor leads together, the Induced Voltage at the Rotor will cause Current to flow in the Rotor; which will be reflected in the Primary (Stator) Coil(s).
The Current flow in the Coils creates Magnetic Flux Fields in the interacting Poles of the Stator and Rotor, thus creating rotation in the Rotor.

As the Rotor picks up speed, the Voltage in the Rotor decreases.
Rotor Voltage at any given speed will be:
Er= Esr*S%
(Rotor Voltage = Stalled Rotor Voltage * % slip)

Examples:

a.: @25% speed (75% slip): 160 *.75 = 120 Volts
(Er = Esr*.75)

b.: @50% speed (50% slip): 160* 50% = 80 Volts
(Er = Esr*.5)

c.: @75% speed (25% slip): 160* 25% = 40 Volts
(Er = Esr*.25)

As the speed of the Rotor increases, the decreasing Rotor Voltage will reduce the Current flow in the Rotor
(I=E/R)
Since the Stator is the Primary, the Current in it will also decrease
(with a 3:1 Turns Ratio, the Rotor Current will be about 3 times higher than the Current it reflects to the Primary).

For example: a Machine which required 40 amps per Rotor Phase to develop sufficient Torque, for handling a given Load.
This would reflect into the Stator as 40/3, or 15 Amps /Phase in the Stator, for our 3:1 Ratio Machine.

In normal operation, the Rotor will increase to a speed where the decreasing Rotor Voltage causes both, Rotor and Stator Currents, to reduce the Magnetic Fields to a value where the Rotor provides sufficient Torque to maintain the Load. This is where the speed will become regulated.

By adding Resistance to the Rotor Circuit, the Wound Rotor Motor can be forced to run at a high degree of slip (slow speed).
The Resistance in the Rotor reduces Rotor Current for any given Rotor Voltage (or speed). This reduced Secondary Current will also reduce the Stator Current - which once again will reduce the Magnetic Fields, which in turn, will reduce the available Torque; causing the Motor to slow down to a point where it is running at a Percentage of slip that will supply sufficient Torque for operating the Load.

In our previous example, we have determined that it would take 40 amps of Rotor Current to lift a given Load.
If we wanted to operate at 10% speed (90% slip), we could use the following Formula:

R = (Esr*S%)/ (1.73 Ir)
Where R is external Resistance per Phase.

R = (160*.9)/ (1.73*40)
so R = 2.08 Ohms in this example.
We would need a Grid Resistance of 2.08 Ohms in each Phase, to limit the Torque, so as to slow the speed of the Motor to approximately 10% of Synchronous speed, with a Rotor Load of 40 Amps per Phase.


When testing the Motor while it is still mounted in the Machine, precautions must be observed before Testing, so that all stored or potential Energy has been released or blocked.
For an example on a Hoist Application:
lower the Hook / Grapple to a resting position (or block the Drum from turning), as some of the testing procedures will release the Brake at a time when Torque is not being developed in the Motor.

Where Duplicate Motors are involved, a good idea would be to Base-Line the results of the Tests on a known good Unit, so as to establish Target Parameters.

With the Rotor Circuit open, apply Line Voltage to the Stator.
Check the Open Rotor Voltage in a stalled Rotor condition.
Check for Phase to Phase Voltage balance at the Rotor.
Check the Voltage and Current balance at the Stator.

Using an appropriate Tool, note how much Torque it takes to stall the Rotor.

Short the Rotor Leads together, and allow the Motor to reach full speed.
If the Motor is in the Machine, disconnect the Drive Coupling to separate the Motor from the Drive,
before performing this Test.

Check the RPM.
If lightly loaded, the Motor should run close to Synchronous speed.

Check the Motor with the Controller connected, and the Machine in operational condition.

With the Load and Controller connected, check both the Stator and Rotor's Voltages and Currents, for balance.

The Controller may be a Symmetrical Design, where on every step or stage of the speed control, each of the 3 Phase Rotor Windings will have the same Resistance. If this is the case, the Rotor Current should be closely balanced in all three phases.

A less often used scheme is the "Non-Symmetrical" Design, where; in some stages, one Winding may have more or less Resistance than the other 2 Windings.
In this scheme, the Rotor Currents will not balance as closely on the steps which present a non-symmetrical Resistance value.

Check the operation for each step of the Controller, where ever possible.


Note;
The Formulas given are for Motors operating with Currents in the normal operational range of the Motor, and assume that the Core Materials of the Rotor and Stator are in the
Linear Region of the Magnetization Curve, and have not reached saturation.

posted 10/17/2002 by S.E.T.
Edited 11/07/2013 by S.E.T. to fix Spelling, Punctuation, Capital Letter usage, and correct a few "hard to follow" Statements...