Handbook for selection of motors


Handbook for selection of motors

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Attention! Following information only apply for direct current motors of the series BG and GR/G.

DC motors

For many applications, it is sufficiently accurate to take the most important data from the motor characteristic diagrams and data tables. Although tolerances and temperature influences are not taken into account, the data is accurate enough for approximate calculations. The degree of protection quoted relates only to the housing – adequate sealing of the shaft is the responsibility of the customer.

The values in the data tables are according to EN60034 and base on a „detached“ motor, which means mounted thermally insulated to a flange. Under real operating conditions, the nominal torque of the motor is much higher in most cases because a direct connection to the flange results in better heat dissipation.

Nominal voltage UN (VDC)
The DC voltage that is applied to the commutation electronics as a system supply voltage. All rated data in our catalogues are with reference to this voltage. Motor applications are, however,    not restricted to this voltage.

Rated current IN (A)
The current drawn from a DC source when the motor is operating at rated torque. (1)

Rated torque MN (Ncm)
The torque that can be produced by the motor, operating continuously, in an ambient temperature of 20°C. (2)

Rated speed nN (min-1)
The speed of the motor when it is operating at rated torque. (3)

Friction torque MR (Ncm)
Torque threshold that needs to be overcome, in case the non-powered has to be moved (is only specified for G/GR motors – relevant for self-locking)
Holding torque MA (Ncm)
Maximum Torque that is generated at speed 0. (4)
In some cases this is a theoretical value, e.g. if the integrated electronics limits the torque or if the motor is demagnetised at lower currents. Then, the maximum possible Value is specified. (4a)


No-load speed (rpm)
Speed that is reached if the corresponding nominal voltage is applied to the motor without mechanical load. (5)

Rated power PN (W)
The output power which the motor can produce continuously; it is calculated from rated speed and rated torque.

Maximum output power Pmax (W)
Maximum mechanical output power the motor can generate at nominal voltage. This power can only be generated for a limited period.

Torque constant Ra (N/A)
Represents the correlation between input current and output torque.

Connecting resistance Ra (Ω)
Typical ohmic connecting resistance phase to phase (BG motors) or between (+) and (-) (G / GR motors).

Connecting inductance La (mH)
Typical connecting inductance phase to phase (BG motors) or between (+) and (-) (G / GR motors).

Peak current Imax (A)
The maximum current for electronics or motors with integral electronics. (6a)

Starting current Imax (A)
The current required to produce the starting torque. For motors with electronics, the starting current may be higher than the permissible peak current.

Rotor moment of inertia JR (gcm2)
The moment of inertia of the rotor is the factor that determines the dynamic properties of a motor.

Motor weight mM (kg)
Motor weight without gearbox and connecting cable.

Speed curve (blue)
This curve shows the speed characteristic at constant voltage. Its end points are the no-load speed n0 (5) and the theoretical starting torque MA (4).

Current curve (black)
The current curve shows the relationship between current and torque. Its end points are the no-load current I0 (7) and the starting current IA (6).

Efficiency curve (green) η
The efficiency is the relationship between the mechanical power output and the electrical power input. The curve shows the efficiency with the motor in cold condition; as the motor warms up, the curve shifts accordingly.

Rated torque MN; Starting torque Mmax
The rated torque (red) is the limit of the continuous operation region (shaded blue). In the region between the rated torque and the maximum permissible torque, the motor must only be used intermittently (shaded orange). Operating conditions above the maximum permissible torque result in demagnetization of the permanent magnets (shaded red).


The specified torques of the gearboxes with metal housing base on a typical lifetime of 3000 h of effective operation at 3000 rpm input speed and the corresponding specified operating mode. In practice, this value may deviate considerably up or down, depending on temperature, acceleration torque and -time and impact forces from outside, etc.

When sizing a motor gearbox combination, we have to make sure that the specified gearbox torque is not exceeded. This is important for the nominal torque and also for the acceleration torque. In the event that a peek torque occurs, the emergency stop torque ME-Stop must not be acceded.

Applicable is:

MN-Mo x i x ηGe ≤ MN-Ge
MAcc-Mo x i x ηGe ≤ MAcc-Ge

MN-Mo = Nominal Torque of Motor
i = Ratio of Gearbox
ηGe = Efficiency of Gearbox
MN-Ge = Nominal Torque of Gearbox
MAcc-Mo = Acceleration Torque of Motor
MAcc-Ge = Acceleration Torque of Gearbox

Based on the calculations it might be necessary to reduce the motor torque by limiting the motor current in order to bring it into the specified range. Another option is to add a coupling to the output-shaft of the gearbox and therefore make sure the gearbox is not overloaded.

Drive dimensioning
In the wide range of Dunkermotoren products, you will find a suitable drive for almost any requirement in the power range of 1 - 5000 Watt.

The following points should be taken into account when selecting motors and gearboxes:

» Which operating mode is used (continuous operation = S1 or periodic operation = S5)?

» What is the expected working life of the motor?

» Which torque and which speed are required?

» How much space is available for the motor?

» What is the available supply voltage? DC or AC?

» Are there special environmental conditions (temperature, humidity, vibration, ...)?

» To what degree can heat be conducted away from the motor?

» Are there exceptional axial and radial shaft loads to be considered?

» What are the requirements for the motor control electronics?

» Is the motor to be controlled online via a bus system?

» Do you need a brake or an encoder?

For dimensioning a suitable motor, determining the required torque plays a decisive role in order to avoid thermal overload of the motor. For combining a drive system consisting of motor and control electronics, it is important to ensure that permissible values for the motor are not exceeded by the electronics. Depending on the required output speed, either a motor or a motor-gearbox combination may be selected. The choice of a reduction gearbox will largely depend on the recommended maximum torque in continuous operation. For intermittent duty, loading above the rated torque is possible.

We will be pleased to carry out a precise adaptation of a motor to your operating conditions.